. Preface . An Introduction to the Human Body 1. Overview 2. Structural Organization of the Human Body 3. Anatomical Terminology 4. Medical Imaging . Body Tissues 1. Types of Tissues 2. Epithelial Tissue 3. Connective Tissue Supports and Protects 4. Muscle Tissue and Motion 5. Nervous Tissue Mediates Perception and Response 6. Tissue Injury and Aging . Integumentary System 1. Layers of the Skin 2. Accessory Structures of the Skin 3. Diseases, Disorders, and Injuries of the Integumentary System . Bone 1. The Functions of the Skeletal System 2. Bone Classification 3. Bone Structure 4. Bone Formation and Development o. Fractures: Bone Repair . Axial Skeleton 1. Divisions of the Skeletal System 2. The Skull 3. The Vertebral Column 4. The Thoracic Cage . Appendicular Skeleton 1. The Pectoral Girdle 2. Bones of the Upper Limb 3. The Pelvic Girdle and Pelvis 4. Bones of the Lower Limb 8. Articulations 1. Classification of Joints 2. Fibrous Joints 3. Cartilaginous Joints 4. Synovial Joints 5. Types of Body Movements 6. Anatomy of Selected Synovial Joints 9. Skeletal Muscle 1. Overview of Muscle Tissues 2. Skeletal Muscle 3. Types of Muscle Fibers 4. Exercise and Muscle Performance 5. Cardiac Muscle Tissue 6. Smooth Muscle 7. Development and Regeneration of Muscle Tissue 8. Interactions of Skeletal Muscles, Their Fascicle Arrangement, and Their Lever Systems 9. Naming Skeletal Muscles 10. Axial Musculature 1. Muscles of the Head, Neck, and Back 2. Muscles of the Abdominal Wall and Thorax 11. Appendicular Musculature 1. Muscles of the Pectoral Girdle and Upper Limbs 2. Muscles of the Pelvic Girdle and Lower Limbs 12. Heart 1. Heart Anatomy 2. Cardiac Muscle and Electrical Activity 3. Cardiac Cycle 13. Blood Vessels 14. 15. 16. 17. 18. 19. 20. 21; 22. 1. Structure and Function of Blood Vessels 2. Circulatory Pathways Blood 1. An Overview of Blood 2. Production of the Formed Elements 3. Erythrocytes 4. Leukocytes and Platelets Nervous Tissue 1. Basic Structure and Function of the Nervous System 2. Nervous ‘Tissue Central Nervous System 1. Anatomy of the CNS 2. Circulation and the Central Nervous System Peripheral Nervous System 1. Nerves and ganglia Senses 1. Sensory Perception Autonomic Nervous System 1. Divisions of the ANS 2. Central Control Respiratory System 2. The Lungs Digestive System 1. Overview of the Digestive System 3. The Stomach 4. The Small and Large Intestines 5. Accessory Organs in Digestion: The Liver, Pancreas, and Gallbladder Lymphatic System 23. Urinary System 1. 2. 3: Gross Anatomy of Urine Transport Gross Anatomy of the Kidney Microscopic Anatomy of the Kidney 24. Endocrine System 1; . The Pituitary Gland and Hypothalamus . The Thyroid Gland . The Parathyroid Glands . The Adrenal Glands . The Pineal Gland . Gonadal and Placental Hormones . The Endocrine Pancreas 9. CON MU BW NHN An Overview of the Endocrine System Organs with Secondary Endocrine Functions 25. Reproductive System 1. 2. Male Anatomy Female Anatomy Preface Human Anatomy is designed for a semester-long course taken by life science and allied health students. The textbook is derived from OpenStax Human Anatomy and Physiology, and its coverage and organization were informed by hundreds of instructors who teach the course. Instructors can customize the book, adapting it to the approach that works best in their classroom. The artwork for this textbook is aimed focusing student learning through a powerful blend of traditional depictions and instructional innovations. Color is used sparingly, to emphasize the most important aspects of any given illustration. Significant use of micrographs from the University of Michigan complement the illustrations, and provide the students with a meaningful alternate depiction of each concept. Finally, enrichment elements provide relevance and deeper context for students, particularly in the areas of health, disease, and information relevant to their intended careers. Welcome to Human Anatomy, a resource designed for a semester-long course aimed at preparing undergraduate students for health-related programs. This book is derived from Human Anatomy and Physiology by OpenStax College. The source materials were created with several goals in mind: accessibility, customization, and student engagement—helping students reach high levels of academic scholarship. Instructors and students alike will find that this textbook offers a thorough introduction to the content in an accessible format. About OpenStax College OpenStax College is a nonprofit organization committed to improving student access to quality learning materials. Our free textbooks are developed and peer-reviewed by educators to ensure that they are readable, accurate, and organized in accordance with the scope and sequence requirements of today’s college courses. Unlike traditional textbooks, OpenStax College resources live online and are owned by the community of educators using them. Through partnerships with companies and foundations committed to reducing costs for students, we are working to improve access to higher education for all. OpenStax College is an initiative of Rice University and is made possible through the generous support of several philanthropic foundations. About OpenStax College’s Resources OpenStax College resources provide quality academic instruction. Three key features set our materials apart from others: 1) They can be easily customized by instructors for each class, 2) they are “living” resources that grow online through contributions from science educators, and 3) they are available for free or for a minimal cost. Customization OpenStax College learning resources are conceived and written with flexibility in mind so that they can be customized for each course. Our textbooks provide a solid foundation on which instructors can build their own texts. Instructors can select the sections that are most relevant to their curricula and create a textbook that speaks directly to the needs of their students. Instructors are encouraged to expand on existing examples in the text by adding unique context via geographically localized applications and topical connections. Human Anatomy and Physiology can be easily customized using our online platform (https://openstaxcollege.org/textbooks/anatomy-and- physiology/adapt). The text is arranged in a modular chapter format. Simply select the content most relevant to your syllabus and create a textbook that addresses the needs of your class. This customization feature will ensure that your textbook reflects the goals of your course. Curation To broaden access and encourage community curation, Human Anatomy and Physiology is “open source” under a Creative Commons Attribution (CC BY) license. Members of the scientific community are invited to submit examples, emerging research, and other feedback to enhance and strengthen the material, keeping it current and relevant for today’s students. Submit your suggestions to info@openstaxcollege.org, and check in on edition status, alternate versions, errata, and news on the StaxDash at http://openstaxcollege.org. Cost Our textbooks are available for free online, and in low-cost print and tablet editions. Contents 01. 02. 03. 04. 05. 06. 07. 08. 09. Preface An Introduction to the Human Body Body Tissues Integumentary System Bone Axial Skeleton Appendicular Skeleton Articulations Skeletal Muscle 10. Axial Musculature 11. Appendicular Musculature 12. Heart 13. Blood Vessels 14. Blood 15. Nervous Tissue 16. Central Nervous System 17. Peripheral Nervous System 18. Senses 19. Autonomic Nervous System 20. Respiratory System 21. Digestive System 22. Lymphatic System 23. Urinary System 24. Endocrine System 25. Reproductive System About Our Team Customization for One-Semester Human Anatomy Marcos Gridi-Papp University of the Pacific Senior Contributors J. Gordon Betts Tyler Junior College Peter Desaix University of North Carolina at Chapel Hill Eddie Johnson Central Oregon Community College Jody E. Johnson Arapahoe Community College Oksana Korol Aims Community College Dean Kruse Portland Community College Brandon Poe Springfield Technical Community College James A. Wise Hampton University Mark Womble Youngstown State University Kelly A. Young California State University, Long Beach Advisor Robin J. Heyden Other Contributors Kim Aquarius Institute; Triton College Aaronson Lopamudra Augusta Technical College Agarwal Gary Allen Dalhousie University Robert i ‘allison McLennan Community College Heather Southern Union State Community College Armbruster Timothy University of North Carolina Wilmington Ballard Matthew Eastern New Mexico University Barlow William Furman Universit Blaker Julie Bowers East Tennessee State University Emily Florida Southern College Bradshaw Nishi Bryska University of North Carolina, Charlotte Susan Caley Opsal Boyd Campbell Ann Caplea Marnie Chapman Barbara Christie-Pope Kenneth Crane Maurice Culver Heather Cushman Noelle Cutter Lynnette Danzl-Tauer Jane Davis AnnMarie DelliPizzi Susan Dentel Pamela Dobbins Illinois Valley Community College Southwest College of Naturopathic Medicine and Health Sciences Walsh University University of Alaska, Sitka Cornell College Texarkana College Florida State College at Jacksonville Tacoma Community College Molloy College Rock Valley College Aurora University Dominican College Washtenaw Community College Shelton State Community College Patty Dolan Sondra Dubowsky Peter Dukehart Ellen DuPré Elizabeth DuPriest Pam Elf Sharon Ellerton Carla Endres Myriam Feldman Greg Fitch Lynn Gargan Michael Giangrande Chaya Gopalan Victor Greco Susanna Pacific Lutheran University McLennan Community College Three Rivers Community College Central College Warner Pacific College University of Minnesota Queensborough Community College Utah State University - College of Eastern Utah: San Juan Campus Lake Washington Institute of Technology; Cascadia Community College Avila University Tarant County College Oakland Community College St. Louis College of Pharmacy Chattahoochee Technical College Skagit Valley College Heinze Ann Henninger Dale Horeth Michael Hortsch Rosemary Hubbard Mark Hubley Branko Jablanovic Norman Johnson Mark Jonasson Jeff Keyte William Kleinelp Leigh Kleinert Brenda Leady John Lepri Sarah Leupen Wartburg College Tidewater Community College University of Michigan Marymount University Prince George's Community College College of Lake County University of Massachusetts Amherst North Arkansas College College of Saint Mary Middlesex County College Grand Rapids Community College University of Toledo University of North Carolina, Greensboro University of Maryland, Baltimore County Lihua Liang Robert Mallet Bruce Maring Elisabeth Martin Natalie Maxwell Julie May Debra McLaughlin Nicholas Mitchell Shobhana Natarajan Phillip Nicotera Mary Jane Niles Ikemefuna Nwosu Betsy Ott Ivan Paul Aaron Payette Johns Hopkins University University of North Texas Health Science Center Daytona State College College of Lake County Carl Albert State College, Sallisaw William Carey University University of Maryland University College St. Bonaventure University Brookhaven College St. Petersburg College University of San Francisco Parkland College; Lake Land College Tyler Junior College John Wood Community College College of Southern Nevada Scott Payne Cameron Perkins David Pfeiffer Thomas Pilat Eileen Preston Mike Pyle Robert Rawding Jason Schreer Laird Sheldahl Brian Shmaefsky Douglas Sizemore Susan Spencer Cynthia Standley Robert Sullivan Kentucky Wesleyan College South Georgia College University of Alaska, Anchorage Illinois Central College Tarrant County College Olivet Nazarene University Gannon University State University of New York at Potsdam Mt. Hood Community College Lone Star College System Bevill State Community College Mount Hood Community College University of Arizona Marist College Eric Sun Tom Swenson Kathleen Tallman Rohinton Tarapore Elizabeth Tattersall Mark Thomas Janis Thompson Rita Thrasher David Van Wylen Lynn Wandrey Margaret Weck Kathleen Weiss Neil Westergaard David Wortham Middle Georgia State College Ithaca College Azusa Pacific University University of Pennsylvania Western Nevada College University of Northern Colorado Lorain County Community College Pensacola State College St. Olaf College Mott Community College St. Louis College of Pharmacy George Fox University Williston State College West Georgia Technical College Umesh Yadav Tony Yates Justin York Cheri Zao Elena Zoubina Shobhana Natarajan Special Thanks University of Texas Medical Branch Oklahoma Baptist University Glendale Community College North Idaho College Bridgewater State University; Massasoit Community College Alcon Laboratories, Inc. OpenStax College wishes to thank the Regents of University of Michigan Medical School for the use of their extensive micrograph collection. Many of the UM micrographs that appear in Human Anatomy and Physiology are interactive WebScopes, which students can explore by zooming in and out. We also wish to thank the Open Learning Initiative at Carnegie Mellon University, with whom we shared and exchanged resources during the development of Human Anatomy and Physiology. Overview By the end of this section, you will be able to: ¢ Compare and contrast anatomy and physiology, including their specializations and methods of study e Discuss the fundamental relationship between anatomy and physiology Human anatomy is the scientific study of the body’s structures. Some of these structures are very small and can only be observed and analyzed with the assistance of a microscope. Other larger structures can readily be seen, manipulated, measured, and weighed. The word “anatomy” comes from a Greek root that means “to cut apart.” Human anatomy was first studied by observing the exterior of the body and observing the wounds of soldiers and other injuries. Later, physicians were allowed to dissect bodies of the dead to augment their knowledge. When a body is dissected, its structures are cut apart in order to observe their physical attributes and their relationships to one another. Dissection is still used in medical schools, anatomy courses, and in pathology labs. In order to observe structures in living people, however, a number of imaging techniques have been developed. These techniques allow clinicians to visualize structures inside the living body such as a cancerous tumor or a fractured bone. Like most scientific disciplines, anatomy has areas of specialization. Gross anatomy is the study of the larger structures of the body, those visible without the aid of magnification ([link]a). Macro- means “large,” thus, gross anatomy is also referred to as macroscopic anatomy. In contrast, micro- means “small,” and microscopic anatomy is the study of structures that can be observed only with the use of a microscope or other magnification devices ({link]b). Microscopic anatomy includes cytology, the study of cells and histology, the study of tissues. As the technology of microscopes has advanced, anatomists have been able to observe smaller and smaller structures of the body, from slices of large structures like the heart, to the three-dimensional structures of large molecules in the body. Gross and Microscopic Anatomy (a) Gross anatomy considers large structures such as the brain. (b) Microscopic anatomy can deal with the same structures, though at a different scale. This is a micrograph of nerve cells from the brain. LM x 1600. (credit a: “WriterHound”/Wikimedia Commons; credit b: Micrograph provided by the Regents of University of Michigan Medical School © 2012) Anatomists take two general approaches to the study of the body’s structures: regional and systemic. Regional anatomy is the study of the interrelationships of all of the structures in a specific body region, such as the abdomen. Studying regional anatomy helps us appreciate the interrelationships of body structures, such as how muscles, nerves, blood vessels, and other structures work together to serve a particular body region. In contrast, systemic anatomy is the study of the structures that make up a discrete body system—that is, a group of structures that work together to perform a unique body function. For example, a systemic anatomical study of the muscular system would consider all of the skeletal muscles of the body. Whereas anatomy is about structure, physiology is about function. Human physiology is the scientific study of the chemistry and physics of the structures of the body and the ways in which they work together to support the functions of life. Much of the study of physiology centers on the body’s tendency toward homeostasis. Homeostasis is the state of steady internal conditions maintained by living things. The study of physiology certainly includes observation, both with the naked eye and with microscopes, as well as manipulations and measurements. However, current advances in physiology usually depend on carefully designed laboratory experiments that reveal the functions of the many structures and chemical compounds that make up the human body. Like anatomists, physiologists typically specialize in a particular branch of physiology. For example, neurophysiology is the study of the brain, spinal cord, and nerves and how these work together to perform functions as complex and diverse as vision, movement, and thinking. Physiologists may work from the organ level (exploring, for example, what different parts of the brain do) to the molecular level (such as exploring how an electrochemical signal travels along nerves). Form is closely related to function in all living things. For example, the thin flap of your eyelid can snap down to clear away dust particles and almost instantaneously slide back up to allow you to see again. At the microscopic level, the arrangement and function of the nerves and muscles that serve the eyelid allow for its quick action and retreat. At a smaller level of analysis, the function of these nerves and muscles likewise relies on the interactions of specific molecules and ions. Even the three-dimensional structure of certain molecules is essential to their function. Your study of anatomy and physiology will make more sense if you continually relate the form of the structures you are studying to their function. In fact, it can be somewhat frustrating to attempt to study anatomy without an understanding of the physiology that a body structure supports. Imagine, for example, trying to appreciate the unique arrangement of the bones of the human hand if you had no conception of the function of the hand. Fortunately, your understanding of how the human hand manipulates tools—from pens to cell phones—helps you appreciate the unique alignment of the thumb in opposition to the four fingers, making your hand a structure that allows you to pinch and grasp objects and type text messages. Chapter Review Human anatomy is the scientific study of the body’s structures. In the past, anatomy has primarily been studied via observing injuries, and later by the dissection of anatomical structures of cadavers, but in the past century, computer-assisted imaging techniques have allowed clinicians to look inside the living body. Human physiology is the scientific study of the chemistry and physics of the structures of the body. Physiology explains how the structures of the body work together to maintain life. It is difficult to study structure (anatomy) without knowledge of function (physiology). The two disciplines are typically studied together because form and function are closely related in all living things. Review Questions Exercise: Problem: Which of the following specialties might focus on studying all of the structures of the ankle and foot? a. microscopic anatomy b. muscle anatomy c. regional anatomy d. systemic anatomy Solution: C Exercise: Problem: A scientist wants to study how the body uses foods and fluids during a marathon run. This scientist is most likely a(n) a. exercise physiologist b. microscopic anatomist c. regional physiologist d. systemic anatomist Solution: A CRITICAL THINKING QUESTIONS Exercise: Problem: Name at least three reasons to study anatomy and physiology. Solution: An understanding of anatomy and physiology is essential for any career in the health professions. It can also help you make choices that promote your health, respond appropriately to signs of illness, make sense of health-related news, and help you in your roles as a parent, spouse, partner, friend, colleague, and caregiver. Exercise: Problem: For whom would an appreciation of the structural characteristics of the human heart come more easily: an alien who lands on Earth, abducts a human, and dissects his heart, or an anatomy and physiology student performing a dissection of the heart on her very first day of class? Why? Solution: A student would more readily appreciate the structures revealed in the dissection. Even though the student has not yet studied the workings of the heart and blood vessels in her class, she has experienced her heart beating every moment of her life, has probably felt her pulse, and likely has at least a basic understanding of the role of the heart in pumping blood throughout her body. This understanding of the heart’s function (physiology) would support her study of the heart’s form (anatomy). Glossary anatomy science that studies the form and composition of the body’s structures gross anatomy study of the larger structures of the body, typically with the unaided eye; also referred to macroscopic anatomy homeostasis steady state of body systems that living organisms maintain microscopic anatomy study of very small structures of the body using magnification physiology science that studies the chemistry, biochemistry, and physics of the body’s functions regional anatomy study of the structures that contribute to specific body regions systemic anatomy study of the structures that contribute to specific body systems Structural Organization of the Human Body By the end of this section, you will be able to: ¢ Describe the structure of the human body in terms of six levels of organization e List the eleven organ systems of the human body and identify at least one organ and one major function of each Before you begin to study the different structures and functions of the human body, it is helpful to consider its basic architecture; that is, how its smallest parts are assembled into larger structures. It is convenient to consider the structures of the body in terms of fundamental levels of organization that increase in complexity: subatomic particles, atoms, molecules, organelles, cells, tissues, organs, organ systems, organisms and biosphere ({link]). Levels of Structural Organization of the Human Body Oxygen atom Chemical level Atoms bond to form molecules with three- dimensional structures. Jd Water molecule Smooth muscle cell Cellular level A variety of molecules combine to form the fluid and organelles of a body cell. Organelle Cell fluid Organelle Smooth muscle tissue Tissue level A community of similar cells form a body tissue. Bladder Organ level Two or more different tissues combine to form an organ. Skeletal muscle Urinary tract system Kidney Organ system level Ureter Two or more organs work closely together to perform he functi \f . Bladder the functions of a body system. A, Ureth ra 1" Organismal level Many organ system work harmoniously together to perform the functions of an J independent organism. fo The organization of the body often is discussed in terms of six distinct levels of increasing complexity, from the smallest chemical building blocks to a unique human organism. The Levels of Organization To study the chemical level of organization, scientists consider the simplest building blocks of matter: subatomic particles, atoms and molecules. All matter in the universe is composed of one or more unique pure substances called elements, familiar examples of which are hydrogen, oxygen, carbon, nitrogen, calcium, and iron. The smallest unit of any of these pure substances (elements) is an atom. Atoms are made up of subatomic particles such as the proton, electron and neutron. Two or more atoms combine to form a molecule, such as the water molecules, proteins, and sugars found in living things. Molecules are the chemical building blocks of all body structures. A cell is the smallest independently functioning unit of a living organism. Even bacteria, which are extremely small, independently-living organisms, have a cellular structure. Each bacterium is a single cell. All living structures of human anatomy contain cells, and almost all functions of human physiology are performed in cells or are initiated by cells. A human cell typically consists of flexible membranes that enclose cytoplasm, a water-based cellular fluid together with a variety of tiny functioning units called organelles. In humans, as in all organisms, cells perform all functions of life. A tissue is a group of many similar cells (though sometimes composed of a few related types) that work together to perform a specific function. An organ is an anatomically distinct structure of the body composed of two or more tissue types. Each organ performs one or more specific physiological functions. An organ system is a group of organs that work together to perform major functions or meet physiological needs of the body. This book covers eleven distinct organ systems in the human body ([Link] and [link]). Assigning organs to organ systems can be imprecise since organs that “belong” to one system can also have functions integral to another system. In fact, most organs contribute to more than one system. Organ Systems of the Human Body Integumentary System Skeletal System ¢ Encloses internal * Supports the body body structures ¢ Enables movement * Site of many (with muscular sensory receptors system) Muscular System Nervous System ¢ Enables movement * Detects and (with skeletal system) processes sensory ¢ Helps maintain body information temperature * Activates bodily responses Skeletal muscles Endocrine System Cardiovascular System Pituitary gland * Secretes hormones * Delivers oxygen . ¢ Regulates bodily and nutrients to Thyroid processes tissues gland * Equalizes temperature in the body Pancreas Adrenal glands Blood vessels Ovaries Organs that work together are grouped into organ systems. Organ Systems of the Human Body (continued) Lymphatic System ¢ Returns fluid to blood ¢ Defends against pathogens Digestive System ¢ Processes food for use by the body « Removes wastes from undigested food Stomach Liver Gall bladder Large intestine Small intestine Male Reproductive System ¢ Produces sex hormones and gametes * Delivers gametes to female Epididymis Respiratory System * Removes carbon dioxide from the body * Delivers oxygen to blood Nasal passage Trachea Lungs Urinary System * Controls water balance in the body « Removes wastes from blood and Kidneys excretes them Urinary bladder Female Reproductive System * Produces sex Mammary hormones glands and gametes * Supports embryo/ fetus until birth * Produces milk for : infant Ovaries Organs that work together are grouped into organ systems. The organism level is the highest level of organization. An organism is a living being that has a cellular structure and that can independently perform all physiologic functions necessary for life. In multicellular organisms, including humans, all cells, tissues, organs, and organ systems of the body work together to maintain the life and health of the organism. Chapter Review Life processes of the human body are maintained at several levels of structural organization. These include the chemical, cellular, tissue, organ, organ system, and the organism level. Higher levels of organization are built from lower levels. Therefore, molecules combine to form cells, cells combine to form tissues, tissues combine to form organs, organs combine to form organ systems, and organ systems combine to form organisms. Review Questions Exercise: Problem: The smallest independently functioning unit of an organism is a(n) a. cell b. molecule c. organ d. tissue Solution: A Exercise: Problem: A collection of similar tissues that performs a specific function is an a. organ b. organelle c. organism d. organ system Solution: A Exercise: Problem: The body system responsible for structural support and movement is the a. cardiovascular system b. endocrine system c. muscular system d. skeletal system Solution: D CRITICAL THINKING QUESTIONS Exercise: Problem:Name the six levels of organization of the human body. Solution: Chemical, cellular, tissue, organ, organ system, organism. Exercise: Problem: The female ovaries and the male testes are a part of which body system? Can these organs be members of more than one organ system? Why or why not? Solution: The female ovaries and the male testes are parts of the reproductive system. But they also secrete hormones, as does the endocrine system, therefore ovaries and testes function within both the endocrine and reproductive systems. Glossary cell smallest independently functioning unit of all organisms; in animals, a cell contains cytoplasm, composed of fluid and organelles organ functionally distinct structure composed of two or more types of tissues organ system group of organs that work together to carry out a particular function organism living being that has a cellular structure and that can independently perform all physiologic functions necessary for life tissue group of similar or closely related cells that act together to perform a specific function Anatomical Terminology By the end of this section, you will be able to: e Demonstrate the anatomical position e Describe the human body using directional and regional terms e Identify three planes most commonly used in the study of anatomy e Distinguish between the posterior (dorsal) and the anterior (ventral) body cavities, identifying their subdivisions and representative organs found in each e Describe serous membrane and explain its function Anatomists and health care providers use terminology that can be bewildering to the uninitiated. However, the purpose of this language is not to confuse, but rather to increase precision and reduce medical errors. For example, is a scar “above the wrist” located on the forearm two or three inches away from the hand? Or is it at the base of the hand? Is it on the palm-side or back-side? By using precise anatomical terminology, we eliminate ambiguity. Anatomical terms derive from ancient Greek and Latin words. Because these languages are no longer used in everyday conversation, the meaning of their words does not change. Anatomical terms are made up of roots, prefixes, and suffixes. The root of a term often refers to an organ, tissue, or condition, whereas the prefix or suffix often describes the root. For example, in the disorder hypertension, the prefix “hyper-” means “high” or “over,” and the root word “tension” refers to pressure, so the word “hypertension” refers to abnormally high blood pressure. Anatomical Position To further increase precision, anatomists standardize the way in which they view the body. Just as maps are normally oriented with north at the top, the standard body “map,” or anatomical position, is that of the body standing upright, with the feet at shoulder width and parallel, toes forward. The upper limbs are held out to each side, and the palms of the hands face forward as illustrated in [link]. Using this standard position reduces confusion. It does not matter how the body being described is oriented, the terms are used as if it is in anatomical position. For example, a scar in the “anterior (front) carpal (wrist) region” would be present on the palm side of the wrist. The term “anterior” would be used even if the hand were palm down on a table. Regions of the Human Body Frons or forehead (frontal). Oculus or eye (orbital or ocular) Cranium or skull (cranial) Bucca or cheek (buccal) Cephalon or head Shoulder (cephalic) (acromial) Facies or face (facial) Auris or ear (otic) Nasus or nose (nasal) Dorsum or back F Cervicis or neck Oris or mouth (oral) Cervicis or neck (cervical) Mentis or chin (dorsal) (mental) ; : Seaeun Axilla or armpit (thoracic) (brachial) (axillary) Mamma Olecranon . or breast or back bslhoral o (mammary) of elbow arm (brachial) ‘abdanner Trunk (olecranal) Antecubitis (abdominal) Lumbus or front of elbow or loin (antecubital) Umbilicus (lumbar) Upper or navel limb Antebrachium Sacrum (umbilical) or forearm (antebrachial) Carpus or wrist (carpal) Pollex Pelvis or thumb (pelvic) (manual) Palma or Inguen or groin Gluteus palm (palmar) ite inal) 9g sbatiock Digits (phalanges) (gluteal) ig or fingers (digital or phalangeal) aie or thigh Patella or (pubic) tone) ineecap Femur Poplit atellar) A opliteus or P ) . Naty back of knee Crus or (femoral) (popliteal) leg (crural) Sura or calf Tarsus (sural) or ankle (tarsal) Calcaneus or heel of foot Digits (phalanges) (calacaneal) or toes (digital or Pes or foot phalangeal) (pedal) Planta or sole ~ of foot (plantar) (a) Anterior view Antebrachium or forearm (antebrachial) (sacral) Manus or hand (b) Posterior view The human body is shown in anatomical position in an (a) anterior view and a (b) posterior view. The regions of the body are labeled in boldface. A body that is lying down is described as either prone or supine. Prone describes a face-down orientation, and supine describes a face up orientation. These terms are sometimes used in describing the position of the body during specific physical examinations or surgical procedures. Regional Terms The human body’s numerous regions have specific terms to help increase precision (see [link]). Notice that the term “brachium” or “arm” is reserved for the “upper arm” and “antebrachium” or “forearm” is used rather than “lower arm.” Similarly, “femur” or “thigh” is correct, and “leg” or “crus” is reserved for the portion of the lower limb between the knee and the ankle. You will be able to describe the body’s regions using the terms from the figure. Directional Terms Certain directional anatomical terms appear throughout this and any other anatomy textbook ({link]). These terms are essential for describing the relative locations of different body structures. For instance, an anatomist might describe one band of tissue as “inferior to” another or a physician might describe a tumor as “superficial to” a deeper body structure. Commit these terms to memory to avoid confusion when you are studying or describing the locations of particular body parts. e Anterior (or ventral) Describes the front or direction toward the front of the body. The toes are anterior to the foot. ¢ Posterior (or dorsal) Describes the back or direction toward the back of the body. The popliteus is posterior to the patella. e Superior (or cranial) describes a position above or higher than another part of the body proper. The orbits are superior to the oris. ¢ Inferior (or caudal) describes a position below or lower than another part of the body proper; near or toward the tail (in humans, the coccyx, or lowest part of the spinal column). The pelvis is inferior to the abdomen. e Lateral describes the side or direction toward the side of the body. The thumb (pollex) is lateral to the digits. e Medial describes the middle or direction toward the middle of the body. The hallux is the medial toe. ¢ Proximal describes a position in a limb that is nearer to the point of attachment or the trunk of the body. The brachium is proximal to the antebrachium. ¢ Distal describes a position in a limb that is farther from the point of attachment or the trunk of the body. The crus is distal to the femur. e Superficial describes a position closer to the surface of the body. The skin is superficial to the bones. ¢ Deep describes a position farther from the surface of the body. The brain is deep to the skull. Directional Terms Applied to the Human Body Superior Cranial -> Anterior or ventral Posterior +-7---- or dorsal Caudal Inferior Paired directional terms are shown as applied to the human body. Body Planes A section is a two-dimensional surface of a three-dimensional structure that has been cut. Modern medical imaging devices enable clinicians to obtain “virtual sections” of living bodies. We call these scans. Body sections and scans can be correctly interpreted, however, only if the viewer understands the plane along which the section was made. A plane is an imaginary two- dimensional surface that passes through the body. There are three planes commonly referred to in anatomy and medicine, as illustrated in [link]. e The sagittal plane is the plane that divides the body or an organ vertically into right and left sides. If this vertical plane runs directly down the middle of the body, it is called the midsagittal or median plane. If it divides the body into unequal right and left sides, it is called a parasagittal plane or less commonly a longitudinal section. e The frontal plane is the plane that divides the body or an organ into an anterior (front) portion and a posterior (rear) portion. The frontal plane is often referred to as a coronal plane. (“Corona” is Latin for “crown.”) e The transverse plane is the plane that divides the body or organ horizontally into upper and lower portions. Transverse planes produce images referred to as cross sections. Planes of the Body Frontal (coronal) plane Transverse The three planes most commonly used in anatomical and medical imaging are the sagittal, frontal (or coronal), and transverse plane. Body Cavities and Serous Membranes The body maintains its internal organization by means of membranes, sheaths, and other structures that separate compartments. The dorsal (posterior) cavity and the ventral (anterior) cavity are the largest body compartments ({link]). These cavities contain and protect delicate internal organs, and the ventral cavity allows for significant changes in the size and shape of the organs as they perform their functions. The lungs, heart, stomach, and intestines, for example, can expand and contract without distorting other tissues or disrupting the activity of nearby organs. Dorsal and Ventral Body Cavities Cranial cavity Vertebral cavity Thoracic cavity: Superior mediastinum Pleural cavity Pericardial cavity within the mediastinum Diaphragm Vertebral cavity Ventral body cavity (both thoracic and i i — 7 abdominopelvic Abdominal cavity Abdomino- | cavities) pelvic cavity Pelvic cavity Lateral view Anterior view The ventral cavity includes the thoracic and abdominopelvic cavities and their subdivisions. The dorsal cavity includes the cranial and spinal cavities. Subdivisions of the Posterior (Dorsal) and Anterior (Ventral) Cavities The posterior (dorsal) and anterior (ventral) cavities are each subdivided into smaller cavities. In the posterior (dorsal) cavity, the cranial cavity houses the brain, and the spinal cavity (or vertebral cavity) encloses the spinal cord. Just as the brain and spinal cord make up a continuous, uninterrupted structure, the cranial and spinal cavities that house them are also continuous. The brain and spinal cord are protected by the bones of the skull and vertebral column and by cerebrospinal fluid, a colorless fluid produced by the brain, which cushions the brain and spinal cord within the posterior (dorsal) cavity. The anterior (ventral) cavity has two main subdivisions: the thoracic cavity and the abdominopelvic cavity (see [link]). The thoracic cavity is the more superior subdivision of the anterior cavity, and it is enclosed by the rib cage. The thoracic cavity contains the lungs and the heart, which is located in the mediastinum. The diaphragm forms the floor of the thoracic cavity and separates it from the more inferior abdominopelvic cavity. The abdominopelvic cavity is the largest cavity in the body. Although no membrane physically divides the abdominopelvic cavity, it can be useful to distinguish between the abdominal cavity, the division that houses the digestive organs, and the pelvic cavity, the division that houses the organs of reproduction. Abdominal Regions and Quadrants To promote clear communication, for instance about the location of a patient’s abdominal pain or a suspicious mass, health care providers typically divide up the cavity into either nine regions or four quadrants ({link]). Regions and Quadrants of the Peritoneal Cavity (a) Abdominopelvic regions (b) Abdominopelvic quandrants There are (a) nine abdominal regions and (b) four abdominal quadrants in the peritoneal cavity. The more detailed regional approach subdivides the cavity with one horizontal line immediately inferior to the ribs and one immediately superior to the pelvis, and two vertical lines drawn as if dropped from the midpoint of each clavicle (collarbone). There are nine resulting regions. The simpler quadrants approach, which is more commonly used in medicine, subdivides the cavity with one horizontal and one vertical line that intersect at the patient’s umbilicus (navel). Membranes of the Anterior (Ventral) Body Cavity A serous membrane (also referred to a serosa) is one of the thin membranes that cover the walls and organs in the thoracic and abdominopelvic cavities. The parietal layers of the membranes line the walls of the body cavity (pariet- refers to a cavity wall). The visceral layer of the membrane covers the organs (the viscera). Between the parietal and visceral layers is a very thin, fluid-filled serous space, or cavity ([link]). Serous Membrane Visceral pericardium Pericardial cavity Parietal pericardium \/— Air space Balloon Serous membrane lines the pericardial cavity and reflects back to cover the heart—much the same way that an underinflated balloon would form two layers surrounding a fist. There are three serous cavities and their associated membranes. The pleura is the serous membrane that encloses the pleural cavity; the pleural cavity surrounds the lungs. The pericardium is the serous membrane that encloses the pericardial cavity; the pericardial cavity surrounds the heart. The peritoneum is the serous membrane that encloses the peritoneal cavity; the peritoneal cavity surrounds several organs in the abdominopelvic cavity. The serous membranes form fluid-filled sacs, or cavities, that are meant to cushion and reduce friction on internal organs when they move, such as when the lungs inflate or the heart beats. Both the parietal and visceral serosa secrete the thin, slippery serous fluid located within the serous cavities. The pleural cavity reduces friction between the lungs and the body wall. Likewise, the pericardial cavity reduces friction between the heart and the wall of the pericardium. The peritoneal cavity reduces friction between the abdominal and pelvic organs and the body wall. Therefore, serous membranes provide additional protection to the viscera they enclose by reducing friction that could lead to inflammation of the organs. Chapter Review Ancient Greek and Latin words are used to build anatomical terms. A standard reference position for mapping the body’s structures is the normal anatomical position. Regions of the body are identified using terms such as “occipital” that are more precise than common words and phrases such as “the back of the head.” Directional terms such as anterior and posterior are essential for accurately describing the relative locations of body structures. Images of the body’s interior commonly align along one of three planes: the sagittal, frontal, or transverse. The body’s organs are organized in one of two main cavities—dorsal (also referred to posterior) and ventral (also referred to anterior)—which are further sub-divided according to the structures present in each area. The serous membranes have two layers— parietal and visceral—surrounding a fluid filled space. Serous membranes cover the lungs (pleural serosa), heart (pericardial serosa), and some abdominopelvic organs (peritoneal serosa). Review Chapter Exercise: Problem: What is the position of the body when it is in the “normal anatomical position?” a. The person is prone with upper limbs, including palms, touching sides and lower limbs touching at sides. b. The person is standing facing the observer, with upper limbs extended out at a ninety-degree angle from the torso and lower limbs in a wide stance with feet pointing laterally c. The person is supine with upper limbs, including palms, touching sides and lower limbs touching at sides. d. None of the above Solution: D Exercise: Problem: To make a banana split, you halve a banana into two long, thin, right and left sides along the a. coronal plane b. longitudinal plane c. midsagittal plane d. transverse plane Solution: C Exercise: Problem: The lumbar region is a. inferior to the gluteal region b. inferior to the umbilical region c. superior to the cervical region d. superior to the popliteal region Solution: D Exercise: Problem: The heart is within the a. Cranial cavity b. mediastinum c. posterior (dorsal) cavity d. All of the above Solution: B Critical Thinking Question Exercise: Problem: In which direction would an MRI scanner move to produce sequential images of the body in the frontal plane, and in which direction would an MRI scanner move to produce sequential images of the body in the sagittal plane? Solution: If the body were supine or prone, the MRI scanner would move from top to bottom to produce frontal sections, which would divide the body into anterior and posterior portions, as in “cutting” a deck of cards. Again, if the body were supine or prone, to produce sagittal sections, the scanner would move from left to right or from right to left to divide the body lengthwise into left and right portions. Exercise: Problem: If a bullet were to penetrate a lung, which three anterior thoracic body cavities would it enter, and which layer of the serous membrane would it encounter first? Solution: The bullet would enter the ventral, thoracic, and pleural cavities, and it would encounter the parietal layer of serous membrane first. Glossary abdominopelvic cavity division of the anterior (ventral) cavity that houses the abdominal and pelvic viscera anatomical position standard reference position used for describing locations and directions on the human body anterior describes the front or direction toward the front of the body; also referred to as ventral anterior cavity larger body cavity located anterior to the posterior (dorsal) body cavity; includes the serous membrane-lined pleural cavities for the lungs, pericardial cavity for the heart, and peritoneal cavity for the abdominal and pelvic organs; also referred to as ventral cavity caudal describes a position below or lower than another part of the body proper; near or toward the tail (in humans, the coccyx, or lowest part of the spinal column); also referred to as inferior cranial describes a position above or higher than another part of the body proper; also referred to as superior cranial cavity division of the posterior (dorsal) cavity that houses the brain deep describes a position farther from the surface of the body distal describes a position farther from the point of attachment or the trunk of the body dorsal describes the back or direction toward the back of the body; also referred to as posterior dorsal cavity posterior body cavity that houses the brain and spinal cord; also referred to the posterior body cavity frontal plane two-dimensional, vertical plane that divides the body or organ into anterior and posterior portions inferior describes a position below or lower than another part of the body proper; near or toward the tail (in humans, the coccyx, or lowest part of the spinal column); also referred to as caudal lateral describes the side or direction toward the side of the body medial describes the middle or direction toward the middle of the body pericardium sac that encloses the heart peritoneum serous membrane that lines the abdominopelvic cavity and covers the organs found there plane imaginary two-dimensional surface that passes through the body pleura serous membrane that lines the pleural cavity and covers the lungs posterior describes the back or direction toward the back of the body; also referred to as dorsal posterior cavity posterior body cavity that houses the brain and spinal cord; also referred to as dorsal cavity prone face down proximal describes a position nearer to the point of attachment or the trunk of the body sagittal plane two-dimensional, vertical plane that divides the body or organ into right and left sides section in anatomy, a single flat surface of a three-dimensional structure that has been cut through serous Membrane membrane that covers organs and reduces friction; also referred to as serosa serosa membrane that covers organs and reduces friction; also referred to as serous Membrane spinal cavity division of the dorsal cavity that houses the spinal cord; also referred to as vertebral cavity superficial describes a position nearer to the surface of the body superior describes a position above or higher than another part of the body proper; also referred to as cranial supine face up thoracic cavity division of the anterior (ventral) cavity that houses the heart, lungs, esophagus, and trachea transverse plane two-dimensional, horizontal plane that divides the body or organ into superior and inferior portions ventral describes the front or direction toward the front of the body; also referred to as anterior ventral cavity larger body cavity located anterior to the posterior (dorsal) body cavity; includes the serous membrane-lined pleural cavities for the lungs, pericardial cavity for the heart, and peritoneal cavity for the abdominal and pelvic organs; also referred to as anterior body cavity Medical Imaging By the end of this section, you will be able to: e Discuss the uses and drawbacks of X-ray imaging e Identify four modern medical imaging techniques and how they are used For thousands of years, fear of the dead and legal sanctions limited the ability of anatomists and physicians to study the internal structures of the human body. An inability to control bleeding, infection, and pain made surgeries infrequent, and those that were performed—such as wound suturing, amputations, tooth and tumor removals, skull drilling, and cesarean births—did not greatly advance knowledge about internal anatomy. Theories about the function of the body and about disease were therefore largely based on external observations and imagination. During the fourteenth and fifteenth centuries, however, the detailed anatomical drawings of Italian artist and anatomist Leonardo da Vinci and Flemish anatomist Andreas Vesalius were published, and interest in human anatomy began to increase. Medical schools began to teach anatomy using human dissection; although some resorted to grave robbing to obtain corpses. Laws were eventually passed that enabled students to dissect the corpses of criminals and those who donated their bodies for research. Still, it was not until the late nineteenth century that medical researchers discovered non- surgical methods to look inside the living body. X-Rays German physicist Wilhelm R6éntgen (1845-1923) was experimenting with electrical current when he discovered that a mysterious and invisible “ray” would pass through his flesh but leave an outline of his bones on a screen coated with a metal compound. In 1895, R6ntgen made the first durable record of the internal parts of a living human: an “X-ray” image (as it came to be called) of his wife’s hand. Scientists around the world quickly began their own experiments with X-rays, and by 1900, X-rays were widely used to detect a variety of injuries and diseases. In 1901, R6ntgen was awarded the first Nobel Prize for physics for his work in this field. The X-ray is a form of high energy electromagnetic radiation with a short wavelength capable of penetrating solids and ionizing gases. As they are used in medicine, X-rays are emitted from an X-ray machine and directed toward a specially treated metallic plate placed behind the patient’s body. The beam of radiation results in darkening of the X-ray plate. X-rays are slightly impeded by soft tissues, which show up as gray on the X-ray plate, whereas hard tissues, such as bone, largely block the rays, producing a light-toned “shadow.” Thus, X-rays are best used to visualize hard body structures such as teeth and bones ({link]). Like many forms of high energy radiation, however, X-rays are capable of damaging cells and initiating changes that can lead to cancer. This danger of excessive exposure to X- rays was not fully appreciated for many years after their widespread use. X-Ray of a Hand High energy electromagnetic radiation allows the internal structures of the body, such as bones, to be seen in X- rays like these. (credit: Trace Meek/flickr) Refinements and enhancements of X-ray techniques have continued throughout the twentieth and twenty-first centuries. Although often supplanted by more sophisticated imaging techniques, the X-ray remains a “workhorse” in medical imaging, especially for viewing fractures and for dentistry. The disadvantage of irradiation to the patient and the operator is now attenuated by proper shielding and by limiting exposure. Modern Medical Imaging X-rays can depict a two-dimensional image of a body region, and only from a single angle. In contrast, more recent medical imaging technologies produce data that is integrated and analyzed by computers to produce three- dimensional images or images that reveal aspects of body functioning. Computed Tomography Tomography refers to imaging by sections. Computed tomography (CT) is a noninvasive imaging technique that uses computers to analyze several cross-sectional X-rays in order to reveal minute details about structures in the body ({link]a). The technique was invented in the 1970s and is based on the principle that, as X-rays pass through the body, they are absorbed or reflected at different levels. In the technique, a patient lies on a motorized platform while a computerized axial tomography (CAT) scanner rotates 360 degrees around the patient, taking X-ray images. A computer combines these images into a two-dimensional view of the scanned area, or “slice.” Medical Imaging Techniques (a) The results of a CT scan of the head are shown as successive transverse sections. (b) An MRI machine generates a magnetic field around a patient. (c) PET scans use radiopharmaceuticals to create images of active blood flow and physiologic activity of the organ or organs being targeted. (d) Ultrasound technology is used to monitor pregnancies because it is the least invasive of imaging techniques and uses no electromagnetic radiation. (credit a: Akira Ohgaki/flickr; credit b: “Digital Cate”/flickr; credit c: “Raziel”/Wikimedia Commons; credit d: “Tsis”/Wikimedia Commons) Since 1970, the development of more powerful computers and more sophisticated software has made CT scanning routine for many types of diagnostic evaluations. It is especially useful for soft tissue scanning, such as of the brain and the thoracic and abdominal viscera. Its level of detail is so precise that it can allow physicians to measure the size of a mass down to a millimeter. The main disadvantage of CT scanning is that it exposes patients to a dose of radiation many times higher than that of X-rays. In fact, children who undergo CT scans are at increased risk of developing cancer, as are adults who have multiple CT scans. Note: wae — : mess OPenstax COLLEGE A CT or CAT scan relies on a circling scanner that revolves around the patient’s body. Watch this video to learn more about CT and CAT scans. What type of radiation does a CT scanner use? Magnetic Resonance Imaging Magnetic resonance imaging (MRI) is a noninvasive medical imaging technique based on a phenomenon of nuclear physics discovered in the 1930s, in which matter exposed to magnetic fields and radio waves was found to emit radio signals. In 1970, a physician and researcher named Raymond Damadian noticed that malignant (cancerous) tissue gave off different signals than normal body tissue. He applied for a patent for the first MRI scanning device, which was in use clinically by the early 1980s. The early MRI scanners were crude, but advances in digital computing and electronics led to their advancement over any other technique for precise imaging, especially to discover tumors. MRI also has the major advantage of not exposing patients to radiation. Drawbacks of MRI scans include their much higher cost, and patient discomfort with the procedure. The MRI scanner subjects the patient to such powerful electromagnets that the scan room must be shielded. The patient must be enclosed in a metal tube-like device for the duration of the scan (see [link]b), sometimes as long as thirty minutes, which can be uncomfortable and impractical for ill patients. The device is also so noisy that, even with earplugs, patients can become anxious or even fearful. These problems have been overcome somewhat with the development of “open” MRI scanning, which does not require the patient to be entirely enclosed in the metal tube. Patients with iron-containing metallic implants (internal sutures, some prosthetic devices, and so on) cannot undergo MRI scanning because it can dislodge these implants. Functional MRIs ({MRIs), which detect the concentration of blood flow in certain parts of the body, are increasingly being used to study the activity in parts of the brain during various body activities. This has helped scientists learn more about the locations of different brain functions and more about brain abnormalities and diseases. Note: zis A patient undergoing an MRI is surrounded by a tube-shaped scanner. Watch this video to learn more about MRIs. What is the function of magnets in an MRI? Positron Emission Tomography Positron emission tomography (PET) is a medical imaging technique involving the use of so-called radiopharmaceuticals, substances that emit radiation that is short-lived and therefore relatively safe to administer to the body. Although the first PET scanner was introduced in 1961, it took 15 more years before radiopharmaceuticals were combined with the technique and revolutionized its potential. The main advantage is that PET (see [link]c) can illustrate physiologic activity—including nutrient metabolism and blood flow—of the organ or organs being targeted, whereas CT and MRI scans can only show static images. PET is widely used to diagnose a multitude of conditions, such as heart disease, the spread of cancer, certain forms of infection, brain abnormalities, bone disease, and thyroid disease. PET relies on radioactive substances administered several minutes before the scan. Watch this video to learn more about PET. How is PET used in chemotherapy? Ultrasonography Ultrasonography is an imaging technique that uses the transmission of high-frequency sound waves into the body to generate an echo signal that is converted by a computer into a real-time image of anatomy and physiology (see [link]d). Ultrasonography is the least invasive of all imaging techniques, and it is therefore used more freely in sensitive situations such as pregnancy. The technology was first developed in the 1940s and 1950s. Ultrasonography is used to study heart function, blood flow in the neck or extremities, certain conditions such as gallbladder disease, and fetal growth and development. The main disadvantages of ultrasonography are that the image quality is heavily operator-dependent and that it is unable to penetrate bone and gas. Chapter Review Detailed anatomical drawings of the human body first became available in the fifteenth and sixteenth centuries; however, it was not until the end of the nineteenth century, and the discovery of X-rays, that anatomists and physicians discovered non-surgical methods to look inside a living body. Since then, many other techniques, including CT scans, MRI scans, PET scans, and ultrasonography, have been developed, providing more accurate and detailed views of the form and function of the human body. Interactive Link Questions Exercise: Problem: A CT or CAT scan relies on a circling scanner that revolves around the patient’s body. Watch this video to learn more about CT and CAT scans. What type of radiation does a CT scanner use? Solution: X-rays. Exercise: Problem: A patient undergoing an MRI is surrounded by a tube-shaped scanner. Watch this video to learn more about MRIs. What is the function of magnets in an MRI? Solution: The magnets induce tissue to emit radio signals that can show differences between different types of tissue. Exercise: Problem: PET relies on radioactive substances administered several minutes before the scan. Watch this video to learn more about PET. How is PET used in chemotherapy? Solution: PET scans can indicate how patients are responding to chemotherapy. Review Questions Exercise: Problem: In 1901, Wilhelm Réntgen was the first person to win the Nobel Prize for physics. For what discovery did he win? a. nuclear physics b. radiopharmaceuticals c. the link between radiation and cancer d. X-rays Solution: D Exercise: Problem: Which of the following imaging techniques would be best to use to study the uptake of nutrients by rapidly multiplying cancer cells? a. CT b. MRI c. PET d. ultrasonography Solution: C Exercise: Problem: Which of the following imaging studies can be used most safely during pregnancy? a. CT scans b. PET scans c. ultrasounds d. X-rays Solution: C Exercise: Problem: What are two major disadvantages of MRI scans? a. release of radiation and poor quality images b. high cost and the need for shielding from the magnetic signals c. can only view metabolically active tissues and inadequate availability of equipment d. release of radiation and the need for a patient to be confined to metal tube for up to 30 minutes Solution: Critical Thinking Questions Exercise: Problem: Which medical imaging technique is most dangerous to use repeatedly, and why? Solution: CT scanning subjects patients to much higher levels of radiation than X-rays, and should not be performed repeatedly. Exercise: Problem: Explain why ultrasound imaging is the technique of choice for studying fetal growth and development. Solution: Ultrasonography does not expose a mother or fetus to radiation, to radiopharmaceuticals, or to magnetic fields. At this time, there are no known medical risks of ultrasonography. Glossary computed tomography (CT) medical imaging technique in which a computer-enhanced cross- sectional X-ray image is obtained magnetic resonance imaging (MRI) medical imaging technique in which a device generates a magnetic field to obtain detailed sectional images of the internal structures of the body positron emission tomography (PET) medical imaging technique in which radiopharmaceuticals are traced to reveal metabolic and physiological functions in tissues ultrasonography application of ultrasonic waves to visualize subcutaneous body structures such as tendons and organs X-ray form of high energy electromagnetic radiation with a short wavelength capable of penetrating solids and ionizing gases; used in medicine as a diagnostic aid to visualize body structures such as bones Types of Tissues By the end of this section, you will be able to: e Identify the four main tissue types e Discuss the functions of each tissue type e Relate the structure of each tissue type to their function e Discuss the embryonic origin of tissue e Identify the three major germ layers e Identify the main types of tissue membranes The term tissue is used to describe a group of cells found together in the body. The cells within a tissue share a common embryonic origin. Microscopic observation reveals that the cells in a tissue share morphological features and are arranged in an orderly pattern that achieves the tissue’s functions. From the evolutionary perspective, tissues appear in more complex organisms. For example, multicellular protists, ancient eukaryotes, do not have cells organized into tissues. Although there are many types of cells in the human body, they are organized into four broad categories of tissues: epithelial, connective, muscle, and nervous. Each of these categories is characterized by specific functions that contribute to the overall health and maintenance of the body. A disruption of the structure is a sign of injury or disease. Such changes can be detected through histology, the microscopic study of tissue appearance, organization, and function. The Four Types of Tissues Epithelial tissue, also referred to as epithelium, refers to the sheets of cells that cover exterior surfaces of the body, lines internal cavities and passageways, and forms certain glands. Connective tissue, as its name implies, binds the cells and organs of the body together and functions in the protection, support, and integration of all parts of the body. Muscle tissue is excitable, responding to stimulation and contracting to provide movement, and occurs as three major types: skeletal (voluntary) muscle, smooth muscle, and cardiac muscle in the heart. Nervous tissue is also excitable, allowing the propagation of electrochemical signals in the form of nerve impulses that communicate between different regions of the body ([link]). The next level of organization is the organ, where several types of tissues come together to form a working unit. Just as knowing the structure and function of cells helps you in your study of tissues, knowledge of tissues will help you understand how organs function. The epithelial and connective tissues are discussed in detail in this chapter. Muscle and nervous tissues will be discussed only briefly in this chapter. Four Types of Tissue: Body Nervous tissue Brain Spinal cord Nerves Muscle tissue Cardiac muscle Smooth muscle Skeletal muscle Epithelial tissue Lining of Gl tract organs and other hollow organs Skin surface (epidermis) Connective tissue Fat and other soft padding tissue Bone Tendon The four types of tissues are exemplified in nervous tissue, stratified squamous epithelial tissue, cardiac muscle tissue, and connective tissue in small intestine. Clockwise from nervous tissue, LM x 872, LM x 282, LM x 460, LM x 800. (Micrographs provided by the Regents of University of Michigan Medical School © 2012) Embryonic Origin of Tissues The zygote, or fertilized egg, is a single cell formed by the fusion of an egg and sperm. After fertilization the zygote gives rise to rapid mitotic cycles, generating many cells to form the embryo. The first embryonic cells generated have the ability to differentiate into any type of cell in the body and, as such, are called totipotent, meaning each has the capacity to divide, differentiate, and develop into a new organism. As cell proliferation progresses, three major cell lineages are established within the embryo. As explained in a later chapter, each of these lineages of embryonic cells forms the distinct germ layers from which all the tissues and organs of the human body eventually form. Each germ layer is identified by its relative position: ectoderm (ecto- = “outer”), mesoderm (meso- = “middle”), and endoderm (endo- = “inner’’). [link] shows the types of tissues and organs associated with the each of the three germ layers. Note that epithelial tissue originates in all three layers, whereas nervous tissue derives primarily from the ectoderm and muscle tissue from mesoderm. Embryonic Origin of Tissues and Major Organs Ectoderm Epidermis, glands on skin, some cranial bones, pituitary and adrenal medulla, the nervous system, the mouth between cheek and gums, the anus = ue, ANZ 1Q( | a = is S P= y =. ] : y/ | . = Nos / Skin cells Neurons Pigment cell Mesoderm Connective tissues proper, bone, cartilage, blood, endothelium of blood vessels, muscle, synovial membranes, serous membranes lining body cavities, kidneys, lining of gonads _——— —— = — = — be _< o ix y ing a - Salivary glands; glands of respiratory passages; and pancreas Mammary glands Simple tubular — Simple branched tubular aTIr- - AP Intestinal glands Merocrine sweat glands Gastric glands; and mucous glands of esophagus, tongue, duodenum Compound tubular DOOUDWODLE ot ee oes Mucous glands (in mouth); bulbourethral glands (in male reproductive system); and testes (seminiferous tubules) ) Compound ducts 4 Exocrine glands are classified by their structure. Methods and Types of Secretion Exocrine glands can be classified by their mode of secretion and the nature Tubular of the substances released, as well as by the structure of the glands and shape of ducts ([link]). Merocrine secretion is the most common type of exocrine secretion. The secretions are enclosed in vesicles that move to the apical surface of the cell where the contents are released by exocytosis. For example, watery mucous containing the glycoprotein mucin, a lubricant that offers some pathogen protection is a merocrine secretion. The eccrine glands that produce and secrete sweat are another example. Modes of Glandular Secretion Secretion Secretory vesicle (a) Merocrine Golgi complex secretion Nucleus Pinched off portion of cell is the secretion (b) Apocrine secretion Mature cell dies and becomes secretory product (c) Holocrine secretion (a) In merocrine secretion, the cell remains intact. (b) In apocrine secretion, the apical portion of the cell is released, as well. (c) In holocrine secretion, the cell is destroyed as it releases its product and the cell itself becomes part of the secretion. Apocrine secretion accumulates near the apical portion of the cell. That portion of the cell and its secretory contents pinch off from the cell and are released. Apocrine sweat glands in the axillary and genital areas release fatty secretions that local bacteria break down; this causes body odor. Both merocrine and apocrine glands continue to produce and secrete their contents with little damage caused to the cell because the nucleus and golgi regions remain intact after secretion. In contrast, the process of holocrine secretion involves the rupture and destruction of the entire gland cell. The cell accumulates its secretory products and releases them only when it bursts. New gland cells differentiate from cells in the surrounding tissue to replace those lost by secretion. The sebaceous glands that produce the oils on the skin and hair are holocrine glands/cells ({link]). Sebaceous Glands Hair Sebaceous gland These glands secrete oils that lubricate and protect the skin. They are holocrine glands and they are destroyed after releasing their contents. New glandular cells form to replace the cells that are lost. LM x 400. (Micrograph provided by the Regents of University of Michigan Medical School © 2012) Glands are also named after the products they produce. The serous gland produces watery, blood-plasma-like secretions rich in enzymes such as alpha amylase, whereas the mucous gland releases watery to viscous products rich in the glycoprotein mucin. Both serous and mucous glands are common in the salivary glands of the mouth. Mixed exocrine glands contain both serous and mucous glands and release both types of secretions. Chapter Review In epithelial tissue, cells are closely packed with little or no extracellular matrix except for the basal lamina that separates the epithelium from underlying tissue. The main functions of epithelia are protection from the environment, coverage, secretion and excretion, absorption, and filtration. Cells are bound together by tight junctions that form an impermeable barrier. They can also be connected by gap junctions, which allow free exchange of soluble molecules between cells, and anchoring junctions, which attach cell to cell or cell to matrix. The different types of epithelial tissues are characterized by their cellular shapes and arrangements: squamous, cuboidal, or columnar epithelia. Single cell layers form simple epithelia, whereas stacked cells form stratified epithelia. Very few capillaries penetrate these tissues. Glands are secretory tissues and organs that are derived from epithelial tissues. Exocrine glands release their products through ducts. Endocrine glands secrete hormones directly into the interstitial fluid and blood stream. Glands are classified both according to the type of secretion and by their structure. Merocrine glands secrete products as they are synthesized. Apocrine glands release secretions by pinching off the apical portion of the cell, whereas holocrine gland cells store their secretions until they rupture and release their contents. In this case, the cell becomes part of the secretion. Interactive Link Questions Exercise: Problem: Watch this video to find out more about the anatomy of epithelial tissues. Where in the body would one find non-keratinizing stratified squamous epithelium? Solution: The inside of the mouth, esophagus, vaginal canal, and anus. Review Questions Exercise: Problem: In observing epithelial cells under a microscope, the cells are arranged in a single layer and look tall and narrow, and the nucleus is located close to the basal side of the cell. The specimen is what type of epithelial tissue? a. columnar b. stratified c. squamous d. transitional Solution: A Exercise: Problem: Which of the following is the epithelial tissue that lines the interior of blood vessels? a. columnar b. pseudostratified c. simple squamous d. transitional Solution: C Exercise: Problem: Which type of epithelial tissue specializes in moving particles across its surface and is found in airways and lining of the oviduct? a. transitional b. stratified columnar c. pseudostratified ciliated columnar d. stratified squamous Solution: B Exercise: Problem: The exocrine gland stores its secretion until the glandular cell ruptures, whereas the gland releases its apical region and reforms. a. holocrine; apocrine b. eccrine; endocrine c. apocrine; holocrine d. eccrine; apocrine Solution: A Critical Thinking Questions Exercise: Problem: The structure of a tissue usually is optimized for its function. Describe how the structure of the mucosa and its cells match its function of nutrient absorption. Solution: The mucosa of the intestine is highly folded, increasing the surface area for nutrient absorption. A greater surface area for absorption allows more nutrients to be absorbed per unit time. In addition, the nutrient-absorbing cells of the mucosa have finger-like projections called microvilli that further increase the surface area for nutrient absorption. Glossary anchoring junction mechanically attaches adjacent cells to each other or to the basement membrane apical that part of a cell or tissue which, in general, faces an open space apocrine secretion release of a substance along with the apical portion of the cell basal lamina thin extracellular layer that lies underneath epithelial cells and separates them from other tissues basement membrane in epithelial tissue, a thin layer of fibrous material that anchors the epithelial tissue to the underlying connective tissue; made up of the basal lamina and reticular lamina cell junction point of cell-to-cell contact that connects one cell to another in a tissue endocrine gland groups of cells that release chemical signals into the intercellular fluid to be picked up and transported to their target organs by blood endothelium tissue that lines vessels of the lymphatic and cardiovascular system, made up of a simple squamous epithelium exocrine gland group of epithelial cells that secrete substances through ducts that open to the skin or to internal body surfaces that lead to the exterior of the body gap junction allows cytoplasmic communications to occur between cells goblet cell unicellular gland found in columnar epithelium that secretes mucous holocrine secretion release of a substance caused by the rupture of a gland cell, which becomes part of the secretion merocrine secretion release of a substance from a gland via exocytosis mesothelium simple squamous epithelial tissue which covers the major body cavities and is the epithelial portion of serous membranes mucous gland group of cells that secrete mucous, a thick, slippery substance that keeps tissues moist and acts as a lubricant pseudostratified columnar epithelium tissue that consists of a single layer of irregularly shaped and sized cells that give the appearance of multiple layers; found in ducts of certain glands and the upper respiratory tract reticular lamina matrix containing collagen and elastin secreted by connective tissue; a component of the basement membrane serous gland group of cells within the serous membrane that secrete a lubricating substance onto the surface simple columnar epithelium tissue that consists of a single layer of column-like cells; promotes secretion and absorption in tissues and organs simple cuboidal epithelium tissue that consists of a single layer of cube-shaped cells; promotes secretion and absorption in ducts and tubules simple squamous epithelium tissue that consists of a single layer of flat scale-like cells; promotes diffusion and filtration across surface stratified columnar epithelium tissue that consists of two or more layers of column-like cells, contains glands and is found in some ducts stratified cuboidal epithelium tissue that consists of two or more layers of cube-shaped cells, found in some ducts stratified squamous epithelium tissue that consists of multiple layers of cells with the most apical being flat scale-like cells; protects surfaces from abrasion tight junction forms an impermeable barrier between cells transitional epithelium form of stratified epithelium found in the urinary tract, characterized by an apical layer of cells that change shape in response to the presence of urine Connective Tissue Supports and Protects By the end of this section, you will be able to: e Identify and distinguish between the types of connective tissue: proper, supportive, and fluid e Explain the functions of connective tissues As may be obvious from its name, one of the major functions of connective tissue is to connect tissues and organs. Unlike epithelial tissue, which is composed of cells closely packed with little or no extracellular space in between, connective tissue cells are dispersed in a matrix. The matrix usually includes a large amount of extracellular material produced by the connective tissue cells that are embedded within it. The matrix plays a major role in the functioning of this tissue. The major component of the matrix is a ground substance often crisscrossed by protein fibers. This ground substance is usually a fluid, but it can also be mineralized and solid, as in bones. Connective tissues come in a vast variety of forms, yet they typically have in common three characteristic components: cells, large amounts of amorphous ground substance, and protein fibers. The amount and structure of each component correlates with the function of the tissue, from the rigid ground substance in bones supporting the body to the inclusion of specialized cells; for example, a phagocytic cell that engulfs pathogens and also rids tissue of cellular debris. Functions of Connective Tissues Connective tissues perform many functions in the body, but most importantly, they support and connect other tissues; from the connective tissue sheath that surrounds muscle cells, to the tendons that attach muscles to bones, and to the skeleton that supports the positions of the body. Protection is another major function of connective tissue, in the form of fibrous capsules and bones that protect delicate organs and, of course, the skeletal system. Specialized cells in connective tissue defend the body from microorganisms that enter the body. Transport of fluid, nutrients, waste, and chemical messengers is ensured by specialized fluid connective tissues, such as blood and lymph. Adipose cells store surplus energy in the form of fat and contribute to the thermal insulation of the body. Embryonic Connective Tissue All connective tissues derive from the mesodermal layer of the embryo (see [link]). The first connective tissue to develop in the embryo is mesenchyme, the stem cell line from which all connective tissues are later derived. Clusters of mesenchymal cells are scattered throughout adult tissue and supply the cells needed for replacement and repair after a connective tissue injury. A second type of embryonic connective tissue forms in the umbilical cord, called mucous connective tissue or Wharton’s jelly. This tissue is no longer present after birth, leaving only scattered mesenchymal cells throughout the body. Classification of Connective Tissues The three broad categories of connective tissue are classified according to the characteristics of their ground substance and the types of fibers found within the matrix ({link]). Connective tissue proper includes loose connective tissue and dense connective tissue. Both tissues have a variety of cell types and protein fibers suspended in a viscous ground substance. Dense connective tissue is reinforced by bundles of fibers that provide tensile strength, elasticity, and protection. In loose connective tissue, the fibers are loosely organized, leaving large spaces in between. Supportive connective tissue—bone and cartilage—provide structure and strength to the body and protect soft tissues. A few distinct cell types and densely packed fibers in a matrix characterize these tissues. In bone, the matrix is rigid and described as calcified because of the deposited calcium salts. In fluid connective tissue, in other words, lymph and blood, various specialized cells circulate in a watery fluid containing salts, nutrients, and dissolved proteins. Connective Tissue Examples Connective tisssiee Exaifipjasortive Fluid connective proper connective tissue tissue Connective tissue Supportive Fluid connective proper connective tissue tissue Loose connective : an Cartilage e Hyaline e Areolar y Blood a aadinose e Fibrocartilage e Reticular Pane Dense connective tissue Bones Regular ; ere e Compact bone Lymph eicsalat e Cancellous bone elastic Connective Tissue Proper Fibroblasts are present in all connective tissue proper ([link]). Fibrocytes, adipocytes, and mesenchymal cells are fixed cells, which means they remain within the connective tissue. Other cells move in and out of the connective tissue in response to chemical signals. Macrophages, mast cells, lymphocytes, plasma cells, and phagocytic cells are found in connective tissue proper but are actually part of the immune system protecting the body. Connective Tissue Proper Reticular fibers Adipocytes Mesenchymal cell Elastic fibers Collagen fibers Fibroblast Macrophage Fibroblasts produce this fibrous tissue. Connective tissue proper includes the fixed cells fibrocytes, adipocytes, and mesenchymal cells. LM x 400. (Micrograph provided by the Regents of University of Michigan Medical School © 2012) Cell Types The most abundant cell in connective tissue proper is the fibroblast. Polysaccharides and proteins secreted by fibroblasts combine with extra- cellular fluids to produce a viscous ground substance that, with embedded fibrous proteins, forms the extra-cellular matrix. As you might expect, a fibrocyte, a less active form of fibroblast, is the second most common cell type in connective tissue proper. Adipocytes are cells that store lipids as droplets that fill most of the cytoplasm. There are two basic types of adipocytes: white and brown. The brown adipocytes store lipids as many droplets, and have high metabolic activity. In contrast, white fat adipocytes store lipids as a single large drop and are metabolically less active. Their effectiveness at storing large amounts of fat is witnessed in obese individuals. The number and type of adipocytes depends on the tissue and location, and vary among individuals in the population. The mesenchymal cell is a multipotent adult stem cell. These cells can differentiate into any type of connective tissue cells needed for repair and healing of damaged tissue. The macrophage cell is a large cell derived from a monocyte, a type of blood cell, which enters the connective tissue matrix from the blood vessels. The macrophage cells are an essential component of the immune system, which is the body’s defense against potential pathogens and degraded host cells. When stimulated, macrophages release cytokines, small proteins that act as chemical messengers. Cytokines recruit other cells of the immune system to infected sites and stimulate their activities. Roaming, or free, macrophages move rapidly by amoeboid movement, engulfing infectious agents and cellular debris. In contrast, fixed macrophages are permanent residents of their tissues. The mast cell, found in connective tissue proper, has many cytoplasmic granules. These granules contain the chemical signals histamine and heparin. When irritated or damaged, mast cells release histamine, an inflammatory mediator, which causes vasodilation and increased blood flow at a site of injury or infection, along with itching, swelling, and redness you recognize as an allergic response. Like blood cells, mast cells are derived from hematopoietic stem cells and are part of the immune system. Connective Tissue Fibers and Ground Substance Three main types of fibers are secreted by fibroblasts: collagen fibers, elastic fibers, and reticular fibers. Collagen fiber is made from fibrous protein subunits linked together to form a long and straight fiber. Collagen fibers, while flexible, have great tensile strength, resist stretching, and give ligaments and tendons their characteristic resilience and strength. These fibers hold connective tissues together, even during the movement of the body. Elastic fiber contains the protein elastin along with lesser amounts of other proteins and glycoproteins. The main property of elastin is that after being stretched or compressed, it will return to its original shape. Elastic fibers are prominent in elastic tissues found in skin and the elastic ligaments of the vertebral column. Reticular fiber is also formed from the same protein subunits as collagen fibers; however, these fibers remain narrow and are arrayed in a branching network. They are found throughout the body, but are most abundant in the reticular tissue of soft organs, such as liver and spleen, where they anchor and provide structural support to the parenchyma (the functional cells, blood vessels, and nerves of the organ). All of these fiber types are embedded in ground substance. Secreted by fibroblasts, ground substance is made of polysaccharides, specifically hyaluronic acid, and proteins. These combine to form a proteoglycan with a protein core and polysaccharide branches. The proteoglycan attracts and traps available moisture forming the clear, viscous, colorless matrix you now know as ground substance. Loose Connective Tissue Loose connective tissue is found between many organs where it acts both to absorb shock and bind tissues together. It allows water, salts, and various nutrients to diffuse through to adjacent or imbedded cells and tissues. Adipose tissue consists mostly of fat storage cells, with little extracellular matrix ({link]). A large number of capillaries allow rapid storage and mobilization of lipid molecules. White adipose tissue is most abundant. It can appear yellow and owes its color to carotene and related pigments from plant food. White fat contributes mostly to lipid storage and can serve as insulation from cold temperatures and mechanical injuries. White adipose tissue can be found protecting the kidneys and cushioning the back of the eye. Brown adipose tissue is more common in infants, hence the term “baby fat.” In adults, there is a reduced amount of brown fat and it is found mainly in the neck and clavicular regions of the body. The many mitochondria in the cytoplasm of brown adipose tissue help explain its efficiency at metabolizing stored fat. Brown adipose tissue is thermogenic, meaning that as it breaks down fats, it releases metabolic heat, rather than producing adenosine triphosphate (ATP), a key molecule used in metabolism. Adipose Tissue | es ————— This is a loose connective tissue that consists of fat cells with little extracellular matrix. It stores fat for energy and provides insulation. LM x 800. (Micrograph provided by the Regents of University of Michigan Medical School © 2012) Areolar tissue shows little specialization. It contains all the cell types and fibers previously described and is distributed in a random, web-like fashion. It fills the spaces between muscle fibers, surrounds blood and lymph vessels, and supports organs in the abdominal cavity. Areolar tissue underlies most epithelia and represents the connective tissue component of epithelial membranes, which are described further in a later section. Reticular tissue is a mesh-like, supportive framework for soft organs such as lymphatic tissue, the spleen, and the liver ([link]). Reticular cells produce the reticular fibers that form the network onto which other cells attach. It derives its name from the Latin reticulus, which means “little net.” Reticular Tissue This is a loose connective tissue made up of a network of reticular fibers that provides a supportive framework for soft organs. LM x 1600. (Micrograph provided by the Regents of University of Michigan Medical School © 2012) Dense Connective Tissue Dense connective tissue contains more collagen fibers than does loose connective tissue. As a consequence, it displays greater resistance to stretching. There are two major categories of dense connective tissue: regular and irregular. Dense regular connective tissue fibers are parallel to each other, enhancing tensile strength and resistance to stretching in the direction of the fiber orientations. Ligaments and tendons are made of dense regular connective tissue, but in ligaments not all fibers are parallel. Dense regular elastic tissue contains elastin fibers in addition to collagen fibers, which allows the ligament to return to its original length after stretching. The ligaments in the vocal folds and between the vertebrae in the vertebral column are elastic. In dense irregular connective tissue, the direction of fibers is random. This arrangement gives the tissue greater strength in all directions and less strength in one particular direction. In some tissues, fibers crisscross and form a mesh. In other tissues, stretching in several directions is achieved by alternating layers where fibers run in the same orientation in each layer, and it is the layers themselves that are stacked at an angle. The dermis of the skin is an example of dense irregular connective tissue rich in collagen fibers. Dense irregular elastic tissues give arterial walls the strength and the ability to regain original shape after stretching ([link]). Dense Connective Tissue Fibroblast nuclei Fibroblast nuclei Collagen fiber bundles (b) Irregular dense (a) Dense regular connective tissue consists of collagenous fibers packed into parallel bundles. (b) Dense irregular connective tissue consists of collagenous fibers interwoven into a mesh-like network. From top, LM x 1000, LM x 200. (Micrographs provided by the Regents of University of Michigan Medical School © 2012) Note: Disorders of the... Connective Tissue: Tendinitis Your opponent stands ready as you prepare to hit the serve, but you are confident that you will smash the ball past your opponent. As you toss the ball high in the air, a bumming pain shoots across your wrist and you drop the tennis racket. That dull ache in the wrist that you ignored through the summer is now an unbearable pain. The game is over for now. After examining your swollen wrist, the doctor in the emergency room announces that you have developed wrist tendinitis. She recommends icing the tender area, taking non-steroidal anti-inflammatory medication to ease the pain and to reduce swelling, and complete rest for a few weeks. She interrupts your protests that you cannot stop playing. She issues a stern warning about the risk of aggravating the condition and the possibility of surgery. She consoles you by mentioning that well known tennis players such as Venus and Serena Williams and Rafael Nadal have also suffered from tendinitis related injuries. What is tendinitis and how did it happen? Tendinitis is the inflammation of a tendon, the thick band of fibrous connective tissue that attaches a muscle to a bone. The condition causes pain and tenderness in the area around a joint. On rare occasions, a sudden serious injury will cause tendinitis. Most often, the condition results from repetitive motions over time that strain the tendons needed to perform the tasks. Persons whose jobs and hobbies involve performing the same movements over and over again are often at the greatest risk of tendinitis. You hear of tennis and golfer’s elbow, jumper's knee, and swimmer’s shoulder. In all cases, overuse of the joint causes a microtrauma that initiates the inflammatory response. Tendinitis is routinely diagnosed through a clinical examination. In case of severe pain, X-rays can be examined to rule out the possibility of a bone injury. Severe cases of tendinitis can even tear loose a tendon. Surgical repair of a tendon is painful. Connective tissue in the tendon does not have abundant blood supply and heals slowly. While older adults are at risk for tendinitis because the elasticity of tendon tissue decreases with age, active people of all ages can develop tendinitis. Young athletes, dancers, and computer operators; anyone who performs the same movements constantly is at risk for tendinitis. Although repetitive motions are unavoidable in many activities and may lead to tendinitis, precautions can be taken that can lessen the probability of developing tendinitis. For active individuals, stretches before exercising and cross training or changing exercises are recommended. For the passionate athlete, it may be time to take some lessons to improve technique. All of the preventive measures aim to increase the strength of the tendon and decrease the stress put on it. With proper rest and managed care, you will be back on the court to hit that slice-spin serve over the net. Note: Pelee Le Watch this animation to learn more about tendonitis, a painful condition caused by swollen or injured tendons. Supportive Connective Tissues Two major forms of supportive connective tissue, cartilage and bone, allow the body to maintain its posture and protect internal organs. Cartilage The distinctive appearance of cartilage is due to polysaccharides called chondroitin sulfates, which bind with ground substance proteins to form proteoglycans. Embedded within the cartilage matrix are chondrocytes, or cartilage cells, and the space they occupy are called lacunae (singular = lacuna). A layer of dense irregular connective tissue, the perichondrium, encapsulates the cartilage. Cartilaginous tissue is avascular, thus all nutrients need to diffuse through the matrix to reach the chondrocytes. This is a factor contributing to the very slow healing of cartilaginous tissues. The three main types of cartilage tissue are hyaline cartilage, fibrocartilage, and elastic cartilage ([{link]). Hyaline cartilage, the most common type of cartilage in the body, consists of short and dispersed collagen fibers and contains large amounts of proteoglycans. Under the microscope, tissue samples appear clear. The surface of hyaline cartilage is smooth. Both strong and flexible, it is found in the rib cage and nose and covers bones where they meet to form moveable joints. It makes up a template of the embryonic skeleton before bone formation. A plate of hyaline cartilage at the ends of bone allows continued growth until adulthood. Fibrocartilage is tough because it has thick bundles of collagen fibers dispersed through its matrix. Menisci in the knee joint and the intervertebral discs are examples of fibrocartilage. Elastic cartilage contains elastic fibers as well as collagen and proteoglycans. This tissue gives rigid support as well as elasticity. Tug gently at your ear lobes, and notice that the lobes return to their initial shape. The external ear contains elastic cartilage. Types of Cartilage (a) Hyaline cartilage Chondrocytes in lacunae Matrix (b) Fibrocartilage Chondrocyte in lacuna Collagen fiber in matrix (c) Elastic cartilage Chondrocyte in lacuna Elastic fibers in matrix Cartilage is a connective tissue consisting of collagenous fibers embedded in a firm matrix of chondroitin sulfates. (a) Hyaline cartilage provides support with some flexibility. The example is from dog tissue. (b) Fibrocartilage provides some compressibility and can absorb pressure. (c) Elastic cartilage provides firm but elastic support. From top, LM x 300, LM x 1200, LM x 1016. (Micrographs provided by the Regents of University of Michigan Medical School © 2012) Bone Bone is the hardest connective tissue. It provides protection to internal organs and supports the body. Bone’s rigid extracellular matrix contains mostly collagen fibers embedded in a mineralized ground substance containing hydroxyapatite, a form of calcium phosphate. Both components of the matrix, organic and inorganic, contribute to the unusual properties of bone. Without collagen, bones would be brittle and shatter easily. Without mineral crystals, bones would flex and provide little support. Osteocytes, bone cells like chondrocytes, are located within lacunae. The histology of transverse tissue from long bone shows a typical arrangement of osteocytes in concentric circles around a central canal. Bone is a highly vascularized tissue. Unlike cartilage, bone tissue can recover from injuries in a relatively short time. Cancellous bone looks like a sponge under the microscope and contains empty spaces between trabeculae, or arches of bone proper. It is lighter than compact bone and found in the interior of some bones and at the end of long bones. Compact bone is solid and has greater structural strength. Fluid Connective Tissue Blood and lymph are fluid connective tissues. Cells circulate in a liquid extracellular matrix. The formed elements circulating in blood are all derived from hematopoietic stem cells located in bone marrow ([link]). Erythrocytes, red blood cells, transport oxygen and some carbon dioxide. Leukocytes, white blood cells, are responsible for defending against potentially harmful microorganisms or molecules. Platelets are cell fragments involved in blood clotting. Some white blood cells have the ability to cross the endothelial layer that lines blood vessels and enter adjacent tissues. Nutrients, salts, and wastes are dissolved in the liquid matrix and transported through the body. Lymph contains a liquid matrix and white blood cells. Lymphatic capillaries are extremely permeable, allowing larger molecules and excess fluid from interstitial spaces to enter the lymphatic vessels. Lymph drains into blood vessels, delivering molecules to the blood that could not otherwise directly enter the bloodstream. In this way, specialized lymphatic capillaries transport absorbed fats away from the intestine and deliver these molecules to the blood. Blood: A Fluid Connective Tissue —, o AY q fe) € Ne) G e oe ° ie 3 =e) ot 38 oe 8% 0 266.0 0 206 Blood is a fluid connective tissue containing erythrocytes and various types of leukocytes that circulate in a liquid extracellular matrix. LM x 1600. (Micrograph provided by the Regents of University of Michigan Medical School © 2012) Note: ac| openstax COLLEGE” Wii ae View the University of Michigan Webscope to explore the tissue sample in greater detail. Note: =" meee OFENStAX COLLEGE” Visit this link to test your connective tissue knowledge with this 10- question quiz. Can you name the 10 tissue types shown in the histology slides? Chapter Review Connective tissue is a heterogeneous tissue with many cell shapes and tissue architecture. Structurally, all connective tissues contain cells that are embedded in an extracellular matrix stabilized by proteins. The chemical nature and physical layout of the extracellular matrix and proteins vary enormously among tissues, reflecting the variety of functions that connective tissue fulfills in the body. Connective tissues separate and cushion organs, protecting them from shifting or traumatic injury. Connect tissues provide support and assist movement, store and transport energy molecules, protect against infections, and contribute to temperature homeostasis. Many different cells contribute to the formation of connective tissues. They originate in the mesodermal germ layer and differentiate from mesenchyme and hematopoietic tissue in the bone marrow. Fibroblasts are the most abundant and secrete many protein fibers, adipocytes specialize in fat storage, hematopoietic cells from the bone marrow give rise to all the blood cells, chondrocytes form cartilage, and osteocytes form bone. The extracellular matrix contains fluid, proteins, polysaccharide derivatives, and, in the case of bone, mineral crystals. Protein fibers fall into three major groups: collagen fibers that are thick, strong, flexible, and resist stretch; reticular fibers that are thin and form a supportive mesh; and elastin fibers that are thin and elastic. The major types of connective tissue are connective tissue proper, supportive tissue, and fluid tissue. Loose connective tissue proper includes adipose tissue, areolar tissue, and reticular tissue. These serve to hold organs and other tissues in place and, in the case of adipose tissue, isolate and store energy reserves. The matrix is the most abundant feature for loose tissue although adipose tissue does not have much extracellular matrix. Dense connective tissue proper is richer in fibers and may be regular, with fibers oriented in parallel as in ligaments and tendons, or irregular, with fibers oriented in several directions. Organ capsules (collagenous type) and walls of arteries (elastic type) contain dense irregular connective tissue. Cartilage and bone are supportive tissue. Cartilage contains chondrocytes and is somewhat flexible. Hyaline cartilage is smooth and clear, covers joints, and is found in the growing portion of bones. Fibrocartilage is tough because of extra collagen fibers and forms, among other things, the intervertebral discs. Elastic cartilage can stretch and recoil to its original shape because of its high content of elastic fibers. The matrix contains very few blood vessels. Bones are made of a rigid, mineralized matrix containing calcium salts, crystals, and osteocytes lodged in lacunae. Bone tissue is highly vascularized. Cancellous bone is spongy and less solid than compact bone. Fluid tissue, for example blood and lymph, is characterized by a liquid matrix and no supporting fibers. Interactive Link Questions Exercise: Problem: Visit this link to test your connective tissue knowledge with this 10- question quiz. Can you name the 10 tissue types shown in the histology slides? Solution: Click at the bottom of the quiz for the answers. Review Questions Exercise: Problem: Connective tissue is made of which three essential components? a. cells, ground substance, and carbohydrate fibers b. cells, ground substance, and protein fibers c. collagen, ground substance, and protein fibers d. matrix, ground substance, and fluid Solution: B Exercise: Problem: Under the microscope, a tissue specimen shows cells located in spaces scattered in a transparent background. This is probably a. loose connective tissue b. a tendon c. bone d. hyaline cartilage Solution: D Exercise: Problem: Which connective tissue specializes in storage of fat? a. tendon b. adipose tissue c. reticular tissue d. dense connective tissue Solution: B Exercise: Problem: Ligaments connect bones together and withstand a lot of stress. What type of connective tissue should you expect ligaments to contain? a. areolar tissue b. adipose tissue c. dense regular connective tissue d. dense irregular connective tissue Solution: C Exercise: Problem: In adults, new connective tissue cells originate from the a. mesoderm b. mesenchyme c. ectoderm d. endoderm Solution: B Exercise: Problem:In bone, the main cells are a. fibroblasts b. chondrocytes c. lymphocytes d. osteocytes Solution: D Critical Thinking Questions Exercise: Problem: One of the main functions of connective tissue is to integrate organs and organ systems in the body. Discuss how blood fulfills this role. Solution: Blood is a fluid connective tissue, a variety of specialized cells that circulate in a watery fluid containing salts, nutrients, and dissolved proteins in a liquid extracellular matrix. Blood contains formed elements derived from bone marrow. Erythrocytes, or red blood cells, transport the gases oxygen and carbon dioxide. Leukocytes, or white blood cells, are responsible for the defense of the organism against potentially harmful microorganisms or molecules. Platelets are cell fragments involved in blood clotting. Some cells have the ability to cross the endothelial layer that lines vessels and enter adjacent tissues. Nutrients, salts, and waste are dissolved in the liquid matrix and transported through the body. Exercise: Problem: Why does an injury to cartilage, especially hyaline cartilage, heal much more slowly than a bone fracture? Solution: A layer of dense irregular connective tissue covers cartilage. No blood vessels supply cartilage tissue. Injuries to cartilage heal very slowly because cells and nutrients needed for repair diffuse slowly to the injury site. Glossary adipocytes lipid storage cells adipose tissue specialized areolar tissue rich in stored fat areolar tissue (also, loose connective tissue) a type of connective tissue proper that shows little specialization with cells dispersed in the matrix chondrocytes cells of the cartilage collagen fiber flexible fibrous proteins that give connective tissue tensile strength connective tissue proper connective tissue containing a viscous matrix, fibers, and cells. dense connective tissue connective tissue proper that contains many fibers that provide both elasticity and protection elastic cartilage type of cartilage, with elastin as the major protein, characterized by rigid support as well as elasticity elastic fiber fibrous protein within connective tissue that contains a high percentage of the protein elastin that allows the fibers to stretch and return to original size fibroblast most abundant cell type in connective tissue, secretes protein fibers and matrix into the extracellular space fibrocartilage tough form of cartilage, made of thick bundles of collagen fibers embedded in chondroitin sulfate ground substance fibrocyte less active form of fibroblast fluid connective tissue specialized cells that circulate in a watery fluid containing salts, nutrients, and dissolved proteins ground substance fluid or semi-fluid portion of the matrix hyaline cartilage most common type of cartilage, smooth and made of short collagen fibers embedded in a chondroitin sulfate ground substance lacunae (singular = lacuna) small spaces in bone or cartilage tissue that cells occupy loose connective tissue (also, areolar tissue) type of connective tissue proper that shows little specialization with cells dispersed in the matrix matrix extracellular material which is produced by the cells embedded in it, containing ground substance and fibers mesenchymal cell adult stem cell from which most connective tissue cells are derived mesenchyme embryonic tissue from which connective tissue cells derive mucous connective tissue specialized loose connective tissue present in the umbilical cord parenchyma functional cells of a gland or organ, in contrast with the supportive or connective tissue of a gland or organ reticular fiber fine fibrous protein, made of collagen subunits, which cross-link to form supporting “nets” within connective tissue reticular tissue type of loose connective tissue that provides a supportive framework to soft organs, such as lymphatic tissue, spleen, and the liver supportive connective tissue type of connective tissue that provides strength to the body and protects soft tissue Muscle Tissue and Motion By the end of this section, you will be able to: e Identify the three types of muscle tissue ¢ Compare and contrast the functions of each muscle tissue type e Explain how muscle tissue can enable motion Muscle tissue is characterized by properties that allow movement. Muscle cells are excitable; they respond to a stimulus. They are contractile, meaning they can shorten and generate a pulling force. When attached between two movable objects, in other words, bones, contractions of the muscles cause the bones to move. Some muscle movement is voluntary, which means it is under conscious control. For example, a person decides to open a book and read a chapter on anatomy. Other movements are involuntary, meaning they are not under conscious control, such as the contraction of your pupil in bright light. Muscle tissue is classified into three types according to structure and function: skeletal, cardiac, and smooth ({link]). Comparison of Structure and Properties of Muscle Tissue Types Tissue Skeletal Histology Long cylindrical fiber, Striated, many peripherally located nuclei Function Voluntary movement, produces heat, protects organs Location Attached to bones and around entrance points to body (e.g., mouth, anus) Comparison of Structure and Properties of Muscle Tissue Types Tissue Histology Function Location Short, branched, : rl ntr m aie st ated, Contracts to pump ean single blood central nucleus Involuntar Short, vey : movement, moves ae food, involuntar shaped, no ‘ y Walls of : control of : evident Pea major Smooth ihe respiration, moves Striation, organs and single secretions, regulates passageways F flow of blood in SSA eae arteries b each fiber y contraction Skeletal muscle is attached to bones and its contraction makes possible locomotion, facial expressions, posture, and other voluntary movements of the body. Forty percent of your body mass is made up of skeletal muscle. Skeletal muscles generate heat as a byproduct of their contraction and thus participate in thermal homeostasis. Shivering is an involuntary contraction of skeletal muscles in response to perceived lower than normal body temperature. The muscle cell, or myocyte, develops from myoblasts derived from the mesoderm. Myocytes and their numbers remain relatively constant throughout life. Skeletal muscle tissue is arranged in bundles surrounded by connective tissue. Under the light microscope, muscle cells appear striated with many nuclei squeezed along the membranes. The striation is due to the regular alternation of the contractile proteins actin and myosin, along with the structural proteins that couple the contractile proteins to connective tissues. The cells are multinucleated as a result of the fusion of the many myoblasts that fuse to form each long muscle fiber. Cardiac muscle forms the contractile walls of the heart. The cells of cardiac muscle, known as cardiomyocytes, also appear striated under the microscope. Unlike skeletal muscle fibers, cardiomyocytes are single cells typically with a single centrally located nucleus. A principal characteristic of cardiomyocytes is that they contract on their own intrinsic rhythms without any external stimulation. Cardiomyocyte attach to one another with specialized cell junctions called intercalated discs. Intercalated discs have both anchoring junctions and gap junctions. Attached cells form long, branching cardiac muscle fibers that are, essentially, a mechanical and electrochemical syncytium allowing the cells to synchronize their actions. The cardiac muscle pumps blood through the body and is under involuntary control. The attachment junctions hold adjacent cells together across the dynamic pressures changes of the cardiac cycle. Smooth muscle tissue contraction is responsible for involuntary movements in the internal organs. It forms the contractile component of the digestive, urinary, and reproductive systems as well as the airways and arteries. Each cell is spindle shaped with a single nucleus and no visible striations ((link]). Muscle Tissue (a) Skeletal muscle cells have prominent striation and nuclei on their periphery. (b) Smooth muscle cells have a single nucleus and no visible striations. (c) Cardiac muscle cells appear striated and have a single nucleus. From top, LM x 1600, LM x 1600, LM x 1600. (Micrographs provided by the Regents of University of Michigan Medical School © 2012) Note: | acl ele, "a —: mess Openstax COLLEGE —— . . Watch this video to learn more about muscle tissue. In looking through a microscope how could you distinguish skeletal muscle tissue from smooth muscle? Chapter Review The three types of muscle cells are skeletal, cardiac, and smooth. Their morphologies match their specific functions in the body. Skeletal muscle is voluntary and responds to conscious stimuli. The cells are striated and multinucleated appearing as long, unbranched cylinders. Cardiac muscle is involuntary and found only in the heart. Each cell is striated with a single nucleus and they attach to one another to form long fibers. Cells are attached to one another at intercalated disks. The cells are interconnected physically and electrochemically to act as a syncytium. Cardiac muscle cells contract autonomously and involuntarily. Smooth muscle is involuntary. Each cell is a spindle-shaped fiber and contains a single nucleus. No striations are evident because the actin and myosin filaments do not align in the cytoplasm. Interactive Link Questions Exercise: Problem: Watch this video to learn more about muscle tissue. In looking through a microscope how could you distinguish skeletal muscle tissue from smooth muscle? Solution: Skeletal muscle cells are striated. Review Questions Exercise: Problem: Striations, cylindrical cells, and multiple nuclei are observed in a. Skeletal muscle only b. cardiac muscle only c. smooth muscle only d. skeletal and cardiac muscles Solution: A Exercise: Problem:The cells of muscles, myocytes, develop from a. myoblasts b. endoderm c. fibrocytes d. chondrocytes Solution: A Exercise: Problem: Skeletal muscle is composed of very hard working cells. Which organelles do you expect to find in abundance in skeletal muscle cell? a. nuclei b. striations c. golgi bodies d. mitochondria Solution: D Critical Thinking Questions Exercise: Problem: You are watching cells in a dish spontaneously contract. They are all contracting at different rates; some fast, some slow. After a while, several cells link up and they begin contracting in synchrony. Discuss what is going on and what type of cells you are looking at. Solution: The cells in the dish are cardiomyocytes, cardiac muscle cells. They have an intrinsic ability to contract. When they link up, they form intercalating discs that allow the cells to communicate with each other and begin contracting in synchrony. Exercise: Problem: Why does skeletal muscle look striated? Solution: Under the light microscope, cells appear striated due to the arrangement of the contractile proteins actin and myosin. Glossary cardiac muscle heart muscle, under involuntary control, composed of striated cells that attach to form fibers, each cell contains a single nucleus, contracts autonomously myocyte muscle cells skeletal muscle usually attached to bone, under voluntary control, each cell is a fiber that is multinucleated and striated smooth muscle under involuntary control, moves internal organs, cells contain a single nucleus, are spindle-shaped, and do not appear striated; each cell is a fiber striation alignment of parallel actin and myosin filaments which form a banded pattern Nervous Tissue Mediates Perception and Response By the end of this section, you will be able to: e Identify the classes of cells that make up nervous tissue e Discuss how nervous tissue mediates perception and response Nervous tissue is characterized as being excitable and capable of sending and receiving electrochemical signals that provide the body with information. Two main classes of cells make up nervous tissue: the neuron and neuroglia ([link]). Neurons propagate information via electrochemical impulses, called action potentials, which are biochemically linked to the release of chemical signals. Neuroglia play an essential role in supporting neurons and modulating their information propagation. The Neuron Contact with other cells a Nucleus Microfibrils and microtubules Axon Dendrites The cell body of a neuron, also called the soma, contains the nucleus and mitochondria. The dendrites transfer the nerve impulse to the soma. The axon carries the action potential away to another excitable cell. LM x 1600. (Micrograph provided by the Regents of University of Michigan Medical School © 2012) Note: = mss Openstax COLLEGE Follow this link to learn more about nervous tissue. What are the main parts of a nerve cell? Neurons display distinctive morphology, well suited to their role as conducting cells, with three main parts. The cell body includes most of the cytoplasm, the organelles, and the nucleus. Dendrites branch off the cell body and appear as thin extensions. A long “tail,” the axon, extends from the neuron body and can be wrapped in an insulating layer known as myelin, which is formed by accessory cells. The synapse is the gap between nerve cells, or between a nerve cell and its target, for example, a muscle or a gland, across which the impulse is transmitted by chemical compounds known as neurotransmitters. Neurons categorized as multipolar neurons have several dendrites and a single prominent axon. Bipolar neurons possess a single dendrite and axon with the cell body, while unipolar neurons have only a single process extending out from the cell body, which divides into a functional dendrite and into a functional axon. When a neuron is sufficiently stimulated, it generates an action potential that propagates down the axon towards the synapse. If enough neurotransmitters are released at the synapse to stimulate the next neuron or target, a response is generated. The second class of neural cells comprises the neuroglia or glial cells, which have been characterized as having a simple support role. The word “glia” comes from the Greek word for glue. Recent research is shedding light on the more complex role of neuroglia in the function of the brain and nervous system. Astrocyte cells, named for their distinctive star shape, are abundant in the central nervous system. The astrocytes have many functions, including regulation of ion concentration in the intercellular space, uptake and/or breakdown of some neurotransmitters, and formation of the blood-brain barrier, the membrane that separates the circulatory system from the brain. Microglia protect the nervous system against infection but are not nervous tissue because they are related to macrophages. Oligodendrocyte cells produce myelin in the central nervous system (brain and spinal cord) while the Schwann cell produces myelin in the peripheral nervous system ([link]). Nervous Tissue Neurons Microglial cell Sa Astrocytes y ' “s aK. OZ Oligodendrocyte Nervous tissue is made up of neurons and neuroglia. The cells of nervous tissue are specialized to transmit and receive impulses. LM x 872. (Micrograph provided by the Regents of University of Michigan Medical School © 2012) Chapter Review The most prominent cell of the nervous tissue, the neuron, is characterized mainly by its ability to receive stimuli and respond by generating an electrical signal, known as an action potential, which can travel rapidly over great distances in the body. A typical neuron displays a distinctive morphology: a large cell body branches out into short extensions called dendrites, which receive chemical signals from other neurons, and a long tail called an axon, which relays signals away from the cell to other neurons, muscles, or glands. Many axons are wrapped by a myelin sheath, a lipid derivative that acts as an insulator and speeds up the transmission of the action potential. Other cells in the nervous tissue, the neuroglia, include the astrocytes, microglia, oligodendrocytes, and Schwann cells. Interactive Link Questions Exercise: Problem: Follow this link to learn more about nervous tissue. What are the main parts of a nerve cell? Solution: Dendrites, cell body, and the axon. Review Questions Exercise: Problem: The cells responsible for the transmission of the nerve impulse are a. Neurons b. oligodendrocytes c. astrocytes d. microglia Solution: A Exercise: Problem: The nerve impulse travels down a(n) , away from the cell body. a. dendrite b. axon c. microglia d. collagen fiber Solution: B Exercise: Problem: Which of the following central nervous system cells regulate ions, regulate the uptake and/or breakdown of some neurotransmitters, and contribute to the formation of the blood-brain barrier? a. microglia b. neuroglia c. oligodendrocytes d. astrocytes Solution: D Critical Thinking Questions Exercise: Problem: Which morphological adaptations of neurons make them suitable for the transmission of nerve impulse? Solution: Neurons are well suited for the transmission of nerve impulses because short extensions, dendrites, receive impulses from other neurons, while a long tail extension, an axon, carries electrical impulses away from the cell to other neurons. Exercise: Problem: What are the functions of astrocytes? Solution: Astrocytes regulate ions and uptake and/or breakdown of some neurotransmitters and contribute to the formation of the blood-brain- barrier. References Stern, P. Focus issue: getting excited about glia. Science [Internet]. 2010 [cited 2012 Dec 4]; 3(147):330-773. Available from: http://stke.sciencemag,org/cgi/content/abstract/sigtrans;3/147/eg11 Ming GL, Song H. Adult neurogenesis in the mammalian central nervous system. Annu. Rev. Neurosci. 2005 [cited 2012 Dec 4]; 28:223—250. Glossary astrocyte star-shaped cell in the central nervous system that regulates ions and uptake and/or breakdown of some neurotransmitters and contributes to the formation of the blood-brain barrier myelin layer of lipid inside some neuroglial cells that wraps around the axons of some neurons neuroglia supportive neural cells neuron excitable neural cell that transfer nerve impulses oligodendrocyte neuroglial cell that produces myelin in the brain Schwann cell neuroglial cell that produces myelin in the peripheral nervous system Tissue Injury and Aging By the end of this section, you will be able to: e Identify the cardinal signs of inflammation List the body’s response to tissue injury e Explain the process of tissue repair Discuss the progressive impact of aging on tissue e Describe cancerous mutations’ effect on tissue Tissues of all types are vulnerable to injury and, inevitably, aging. In the former case, understanding how tissues respond to damage can guide strategies to aid repair. In the latter case, understanding the impact of aging can help in the search for ways to diminish its effects. Tissue Injury and Repair Inflammation is the standard, initial response of the body to injury. Whether biological, chemical, physical, or radiation burns, all injuries lead to the same sequence of physiological events. Inflammation limits the extent of injury, partially or fully eliminates the cause of injury, and initiates repair and regeneration of damaged tissue. Necrosis, or accidental cell death, causes inflammation. Apoptosis is programmed cell death, a normal step-by-step process that destroys cells no longer needed by the body. By mechanisms still under investigation, apoptosis does not initiate the inflammatory response. Acute inflammation resolves over time by the healing of tissue. If inflammation persists, it becomes chronic and leads to diseased conditions. Arthritis and tuberculosis are examples of chronic inflammation. The suffix “-itis” denotes inflammation of a specific organ or type, for example, peritonitis is the inflammation of the peritoneum, and meningitis refers to the inflammation of the meninges, the tough membranes that surround the central nervous system The four cardinal signs of inflammation—redness, swelling, pain, and local heat—were first recorded in antiquity. Cornelius Celsus is credited with documenting these signs during the days of the Roman Empire, as early as the first century AD. A fifth sign, loss of function, may also accompany inflammation. Upon tissue injury, damaged cells release inflammatory chemical signals that evoke local vasodilation, the widening of the blood vessels. Increased blood flow results in apparent redness and heat. In response to injury, mast cells present in tissue degranulate, releasing the potent vasodilator histamine. Increased blood flow and inflammatory mediators recruit white blood cells to the site of inflammation. The endothelium lining the local blood vessel becomes “leaky” under the influence of histamine and other inflammatory mediators allowing neutrophils, macrophages, and fluid to move from the blood into the interstitial tissue spaces. The excess liquid in tissue causes swelling, more properly called edema. The swollen tissues squeezing pain receptors cause the sensation of pain. Prostaglandins released from injured cells also activate pain neurons. Non-steroidal anti- inflammatory drugs (NSAIDs) reduce pain because they inhibit the synthesis of prostaglandins. High levels of NSAIDs reduce inflammation. Antihistamines decrease allergies by blocking histamine receptors and as a result the histamine response. After containment of an injury, the tissue repair phase starts with removal of toxins and waste products. Clotting (coagulation) reduces blood loss from damaged blood vessels and forms a network of fibrin proteins that trap blood cells and bind the edges of the wound together. A scab forms when the clot dries, reducing the risk of infection. Sometimes a mixture of dead leukocytes and fluid called pus accumulates in the wound. As healing progresses, fibroblasts from the surrounding connective tissues replace the collagen and extracellular material lost by the injury. Angiogenesis, the growth of new blood vessels, results in vascularization of the new tissue known as granulation tissue. The clot retracts pulling the edges of the wound together, and it slowly dissolves as the tissue is repaired. When a large amount of granulation tissue forms and capillaries disappear, a pale scar is often visible in the healed area. A primary union describes the healing of a wound where the edges are close together. When there is a gaping wound, it takes longer to refill the area with cells and collagen. The process called secondary union occurs as the edges of the wound are pulled together by what is called wound contraction. When a wound is more than one quarter of an inch deep, sutures (stitches) are recommended to promote a primary union and avoid the formation of a disfiguring scar. Regeneration is the addition of new cells of the same type as the ones that were injured ((link]). Tissue Healing Clotting occurs, caused by clotting proteins Epithelial cells multiply and fill Restored epthelium thickens; the and plasma proteins, and a scab is formed in over the granulation tissue area matures and contracts Inflammatory chemicals White blood cells seep Granulation tissue restores Underlying area are released from injury _ into the injured area the vascular supply of scar tissue During wound repair, collagen fibers are laid down randomly by fibroblasts that move into repair the area. Note: Oa ec openstax COLLEGE” . 75 . O) a r Watch this video to see a hand heal. Over what period of time do you think these images were taken? Tissue and Aging According to poet Ralph Waldo Emerson, “The surest poison is time.” In fact, biology confirms that many functions of the body decline with age. All the cells, tissues, and organs are affected by senescence, with noticeable variability between individuals owing to different genetic makeup and lifestyles. The outward signs of aging are easily recognizable. The skin and other tissues become thinner and drier, reducing their elasticity, contributing to wrinkles and high blood pressure. Hair turns gray because follicles produce less melanin, the brown pigment of hair and the iris of the eye. The face looks flabby because elastic and collagen fibers decrease in connective tissue and muscle tone is lost. Glasses and hearing aids may become parts of life as the senses slowly deteriorate, all due to reduced elasticity. Overall height decreases as the bones lose calcium and other minerals. With age, fluid decreases in the fibrous cartilage disks intercalated between the vertebrae in the spine. Joints lose cartilage and stiffen. Many tissues, including those in muscles, lose mass through a process called atrophy. Lumps and rigidity become more widespread. As a consequence, the passageways, blood vessels, and airways become more rigid. The brain and spinal cord lose mass. Nerves do not transmit impulses with the same speed and frequency as in the past. Some loss of thought clarity and memory can accompany aging. More severe problems are not necessarily associated with the aging process and may be symptoms of underlying illness. As exterior signs of aging increase, so do the interior signs, which are not as noticeable. The incidence of heart diseases, respiratory syndromes, and type 2 diabetes increases with age, though these are not necessarily age- dependent effects. Wound healing is slower in the elderly, accompanied by a higher frequency of infection as the capacity of the immune system to fend off pathogen declines. Aging is also apparent at the cellular level because all cells experience changes with aging. Telomeres, regions of the chromosomes necessary for cell division, shorten each time cells divide. As they do, cells are less able to divide and regenerate. Because of alterations in cell membranes, transport of oxygen and nutrients into the cell and removal of carbon dioxide and waste products from the cell are not as efficient in the elderly. Cells may begin to function abnormally, which may lead to diseases associated with aging, including arthritis, memory issues, and some cancers. The progressive impact of aging on the body varies considerably among individuals, but Studies indicate, however, that exercise and healthy lifestyle choices can slow down the deterioration of the body that comes with old age. Note: Homeostatic Imbalances Tissues and Cancer Cancer is a generic term for many diseases in which cells escape regulatory signals. Uncontrolled growth, invasion into adjacent tissues, and colonization of other organs, if not treated early enough, are its hallmarks. Health suffers when tumors “rob” blood supply from the “normal” organs. A mutation is defined as a permanent change in the DNA of a cell. Epigenetic modifications, changes that do not affect the code of the DNA but alter how the DNA is decoded, are also known to generate abnormal cells. Alterations in the genetic material may be caused by environmental agents, infectious agents, or errors in the replication of DNA that accumulate with age. Many mutations do not cause any noticeable change in the functions of a cell. However, if the modification affects key proteins that have an impact on the cell’s ability to proliferate in an orderly fashion, the cell starts to divide abnormally. As changes in cells accumulate, they lose their ability to form regular tissues. A tumor, a mass of cells displaying abnormal architecture, forms in the tissue. Many tumors are benign, meaning they do not metastasize nor cause disease. A tumor becomes malignant, or cancerous, when it breaches the confines of its tissue, promotes angiogenesis, attracts the growth of capillaries, and metastasizes to other organs ((link]). The specific names of cancers reflect the tissue of origin. Cancers derived from epithelial cells are referred to as carcinomas. Cancer in myeloid tissue or blood cells form myelomas. Leukemias are cancers of white blood cells, whereas sarcomas derive from connective tissue. Cells in tumors differ both in structure and function. Some cells, called cancer stem cells, appear to be a subtype of cell responsible for uncontrolled growth. Recent research shows that contrary to what was previously assumed, tumors are not disorganized masses of cells, but have their own structures. Development of Cancer Cell division takes place to replace lost tissue Cell division accelerates .—\— WSS SS SW: nN Carcinoma breaks into underlying tissue Underlying tissue Note the change in cell size, nucleus size, and organization in the tissue. Note: Coker aa) ae — wm, OPENSTAX COLLEGE — . - tO) hs Watch this video to learn more about tumors. What is a tumor? Cancer treatments vary depending on the disease’s type and stage. Traditional approaches, including surgery, radiation, chemotherapy, and hormonal therapy, aim to remove or kill rapidly dividing cancer cells, but these strategies have their limitations. Depending on a tumor’s location, for example, cancer surgeons may be unable to remove it. Radiation and chemotherapy are difficult, and it is often impossible to target only the cancer cells. The treatments inevitably destroy healthy tissue as well. To address this, researchers are working on pharmaceuticals that can target specific proteins implicated in cancer-associated molecular pathways. Chapter Review Inflammation is the classic response of the body to injury and follows a common sequence of events. The area is red, feels warm to the touch, swells, and is painful. Injured cells, mast cells, and resident macrophages release chemical signals that cause vasodilation and fluid leakage in the surrounding tissue. The repair phase includes blood clotting, followed by regeneration of tissue as fibroblasts deposit collagen. Some tissues regenerate more readily than others. Epithelial and connective tissues replace damaged or dead cells from a supply of adult stem cells. Muscle and nervous tissues undergo either slow regeneration or do not repair at all. Age affects all the tissues and organs of the body. Damaged cells do not regenerate as rapidly as in younger people. Perception of sensation and effectiveness of response are lost in the nervous system. Muscles atrophy, and bones lose mass and become brittle. Collagen decreases in some connective tissue, and joints stiffen. Interactive Link Questions Exercise: Problem: Watch this video to see a hand heal. Over what period of time do you think these images were taken? Solution: Approximately one month. Exercise: Problem: Watch this video to learn more about tumors. What is a tumor? Solution: A mass of cancer cells that continue to grow and divide. Review Questions Exercise: Problem: Which of the following processes is not a cardinal sign of inflammation? a. redness b. heat c. fever d. swelling Solution: C Exercise: Problem: When a mast cell reacts to an irritation, which of the following chemicals does it release? a. collagen b. histamine c. hyaluronic acid d. meylin Solution: B Exercise: Problem: Atrophy refers to a. loss of elasticity b. loss of mass c. loss of rigidity d. loss of permeability Solution: B Exercise: Problem: Individuals can slow the rate of aging by modifying all of these lifestyle aspects except for a. diet b. exercise c. genetic factors d. stress Solution: C Critical Thinking Questions Exercise: Problem: Why is it important to watch for increased redness, swelling and pain after a cut or abrasion has been cleaned and bandaged? Solution: These symptoms would indicate that infection is present. Exercise: Problem: Aspirin is a non-steroidal anti-inflammatory drug (NSAID) that inhibits the formation of blood clots and is taken regularly by individuals with a heart condition. Steroids such as cortisol are used to control some autoimmune diseases and severe arthritis by down- regulating the inflammatory response. After reading the role of inflammation in the body’s response to infection, can you predict an undesirable consequence of taking anti-inflammatory drugs on a regular basis? Solution: Since NSAIDs or other anti-inflammatory drugs inhibit the formation of blood clots, regular and prolonged use of these drugs may promote internal bleeding, such as bleeding in the stomach. Excessive levels of cortisol would suppress inflammation, which could slow the wound healing process. Exercise: Problem: As an individual ages, a constellation of symptoms begins the decline to the point where an individual’s functioning is compromised. Identify and discuss two factors that have a role in factors leading to the compromised situation. Solution: The genetic makeup and the lifestyle of each individual are factors which determine the degree of decline in cells, tissues, and organs as an individual ages. Exercise: Problem: Discuss changes that occur in cells as a person ages. Solution: All cells experience changes with aging. They become larger, and many cannot divide and regenerate. Because of alterations in cell membranes, transport of oxygen and nutrients into the cell and removal of carbon dioxide and waste products are not as efficient in the elderly. Cells lose their ability to function, or they begin to function abnormally, leading to disease and cancer. References Emerson, RW. Old age. Atlantic. 1862 [cited 2012 Dec 4]; 9(51):134—140. Glossary apoptosis programmed cell death atrophy loss of mass and function clotting also called coagulation; complex process by which blood components form a plug to stop bleeding histamine chemical compound released by mast cells in response to injury that causes vasodilation and endothelium permeability inflammation response of tissue to injury necrosis accidental death of cells and tissues primary union condition of a wound where the wound edges are close enough to be brought together and fastened if necessary, allowing quicker and more thorough healing secondary union wound healing facilitated by wound contraction vasodilation widening of blood vessels wound contraction process whereby the borders of a wound are physically drawn together Layers of the Skin By the end of this section, you will be able to: e Identify the components of the integumentary system Describe the layers of the skin and the functions of each layer e Identify and describe the hypodermis and deep fascia Describe the role of keratinocytes and their life cycle e Describe the role of melanocytes in skin pigmentation Although you may not typically think of the skin as an organ, it is in fact made of tissues that work together as a single structure to perform unique and critical functions. The skin and its accessory structures make up the integumentary system, which provides the body with overall protection. The skin is made of multiple layers of cells and tissues, which are held to underlying structures by connective tissue ({link]). The deeper layer of skin is well vascularized (has numerous blood vessels). It also has numerous sensory, and autonomic and sympathetic nerve fibers ensuring communication to and from the brain. Layers of Skin Hair shaft Pore of sweat gland duct Epidermis | Arrector pili muscle Hair follicle Sebaceous (oil) gland Hypodermis Hair root Hair follicle receptor Eccrine sweat gland Pacinian corpuscle Cutaneous vascular plexus Adipose tissue Sensory nerve fiber The skin is composed of two main layers: the epidermis, made of closely packed epithelial cells, and the dermis, made of dense, irregular connective tissue that houses blood vessels, hair follicles, sweat glands, and other structures. Beneath the dermis lies the hypodermis, which is composed mainly of loose connective and fatty tissues. ee mess’ OPENStAX COLLEGE” i io ae T The skin consists of two main layers and a closely associated layer. View this animation to learn more about layers of the skin. What are the basic functions of each of these layers? The Epidermis The epidermis is composed of keratinized, stratified squamous epithelium. It is made of four or five layers of epithelial cells, depending on its location in the body. It does not have any blood vessels within it (i.e., it is avascular). Skin that has four layers of cells is referred to as “thin skin.” From deep to superficial, these layers are the stratum basale, stratum spinosum, stratum granulosum, and stratum corneum. Most of the skin can be classified as thin skin. “Thick skin” is found only on the palms of the hands and the soles of the feet. It has a fifth layer, called the stratum lucidum, located between the stratum corneum and the stratum granulosum (Llink]). Thin Skin versus Thick Skin These slides show cross- sections of the epidermis and dermis of (a) thin and (b) thick skin. Note the significant difference in the thickness of the epithelial layer of the thick skin. From top, LM x 40, LM ~x 40. (Micrographs provided by the Regents of University of Michigan Medical School © 2012) The cells in all of the layers except the stratum basale are called keratinocytes. A keratinocyte is a cell that manufactures and stores the protein keratin. Keratin is an intracellular fibrous protein that gives hair, nails, and skin their hardness and water-resistant properties. The keratinocytes in the stratum corneum are dead and regularly slough away, being replaced by cells from the deeper layers ({link]). Epidermis The epidermis is epithelium composed of multiple layers of cells. The basal layer consists of cuboidal cells, whereas the outer layers are squamous, keratinized cells, so the whole epithelium is often described as being keratinized stratified squamous epithelium. LM x 40. (Micrograph provided by the Regents of University of Michigan Medical School © 2012) Note: . . openstax COLLEGE View the University of Michigan WebScope to explore the tissue sample in greater detail. If you zoom on the cells at the outermost layer of this section of skin, what do you notice about the cells? Stratum Basale The stratum basale (also called the stratum germinativum) is the deepest epidermal layer and attaches the epidermis to the basal lamina, below which lie the layers of the dermis. The cells in the stratum basale bond to the dermis via intertwining collagen fibers, referred to as the basement membrane. A finger-like projection, or fold, known as the dermal papilla (plural = dermal papillae) is found in the superficial portion of the dermis. Dermal papillae increase the strength of the connection between the epidermis and dermis; the greater the folding, the stronger the connections made ([link]). Layers of the Epidermis Dead cells filled with keratin 5 = Stratum corneum = eS =o SSSqNqBPHOO Stratum lucidum ——_{_ Stratum granulosum : ——_,_ Stratum spinosum = Stratum basale — j Melanocyte Lamellar granules Keratinocyte Merkel cell : Sensory neuron Dermis The epidermis of thick skin has five layers: stratum basale, stratum spinosum, stratum granulosum, stratum lucidum, and stratum corneum. The stratum basale is a single layer of cells primarily made of basal cells. A basal cell is a cuboidal-shaped stem cell that is a precursor of the keratinocytes of the epidermis. All of the keratinocytes are produced from this single layer of cells, which are constantly going through mitosis to produce new cells. As new cells are formed, the existing cells are pushed superficially away from the stratum basale. Two other cell types are found dispersed among the basal cells in the stratum basale. The first is a Merkel cell, which functions as a receptor and is responsible for stimulating sensory nerves that the brain perceives as touch. These cells are especially abundant on the surfaces of the hands and feet. The second is a melanocyte, a cell that produces the pigment melanin. Melanin gives hair and skin its color, and also helps protect the living cells of the epidermis from ultraviolet (UV) radiation damage. In a growing fetus, fingerprints form where the cells of the stratum basale meet the papillae of the underlying dermal layer (papillary layer), resulting in the formation of the ridges on your fingers that you recognize as fingerprints. Fingerprints are unique to each individual and are used for forensic analyses because the patterns do not change with the growth and aging processes. Stratum Spinosum As the name suggests, the stratum spinosum is spiny in appearance due to the protruding cell processes that join the cells via a structure called a desmosome. The desmosomes interlock with each other and strengthen the bond between the cells. It is interesting to note that the “spiny” nature of this layer is an artifact of the staining process. Unstained epidermis samples do not exhibit this characteristic appearance. The stratum spinosum is composed of eight to 10 layers of keratinocytes, formed as a result of cell division in the stratum basale ([link]). Interspersed among the keratinocytes of this layer is a type of dendritic cell called the Langerhans cell, which functions as a macrophage by engulfing bacteria, foreign particles, and damaged cells that occur in this layer. Cells of the Epidermis 065 - Epidermis_001.svs WebScope 1 16013 x 16013 size 733.61MB mag 20X Return to image directory (© fale "SSB Oo Gm O OVE | Epiaermis Stratum ,, corneum i Stratum ~ granulosum Remnants of cross-sectioned Stratum shed hair and spinosum its follicle Stratum basal or germinativum Capillai Dermi poy EJM Mag. 2,700 X The cells in the different layers of the epidermis originate from basal cells located in the stratum basale, yet the cells of each layer are distinctively different. EM x 2700. (Micrograph provided by the Regents of University of Michigan Medical School © 2012) Note: poe openstax COLLEGE” View the University of Michigan WebScope to explore the tissue sample in greater detail. If you zoom on the cells at the outermost layer of this section of skin, what do you notice about the cells? The keratinocytes in the stratum spinosum begin the synthesis of keratin and release a water-repelling glycolipid that helps prevent water loss from the body, making the skin relatively waterproof. As new keratinocytes are produced atop the stratum basale, the keratinocytes of the stratum spinosum are pushed into the stratum granulosum. Stratum Granulosum The stratum granulosum has a grainy appearance due to further changes to the keratinocytes as they are pushed from the stratum spinosum. The cells (three to five layers deep) become flatter, their cell membranes thicken, and they generate large amounts of the proteins keratin, which is fibrous, and keratohyalin, which accumulates as lamellar granules within the cells (see [link]). These two proteins make up the bulk of the keratinocyte mass in the stratum granulosum and give the layer its grainy appearance. The nuclei and other cell organelles disintegrate as the cells die, leaving behind the keratin, keratohyalin, and cell membranes that will form the stratum lucidum, the stratum comeum, and the accessory structures of hair and nails. Stratum Lucidum The stratum lucidum is a smooth, seemingly translucent layer of the epidermis located just above the stratum granulosum and below the stratum corneum. This thin layer of cells is found only in the thick skin of the palms, soles, and digits. The keratinocytes that compose the stratum lucidum are dead and flattened (see [link]). These cells are densely packed with eleiden, a clear protein rich in lipids, derived from keratohyalin, which gives these cells their transparent (i.e., lucid) appearance and provides a barrier to water. Stratum Corneum The stratum corneum is the most superficial layer of the epidermis and is the layer exposed to the outside environment (see [link]). The increased keratinization (also called cornification) of the cells in this layer gives it its name. There are usually 15 to 30 layers of cells in the stratum corneum. This dry, dead layer helps prevent the penetration of microbes and the dehydration of underlying tissues, and provides a mechanical protection against abrasion for the more delicate, underlying layers. Cells in this layer are shed periodically and are replaced by cells pushed up from the stratum granulosum (or stratum lucidum in the case of the palms and soles of feet). The entire layer is replaced during a period of about 4 weeks. Cosmetic procedures, such as microdermabrasion, help remove some of the dry, upper layer and aim to keep the skin looking “fresh” and healthy. Dermis The dermis might be considered the “core” of the integumentary system (derma- = “skin”), as distinct from the epidermis (epi- = “upon” or “over” and hypodermis (hypo- = “below”). It contains blood and lymph vessels, nerves, and other structures, such as hair follicles and sweat glands. The dermis is made of two layers of connective tissue that compose an interconnected mesh of elastin and collagenous fibers, produced by fibroblasts ({link]). Layers of the Dermis This stained slide shows the two components of the dermis—the papillary layer and the reticular layer. Both are made of connective tissue with fibers of collagen extending from one to the other, making the border between the two somewhat indistinct. The dermal papillae extending into the epidermis belong to the papillary layer, whereas the dense collagen fiber bundles below belong to the reticular layer. LM x 10. (credit: modification of work by “kilbad”/Wikimedia Commons) Papillary Layer The papillary layer is made of loose, areolar connective tissue, which means the collagen and elastin fibers of this layer form a loose mesh. This superficial layer of the dermis projects into the stratum basale of the epidermis to form finger-like dermal papillae (see [link]). Within the papillary layer are fibroblasts, a small number of fat cells (adipocytes), and an abundance of small blood vessels. In addition, the papillary layer contains phagocytes, defensive cells that help fight bacteria or other infections that have breached the skin. This layer also contains lymphatic capillaries, nerve fibers, and touch receptors called the Meissner corpuscles. Reticular Layer Underlying the papillary layer is the much thicker reticular layer, composed of dense, irregular connective tissue. This layer is well vascularized and has a rich sensory and sympathetic nerve supply. The reticular layer appears reticulated (net-like) due to a tight meshwork of fibers. Elastin fibers provide some elasticity to the skin, enabling movement. Collagen fibers provide structure and tensile strength, with strands of collagen extending into both the papillary layer and the hypodermis. In addition, collagen binds water to keep the skin hydrated. Collagen injections and Retin-A creams help restore skin turgor by either introducing collagen externally or stimulating blood flow and repair of the dermis, respectively. Hypodermis The hypodermis (also called the subcutaneous layer or superficial fascia) is a layer directly below the dermis and serves to connect the skin to the underlying fascia (fibrous tissue) of the bones and muscles. It is not strictly a part of the skin, although the border between the hypodermis and dermis can be difficult to distinguish. The hypodermis consists of well- vascularized, loose, areolar connective tissue and adipose tissue, which functions as a mode of fat storage and provides insulation and cushioning for the integument. Note: Everyday Connection Lipid Storage The hypodermis is home to most of the fat that concerns people when they are trying to keep their weight under control. Adipose tissue present in the hypodermis consists of fat-storing cells called adipocytes. This stored fat can serve as an energy reserve, insulate the body to prevent heat loss, and act as a cushion to protect underlying structures from trauma. Where the fat is deposited and accumulates within the hypodermis depends on hormones (testosterone, estrogen, insulin, glucagon, leptin, and others), as well as genetic factors. Fat distribution changes as our bodies mature and age. Men tend to accumulate fat in different areas (neck, arms, lower back, and abdomen) than do women (breasts, hips, thighs, and buttocks). The body mass index (BMI) is often used as a measure of fat, although this measure is, in fact, derived from a mathematical formula that compares body weight (mass) to height. Therefore, its accuracy as a health indicator can be called into question in individuals who are extremely physically fit. In many animals, there is a pattern of storing excess calories as fat to be used in times when food is not readily available. In much of the developed world, insufficient exercise coupled with the ready availability and consumption of high-calorie foods have resulted in unwanted accumulations of adipose tissue in many people. Although periodic accumulation of excess fat may have provided an evolutionary advantage to our ancestors, who experienced unpredictable bouts of famine, it is now becoming chronic and considered a major health threat. Recent studies indicate that a distressing percentage of our population is overweight and/or clinically obese. Not only is this a problem for the individuals affected, but it also has a severe impact on our healthcare system. Changes in lifestyle, specifically in diet and exercise, are the best ways to control body fat accumulation, especially when it reaches levels that increase the risk of heart disease and diabetes. Pigmentation The color of skin is influenced by a number of pigments, including melanin, carotene, and hemoglobin. Recall that melanin is produced by cells called melanocytes, which are found scattered throughout the stratum basale of the epidermis. The melanin is transferred into the keratinocytes via a cellular vesicle called a melanosome ((link]). Skin Pigmentation Surface Upper keratinocytes Melanosomes Basal keratinocytes Melanocytes The relative coloration of the skin depends of the amount of melanin produced by melanocytes in the stratum basale and taken up by keratinocytes. Melanin occurs in two primary forms. Eumelanin exists as black and brown, whereas pheomelanin provides a red color. Dark-skinned individuals produce more melanin than those with pale skin. Exposure to the UV rays of the sun or a tanning salon causes melanin to be manufactured and built up in keratinocytes, as sun exposure stimulates keratinocytes to secrete chemicals that stimulate melanocytes. The accumulation of melanin in keratinocytes results in the darkening of the skin, or a tan. This increased melanin accumulation protects the DNA of epidermal cells from UV ray damage and the breakdown of folic acid, a nutrient necessary for our health and well-being. In contrast, too much melanin can interfere with the production of vitamin D, an important nutrient involved in calcium absorption. Thus, the amount of melanin present in our skin is dependent on a balance between available sunlight and folic acid destruction, and protection from UV radiation and vitamin D production. It requires about 10 days after initial sun exposure for melanin synthesis to peak, which is why pale-skinned individuals tend to suffer sunburns of the epidermis initially. Dark-skinned individuals can also get sunburns, but are more protected than are pale-skinned individuals. Melanosomes are temporary structures that are eventually destroyed by fusion with lysosomes; this fact, along with melanin-filled keratinocytes in the stratum corneum sloughing off, makes tanning impermanent. Too much sun exposure can eventually lead to wrinkling due to the destruction of the cellular structure of the skin, and in severe cases, can cause sufficient DNA damage to result in skin cancer. When there is an irregular accumulation of melanocytes in the skin, freckles appear. Moles are larger masses of melanocytes, and although most are benign, they should be monitored for changes that might indicate the presence of cancer ({link]). Moles Moles range from benign accumulations of melanocytes to melanomas. These structures populate the landscape of our skin. (credit: the National Cancer Institute) Note: Disorders of the... Integumentary System The first thing a clinician sees is the skin, and so the examination of the skin should be part of any thorough physical examination. Most skin disorders are relatively benign, but a few, including melanomas, can be fatal if untreated. A couple of the more noticeable disorders, albinism and vitiligo, affect the appearance of the skin and its accessory organs. Although neither is fatal, it would be hard to claim that they are benign, at least to the individuals so afflicted. Albinism is a genetic disorder that affects (completely or partially) the coloring of skin, hair, and eyes. The defect is primarily due to the inability of melanocytes to produce melanin. Individuals with albinism tend to appear white or very pale due to the lack of melanin in their skin and hair. Recall that melanin helps protect the skin from the harmful effects of UV radiation. Individuals with albinism tend to need more protection from UV radiation, as they are more prone to sunburns and skin cancer. They also tend to be more sensitive to light and have vision problems due to the lack of pigmentation on the retinal wall. Treatment of this disorder usually involves addressing the symptoms, such as limiting UV light exposure to the skin and eyes. In vitiligo, the melanocytes in certain areas lose their ability to produce melanin, possibly due to an autoimmune reaction. This leads to a loss of color in patches ({link]). Neither albinism nor vitiligo directly affects the lifespan of an individual. Vitiligo Individuals with vitiligo experience depigmentation that results in lighter colored patches of skin. The condition is especially noticeable on darker skin. (credit: Klaus D. Peter) Other changes in the appearance of skin coloration can be indicative of diseases associated with other body systems. Liver disease or liver cancer can cause the accumulation of bile and the yellow pigment bilirubin, leading to the skin appearing yellow or jaundiced (jaune is the French word for “yellow”). Tumors of the pituitary gland can result in the secretion of large amounts of melanocyte-stimulating hormone (MSH), which results in a darkening of the skin. Similarly, Addison’s disease can stimulate the release of excess amounts of adrenocorticotropic hormone (ACTH), which can give the skin a deep bronze color. A sudden drop in oxygenation can affect skin color, causing the skin to initially turn ashen (white). With a prolonged reduction in oxygen levels, dark red deoxyhemoglobin becomes dominant in the blood, making the skin appear blue, a condition referred to as cyanosis (kyanos is the Greek word for “blue”). This happens when the oxygen supply is restricted, as when someone is experiencing difficulty in breathing because of asthma or a heart attack. However, in these cases the effect on skin color has nothing do with the skin’s pigmentation. mess’ OPENStAX COLLEGE” seh : T This ABC video follows the story of a pair of fraternal African-American twins, one of whom is albino. Watch this video to learn about the challenges these children and their family face. Which ethnicities do you think are exempt from the possibility of albinism? Chapter Review The skin is composed of two major layers: a superficial epidermis and a deeper dermis. The epidermis consists of several layers beginning with the innermost (deepest) stratum basale (germinatum), followed by the stratum spinosum, stratum granulosum, stratum lucidum (when present), and ending with the outermost layer, the stratum corneum. The topmost layer, the stratum corneum, consists of dead cells that shed periodically and is progressively replaced by cells formed from the basal layer. The stratum basale also contains melanocytes, cells that produce melanin, the pigment primarily responsible for giving skin its color. Melanin is transferred to keratinocytes in the stratum spinosum to protect cells from UV rays. The dermis connects the epidermis to the hypodermis, and provides strength and elasticity due to the presence of collagen and elastin fibers. It has only two layers: the papillary layer with papillae that extend into the epidermis and the lower, reticular layer composed of loose connective tissue. The hypodermis, deep to the dermis of skin, is the connective tissue that connects the dermis to underlying structures; it also harbors adipose tissue for fat storage and protection. Interactive Link Questions Exercise: Problem: The skin consists of two layers and a closely associated layer. View this animation to learn more about layers of the skin. What are the basic functions of each of these layers? Solution: The epidermis provides protection, the dermis provides support and flexibility, and the hypodermis (fat layer) provides insulation and padding. Exercise: Problem: [link] If you zoom on the cells at the outermost layer of this section of skin, what do you notice about the cells? Solution: [link] These cells do not have nuclei, so you can deduce that they are dead. They appear to be sloughing off. Exercise: Problem: [link] If you zoom on the cells of the stratum spinosum, what is distinctive about them? Solution: [link] These cells have desmosomes, which give the cells their spiny appearance. Exercise: Problem: This ABC video follows the story of a pair of fraternal African- American twins, one of whom is albino. Watch this video to learn about the challenges these children and their family face. Which ethnicities do you think are exempt from the possibility of albinism? Solution: There are none. Review Questions Exercise: Problem: The papillary layer of the dermis is most closely associated with which layer of the epidermis? a. stratum spinosum b. stratum corneum c. stratum granulosum d. stratum basale Solution: D Exercise: Problem:Langerhans cells are commonly found in the a. stratum spinosum b. stratum corneum c. stratum granulosum d. stratum basale Solution: A Exercise: Problem: The papillary and reticular layers of the dermis are composed mainly of a. melanocytes b. keratinocytes c. connective tissue d. adipose tissue Solution: C Exercise: Problem: Collagen lends to the skin. a. elasticity b. structure c. color d. UV protection Solution: B Exercise: Problem: Which of the following is not a function of the hypodermis? a. protects underlying organs b. helps maintain body temperature c. source of blood vessels in the epidermis d. a site to long-term energy storage Solution: C Critical Thinking Questions Exercise: Problem: What determines the color of skin, and what is the process that darkens skin when it is exposed to UV light? Solution: The pigment melanin, produced by melanocytes, is primarily responsible for skin color. Melanin comes in different shades of brown and black. Individuals with darker skin have darker, more abundant melanin, whereas fair-skinned individuals have a lighter shade of skin and less melanin. Exposure to UV irradiation stimulates the melanocytes to produce and secrete more melanin. Exercise: Problem: Cells of the epidermis derive from stem cells of the stratum basale. Describe how the cells change as they become integrated into the different layers of the epidermis. Solution: As the cells move into the stratum spinosum, they begin the synthesis of keratin and extend cell processes, desmosomes, which link the cells. As the stratum basale continues to produce new cells, the keratinocytes of the stratum spinosum are pushed into the stratum granulosum. The cells become flatter, their cell membranes thicken, and they generate large amounts of the proteins keratin and keratohyalin. The nuclei and other cell organelles disintegrate as the cells die, leaving behind the keratin, keratohyalin, and cell membranes that form the stratum lucidum and the stratum corneum. The keratinocytes in these layers are mostly dead and flattened. Cells in the stratum corneum are periodically shed. Glossary albinism genetic disorder that affects the skin, in which there is no melanin production basal cell type of stem cell found in the stratum basale and in the hair matrix that continually undergoes cell division, producing the keratinocytes of the epidermis dermal papilla (plural = dermal papillae) extension of the papillary layer of the dermis that increases surface contact between the epidermis and dermis dermis layer of skin between the epidermis and hypodermis, composed mainly of connective tissue and containing blood vessels, hair follicles, sweat glands, and other structures desmosome structure that forms an impermeable junction between cells elastin fibers fibers made of the protein elastin that increase the elasticity of the dermis eleiden clear protein-bound lipid found in the stratum lucidum that is derived from keratohyalin and helps to prevent water loss epidermis outermost tissue layer of the skin hypodermis connective tissue connecting the integument to the underlying bone and muscle integumentary system skin and its accessory structures keratin type of structural protein that gives skin, hair, and nails its hard, water- resistant properties keratinocyte cell that produces keratin and is the most predominant type of cell found in the epidermis keratohyalin granulated protein found in the stratum granulosum Langerhans cell specialized dendritic cell found in the stratum spinosum that functions as a macrophage melanin pigment that determines the color of hair and skin melanocyte cell found in the stratum basale of the epidermis that produces the pigment melanin melanosome intercellular vesicle that transfers melanin from melanocytes into keratinocytes of the epidermis Merkel cell receptor cell in the stratum basale of the epidermis that responds to the sense of touch papillary layer superficial layer of the dermis, made of loose, areolar connective tissue reticular layer deeper layer of the dermis; it has a reticulated appearance due to the presence of abundant collagen and elastin fibers stratum basale deepest layer of the epidermis, made of epidermal stem cells stratum corneum most superficial layer of the epidermis stratum granulosum layer of the epidermis superficial to the stratum spinosum stratum lucidum layer of the epidermis between the stratum granulosum and stratum corneum, found only in thick skin covering the palms, soles of the feet, and digits stratum spinosum layer of the epidermis superficial to the stratum basale, characterized by the presence of desmosomes vitiligo skin condition in which melanocytes in certain areas lose the ability to produce melanin, possibly due an autoimmune reaction that leads to loss of color in patches Accessory Structures of the Skin By the end of this section, you will be able to: e Identify the accessory structures of the skin e Describe the structure and function of hair and nails e Describe the structure and function of sweat glands and sebaceous glands Accessory structures of the skin include hair, nails, sweat glands, and sebaceous glands. These structures embryologically originate from the epidermis and can extend down through the dermis into the hypodermis. Hair Hair is a keratinous filament growing out of the epidermis. It is primarily made of dead, keratinized cells. Strands of hair originate in an epidermal penetration of the dermis called the hair follicle. The hair shaft is the part of the hair not anchored to the follicle, and much of this is exposed at the skin’s surface. The rest of the hair, which is anchored in the follicle, lies below the surface of the skin and is referred to as the hair root. The hair root ends deep in the dermis at the hair bulb, and includes a layer of mitotically active basal cells called the hair matrix. The hair bulb surrounds the hair papilla, which is made of connective tissue and contains blood capillaries and nerve endings from the dermis ([link]). Hair Medulla Cortex Cuticle Sebaceous gland Inner root sheath Outer root sheath Hair matrix Hair papilla Hair follicles originate in the epidermis and have many different parts. Just as the basal layer of the epidermis forms the layers of epidermis that get pushed to the surface as the dead skin on the surface sheds, the basal cells of the hair bulb divide and push cells outward in the hair root and shaft as the hair grows. The medulla forms the central core of the hair, which is surrounded by the cortex, a layer of compressed, keratinized cells that is covered by an outer layer of very hard, keratinized cells known as the cuticle. These layers are depicted in a longitudinal cross-section of the hair follicle ([link]), although not all hair has a medullary layer. Hair texture (straight, curly) is determined by the shape and structure of the cortex, and to the extent that it is present, the medulla. The shape and structure of these layers are, in turn, determined by the shape of the hair follicle. Hair growth begins with the production of keratinocytes by the basal cells of the hair bulb. As new cells are deposited at the hair bulb, the hair shaft is pushed through the follicle toward the surface. Keratinization is completed as the cells are pushed to the skin surface to form the shaft of hair that is externally visible. The external hair is completely dead and composed entirely of keratin. For this reason, our hair does not have sensation. Furthermore, you can cut your hair or shave without damaging the hair structure because the cut is superficial. Most chemical hair removers also act superficially; however, electrolysis and yanking both attempt to destroy the hair bulb so hair cannot grow. Hair Follicle The slide shows a cross-section of a hair follicle. Basal cells of the hair matrix in the center differentiate into cells of the inner root sheath. Basal cells at the base of the hair root form the outer root sheath. LM x 4. (credit: modification of work by “kilbad”/Wikimedia Commons) The wall of the hair follicle is made of three concentric layers of cells. The cells of the internal root sheath surround the root of the growing hair and extend just up to the hair shaft. They are derived from the basal cells of the hair matrix. The external root sheath, which is an extension of the epidermis, encloses the hair root. It is made of basal cells at the base of the hair root and tends to be more keratinous in the upper regions. The glassy membrane is a thick, clear connective tissue sheath covering the hair root, connecting it to the tissue of the dermis. Note: Dasara — meee, OPENStAX COLLEGE —— . The hair follicle is made of multiple layers of cells that form from basal cells in the hair matrix and the hair root. Cells of the hair matrix divide and differentiate to form the layers of the hair. Watch this video to learn more about hair follicles. Hair serves a variety of functions, including protection, sensory input, thermoregulation, and communication. For example, hair on the head protects the skull from the sun. The hair in the nose and ears, and around the eyes (eyelashes) defends the body by trapping and excluding dust particles that may contain allergens and microbes. Hair of the eyebrows prevents sweat and other particles from dripping into and bothering the eyes. Hair also has a sensory function due to sensory innervation by a hair root plexus surrounding the base of each hair follicle. Hair is extremely sensitive to air movement or other disturbances in the environment, much more so than the skin surface. This feature is also useful for the detection of the presence of insects or other potentially damaging substances on the skin surface. Each hair root is connected to a smooth muscle called the arrector pili that contracts in response to nerve signals from the sympathetic nervous system, making the external hair shaft “stand up.” The primary purpose for this is to trap a layer of air to add insulation. This is visible in humans as goose bumps and even more obvious in animals, such as when a frightened cat raises its fur. Of course, this is much more obvious in organisms with a heavier coat than most humans, such as dogs and cats. Hair Growth Hair grows and is eventually shed and replaced by new hair. This occurs in three phases. The first is the anagen phase, during which cells divide rapidly at the root of the hair, pushing the hair shaft up and out. The length of this phase is measured in years, typically from 2 to 7 years. The catagen phase lasts only 2 to 3 weeks, and marks a transition from the hair follicle’s active growth. Finally, during the telogen phase, the hair follicle is at rest and no new growth occurs. At the end of this phase, which lasts about 2 to 4 months, another anagen phase begins. The basal cells in the hair matrix then produce a new hair follicle, which pushes the old hair out as the growth cycle repeats itself. Hair typically grows at the rate of 0.3 mm per day during the anagen phase. On average, 50 hairs are lost and replaced per day. Hair loss occurs if there is more hair shed than what is replaced and can happen due to hormonal or dietary changes. Hair loss can also result from the aging process, or the influence of hormones. Hair Color Similar to the skin, hair gets its color from the pigment melanin, produced by melanocytes in the hair papilla. Different hair color results from differences in the type of melanin, which is genetically determined. As a person ages, the melanin production decreases, and hair tends to lose its color and becomes gray and/or white. Nails The nail bed is a specialized structure of the epidermis that is found at the tips of our fingers and toes. The nail body is formed on the nail bed, and protects the tips of our fingers and toes as they are the farthest extremities and the parts of the body that experience the maximum mechanical stress ({link]). In addition, the nail body forms a back-support for picking up small objects with the fingers. The nail body is composed of densely packed dead keratinocytes. The epidermis in this part of the body has evolved a specialized structure upon which nails can form. The nail body forms at the nail root, which has a matrix of proliferating cells from the stratum basale that enables the nail to grow continuously. The lateral nail fold overlaps the nail on the sides, helping to anchor the nail body. The nail fold that meets the proximal end of the nail body forms the nail cuticle, also called the eponychium. The nail bed is rich in blood vessels, making it appear pink, except at the base, where a thick layer of epithelium over the nail matrix forms a crescent-shaped region called the lunula (the “little moon”). The area beneath the free edge of the nail, furthest from the cuticle, is called the hyponychium. It consists of a thickened layer of stratum corneum. Nails Free edge Eponychium a ee Proximal nail fold . Lunula Nail / Nail body Lateral nail fold Lunula Eponychium Proximal nail fold Epidermis Dermis Phalanx Hyponychium The nail is an accessory structure of the integumentary system. Note: — e meee ) € y —_—_ Mandible Anterior view An anterior view of the skull shows the bones that form the forehead, orbits (eye sockets), nasal cavity, nasal septum, and upper and lower jaws. Inside the nasal area of the skull, the nasal cavity is divided into halves by the nasal septum. The upper portion of the nasal septum is formed by the perpendicular plate of the ethmoid bone and the lower portion is the vomer bone. Each side of the nasal cavity is triangular in shape, with a broad inferior space that narrows superiorly. When looking into the nasal cavity from the front of the skull, two bony plates are seen projecting from each lateral wall. The larger of these is the inferior nasal concha, an independent bone of the skull. Located just above the inferior concha is the middle nasal concha, which is part of the ethmoid bone. A third bony plate, also part of the ethmoid bone, is the superior nasal concha. It is much smaller and out of sight, above the middle concha. The superior nasal concha is located just lateral to the perpendicular plate, in the upper nasal cavity. Lateral View of Skull A view of the lateral skull is dominated by the large, rounded brain case above and the upper and lower jaws with their teeth below ([link]). Separating these areas is the bridge of bone called the zygomatic arch. The zygomatic arch is the bony arch on the side of skull that spans from the area of the cheek to just above the ear canal. It is formed by the junction of two bony processes: a short anterior component, the temporal process of the zygomatic bone (the cheekbone) and a longer posterior portion, the zygomatic process of the temporal bone, extending forward from the temporal bone. Thus the temporal process (anteriorly) and the zygomatic process (posteriorly) join together, like the two ends of a drawbridge, to form the zygomatic arch. One of the major muscles that pulls the mandible upward during biting and chewing arises from the zygomatic arch. On the lateral side of the brain case, above the level of the zygomatic arch, is a Shallow space called the temporal fossa. Below the level of the zygomatic arch and deep to the vertical portion of the mandible is another space called the infratemporal fossa. Both the temporal fossa and infratemporal fossa contain muscles that act on the mandible during chewing. Lateral View of Skull Zygomatic arch Coronal suture Parietal bone Frontal bone Greater wing of sphenoid bone Si tl quamous suture Ethmoid bone Temporal bone Lacrimal bone Squamous ‘ temporal Lacrimal fossa Zygomatic process ~L_ External acoustic meatus Lambdoid suture Nasal bone Zygomatic bone Mastoid portion Temporal process Styloid process Mastoid process Maxilla Articular tubercle Occipital bone Mandibular fossa Mandible Mental protuberance of mandible Right lateral view The lateral skull shows the large rounded brain case, zygomatic arch, and the upper and lower jaws. The zygomatic arch is formed jointly by the zygomatic process of the temporal bone and the temporal process of the zygomatic bone. The shallow space above the zygomatic arch is the temporal fossa. The space inferior to the zygomatic arch and deep to the posterior mandible is the infratemporal fossa. Bones of the Brain Case The brain case contains and protects the brain. The interior space that is almost completely occupied by the brain is called the cranial cavity. This cavity is bounded superiorly by the rounded top of the skull, which is called the calvaria (skullcap), and the lateral and posterior sides of the skull. The bones that form the top and sides of the brain case are usually referred to as the “flat” bones of the skull. The floor of the brain case is referred to as the base of the skull. This is a complex area that varies in depth and has numerous openings for the passage of cranial nerves, blood vessels, and the spinal cord. Inside the skull, the base is subdivided into three large spaces, called the anterior cranial fossa, middle cranial fossa, and posterior cranial fossa (fossa = “trench or ditch”) ([link]). From anterior to posterior, the fossae increase in depth. The shape and depth of each fossa corresponds to the shape and size of the brain region that each houses. The boundaries and openings of the cranial fossae (singular = fossa) will be described in a later section. Cranial Fossae Anterior cranial fossa Middle cranial fossa Posterior cranial fossa Brain within —____. cranial cavity ; Lateral view The bones of the brain case surround and protect the brain, which occupies the cranial cavity. The base of the brain case, which forms the floor of cranial cavity, is subdivided into the shallow anterior cranial fossa, the middle cranial fossa, and the deep posterior cranial fossa. The brain case consists of eight bones. These include the paired parietal and temporal bones, plus the unpaired frontal, occipital, sphenoid, and ethmoid bones. Parietal Bone The parietal bone forms most of the upper lateral side of the skull (see [link]). These are paired bones, with the right and left parietal bones joining together at the top of the skull. Each parietal bone is also bounded anteriorly by the frontal bone, inferiorly by the temporal bone, and posteriorly by the occipital bone. Temporal Bone The temporal bone forms the lower lateral side of the skull (see [link]). Common wisdom has it that the temporal bone (temporal = “time”’) is so named because this area of the head (the temple) is where hair typically first turns gray, indicating the passage of time. The temporal bone is subdivided into several regions ({link]). The flattened, upper portion is the squamous portion of the temporal bone. Below this area and projecting anteriorly is the zygomatic process of the temporal bone, which forms the posterior portion of the zygomatic arch. Posteriorly is the mastoid portion of the temporal bone. Projecting inferiorly from this region is a large prominence, the mastoid process, which serves as a muscle attachment site. The mastoid process can easily be felt on the side of the head just behind your earlobe. On the interior of the skull, the petrous portion of each temporal bone forms the prominent, diagonally oriented petrous ridge in the floor of the cranial cavity. Located inside each petrous ridge are small cavities that house the structures of the middle and inner ears. Temporal Bone External acoustic Squamous meatus portion Zygomatic process Articular Masicid tubercle pomiat Mandibular fossa Mastoid process Styloid process A lateral view of the isolated temporal bone shows the squamous, mastoid, and zygomatic portions of the temporal bone. Important landmarks of the temporal bone, as shown in [link], include the following: e External acoustic meatus (ear canal)—This is the large opening on the lateral side of the skull that is associated with the ear. ¢ Internal acoustic meatus—This opening is located inside the cranial cavity, on the medial side of the petrous ridge. It connects to the middle and inner ear cavities of the temporal bone. e Mandibular fossa—This is the deep, oval-shaped depression located on the external base of the skull, just in front of the external acoustic meatus. The mandible (lower jaw) joins with the skull at this site as part of the temporomandibular joint, which allows for movements of the mandible during opening and closing of the mouth. e Articular tubercle—The smooth ridge located immediately anterior to the mandibular fossa. Both the articular tubercle and mandibular fossa contribute to the temporomandibular joint, the joint that provides for movements between the temporal bone of the skull and the mandible. e Styloid process—Posterior to the mandibular fossa on the external base of the skull is an elongated, downward bony projection called the styloid process, so named because of its resemblance to a stylus (a pen or writing tool). This structure serves as an attachment site for several small muscles and for a ligament that supports the hyoid bone of the neck. (See also [link].) ¢ Stylomastoid foramen—This small opening is located between the styloid process and mastoid process. This is the point of exit for the cranial nerve that supplies the facial muscles. ¢ Carotid canal—tThe carotid canal is a zig-zag shaped tunnel that provides passage through the base of the skull for one of the major arteries that supplies the brain. Its entrance is located on the outside base of the skull, anteromedial to the styloid process. The canal then runs anteromedially within the bony base of the skull, and then turns upward to its exit in the floor of the middle cranial cavity, above the foramen lacerum. External and Internal Views of Base of Skull Maxilla: Palatine process Zygomatic bone Palatine bone (horizontal plate) Zygomatic arch Medial and lateral pterygoid plates Articular tubercle Sphenoid bone Foramen ovale Foramen spinosum Mandibular fossa Foramen lacerum External auditory meatus Jugular foramen Mastoid process Occipital condyle Styloid process Temporal bone Stylomastoid foramen Foramen magnum F g Entrance to carotid canal Occipital bone Superior nuchal line External occipital protuberance (a) Inferior view Transverse plane Frontal bone Ethmoid bone: Crista galli Superior orbital Cribriform plate fissure Sphenoid bone: Lesser wing Hypophyseal eit eels Foramen rotundum turcica Foramen lacerum and exit of carotid canal Foramen ovale Foramen spinosum Internal acoustic meatus Temporal bone Hypoglossal canal Petrous portion Foramen magnum (petrous ridge) Occipital bone Jugular foramen Parietal bone (b) Superior view (a) The hard palate is formed anteriorly by the palatine processes of the maxilla bones and posteriorly by the horizontal plate of the palatine bones. (b) The complex floor of the cranial cavity is formed by the frontal, ethmoid, sphenoid, temporal, and occipital bones. The lesser wing of the sphenoid bone separates the anterior and middle cranial fossae. The petrous ridge (petrous portion of temporal bone) separates the middle and posterior cranial fossae. Frontal Bone The frontal bone is the single bone that forms the forehead. At its anterior midline, between the eyebrows, there is a slight depression called the glabella (see [link]). The frontal bone also forms the supraorbital margin of the orbit. Near the middle of this margin, is the supraorbital foramen, the opening that provides passage for a sensory nerve to the forehead. The frontal bone is thickened just above each supraorbital margin, forming rounded brow ridges. These are located just behind your eyebrows and vary in size among individuals, although they are generally larger in males. Inside the cranial cavity, the frontal bone extends posteriorly. This flattened region forms both the roof of the orbit below and the floor of the anterior cranial cavity above (see [link]b). Occipital Bone The occipital bone is the single bone that forms the posterior skull and posterior base of the cranial cavity ([link]; see also [link]). On its outside surface, at the posterior midline, is a small protrusion called the external occipital protuberance, which serves as an attachment site for a ligament of the posterior neck. Lateral to either side of this bump is a superior nuchal line (nuchal = “nape” or “posterior neck”). The nuchal lines represent the most superior point at which muscles of the neck attach to the skull, with only the scalp covering the skull above these lines. On the base of the skull, the occipital bone contains the large opening of the foramen magnum, which allows for passage of the spinal cord as it exits the skull. On either side of the foramen magnum is an oval-shaped occipital condyle. These condyles form joints with the first cervical vertebra and thus support the skull on top of the vertebral column. Posterior View of Skull Parietal bones Sagittal suture Lambdoid suture Occipital bone External occipital protuberance Temporal bone Superior nuchal line Mastoid process Occipital condyle Foramen magnum Zygomatic bone Posterior view This view of the posterior skull shows attachment sites for muscles and joints that support the skull. Sphenoid Bone The sphenoid bone is a single, complex bone of the central skull ([{link]). It serves as a “keystone” bone, because it joins with almost every other bone of the skull. The sphenoid forms much of the base of the central skull (see [link]) and also extends laterally to contribute to the sides of the skull (see [link]). Inside the cranial cavity, the right and left lesser wings of the sphenoid bone, which resemble the wings of a flying bird, form the lip of a prominent ridge that marks the boundary between the anterior and middle cranial fossae. The sella turcica (“Turkish saddle”) is located at the midline of the middle cranial fossa. This bony region of the sphenoid bone is named for its resemblance to the horse saddles used by the Ottoman Turks, with a high back and a tall front. The rounded depression in the floor of the sella turcica is the hypophyseal (pituitary) fossa, which houses the pea-sized pituitary (hypophyseal) gland. The greater wings of the sphenoid bone extend laterally to either side away from the sella turcica, where they form the anterior floor of the middle cranial fossa. The greater wing is best seen on the outside of the lateral skull, where it forms a rectangular area immediately anterior to the squamous portion of the temporal bone. On the inferior aspect of the skull, each half of the sphenoid bone forms two thin, vertically oriented bony plates. These are the medial pterygoid plate and lateral pterygoid plate (pterygoid = “wing-shaped”). The right and left medial pterygoid plates form the posterior, lateral walls of the nasal cavity. The somewhat larger lateral pterygoid plates serve as attachment sites for chewing muscles that fill the infratemporal space and act on the mandible. Sphenoid Bone Superior orbital fissure Foramen rotundum < 'y Foramen ovale Ww “| Foramen spinosum Hypophyseal fossa of sella turcica Body of sphenoid (a) Superior view Body of sphenoid Lesser wing Superior orbital fissure Pterygoid plates (b) Posterior view Shown in isolation in (a) superior and (b) posterior views, the sphenoid bone is a single midline bone that forms the anterior walls and floor of the middle cranial fossa. It has a pair of lesser wings and a pair of greater wings. The sella turcica surrounds the hypophyseal fossa. Projecting downward are the medial and lateral pterygoid plates. The sphenoid has multiple openings for the passage of nerves and blood vessels, including the optic canal, superior orbital fissure, foramen rotundum, foramen ovale, and foramen spinosum. Ethmoid Bone The ethmoid bone is a single, midline bone that forms the roof and lateral walls of the upper nasal cavity, the upper portion of the nasal septum, and contributes to the medial wall of the orbit ({link] and [link]). On the interior of the skull, the ethmoid also forms a portion of the floor of the anterior cranial cavity (see [link |b). Within the nasal cavity, the perpendicular plate of the ethmoid bone forms the upper portion of the nasal septum. The ethmoid bone also forms the lateral walls of the upper nasal cavity. Extending from each lateral wall are the superior nasal concha and middle nasal concha, which are thin, curved projections that extend into the nasal cavity ({link]). In the cranial cavity, the ethmoid bone forms a small area at the midline in the floor of the anterior cranial fossa. This region also forms the narrow roof of the underlying nasal cavity. This portion of the ethmoid bone consists of two parts, the crista galli and cribriform plates. The crista galli (“rooster’s comb or crest”) is a small upward bony projection located at the midline. It functions as an anterior attachment point for one of the covering layers of the brain. To either side of the crista galli is the cribriform plate (cribrum = “sieve”), a small, flattened area with numerous small openings termed olfactory foramina. Small nerve branches from the olfactory areas of the nasal cavity pass through these openings to enter the brain. The lateral portions of the ethmoid bone are located between the orbit and upper nasal cavity, and thus form the lateral nasal cavity wall and a portion of the medial orbit wall. Located inside this portion of the ethmoid bone are several small, air-filled spaces that are part of the paranasal sinus system of the skull. Sagittal Section of Skull Sella turcica: Hypophyseal fossa Parietal bone Crista galli Frontal sinus Cribriform plate Perpendicular plate Temporal bone Nasal bone F Sphenoid bone Internal acoustic meatus Sphenoid sinus Hypoglossal canal Inferior nasal concha Occipital bone Vomer Styloid process bs a Maxilla Medial and lateral pterygoid plates Palatine bone Mandibular foramen Mylohyoid line Mandible Hyoid bone This midline view of the sagittally sectioned skull shows the nasal septum. Ethmoid Bone Superior Crista galli Cribriform plate Ethmoid air cells Superior nasal ; concha Nasal cavity Middle nasal concha Medial wall of orbit Perpendicular plate Inferior The unpaired ethmoid bone is located at the midline within the central skull. It has an upward projection, the crista galli, and a downward projection, the perpendicular plate, which forms the upper nasal septum. The cribriform plates form both the roof of the nasal cavity and a portion of the anterior cranial fossa floor. The lateral sides of the ethmoid bone form the lateral walls of the upper nasal cavity, part of the medial orbit wall, and give rise to the superior and middle nasal conchae. The ethmoid bone also contains the ethmoid air cells. Lateral Wall of Nasal Cavity Ethmoid bone: Superior nasal concha Middle nasal concha Inferior nasal concha Sphenoidal sinus Medial view The three nasal conchae are curved bones that project from the lateral walls of the nasal cavity. The superior nasal concha and middle nasal concha are parts of the ethmoid bone. The inferior nasal concha is an independent bone of the skull. Sutures of the Skull A suture is an immobile joint between adjacent bones of the skull. The narrow gap between the bones is filled with dense, fibrous connective tissue that unites the bones. The long sutures located between the bones of the brain case are not straight, but instead follow irregular, tightly twisting paths. These twisting lines serve to tightly interlock the adjacent bones, thus adding strength to the skull for brain protection. The two suture lines seen on the top of the skull are the coronal and sagittal sutures. The coronal suture runs from side to side across the skull, within the coronal plane of section (see [link]). It joins the frontal bone to the right and left parietal bones. The sagittal suture extends posteriorly from the coronal suture, running along the midline at the top of the skull in the Sagittal plane of section (see [link]). It unites the right and left parietal bones. On the posterior skull, the sagittal suture terminates by joining the lambdoid suture. The lambdoid suture extends downward and laterally to either side away from its junction with the sagittal suture. The lambdoid suture joins the occipital bone to the right and left parietal and temporal bones. This suture is named for its upside-down "V" shape, which resembles the capital letter version of the Greek letter lambda (A). The squamous suture is located on the lateral skull. It unites the squamous portion of the temporal bone with the parietal bone (see [link]). At the intersection of four bones is the pterion, a small, capital-H-shaped suture line region that unites the frontal bone, parietal bone, squamous portion of the temporal bone, and greater wing of the sphenoid bone. It is the weakest part of the skull. The pterion is located approximately two finger widths above the zygomatic arch and a thumb’s width posterior to the upward portion of the zygomatic bone. Note: Disorders of the... Skeletal System Head and traumatic brain injuries are major causes of immediate death and disability, with bleeding and infections as possible additional complications. According to the Centers for Disease Control and Prevention (2010), approximately 30 percent of all injury-related deaths in the United States are caused by head injuries. The majority of head injuries involve falls. They are most common among young children (ages 0-4 years), adolescents (15-19 years), and the elderly (over 65 years). Additional causes vary, but prominent among these are automobile and motorcycle accidents. Strong blows to the brain-case portion of the skull can produce fractures. These may result in bleeding inside the skull with subsequent injury to the brain. The most common is a linear skull fracture, in which fracture lines radiate from the point of impact. Other fracture types include a comminuted fracture, in which the bone is broken into several pieces at the point of impact, or a depressed fracture, in which the fractured bone is pushed inward. In a contrecoup (counterblow) fracture, the bone at the point of impact is not broken, but instead a fracture occurs on the opposite side of the skull. Fractures of the occipital bone at the base of the skull can occur in this manner, producing a basilar fracture that can damage the artery that passes through the carotid canal. A blow to the lateral side of the head may fracture the bones of the pterion. The pterion is an important clinical landmark because located immediately deep to it on the inside of the skull is a major branch of an artery that supplies the skull and covering layers of the brain. A strong blow to this region can fracture the bones around the pterion. If the underlying artery is damaged, bleeding can cause the formation of a hematoma (collection of blood) between the brain and interior of the skull. As blood accumulates, it will put pressure on the brain. Symptoms associated with a hematoma may not be apparent immediately following the injury, but if untreated, blood accumulation will exert increasing pressure on the brain and can result in death within a few hours. Note: [= [=] ‘all “eat a ee 5 openstax COLLEGE” View this animation to see how a blow to the head may produce a contrecoup (counterblow) fracture of the basilar portion of the occipital bone on the base of the skull. Why may a basilar fracture be life threatening? Facial Bones of the Skull The facial bones of the skull form the upper and lower jaws, the nose, nasal cavity and nasal septum, and the orbit. The facial bones include 14 bones, with six paired bones and two unpaired bones. The paired bones are the maxilla, palatine, zygomatic, nasal, lacrimal, and inferior nasal conchae bones. The unpaired bones are the vomer and mandible bones. Although classified with the brain-case bones, the ethmoid bone also contributes to the nasal septum and the walls of the nasal cavity and orbit. Maxillary Bone The maxillary bone, often referred to simply as the maxilla (plural = maxillae), is one of a pair that together form the upper jaw, much of the hard palate, the medial floor of the orbit, and the lateral base of the nose (see [link]). The curved, inferior margin of the maxillary bone that forms the upper jaw and contains the upper teeth is the alveolar process of the maxilla ({link]). Each tooth is anchored into a deep socket called an alveolus. On the anterior maxilla, just below the orbit, is the infraorbital foramen. This is the point of exit for a sensory nerve that supplies the nose, upper lip, and anterior cheek. On the inferior skull, the palatine process from each maxillary bone can be seen joining together at the midline to form the anterior three-quarters of the hard palate (see [link]a). The hard palate is the bony plate that forms the roof of the mouth and floor of the nasal cavity, separating the oral and nasal cavities. Maxillary Bone Articulates with frontal bone Infraorbital foramen Alveolar process Right lateral view The maxillary bone forms the upper jaw and supports the upper teeth. Each maxilla also forms the lateral floor of each orbit and the majority of the hard palate. Palatine Bone The palatine bone is one of a pair of irregularly shaped bones that contribute small areas to the lateral walls of the nasal cavity and the medial wall of each orbit. The largest region of each of the palatine bone is the horizontal plate. The plates from the right and left palatine bones join together at the midline to form the posterior quarter of the hard palate (see [link]a). Thus, the palatine bones are best seen in an inferior view of the skull and hard palate. Note: Homeostatic Imbalances Cleft Lip and Cleft Palate During embryonic development, the right and left maxilla bones come together at the midline to form the upper jaw. At the same time, the muscle and skin overlying these bones join together to form the upper lip. Inside the mouth, the palatine processes of the maxilla bones, along with the horizontal plates of the right and left palatine bones, join together to form the hard palate. If an error occurs in these developmental processes, a birth defect of cleft lip or cleft palate may result. Cleft lip is a common development defect that affects approximately 1:1000 births, most of which are male. This defect involves a partial or complete failure of the right and left portions of the upper lip to fuse together, leaving a cleft (gap). A more severe developmental defect is cleft palate, which affects the hard palate. The hard palate is the bony structure that separates the nasal cavity from the oral cavity. It is formed during embryonic development by the midline fusion of the horizontal plates from the right and left palatine bones and the palatine processes of the maxilla bones. Cleft palate affects approximately 1:2500 births and is more common in females. It results from a failure of the two halves of the hard palate to completely come together and fuse at the midline, thus leaving a gap between them. This gap allows for communication between the nasal and oral cavities. In severe cases, the bony gap continues into the anterior upper jaw where the alveolar processes of the maxilla bones also do not properly join together above the front teeth. If this occurs, a cleft lip will also be seen. Because of the communication between the oral and nasal cavities, a cleft palate makes it very difficult for an infant to generate the suckling needed for nursing, thus leaving the infant at risk for malnutrition. Surgical repair is required to correct cleft palate defects. Zygomatic Bone The zygomatic bone is also known as the cheekbone. Each of the paired zygomatic bones forms much of the lateral wall of the orbit and the lateral- inferior margins of the anterior orbital opening (see [link]). The short temporal process of the zygomatic bone projects posteriorly, where it forms the anterior portion of the zygomatic arch (see [link]). Nasal Bone The nasal bone is one of two small bones that articulate (join) with each other to form the bony base (bridge) of the nose. They also support the cartilages that form the lateral walls of the nose (see [link]). These are the bones that are damaged when the nose is broken. Lacrimal Bone Each lacrimal bone is a small, rectangular bone that forms the anterior, medial wall of the orbit (see [link] and [link]). The anterior portion of the lacrimal bone forms a shallow depression called the lacrimal fossa, and extending inferiorly from this is the nasolacrimal canal. The lacrimal fluid (tears of the eye), which serves to maintain the moist surface of the eye, drains at the medial corner of the eye into the nasolacrimal canal. This duct then extends downward to open into the nasal cavity, behind the inferior nasal concha. In the nasal cavity, the lacrimal fluid normally drains posteriorly, but with an increased flow of tears due to crying or eye irritation, some fluid will also drain anteriorly, thus causing a runny nose. Inferior Nasal Conchae The right and left inferior nasal conchae form a curved bony plate that projects into the nasal cavity space from the lower lateral wall (see [link]). The inferior concha is the largest of the nasal conchae and can easily be seen when looking into the anterior opening of the nasal cavity. Vomer Bone The unpaired vomer bone, often referred to simply as the vomer, is triangular-shaped and forms the posterior-inferior part of the nasal septum (see [link]). The vomer is best seen when looking from behind into the posterior openings of the nasal cavity (see [link]a). In this view, the vomer is seen to form the entire height of the nasal septum. A much smaller portion of the vomer can also be seen when looking into the anterior opening of the nasal cavity. Mandible The mandible forms the lower jaw and is the only moveable bone of the skull. At the time of birth, the mandible consists of paired right and left bones, but these fuse together during the first year to form the single U- shaped mandible of the adult skull. Each side of the mandible consists of a horizontal body and posteriorly, a vertically oriented ramus of the mandible (ramus = “branch’). The outside margin of the mandible, where the body and ramus come together is called the angle of the mandible ({link]). The ramus on each side of the mandible has two upward-going bony projections. The more anterior projection is the flattened coronoid process of the mandible, which provides attachment for one of the biting muscles. The posterior projection is the condylar process of the mandible, which is topped by the oval-shaped condyle. The condyle of the mandible articulates (joins) with the mandibular fossa and articular tubercle of the temporal bone. Together these articulations form the temporomandibular joint, which allows for opening and closing of the mouth (see [link]). The broad U- shaped curve located between the coronoid and condylar processes is the mandibular notch. Important landmarks for the mandible include the following: e Alveolar process of the mandible—This is the upper border of the mandibular body and serves to anchor the lower teeth. ¢ Mental protuberance—The forward projection from the inferior margin of the anterior mandible that forms the chin (mental = “chin’”). ¢ Mental foramen—The opening located on each side of the anterior- lateral mandible, which is the exit site for a sensory nerve that supplies the chin. e Mylohyoid line—This bony ridge extends along the inner aspect of the mandibular body (see [link]). The muscle that forms the floor of the oral cavity attaches to the mylohyoid lines on both sides of the mandible. e Mandibular foramen—This opening is located on the medial side of the ramus of the mandible. The opening leads into a tunnel that runs down the length of the mandibular body. The sensory nerve and blood vessels that supply the lower teeth enter the mandibular foramen and then follow this tunnel. Thus, to numb the lower teeth prior to dental work, the dentist must inject anesthesia into the lateral wall of the oral cavity at a point prior to where this sensory nerve enters the mandibular foramen. ¢ Lingula—tThis small flap of bone is named for its shape (lingula = “little tongue”). It is located immediately next to the mandibular foramen, on the medial side of the ramus. A ligament that anchors the mandible during opening and closing of the mouth extends down from the base of the skull and attaches to the lingula. Isolated Mandible Condylar process ; Coronoid process Lingula Mandibular notch Mandibular condyle Mandibular foramen Mylohyoid line Alveolar process Ramus of mandible Mental protuberance Mental foramen Mandibular angle Body of mandible Right lateral view The mandible is the only moveable bone of the skull. The Orbit The orbit is the bony socket that houses the eyeball and contains the muscles that move the eyeball or open the upper eyelid. Each orbit is cone- shaped, with a narrow posterior region that widens toward the large anterior opening. To help protect the eye, the bony margins of the anterior opening are thickened and somewhat constricted. The medial walls of the two orbits are parallel to each other but each lateral wall diverges away from the midline at a 45° angle. This divergence provides greater lateral peripheral vision. The walls of each orbit include contributions from seven skull bones ({link]). The frontal bone forms the roof and the zygomatic bone forms the lateral wall and lateral floor. The medial floor is primarily formed by the maxilla, with a small contribution from the palatine bone. The ethmoid bone and lacrimal bone make up much of the medial wall and the sphenoid bone forms the posterior orbit. At the posterior apex of the orbit is the opening of the optic canal, which allows for passage of the optic nerve from the retina to the brain. Lateral to this is the elongated and irregularly shaped superior orbital fissure, which provides passage for the artery that supplies the eyeball, sensory nerves, and the nerves that supply the muscles involved in eye movements. Bones of the Orbit Frontal bone Supraorbital foramen Supraorbital margin Sphenoid bone Nasal bone Lacrimal bone Ethmoid bone Lacrimal fossa Optic canal Superior orbital fissure Zygomatic bone Palatine bone Infraorbital foramen Seven skull bones contribute to the walls of the orbit. Opening into the posterior orbit from the cranial cavity are the optic canal and superior orbital fissure. The Nasal Septum and Nasal Conchae The nasal septum consists of both bone and cartilage components ([Link]; see also [link]). The upper portion of the septum is formed by the perpendicular plate of the ethmoid bone. The lower and posterior parts of the septum are formed by the triangular-shaped vomer bone. In an anterior view of the skull, the perpendicular plate of the ethmoid bone is easily seen inside the nasal opening as the upper nasal septum, but only a small portion of the vomer is seen as the inferior septum. A better view of the vomer bone is seen when looking into the posterior nasal cavity with an inferior view of the skull, where the vomer forms the full height of the nasal septum. The anterior nasal septum is formed by the septal cartilage, a flexible plate that fills in the gap between the perpendicular plate of the ethmoid and vomer bones. This cartilage also extends outward into the nose where it separates the right and left nostrils. The septal cartilage is not found in the dry skull. Attached to the lateral wall on each side of the nasal cavity are the superior, middle, and inferior nasal conchae (singular = concha), which are named for their positions (see [link]). These are bony plates that curve downward as they project into the space of the nasal cavity. They serve to swirl the incoming air, which helps to warm and moisturize it before the air moves into the delicate air sacs of the lungs. This also allows mucus, secreted by the tissue lining the nasal cavity, to trap incoming dust, pollen, bacteria, and viruses. The largest of the conchae is the inferior nasal concha, which is an independent bone of the skull. The middle concha and the superior conchae, which is the smallest, are both formed by the ethmoid bone. When looking into the anterior nasal opening of the skull, only the inferior and middle conchae can be seen. The small superior nasal concha is well hidden above and behind the middle concha. Nasal Septum Frontal bone Crista galli Frontal sinus Sphenoid sinus Nasal bone Nasal septum: Perpendicular plate of ethmoid bone Vomer bone Septal cartilage Sphenoid bone Qy Palatine process Horizontal plate — of maxilla of palatine bone Sagittal section The nasal septum is formed by the perpendicular plate of the ethmoid bone and the vomer bone. The septal cartilage fills the gap between these bones and extends into the nose. Cranial Fossae Inside the skull, the floor of the cranial cavity is subdivided into three cranial fossae (spaces), which increase in depth from anterior to posterior (see [link], [link]b, and [link]). Since the brain occupies these areas, the shape of each conforms to the shape of the brain regions that it contains. Each cranial fossa has anterior and posterior boundaries and is divided at the midline into right and left areas by a significant bony structure or opening. Anterior Cranial Fossa The anterior cranial fossa is the most anterior and the shallowest of the three cranial fossae. It overlies the orbits and contains the frontal lobes of the brain. Anteriorly, the anterior fossa is bounded by the frontal bone, which also forms the majority of the floor for this space. The lesser wings of the sphenoid bone form the prominent ledge that marks the boundary between the anterior and middle cranial fossae. Located in the floor of the anterior cranial fossa at the midline is a portion of the ethmoid bone, consisting of the upward projecting crista galli and to either side of this, the cribriform plates. Middle Cranial Fossa The middle cranial fossa is deeper and situated posterior to the anterior fossa. It extends from the lesser wings of the sphenoid bone anteriorly, to the petrous ridges (petrous portion of the temporal bones) posteriorly. The large, diagonally positioned petrous ridges give the middle cranial fossa a butterfly shape, making it narrow at the midline and broad laterally. The temporal lobes of the brain occupy this fossa. The middle cranial fossa is divided at the midline by the upward bony prominence of the sella turcica, a part of the sphenoid bone. The middle cranial fossa has several openings for the passage of blood vessels and cranial nerves (see [link]). Openings in the middle cranial fossa are as follows: Optic canal—This opening is located at the anterior lateral comer of the sella turcica. It provides for passage of the optic nerve into the orbit. Superior orbital fissure—This large, irregular opening into the posterior orbit is located on the anterior wall of the middle cranial fossa, lateral to the optic canal and under the projecting margin of the lesser wing of the sphenoid bone. Nerves to the eyeball and associated muscles, and sensory nerves to the forehead pass through this opening. Foramen rotundum—This rounded opening (rotundum = “round”) is located in the floor of the middle cranial fossa, just inferior to the superior orbital fissure. It is the exit point for a major sensory nerve that supplies the cheek, nose, and upper teeth. Foramen ovale of the middle cranial fossa—This large, oval-shaped opening in the floor of the middle cranial fossa provides passage for a major sensory nerve to the lateral head, cheek, chin, and lower teeth. Foramen spinosum—This small opening, located posterior-lateral to the foramen ovale, is the entry point for an important artery that supplies the covering layers surrounding the brain. The branching pattern of this artery forms readily visible grooves on the internal surface of the skull and these grooves can be traced back to their origin at the foramen spinosum. Carotid canal—This is the zig-zag passageway through which a major artery to the brain enters the skull. The entrance to the carotid canal is located on the inferior aspect of the skull, anteromedial to the styloid process (see [link]a). From here, the canal runs anteromedially within the bony base of the skull. Just above the foramen lacerum, the carotid canal opens into the middle cranial cavity, near the posterior- lateral base of the sella turcica. Foramen lacerum—This irregular opening is located in the base of the skull, immediately inferior to the exit of the carotid canal. This opening is an artifact of the dry skull, because in life it is completely filled with cartilage. All the openings of the skull that provide for passage of nerves or blood vessels have smooth margins; the word lacerum (“ragged” or “torn’”) tells us that this opening has ragged edges and thus nothing passes through it. Posterior Cranial Fossa The posterior cranial fossa is the most posterior and deepest portion of the cranial cavity. It contains the cerebellum of the brain. The posterior fossa is bounded anteriorly by the petrous ridges, while the occipital bone forms the floor and posterior wall. It is divided at the midline by the large foramen magnum (“great aperture”), the opening that provides for passage of the spinal cord. Located on the medial wall of the petrous ridge in the posterior cranial fossa is the internal acoustic meatus (see [link]). This opening provides for passage of the nerve from the hearing and equilibrium organs of the inner ear, and the nerve that supplies the muscles of the face. Located at the anterior-lateral margin of the foramen magnum is the hypoglossal canal. These emerge on the inferior aspect of the skull at the base of the occipital condyle and provide passage for an important nerve to the tongue. Immediately inferior to the internal acoustic meatus is the large, irregularly shaped jugular foramen (see [link]a). Several cranial nerves from the brain exit the skull via this opening. It is also the exit point through the base of the skull for all the venous return blood leaving the brain. The venous structures that carry blood inside the skull form large, curved grooves on the inner walls of the posterior cranial fossa, which terminate at each jugular foramen. Paranasal Sinuses The paranasal sinuses are hollow, air-filled spaces located within certain bones of the skull ([link]). All of the sinuses communicate with the nasal cavity (paranasal = “next to nasal cavity”) and are lined with nasal mucosa. They serve to reduce bone mass and thus lighten the skull, and they also add resonance to the voice. This second feature is most obvious when you have a cold or sinus congestion. These produce swelling of the mucosa and excess mucus production, which can obstruct the narrow passageways between the sinuses and the nasal cavity, causing your voice to sound different to yourself and others. This blockage can also allow the sinuses to fill with fluid, with the resulting pressure producing pain and discomfort. The paranasal sinuses are named for the skull bone that each occupies. The frontal sinus is located just above the eyebrows, within the frontal bone (see [link]). This irregular space may be divided at the midline into bilateral spaces, or these may be fused into a single sinus space. The frontal sinus is the most anterior of the paranasal sinuses. The largest sinus is the maxillary sinus. These are paired and located within the right and left maxillary bones, where they occupy the area just below the orbits. The maxillary sinuses are most commonly involved during sinus infections. Because their connection to the nasal cavity is located high on their medial wall, they are difficult to drain. The sphenoid sinus is a single, midline sinus. It is located within the body of the sphenoid bone, just anterior and inferior to the sella turcica, thus making it the most posterior of the paranasal sinuses. The lateral aspects of the ethmoid bone contain multiple small spaces separated by very thin bony walls. Each of these spaces is called an ethmoid air cell. These are located on both sides of the ethmoid bone, between the upper nasal cavity and medial orbit, just behind the superior nasal conchae. Paranasal Sinuses Anterior Lateral The paranasal sinuses are hollow, air-filled spaces named for the skull bone that each occupies. The most anterior is the frontal sinus, located in the frontal bone above the eyebrows. The largest are the maxillary sinuses, located in the right and left maxillary bones below the orbits. The most posterior is the sphenoid sinus, located in the body of the sphenoid bone, under the sella turcica. The ethmoid air cells are multiple small spaces located in the right and left sides of the ethmoid bone, between the medial wall of the orbit and lateral wall of the upper nasal cavity. Hyoid Bone The hyoid bone is an independent bone that does not contact any other bone and thus is not part of the skull ({link]). It is a small U-shaped bone located in the upper neck near the level of the inferior mandible, with the tips of the “U” pointing posteriorly. The hyoid serves as the base for the tongue above, and is attached to the larynx below and the pharynx posteriorly. The hyoid is held in position by a series of small muscles that attach to it either from above or below. These muscles act to move the hyoid up/down or forward/back. Movements of the hyoid are coordinated with movements of the tongue, larynx, and pharynx during swallowing and speaking. Hyoid Bone Mandible Hyoid bone Larynx Greater horn Lesser horn Body Greater horn Lesser horn Right lateral view The hyoid bone is located in the upper neck and does not join with any other bone. It provides attachments for muscles that act on the tongue, larynx, and pharynx. Chapter Review The skull consists of the brain case and the facial bones. The brain case surrounds and protects the brain, which occupies the cranial cavity inside the skull. It consists of the rounded calvaria and a complex base. The brain case is formed by eight bones, the paired parietal and temporal bones plus the unpaired frontal, occipital, sphenoid, and ethmoid bones. The narrow gap between the bones is filled with dense, fibrous connective tissue that unites the bones. The sagittal suture joins the right and left parietal bones. The coronal suture joins the parietal bones to the frontal bone, the lamboid suture joins them to the occipital bone, and the squamous suture joins them to the temporal bone. The facial bones support the facial structures and form the upper and lower jaws. These consist of 14 bones, with the paired maxillary, palatine, zygomatic, nasal, lacrimal, and inferior conchae bones and the unpaired vomer and mandible bones. The ethmoid bone also contributes to the formation of facial structures. The maxilla forms the upper jaw and the mandible forms the lower jaw. The maxilla also forms the larger anterior portion of the hard palate, which is completed by the smaller palatine bones that form the posterior portion of the hard palate. The floor of the cranial cavity increases in depth from front to back and is divided into three cranial fossae. The anterior cranial fossa is located between the frontal bone and lesser wing of the sphenoid bone. A small area of the ethmoid bone, consisting of the crista galli and cribriform plates, is located at the midline of this fossa. The middle cranial fossa extends from the lesser wing of the sphenoid bone to the petrous ridge (petrous portion of temporal bone). The right and left sides are separated at the midline by the sella turcica, which surrounds the shallow hypophyseal fossa. Openings through the skull in the floor of the middle fossa include the optic canal and superior orbital fissure, which open into the posterior orbit, the foramen rotundum, foramen ovale, and foramen spinosum, and the exit of the carotid canal with its underlying foramen lacerum. The deep posterior cranial fossa extends from the petrous ridge to the occipital bone. Openings here include the large foramen magnum, plus the internal acoustic meatus, jugular foramina, and hypoglossal canals. Additional openings located on the external base of the skull include the stylomastoid foramen and the entrance to the carotid canal. The anterior skull has the orbits that house the eyeballs and associated muscles. The walls of the orbit are formed by contributions from seven bones: the frontal, zygomatic, maxillary, palatine, ethmoid, lacrimal, and sphenoid. Located at the superior margin of the orbit is the supraorbital foramen, and below the orbit is the infraorbital foramen. The mandible has two openings, the mandibular foramen on its inner surface and the mental foramen on its external surface near the chin. The nasal conchae are bony projections from the lateral walls of the nasal cavity. The large inferior nasal concha is an independent bone, while the middle and superior conchae are parts of the ethmoid bone. The nasal septum is formed by the perpendicular plate of the ethmoid bone, the vomer bone, and the septal cartilage. The paranasal sinuses are air-filled spaces located within the frontal, maxillary, sphenoid, and ethmoid bones. On the lateral skull, the zygomatic arch consists of two parts, the temporal process of the zygomatic bone anteriorly and the zygomatic process of the temporal bone posteriorly. The temporal fossa is the shallow space located on the lateral skull above the level of the zygomatic arch. The infratemporal fossa is located below the zygomatic arch and deep to the ramus of the mandible. The hyoid bone is located in the upper neck and does not join with any other bone. It is held in position by muscles and serves to support the tongue above, the larynx below, and the pharynx posteriorly. Interactive Link Questions Exercise: Problem: Watch this video to view a rotating and exploded skull with color- coded bones. Which bone (yellow) is centrally located and joins with most of the other bones of the skull? Solution: The sphenoid bone joins with most other bones of the skull. It is centrally located, where it forms portions of the rounded brain case and cranial base. Exercise: Problem: View this animation to see how a blow to the head may produce a contrecoup (counterblow) fracture of the basilar portion of the occipital bone on the base of the skull. Why may a basilar fracture be life threatening? Solution: A basilar fracture may damage an artery entering the skull, causing bleeding in the brain. Review Questions Exercise: Problem: Which of the following is a bone of the brain case? a. parietal bone b. zygomatic bone c. maxillary bone d. lacrimal bone Solution: A Exercise: Problem:The lambdoid suture joins the parietal bone to the a. frontal bone b. occipital bone c. other parietal bone d. temporal bone Solution: B Exercise: Problem:The middle cranial fossa a. is bounded anteriorly by the petrous ridge b. is bounded posteriorly by the lesser wing of the sphenoid bone c. is divided at the midline by a small area of the ethmoid bone d. has the foramen rotundum, foramen ovale, and foramen spinosum Solution: D Exercise: Problem:The paranasal sinuses are a. air-filled spaces found within the frontal, maxilla, sphenoid, and ethmoid bones only b. air-filled spaces found within all bones of the skull c. not connected to the nasal cavity d. divided at the midline by the nasal septum Solution: A Exercise: Problem: Parts of the sphenoid bone include the a. sella turcica b. squamous portion c. glabella d. zygomatic process Solution: A Exercise: Problem:The bony openings of the skull include the a. carotid canal, which is located in the anterior cranial fossa b. superior orbital fissure, which is located at the superior margin of the anterior orbit c. mental foramen, which is located just below the orbit d. hypoglossal canal, which is located in the posterior cranial fossa Solution: D Critical Thinking Questions Exercise: Problem: Define and list the bones that form the brain case or support the facial structures. Solution: The brain case is that portion of the skull that surrounds and protects the brain. It is subdivided into the rounded top of the skull, called the calvaria, and the base of the skull. There are eight bones that form the brain case. These are the paired parietal and temporal bones, plus the unpaired frontal, occipital, sphenoid, and ethmoid bones. The facial bones support the facial structures, and form the upper and lower jaws, nasal cavity, nasal septum, and orbit. There are 14 facial bones. These are the paired maxillary, palatine, zygomatic, nasal, lacrimal, and inferior nasal conchae bones, and the unpaired vomer and mandible bones. Exercise: Problem: Identify the major sutures of the skull, their locations, and the bones united by each. Solution: The coronal suture passes across the top of the anterior skull. It unites the frontal bone anteriorly with the right and left parietal bones. The sagittal suture runs at the midline on the top of the skull. It unites the right and left parietal bones with each other. The squamous suture is a curved suture located on the lateral side of the skull. It unites the squamous portion of the temporal bone to the parietal bone. The lambdoid suture is located on the posterior skull and has an inverted V- shape. It unites the occipital bone with the right and left parietal bones. Exercise: Problem: Describe the anterior, middle, and posterior cranial fossae and their boundaries, and give the midline structure that divides each into right and left areas. Solution: The anterior cranial fossa is the shallowest of the three cranial fossae. It extends from the frontal bone anteriorly to the lesser wing of the sphenoid bone posteriorly. It is divided at the midline by the crista galli and cribriform plates of the ethmoid bone. The middle cranial fossa is located in the central skull, and is deeper than the anterior fossa. The middle fossa extends from the lesser wing of the sphenoid bone anteriorly to the petrous ridge posteriorly. It is divided at the midline by the sella turcica. The posterior cranial fossa is the deepest fossa. It extends from the petrous ridge anteriorly to the occipital bone posteriorly. The large foramen magnum is located at the midline of the posterior fossa. Exercise: Problem: Describe the parts of the nasal septum in both the dry and living skull. Solution: There are two bony parts of the nasal septum in the dry skull. The perpendicular plate of the ethmoid bone forms the superior part of the septum. The vomer bone forms the inferior and posterior parts of the septum. In the living skull, the septal cartilage completes the septum by filling in the anterior area between the bony components and extending outward into the nose. References Centers for Disease Control and Prevention (US). Injury prevention and control: traumatic brain injury [Internet]. Atlanta, GA; [cited 2013 Mar 18]. Glossary alveolar process of the mandible upper border of mandibular body that contains the lower teeth alveolar process of the maxilla curved, inferior margin of the maxilla that supports and anchors the upper teeth angle of the mandible rounded corner located at outside margin of the body and ramus junction anterior cranial fossa shallowest and most anterior cranial fossa of the cranial base that extends from the frontal bone to the lesser wing of the sphenoid bone articular tubercle smooth ridge located on the inferior skull, immediately anterior to the mandibular fossa brain case portion of the skull that contains and protects the brain, consisting of the eight bones that form the cranial base and rounded upper skull calvaria (also, skullcap) rounded top of the skull carotid canal zig-zag tunnel providing passage through the base of the skull for the internal carotid artery to the brain; begins anteromedial to the styloid process and terminates in the middle cranial cavity, near the posterior- lateral base of the sella turcica condylar process of the mandible thickened upward projection from posterior margin of mandibular ramus condyle oval-shaped process located at the top of the condylar process of the mandible coronal suture joint that unites the frontal bone to the right and left parietal bones across the top of the skull coronoid process of the mandible flattened upward projection from the anterior margin of the mandibular ramus cranial cavity interior space of the skull that houses the brain cranium skull cribriform plate small, flattened areas with numerous small openings, located to either side of the midline in the floor of the anterior cranial fossa; formed by the ethmoid bone crista galli small upward projection located at the midline in the floor of the anterior cranial fossa; formed by the ethmoid bone ethmoid air cell one of several small, air-filled spaces located within the lateral sides of the ethmoid bone, between the orbit and upper nasal cavity ethmoid bone unpaired bone that forms the roof and upper, lateral walls of the nasal cavity, portions of the floor of the anterior cranial fossa and medial wall of orbit, and the upper portion of the nasal septum external acoustic meatus ear canal opening located on the lateral side of the skull external occipital protuberance small bump located at the midline on the posterior skull facial bones fourteen bones that support the facial structures and form the upper and lower jaws and the hard palate foramen lacerum irregular opening in the base of the skull, located inferior to the exit of carotid canal foramen magnum large opening in the occipital bone of the skull through which the spinal cord emerges and the vertebral arteries enter the cranium foramen ovale of the middle cranial fossa oval-shaped opening in the floor of the middle cranial fossa foramen rotundum round opening in the floor of the middle cranial fossa, located between the superior orbital fissure and foramen ovale foramen spinosum small opening in the floor of the middle cranial fossa, located lateral to the foramen ovale frontal bone unpaired bone that forms forehead, roof of orbit, and floor of anterior cranial fossa frontal sinus air-filled space within the frontal bone; most anterior of the paranasal sinuses glabella slight depression of frontal bone, located at the midline between the eyebrows greater wings of sphenoid bone lateral projections of the sphenoid bone that form the anterior wall of the middle cranial fossa and an area of the lateral skull hard palate bony structure that forms the roof of the mouth and floor of the nasal cavity, formed by the palatine process of the maxillary bones and the horizontal plate of the palatine bones horizontal plate medial extension from the palatine bone that forms the posterior quarter of the hard palate hypoglossal canal paired openings that pass anteriorly from the anterior-lateral margins of the foramen magnum deep to the occipital condyles hypophyseal (pituitary) fossa shallow depression on top of the sella turcica that houses the pituitary (hypophyseal) gland inferior nasal concha one of the paired bones that project from the lateral walls of the nasal cavity to form the largest and most inferior of the nasal conchae infraorbital foramen opening located on anterior skull, below the orbit infratemporal fossa space on lateral side of skull, below the level of the zygomatic arch and deep (medial) to the ramus of the mandible internal acoustic meatus opening into petrous ridge, located on the lateral wall of the posterior cranial fossa jugular foramen irregularly shaped opening located in the lateral floor of the posterior cranial cavity lacrimal bone paired bones that contribute to the anterior-medial wall of each orbit lacrimal fossa shallow depression in the anterior-medial wall of the orbit, formed by the lacrimal bone that gives rise to the nasolacrimal canal lambdoid suture inverted V-shaped joint that unites the occipital bone to the right and left parietal bones on the posterior skull lateral pterygoid plate paired, flattened bony projections of the sphenoid bone located on the inferior skull, lateral to the medial pterygoid plate lesser wings of the sphenoid bone lateral extensions of the sphenoid bone that form the bony lip separating the anterior and middle cranial fossae lingula small flap of bone located on the inner (medial) surface of mandibular ramus, next to the mandibular foramen mandible unpaired bone that forms the lower jaw bone; the only moveable bone of the skull mandibular foramen opening located on the inner (medial) surface of the mandibular ramus mandibular fossa oval depression located on the inferior surface of the skull mandibular notch large U-shaped notch located between the condylar process and coronoid process of the mandible mastoid process large bony prominence on the inferior, lateral skull, just behind the earlobe maxillary bone (also, maxilla) paired bones that form the upper jaw and anterior portion of the hard palate maxillary sinus air-filled space located with each maxillary bone; largest of the paranasal sinuses medial pterygoid plate paired, flattened bony projections of the sphenoid bone located on the inferior skull medial to the lateral pterygoid plate; form the posterior portion of the nasal cavity lateral wall mental foramen opening located on the anterior-lateral side of the mandibular body mental protuberance inferior margin of anterior mandible that forms the chin middle cranial fossa centrally located cranial fossa that extends from the lesser wings of the sphenoid bone to the petrous ridge middle nasal concha nasal concha formed by the ethmoid bone that is located between the superior and inferior conchae mylohyoid line bony ridge located along the inner (medial) surface of the mandibular body nasal bone paired bones that form the base of the nose nasal cavity opening through skull for passage of air nasal conchae curved bony plates that project from the lateral walls of the nasal cavity; include the superior and middle nasal conchae, which are parts of the ethmoid bone, and the independent inferior nasal conchae bone nasal septum flat, midline structure that divides the nasal cavity into halves, formed by the perpendicular plate of the ethmoid bone, vomer bone, and septal cartilage nasolacrimal canal passage for drainage of tears that extends downward from the medial- anterior orbit to the nasal cavity, terminating behind the inferior nasal conchae occipital bone unpaired bone that forms the posterior portions of the brain case and base of the skull occipital condyle paired, oval-shaped bony knobs located on the inferior skull, to either side of the foramen magnum optic canal opening spanning between middle cranial fossa and posterior orbit orbit bony socket that contains the eyeball and associated muscles palatine bone paired bones that form the posterior quarter of the hard palate and a small area in floor of the orbit palatine process medial projection from the maxilla bone that forms the anterior three quarters of the hard palate paranasal sinuses cavities within the skull that are connected to the conchae that serve to warm and humidify incoming air, produce mucus, and lighten the weight of the skull; consist of frontal, maxillary, sphenoidal, and ethmoidal sinuses parietal bone paired bones that form the upper, lateral sides of the skull perpendicular plate of the ethmoid bone downward, midline extension of the ethmoid bone that forms the superior portion of the nasal septum petrous ridge petrous portion of the temporal bone that forms a large, triangular ridge in the floor of the cranial cavity, separating the middle and posterior cranial fossae; houses the middle and inner ear structures posterior cranial fossa deepest and most posterior cranial fossa; extends from the petrous ridge to the occipital bone pterion H-shaped suture junction region that unites the frontal, parietal, temporal, and sphenoid bones on the lateral side of the skull ramus of the mandible vertical portion of the mandible Sagittal suture joint that unites the right and left parietal bones at the midline along the top of the skull sella turcica elevated area of sphenoid bone located at midline of the middle cranial fossa septal cartilage flat cartilage structure that forms the anterior portion of the nasal septum sphenoid bone unpaired bone that forms the central base of skull sphenoid sinus air-filled space located within the sphenoid bone; most posterior of the paranasal sinuses squamous suture joint that unites the parietal bone to the squamous portion of the temporal bone on the lateral side of the skull styloid process downward projecting, elongated bony process located on the inferior aspect of the skull stylomastoid foramen opening located on inferior skull, between the styloid process and mastoid process superior nasal concha smallest and most superiorly located of the nasal conchae; formed by the ethmoid bone superior nuchal line paired bony lines on the posterior skull that extend laterally from the external occipital protuberance superior orbital fissure irregularly shaped opening between the middle cranial fossa and the posterior orbit supraorbital foramen opening located on anterior skull, at the superior margin of the orbit supraorbital margin superior margin of the orbit suture junction line at which adjacent bones of the skull are united by fibrous connective tissue temporal bone paired bones that form the lateral, inferior portions of the skull, with Squamous, mastoid, and petrous portions temporal fossa shallow space on the lateral side of the skull, above the level of the zygomatic arch temporal process of the zygomatic bone short extension from the zygomatic bone that forms the anterior portion of the zygomatic arch vomer bone unpaired bone that forms the inferior and posterior portions of the nasal septum zygomatic arch elongated, free-standing arch on the lateral skull, formed anteriorly by the temporal process of the zygomatic bone and posteriorly by the zygomatic process of the temporal bone zygomatic bone cheekbone; paired bones that contribute to the lateral orbit and anterior zygomatic arch zygomatic process of the temporal bone extension from the temporal bone that forms the posterior portion of the zygomatic arch The Vertebral Column By the end of this section, you will be able to: Describe each region of the vertebral column and the number of bones in each region Discuss the curves of the vertebral column and how these change after birth Describe a typical vertebra and determine the distinguishing characteristics for vertebrae in each vertebral region and features of the sacrum and the coccyx Define the structure of an intervertebral disc Determine the location of the ligaments that provide support for the vertebral column The vertebral column is also known as the spinal column or spine ([link]). It consists of a sequence of vertebrae (singular = vertebra), each of which is separated and united by an intervertebral disc. Together, the vertebrae and intervertebral discs form the vertebral column. It is a flexible column that supports the head, neck, and body and allows for their movements. It also protects the spinal cord, which passes down the back through openings in the vertebrae. Vertebral Column 7 Cervical vertebrae (C1-C7) form cervical curve 12 Thoracic vertbrae (T1-T12) form thoracic curve 5 Lumbar vertebrae (L1—L5) form lumbar curve Fused vertebrae of sacrum and coccyx form sacrococcygeal curve The adult vertebral column consists of 24 vertebrae, plus the sacrum and coccyx. The vertebrae are divided into three regions: cervical C1—C7 vertebrae, thoracic T1—-T12 vertebrae, and lumbar L1—L5 vertebrae. The vertebral column is curved, with two primary curvatures (thoracic and sacrococcygeal curves) and two secondary curvatures (cervical and lumbar curves). Regions of the Vertebral Column The vertebral column originally develops as a series of 33 vertebrae, but this number is eventually reduced to 24 vertebrae, plus the sacrum and coccyx. The vertebral column is subdivided into five regions, with the vertebrae in each area named for that region and numbered in descending order. In the neck, there are seven cervical vertebrae, each designated with the letter “C” followed by its number. Superiorly, the C1 vertebra articulates (forms a joint) with the occipital condyles of the skull. Inferiorly, C1 articulates with the C2 vertebra, and so on. Below these are the 12 thoracic vertebrae, designated T1—T12. The lower back contains the L1—L5 lumbar vertebrae. The single sacrum, which is also part of the pelvis, is formed by the fusion of five sacral vertebrae. Similarly, the coccyx, or tailbone, results from the fusion of four small coccygeal vertebrae. However, the sacral and coccygeal fusions do not start until age 20 and are not completed until middle age. An interesting anatomical fact is that almost all mammals have seven cervical vertebrae, regardless of body size. This means that there are large variations in the size of cervical vertebrae, ranging from the very small cervical vertebrae of a shrew to the greatly elongated vertebrae in the neck of a giraffe. In a full-grown giraffe, each cervical vertebra is 11 inches tall. Curvatures of the Vertebral Column The adult vertebral column does not form a straight line, but instead has four curvatures along its length (see [link]). These curves increase the vertebral column’s strength, flexibility, and ability to absorb shock. When the load on the spine is increased, by carrying a heavy backpack for example, the curvatures increase in depth (become more curved) to accommodate the extra weight. They then spring back when the weight is removed. The four adult curvatures are classified as either primary or secondary curvatures. Primary curves are retained from the original fetal curvature, while secondary curvatures develop after birth. During fetal development, the body is flexed anteriorly into the fetal position, giving the entire vertebral column a single curvature that is concave anteriorly. In the adult, this fetal curvature is retained in two regions of the vertebral column as the thoracic curve, which involves the thoracic vertebrae, and the sacrococcygeal curve, formed by the sacrum and coccyx. Each of these is thus called a primary curve because they are retained from the original fetal curvature of the vertebral column. A secondary curve develops gradually after birth as the child learns to sit upright, stand, and walk. Secondary curves are concave posteriorly, opposite in direction to the original fetal curvature. The cervical curve of the neck region develops as the infant begins to hold their head upright when sitting. Later, as the child begins to stand and then to walk, the lumbar curve of the lower back develops. In adults, the lumbar curve is generally deeper in females. Disorders associated with the curvature of the spine include kyphosis (an excessive posterior curvature of the thoracic region), lordosis (an excessive anterior curvature of the lumbar region), and scoliosis (an abnormal, lateral curvature, accompanied by twisting of the vertebral column). Note: Disorders of the... Vertebral Column Developmental anomalies, pathological changes, or obesity can enhance the normal vertebral column curves, resulting in the development of abnormal or excessive curvatures ({link]). Kyphosis, also referred to as humpback or hunchback, is an excessive posterior curvature of the thoracic region. This can develop when osteoporosis causes weakening and erosion of the anterior portions of the upper thoracic vertebrae, resulting in their gradual collapse ({link]). Lordosis, or swayback, is an excessive anterior curvature of the lumbar region and is most commonly associated with obesity or late pregnancy. The accumulation of body weight in the abdominal region results an anterior shift in the line of gravity that carries the weight of the body. This causes in an anterior tilt of the pelvis and a pronounced enhancement of the lumbar curve. Scoliosis is an abnormal, lateral curvature, accompanied by twisting of the vertebral column. Compensatory curves may also develop in other areas of the vertebral column to help maintain the head positioned over the feet. Scoliosis is the most common vertebral abnormality among girls. The cause is usually unknown, but it may result from weakness of the back muscles, defects such as differential growth rates in the right and left sides of the vertebral column, or differences in the length of the lower limbs. When present, scoliosis tends to get worse during adolescent growth spurts. Although most individuals do not require treatment, a back brace may be recommended for growing children. In extreme cases, surgery may be required. Excessive vertebral curves can be identified while an individual stands in the anatomical position. Observe the vertebral profile from the side and then from behind to check for kyphosis or lordosis. Then have the person bend forward. If scoliosis is present, an individual will have difficulty in bending directly forward, and the right and left sides of the back will not be level with each other in the bent position. Abnormal Curvatures of the Vertebral Column (a) Scoliosis (b) Kyphosis (c) Lordosis (a) Scoliosis is an abnormal lateral bending of the vertebral column. (b) An excessive curvature of the upper thoracic vertebral column is called kyphosis. (c) Lordosis is an excessive curvature in the lumbar region of the vertebral column. Osteoporosis Normal Bone loss vertebrae amplifies curvature Osteoporosis is an age-related disorder that causes the gradual loss of bone density and strength. When the thoracic vertebrae are affected, there can be a gradual collapse of the vertebrae. This results in kyphosis, an excessive curvature of the thoracic region. Note: — meee OPENStAX COLLEGE Osteoporosis is a common age-related bone disease in which bone density and strength is decreased. Watch this video to get a better understanding of how thoracic vertebrae may become weakened and may fracture due to this disease. How may vertebral osteoporosis contribute to kyphosis? General Structure of a Vertebra Within the different regions of the vertebral column, vertebrae vary in size and shape, but they all follow a similar structural pattern. A typical vertebra will consist of a body, a vertebral arch, and seven processes ({link]). The body is the anterior portion of each vertebra and is the part that supports the body weight. Because of this, the vertebral bodies progressively increase in size and thickness going down the vertebral column. The bodies of adjacent vertebrae are separated and strongly united by an intervertebral disc. The vertebral arch forms the posterior portion of each vertebra. It consists of four parts, the right and left pedicles and the right and left laminae. Each pedicle forms one of the lateral sides of the vertebral arch. The pedicles are anchored to the posterior side of the vertebral body. Each lamina forms part of the posterior roof of the vertebral arch. The large opening between the vertebral arch and body is the vertebral foramen, which contains the spinal cord. In the intact vertebral column, the vertebral foramina of all of the vertebrae align to form the vertebral (spinal) canal, which serves as the bony protection and passageway for the spinal cord down the back. When the vertebrae are aligned together in the vertebral column, notches in the margins of the pedicles of adjacent vertebrae together form an intervertebral foramen, the opening through which a spinal nerve exits from the vertebral column ([link]). Seven processes arise from the vertebral arch. Each paired transverse process projects laterally and arises from the junction point between the pedicle and lamina. The single spinous process (vertebral spine) projects posteriorly at the midline of the back. The vertebral spines can easily be felt as a Series of bumps just under the skin down the middle of the back. The transverse and spinous processes serve as important muscle attachment sites. A superior articular process extends or faces upward, and an inferior articular process faces or projects downward on each side of a vertebrae. The paired superior articular processes of one vertebra join with the corresponding paired inferior articular processes from the next higher vertebra. These junctions form slightly moveable joints between the adjacent vertebrae. The shape and orientation of the articular processes vary in different regions of the vertebral column and play a major role in determining the type and range of motion available in each region. Parts of a Typical Vertebra Anterior Posterior Posterior Spinal cord Spinal cord Spinous process Transverse process Facet of superior articular process Vertebral foramen Intervetebral disc Facet for head Vertebral arch: of rib Lamina Facet of superior articular process Inferior articular process Facet for head of rib Spinal nerve exiting Anterior through intervertebral foramen Spinous process Superior view Left posterolateral view of articulated vertebrae A typical vertebra consists of a body and a vertebral arch. The arch is formed by the paired pedicles and paired laminae. Arising from the vertebral arch are the transverse, spinous, superior articular, and inferior articular processes. The vertebral foramen provides for passage of the spinal cord. Each spinal nerve exits through an intervertebral foramen, located between adjacent vertebrae. Intervertebral discs unite the bodies of adjacent vertebrae. Intervertebral Disc Vertebral body Intervertebral foramen o> Anulus fibrosus ~~ Nucleus pulposus Lateral view Superior view The bodies of adjacent vertebrae are separated and united by an intervertebral disc, which provides padding and allows for movements between adjacent vertebrae. The disc consists of a fibrous outer layer called the anulus fibrosus and a gel-like center called the nucleus pulposus. The intervertebral foramen is the opening formed between adjacent vertebrae for the exit of a spinal nerve. Regional Modifications of Vertebrae In addition to the general characteristics of a typical vertebra described above, vertebrae also display characteristic size and structural features that vary between the different vertebral column regions. Thus, cervical vertebrae are smaller than lumbar vertebrae due to differences in the proportion of body weight that each supports. Thoracic vertebrae have sites for rib attachment, and the vertebrae that give rise to the sacrum and coccyx have fused together into single bones. Cervical Vertebrae Typical cervical vertebrae, such as C4 or C5, have several characteristic features that differentiate them from thoracic or lumbar vertebrae ([link]). Cervical vertebrae have a small body, reflecting the fact that they carry the least amount of body weight. Cervical vertebrae usually have a bifid (Y- shaped) spinous process. The spinous processes of the C3—C6 vertebrae are short, but the spine of C7 is much longer. You can find these vertebrae by running your finger down the midline of the posterior neck until you encounter the prominent C7 spine located at the base of the neck. The transverse processes of the cervical vertebrae are sharply curved (U-shaped) to allow for passage of the cervical spinal nerves. Each transverse process also has an opening called the transverse foramen. An important artery that supplies the brain ascends up the neck by passing through these openings. The superior and inferior articular processes of the cervical vertebrae are flattened and largely face upward or downward, respectively. The first and second cervical vertebrae are further modified, giving each a distinctive appearance. The first cervical (C1) vertebra is also called the atlas, because this is the vertebra that supports the skull on top of the vertebral column (in Greek mythology, Atlas was the god who supported the heavens on his shoulders). The C1 vertebra does not have a body or spinous process. Instead, it is ring-shaped, consisting of an anterior arch and a posterior arch. The transverse processes of the atlas are longer and extend more laterally than do the transverse processes of any other cervical vertebrae. The superior articular processes face upward and are deeply curved for articulation with the occipital condyles on the base of the skull. The inferior articular processes are flat and face downward to join with the superior articular processes of the C2 vertebra. The second cervical (C2) vertebra is called the axis, because it serves as the axis for rotation when turning the head toward the right or left. The axis resembles typical cervical vertebrae in most respects, but is easily distinguished by the dens (odontoid process), a bony projection that extends upward from the vertebral body. The dens joins with the inner aspect of the anterior arch of the atlas, where it is held in place by transverse ligament. Cervical Vertebrae Dens of axis Transverse ligament s Spinous process (bifid) & C4 f SS ers Vertebral foramen C; (atlas) “> ' Cz (axis) —— > Pedicle C3 : Lamina Inferior articular process Superior articular process Bifid spinous process ieee? Transverse process process Transverse C; (vertebra foramen Body Groove prominens) for spinal nerve Structure of a typical cervical vertebra Dens Superior articular Transverse facet fie process Dens - Superior articular Anterior arch Transverse foramen facet Transverse Lamina process Transverse foramen Posterior arch Spinous process Ligament Superior view of atlas Superior view of axis Dens Transverse process Inferior articular Body process Anterior view of axis A typical cervical vertebra has a small body, a bifid spinous process, transverse processes that have a transverse foramen and are curved for spinal nerve passage. The atlas (C1 vertebra) does not have a body or spinous process. It consists of an anterior and a posterior arch and elongated transverse processes. The axis (C2 vertebra) has the upward projecting dens, which articulates with the anterior arch of the atlas. Thoracic Vertebrae The bodies of the thoracic vertebrae are larger than those of cervical vertebrae ({link]). The characteristic feature for a typical midthoracic vertebra is the spinous process, which is long and has a pronounced downward angle that causes it to overlap the next inferior vertebra. The superior articular processes of thoracic vertebrae face anteriorly and the inferior processes face posteriorly. These orientations are important determinants for the type and range of movements available to the thoracic region of the vertebral column. Thoracic vertebrae have several additional articulation sites, each of which is called a facet, where a rib is attached. Most thoracic vertebrae have two facets located on the lateral sides of the body, each of which is called a costal facet (costal = “rib”). These are for articulation with the head (end) of arib. An additional facet is located on the transverse process for articulation with the tubercle of a rib. Thoracic Vertebrae Superior articular process Articular facet for tubercle of rib Transverse process Pedicle Intervertebral Lamina disc Body Spinous rocess ' P Superior costal facet fe vy] // \nferior Inferior costal iff articular facet process A typical thoracic vertebra is distinguished by the spinous process, which is long and projects downward to overlap the next inferior vertebra. It also has articulation sites (facets) on the vertebral body and a transverse process for rib attachment. Rib Articulation in Thoracic Vertebrae Superior L articular facets U a Superior costal facet 7 fi) Crea ZY \ Z - Facet for —_ WAN eS ; \\ YYY tubercle of rib Ay Body of vertebra | we tl Head of rib Zl >: b Ns Intervertebral disc Tranverse r ; processes Fit— Neck of rib él { ! Se Body of vertebra ; 7 = Tubercle of rib BS . . Inferior costal facet Spinous process Thoracic vertebrae have superior and inferior articular facets on the vertebral body for articulation with the head of a rib, and a transverse process facet for articulation with the rib tubercle. Lumbar Vertebrae Lumbar vertebrae carry the greatest amount of body weight and are thus characterized by the large size and thickness of the vertebral body ((link]). They have short transverse processes and a short, blunt spinous process that projects posteriorly. The articular processes are large, with the superior process facing backward and the inferior facing forward. Lumbar Vertebrae Superior articular Transverse process Inferior articular process ; go ST, Spinous ————4—“ : [7 process Intervertebral disc Inferior articular process Lumbar vertebrae are characterized by having a large, thick body and a short, rounded spinous process. Sacrum and Coccyx The sacrum is a triangular-shaped bone that is thick and wide across its superior base where it is weight bearing and then tapers down to an inferior, non-weight bearing apex ([link]). It is formed by the fusion of five sacral vertebrae, a process that does not begin until after the age of 20. On the anterior surface of the older adult sacrum, the lines of vertebral fusion can be seen as four transverse ridges. On the posterior surface, running down the midline, is the median sacral crest, a bumpy ridge that is the remnant of the fused spinous processes (median = “midline”; while medial = “toward, but not necessarily at, the midline”). Similarly, the fused transverse processes of the sacral vertebrae form the lateral sacral crest. The sacral promontory is the anterior lip of the superior base of the sacrum. Lateral to this is the roughened auricular surface, which joins with the ilium portion of the hipbone to form the immobile sacroiliac joints of the pelvis. Passing inferiorly through the sacrum is a bony tunnel called the sacral canal, which terminates at the sacral hiatus near the inferior tip of the sacrum. The anterior and posterior surfaces of the sacrum have a series of paired openings called sacral foramina (singular = foramen) that connect to the sacral canal. Each of these openings is called a posterior (dorsal) sacral foramen or anterior (ventral) sacral foramen. These openings allow for the anterior and posterior branches of the sacral spinal nerves to exit the sacrum. The superior articular process of the sacrum, one of which is found on either side of the superior opening of the sacral canal, articulates with the inferior articular processes from the L5 vertebra. The coccyx, or tailbone, is derived from the fusion of four very small coccygeal vertebrae (see [link]). It articulates with the inferior tip of the sacrum. It is not weight bearing in the standing position, but may receive some body weight when sitting. Sacrum and Coccyx Sacral Body Facet of superior Sacral promontory articular process Body of first sacral vertebra wi T Transverse ridges (sites of vertebral fusion) Auricular surface Lateral sacral crest Posterior sacral foramina Anterior sacral foramina A Sacral hiatus - Coccyx Anterior view Posterior view Apex Coccyx The sacrum is formed from the fusion of five sacral vertebrae, whose lines of fusion are indicated by the transverse ridges. The fused spinous processes form the median sacral crest, while the lateral sacral crest arises from the fused transverse processes. The coccyx is formed by the fusion of four small coccygeal vertebrae. Intervertebral Discs and Ligaments of the Vertebral Column The bodies of adjacent vertebrae are strongly anchored to each other by an intervertebral disc. This structure provides padding between the bones during weight bearing, and because it can change shape, also allows for movement between the vertebrae. Although the total amount of movement available between any two adjacent vertebrae is small, when these movements are summed together along the entire length of the vertebral column, large body movements can be produced. Ligaments that extend along the length of the vertebral column also contribute to its overall support and stability. Intervertebral Disc An intervertebral disc is a fibrocartilaginous pad that fills the gap between adjacent vertebral bodies (see [link]). Each disc is anchored to the bodies of its adjacent vertebrae, thus strongly uniting these. The discs also provide padding between vertebrae during weight bearing. Because of this, intervertebral discs are thin in the cervical region and thickest in the lumbar region, which carries the most body weight. In total, the intervertebral discs account for approximately 25 percent of your body height between the top of the pelvis and the base of the skull. Intervertebral discs are also flexible and can change shape to allow for movements of the vertebral column. Each intervertebral disc consists of two parts. The anulus fibrosus is the tough, fibrous outer layer of the disc. It forms a circle (anulus = “ring” or “circle”) and is firmly anchored to the outer margins of the adjacent vertebral bodies. Inside is the nucleus pulposus, consisting of a softer, more gel-like material. It has a high water content that serves to resist compression and thus is important for weight bearing. With increasing age, the water content of the nucleus pulposus gradually declines. This causes the disc to become thinner, decreasing total body height somewhat, and reduces the flexibility and range of motion of the disc, making bending more difficult. The gel-like nature of the nucleus pulposus also allows the intervertebral disc to change shape as one vertebra rocks side to side or forward and back in relation to its neighbors during movements of the vertebral column. Thus, bending forward causes compression of the anterior portion of the disc but expansion of the posterior disc. If the posterior anulus fibrosus is weakened due to injury or increasing age, the pressure exerted on the disc when bending forward and lifting a heavy object can cause the nucleus pulposus to protrude posteriorly through the anulus fibrosus, resulting in a herniated disc (“ruptured” or “slipped” disc) ({link]). The posterior bulging of the nucleus pulposus can cause compression of a spinal nerve at the point where it exits through the intervertebral foramen, with resulting pain and/or muscle weakness in those body regions supplied by that nerve. The most common sites for disc herniation are the L4/L5 or L5/S1 intervertebral discs, which can cause sciatica, a widespread pain that radiates from the lower back down the thigh and into the leg. Similar injuries of the C5/C6 or C6/C7 intervertebral discs, following forcible hyperflexion of the neck from a collision accident or football injury, can produce pain in the neck, shoulder, and upper limb. Herniated Intervertebral Disc Spinal cord within vertebral canal Herniated disc compresses nerve in intervertebral foramen Nucleus pulposus \ - Anulus fibrosus Herniated portion of disc Superior view Weakening of the anulus fibrosus can result in herniation (protrusion) of the nucleus pulposus and compression of a spinal nerve, resulting in pain and/or muscle weakness in the body regions supplied by that nerve. Note: os 1 ‘all at t 4 openstax COLLEGE er [my: i C Watch this animation to see what it means to “slip” a disk. Watch this second animation to see one possible treatment for a herniated disc, removing and replacing the damaged disc with an artificial one that allows for movement between the adjacent certebrae. How could lifting a heavy object produce pain in a lower limb? Ligaments of the Vertebral Column Adjacent vertebrae are united by ligaments that run the length of the vertebral column along both its posterior and anterior aspects ([link]). These serve to resist excess forward or backward bending movements of the vertebral column, respectively. The anterior longitudinal ligament runs down the anterior side of the entire vertebral column, uniting the vertebral bodies. It serves to resist excess backward bending of the vertebral column. Protection against this movement is particularly important in the neck, where extreme posterior bending of the head and neck can stretch or tear this ligament, resulting in a painful whiplash injury. Prior to the mandatory installation of seat headrests, whiplash injuries were common for passengers involved in a rear-end automobile collision. The supraspinous ligament is located on the posterior side of the vertebral column, where it interconnects the spinous processes of the thoracic and lumbar vertebrae. This strong ligament supports the vertebral column during forward bending motions. In the posterior neck, where the cervical spinous processes are short, the supraspinous ligament expands to become the nuchal ligament (nuchae = “nape” or “back of the neck”). The nuchal ligament is attached to the cervical spinous processes and extends upward and posteriorly to attach to the midline base of the skull, out to the external occipital protuberance. It supports the skull and prevents it from falling forward. This ligament is much larger and stronger in four-legged animals such as cows, where the large skull hangs off the front end of the vertebral column. You can easily feel this ligament by first extending your head backward and pressing down on the posterior midline of your neck. Then tilt your head forward and you will fill the nuchal ligament popping out as it tightens to limit anterior bending of the head and neck. Additional ligaments are located inside the vertebral canal, next to the spinal cord, along the length of the vertebral column. The posterior longitudinal ligament is found anterior to the spinal cord, where it is attached to the posterior sides of the vertebral bodies. Posterior to the spinal cord is the ligamentum flavum (“yellow ligament”). This consists of a series of short, paired ligaments, each of which interconnects the lamina regions of adjacent vertebrae. The ligamentum flavum has large numbers of elastic fibers, which have a yellowish color, allowing it to stretch and then pull back. Both of these ligaments provide important support for the vertebral column when bending forward. Ligaments of Vertebral Column External occipital protuberance Nuchal ligament Spinous process of T1 vertebra Anterior longitudinal Supraspinous ligament ligament The anterior longitudinal ligament runs the length of the vertebral column, uniting the anterior sides of the vertebral bodies. The supraspinous ligament connects the spinous processes of the thoracic and lumbar vertebrae. In the posterior neck, the supraspinous ligament enlarges to form the nuchal ligament, which attaches to the cervical spinous processes and to the base of the skull. Note: oe openstax COLLEGE Use this tool to identify the bones, intervertebral discs, and ligaments of the vertebral column. The thickest portions of the anterior longitudinal ligament and the supraspinous ligament are found in which regions of the vertebral column? Note: Career Connections Chiropractor Chiropractors are health professionals who use nonsurgical techniques to help patients with musculoskeletal system problems that involve the bones, muscles, ligaments, tendons, or nervous system. They treat problems such as neck pain, back pain, joint pain, or headaches. Chiropractors focus on the patient’s overall health and can also provide counseling related to lifestyle issues, such as diet, exercise, or sleep problems. If needed, they will refer the patient to other medical specialists. Chiropractors use a drug-free, hands-on approach for patient diagnosis and treatment. They will perform a physical exam, assess the patient’s posture and spine, and may perform additional diagnostic tests, including taking X- ray images. They primarily use manual techniques, such as spinal manipulation, to adjust the patient’s spine or other joints. They can recommend therapeutic or rehabilitative exercises, and some also include acupuncture, massage therapy, or ultrasound as part of the treatment program. In addition to those in general practice, some chiropractors specialize in sport injuries, neurology, orthopaedics, pediatrics, nutrition, internal disorders, or diagnostic imaging. To become a chiropractor, students must have 3—4 years of undergraduate education, attend an accredited, four-year Doctor of Chiropractic (D.C.) degree program, and pass a licensure examination to be licensed for practice in their state. With the aging of the baby-boom generation, employment for chiropractors is expected to increase. Chapter Review The vertebral column forms the neck and back. The vertebral column originally develops as 33 vertebrae, but is eventually reduced to 24 vertebrae, plus the sacrum and coccyx. The vertebrae are divided into the cervical region (C1—C7 vertebrae), the thoracic region (T1—T12 vertebrae), and the lumbar region (L1—L5 vertebrae). The sacrum arises from the fusion of five sacral vertebrae and the coccyx from the fusion of four small coccygeal vertebrae. The vertebral column has four curvatures, the cervical, thoracic, lumbar, and sacrococcygeal curves. The thoracic and sacrococcygeal curves are primary curves retained from the original fetal curvature. The cervical and lumbar curves develop after birth and thus are secondary curves. The cervical curve develops as the infant begins to hold up the head, and the lumbar curve appears with standing and walking. A typical vertebra consists of an enlarged anterior portion called the body, which provides weight-bearing support. Attached posteriorly to the body is a vertebral arch, which surrounds and defines the vertebral foramen for passage of the spinal cord. The vertebral arch consists of the pedicles, which attach to the vertebral body, and the laminae, which come together to form the roof of the arch. Arising from the vertebral arch are the laterally projecting transverse processes and the posteriorly oriented spinous process. The superior articular processes project upward, where they articulate with the downward projecting inferior articular processes of the next higher vertebrae. A typical cervical vertebra has a small body, a bifid (Y-shaped) spinous process, and U-shaped transverse processes with a transverse foramen. In addition to these characteristics, the axis (C2 vertebra) also has the dens projecting upward from the vertebral body. The atlas (C1 vertebra) differs from the other cervical vertebrae in that it does not have a body, but instead consists of bony ring formed by the anterior and posterior arches. The atlas articulates with the dens from the axis. A typical thoracic vertebra is distinguished by its long, downward projecting spinous process. Thoracic vertebrae also have articulation facets on the body and transverse processes for attachment of the ribs. Lumbar vertebrae support the greatest amount of body weight and thus have a large, thick body. They also have a short, blunt spinous process. The sacrum is triangular in shape. The median sacral crest is formed by the fused vertebral spinous processes and the lateral sacral crest is derived from the fused transverse processes. Anterior (ventral) and posterior (dorsal) sacral foramina allow branches of the sacral spinal nerves to exit the sacrum. The auricular surfaces are articulation sites on the lateral sacrum that anchor the sacrum to the hipbones to form the pelvis. The coccyx is small and derived from the fusion of four small vertebrae. The intervertebral discs fill in the gaps between the bodies of adjacent vertebrae. They provide strong attachments and padding between the vertebrae. The outer, fibrous layer of a disc is called the anulus fibrosus. The gel-like interior is called the nucleus pulposus. The disc can change shape to allow for movement between vertebrae. If the anulus fibrosus is weakened or damaged, the nucleus pulposus can protrude outward, resulting in a herniated disc. The anterior longitudinal ligament runs along the full length of the anterior vertebral column, uniting the vertebral bodies. The supraspinous ligament is located posteriorly and interconnects the spinous processes of the thoracic and lumbar vertebrae. In the neck, this ligament expands to become the nuchal ligament. The nuchal ligament is attached to the cervical spinous processes and superiorly to the base of the skull, out to the external occipital protuberance. The posterior longitudinal ligament runs within the vertebral canal and unites the posterior sides of the vertebral bodies. The ligamentum flavum unites the lamina of adjacent vertebrae. Interactive Link Questions Exercise: Problem: Osteoporosis is a common age-related bone disease in which bone density and strength is decreased. Watch this video to get a better understanding of how thoracic vertebrae may become weakened and may fractured due to this disease. How may vertebral osteoporosis contribute to kyphosis? Solution: Osteoporosis causes thinning and weakening of the vertebral bodies. When this occurs in thoracic vertebrae, the bodies may collapse producing kyphosis, an enhanced anterior curvature of the thoracic vertebral column. Exercise: Problem: Watch this animation to see what it means to “slip” a disk. Watch this second animation to see one possible treatment for a herniated disc, removing and replacing the damaged disc with an artificial one that allows for movement between the adjacent certebrae. How could lifting a heavy object produce pain in a lower limb? Solution: Lifting a heavy object can cause an intervertebral disc in the lower back to bulge and compress a spinal nerve as it exits through the intervertebral foramen, thus producing pain in those regions of the lower limb supplied by that nerve. Exercise: Problem: Use this tool to identify the bones, intervertebral discs, and ligaments of the vertebral column. The thickest portions of the anterior longitudinal ligament and the supraspinous ligament are found in which regions of the vertebral column? Solution: The anterior longitudinal ligament is thickest in the thoracic region of the vertebral column, while the supraspinous ligament is thickest in the lumbar region. Review Questions Exercise: Problem: The cervical region of the vertebral column consists of a. seven vertebrae b. 12 vertebrae c. five vertebrae d. a single bone derived from the fusion of five vertebrae Solution: A Exercise: Problem:The primary curvatures of the vertebral column a. include the lumbar curve b. are remnants of the original fetal curvature c. include the cervical curve d. develop after the time of birth Solution: B Exercise: Problem: A typical vertebra has a. a vertebral foramen that passes through the body b. a superior articular process that projects downward to articulate with the superior portion of the next lower vertebra c. lamina that spans between the transverse process and spinous process d. a pair of laterally projecting spinous processes Solution: C Exercise: Problem:A typical lumbar vertebra has a. a Short, rounded spinous process b. a bifid spinous process c. articulation sites for ribs d. a transverse foramen Solution: A Exercise: Problem: Which is found only in the cervical region of the vertebral column? a. nuchal ligament b. ligamentum flavum c. supraspinous ligament d. anterior longitudinal ligament Solution: A Critical Thinking Questions Exercise: Problem: Describe the vertebral column and define each region. Solution: Answer: The adult vertebral column consists of 24 vertebrae, plus the sacrum and coccyx. The vertebrae are subdivided into cervical, thoracic, and lumbar regions. There are seven cervical vertebrae (C1— C7), 12 thoracic vertebrae (T1—T12), and five lumbar vertebrae (L1— L5). The sacrum is derived from the fusion of five sacral vertebrae and the coccyx is formed by the fusion of four small coccygeal vertebrae. Exercise: Problem: Describe a typical vertebra. Solution: A typical vertebra consists of an anterior body and a posterior vertebral arch. The body serves for weight bearing. The vertebral arch surrounds and protects the spinal cord. The vertebral arch is formed by the pedicles, which are attached to the posterior side of the vertebral body, and the lamina, which come together to form the top of the arch. A pair of transverse processes extends laterally from the vertebral arch, at the junction between each pedicle and lamina. The spinous process extends posteriorly from the top of the arch. A pair of superior articular processes project upward and a pair of inferior articular processes project downward. Together, the notches found in the margins of the pedicles of adjacent vertebrae form an intervertebral foramen. Exercise: Problem: Describe the sacrum. Solution: The sacrum is a single, triangular-shaped bone formed by the fusion of five sacral vertebrae. On the posterior sacrum, the median sacral crest is derived from the fused spinous processes, and the lateral sacral crest results from the fused transverse processes. The sacral canal contains the sacral spinal nerves, which exit via the anterior (ventral) and posterior (dorsal) sacral foramina. The sacral promontory is the anterior lip. The sacrum also forms the posterior portion of the pelvis. Exercise: Problem: Describe the structure and function of an intervertebral disc. Solution: An intervertebral disc fills in the space between adjacent vertebrae, where it provides padding and weight-bearing ability, and allows for movements between the vertebrae. It consists of an outer anulus fibrosus and an inner nucleus pulposus. The anulus fibrosus strongly anchors the adjacent vertebrae to each other, and the high water content of the nucleus pulposus resists compression for weight bearing and can change shape to allow for vertebral column movements. Exercise: Problem: Define the ligaments of the vertebral column. Solution: The anterior longitudinal ligament is attached to the vertebral bodies on the anterior side of the vertebral column. The supraspinous ligament is located on the posterior side, where it interconnects the thoracic and lumbar spinous processes. In the posterior neck, this ligament expands to become the nuchal ligament, which attaches to the cervical spinous processes and the base of the skull. The posterior longitudinal ligament and ligamentum flavum are located inside the vertebral canal. The posterior longitudinal ligament unites the posterior sides of the vertebral bodies. The ligamentum flavum unites the lamina of adjacent vertebrae. Glossary anterior arch anterior portion of the ring-like C1 (atlas) vertebra anterior longitudinal ligament ligament that runs the length of the vertebral column, uniting the anterior aspects of the vertebral bodies anterior (ventral) sacral foramen one of the series of paired openings located on the anterior (ventral) side of the sacrum anulus fibrosus tough, fibrous outer portion of an intervertebral disc, which is strongly anchored to the bodies of the adjacent vertebrae atlas first cervical (C1) vertebra axis second cervical (C2) vertebra cervical curve posteriorly concave curvature of the cervical vertebral column region; a secondary curve of the vertebral column cervical vertebrae seven vertebrae numbered as C1—C7 that are located in the neck region of the vertebral column costal facet site on the lateral sides of a thoracic vertebra for articulation with the head of a rib dens bony projection (odontoid process) that extends upward from the body of the C2 (axis) vertebra facet small, flattened area on a bone for an articulation (joint) with another bone, or for muscle attachment inferior articular process bony process that extends downward from the vertebral arch of a vertebra that articulates with the superior articular process of the next lower vertebra intervertebral disc structure located between the bodies of adjacent vertebrae that strongly joins the vertebrae; provides padding, weight bearing ability, and enables vertebral column movements intervertebral foramen opening located between adjacent vertebrae for exit of a spinal nerve kyphosis (also, humpback or hunchback) excessive posterior curvature of the thoracic vertebral column region lamina portion of the vertebral arch on each vertebra that extends between the transverse and spinous process lateral sacral crest paired irregular ridges running down the lateral sides of the posterior sacrum that was formed by the fusion of the transverse processes from the five sacral vertebrae ligamentum flavum series of short ligaments that unite the lamina of adjacent vertebrae lordosis (also, swayback) excessive anterior curvature of the lumbar vertebral column region lumbar curve posteriorly concave curvature of the lumbar vertebral column region; a secondary curve of the vertebral column lumbar vertebrae five vertebrae numbered as L1—L5 that are located in lumbar region (lower back) of the vertebral column median sacral crest irregular ridge running down the midline of the posterior sacrum that was formed from the fusion of the spinous processes of the five sacral vertebrae nuchal ligament expanded portion of the supraspinous ligament within the posterior neck; interconnects the spinous processes of the cervical vertebrae and attaches to the base of the skull nucleus pulposus gel-like central region of an intervertebral disc; provides for padding, weight-bearing, and movement between adjacent vertebrae pedicle portion of the vertebral arch that extends from the vertebral body to the transverse process posterior arch posterior portion of the ring-like C1 (atlas) vertebra posterior longitudinal ligament ligament that runs the length of the vertebral column, uniting the posterior sides of the vertebral bodies posterior (dorsal) sacral foramen one of the series of paired openings located on the posterior (dorsal) side of the sacrum primary curve anteriorly concave curvatures of the thoracic and sacrococcygeal regions that are retained from the original fetal curvature of the vertebral column sacral canal bony tunnel that runs through the sacrum sacral foramina series of paired openings for nerve exit located on both the anterior (ventral) and posterior (dorsal) aspects of the sacrum sacral hiatus inferior opening and termination of the sacral canal sacral promontory anterior lip of the base (superior end) of the sacrum sacrococcygeal curve anteriorly concave curvature formed by the sacrum and coccyx; a primary curve of the vertebral column scoliosis abnormal lateral curvature of the vertebral column secondary curve posteriorly concave curvatures of the cervical and lumbar regions of the vertebral column that develop after the time of birth spinous process unpaired bony process that extends posteriorly from the vertebral arch of a vertebra superior articular process bony process that extends upward from the vertebral arch of a vertebra that articulates with the inferior articular process of the next higher vertebra superior articular process of the sacrum paired processes that extend upward from the sacrum to articulate (join) with the inferior articular processes from the L5 vertebra supraspinous ligament ligament that interconnects the spinous processes of the thoracic and lumbar vertebrae thoracic curve anteriorly concave curvature of the thoracic vertebral column region; a primary curve of the vertebral column thoracic vertebrae twelve vertebrae numbered as T1—T12 that are located in the thoracic region (upper back) of the vertebral column transverse foramen opening found only in the transverse processes of cervical vertebrae transverse process paired bony processes that extends laterally from the vertebral arch of a vertebra vertebral arch bony arch formed by the posterior portion of each vertebra that surrounds and protects the spinal cord vertebral (spinal) canal bony passageway within the vertebral column for the spinal cord that is formed by the series of individual vertebral foramina vertebral foramen opening associated with each vertebra defined by the vertebral arch that provides passage for the spinal cord The Thoracic Cage By the end of this section, you will be able to: e Discuss the components that make up the thoracic cage e Identify the parts of the sternum and define the sternal angle e Discuss the parts of a rib and rib classifications The thoracic cage (rib cage) forms the thorax (chest) portion of the body. It consists of the 12 pairs of ribs with their costal cartilages and the sternum ({link]). The ribs are anchored posteriorly to the 12 thoracic vertebrae (T1— T12). The thoracic cage protects the heart and lungs. Thoracic Cage Superior Superior Jugular Clavicular a notch notch or —— Jugular notch Clavicular notch Manubrium Clavicle Sternal 4 angle Sternum: Manubrium Scapula Body § Sternal angle Body —_ ) Xiphoid : process Costal cartilages Intercostal space Xiphoid aw process Inferior Inferior (a) Anterior view of sternum (b) Anterior view of skeleton of thorax The thoracic cage is formed by the (a) sternum and (b) 12 pairs of ribs with their costal cartilages. The ribs are anchored posteriorly to the 12 thoracic vertebrae. The sternum consists of the manubrium, body, and xiphoid process. The ribs are classified as true ribs (1—7) and false ribs (8-12). The last two pairs of false ribs are also known as floating ribs (11-12). Sternum The sternum is the elongated bony structure that anchors the anterior thoracic cage. It consists of three parts: the manubrium, body, and xiphoid process. The manubrium is the wider, superior portion of the sternum. The top of the manubrium has a shallow, U-shaped border called the jugular (suprasternal) notch. This can be easily felt at the anterior base of the neck, between the medial ends of the clavicles. The clavicular notch is the shallow depression located on either side at the superior-lateral margins of the manubrium. This is the site of the sternoclavicular joint, between the sternum and clavicle. The first ribs also attach to the manubrium. The elongated, central portion of the sternum is the body. The manubrium and body join together at the sternal angle, so called because the junction between these two components is not flat, but forms a slight bend. The second rib attaches to the sternum at the sternal angle. Since the first rib is hidden behind the clavicle, the second rib is the highest rib that can be identified by palpation. Thus, the sternal angle and second rib are important landmarks for the identification and counting of the lower ribs. Ribs 3—7 attach to the sternal body. The inferior tip of the sternum is the xiphoid process. This small structure is cartilaginous early in life, but gradually becomes ossified starting during middle age. Ribs Each rib is a curved, flattened bone that contributes to the wall of the thorax. The ribs articulate posteriorly with the T1—-T12 thoracic vertebrae, and most attach anteriorly via their costal cartilages to the sternum. There are 12 pairs of ribs. The ribs are numbered 1—12 in accordance with the thoracic vertebrae. Parts of a Typical Rib The posterior end of a typical rib is called the head of the rib (see [link]). This region articulates primarily with the costal facet located on the body of the same numbered thoracic vertebra and to a lesser degree, with the costal facet located on the body of the next higher vertebra. Lateral to the head is the narrowed neck of the rib. A small bump on the posterior rib surface is the tubercle of the rib, which articulates with the facet located on the transverse process of the same numbered vertebra. The remainder of the rib is the body of the rib (shaft). Just lateral to the tubercle is the angle of the rib, the point at which the rib has its greatest degree of curvature. The angles of the ribs form the most posterior extent of the thoracic cage. In the anatomical position, the angles align with the medial border of the scapula. A shallow costal groove for the passage of blood vessels and a nerve is found along the inferior margin of each rib. Rib Classifications The bony ribs do not extend anteriorly completely around to the sternum. Instead, each rib ends in a costal cartilage. These cartilages are made of hyaline cartilage and can extend for several inches. Most ribs are then attached, either directly or indirectly, to the sternum via their costal cartilage (see [link]). The ribs are classified into three groups based on their relationship to the sternum. Ribs 1—7 are classified as true ribs (vertebrosternal ribs). The costal cartilage from each of these ribs attaches directly to the sternum. Ribs 8-12 are called false ribs (vertebrochondral ribs). The costal cartilages from these ribs do not attach directly to the sternum. For ribs 8—10, the costal cartilages are attached to the cartilage of the next higher rib. Thus, the cartilage of rib 10 attaches to the cartilage of rib 9, rib 9 then attaches to rib 8, and rib 8 is attached to rib 7. The last two false ribs (11-12) are also called floating ribs (vertebral ribs). These are short ribs that do not attach to the sternum at all. Instead, their small costal cartilages terminate within the musculature of the lateral abdominal wall. Chapter Review The thoracic cage protects the heart and lungs. It is composed of 12 pairs of ribs with their costal cartilages and the sternum. The ribs are anchored posteriorly to the 12 thoracic vertebrae. The sternum consists of the manubrium, body, and xiphoid process. The manubrium and body are joined at the sternal angle, which is also the site for attachment of the second ribs. Ribs are flattened, curved bones and are numbered 1-12. Posteriorly, the head of the rib articulates with the costal facets located on the bodies of thoracic vertebrae and the rib tubercle articulates with the facet located on the vertebral transverse process. The angle of the ribs forms the most posterior portion of the thoracic cage. The costal groove in the inferior margin of each rib carries blood vessels and a nerve. Anteriorly, each rib ends in a costal cartilage. True ribs (1—7) attach directly to the sternum via their costal cartilage. The false ribs (8-12) either attach to the sternum indirectly or not at all. Ribs 8—10 have their costal cartilages attached to the cartilage of the next higher rib. The floating ribs (11-12) are short and do not attach to the sternum or to another rib. Review Questions Exercise: Problem: The sternum a. consists of only two parts, the manubrium and xiphoid process b. has the sternal angle located between the manubrium and body c. receives direct attachments from the costal cartilages of all 12 pairs of ribs d. articulates directly with the thoracic vertebrae Solution: B Exercise: Problem:The sternal angle is the a. junction between the body and xiphoid process b. site for attachment of the clavicle c. site for attachment of the floating ribs d. junction between the manubrium and body Solution: D Exercise: Problem:The tubercle of a rib a. is for articulation with the transverse process of a thoracic vertebra b. is for articulation with the body of a thoracic vertebra c. provides for passage of blood vessels and a nerve d. is the area of greatest rib curvature Solution: A Exercise: Problem: True ribs are a. ribs 8-12 b. attached via their costal cartilage to the next higher rib c. made entirely of bone, and thus do not have a costal cartilage d. attached via their costal cartilage directly to the sternum Solution: D Critical Thinking Questions Exercise: Problem: Define the parts and functions of the thoracic cage. Solution: The thoracic cage is formed by the 12 pairs of ribs with their costal cartilages and the sternum. The ribs are attached posteriorly to the 12 thoracic vertebrae and most are anchored anteriorly either directly or indirectly to the sternum. The thoracic cage functions to protect the heart and lungs. Exercise: Problem: Describe the parts of the sternum. Solution: The sternum consists of the manubrium, body, and xiphoid process. The manubrium forms the expanded, superior end of the sternum. It has a jugular (suprasternal) notch, a pair of clavicular notches for articulation with the clavicles, and receives the costal cartilage of the first rib. The manubrium is joined to the body of the sternum at the sternal angle, which is also the site for attachment of the second rib costal cartilages. The body receives the costal cartilage attachments for ribs 3-7. The small xiphoid process forms the inferior tip of the sternum. Exercise: Problem: Discuss the parts of a typical rib. Solution: A typical rib is a flattened, curved bone. The head of a rib is attached posteriorly to the costal facets of the thoracic vertebrae. The rib tubercle articulates with the transverse process of a thoracic vertebra. The angle is the area of greatest rib curvature and forms the largest portion of the thoracic cage. The body (shaft) of a rib extends anteriorly and terminates at the attachment to its costal cartilage. The shallow costal groove runs along the inferior margin of a rib and carries blood vessels and a nerve. Exercise: Problem: Define the classes of ribs. Solution: Ribs are classified based on if and how their costal cartilages attach to the sternum. True (vertebrosternal) ribs are ribs 1—7. The costal cartilage for each of these attaches directly to the sternum. False (vertebrochondral) ribs, 8—12, are attached either indirectly or not at all to the sternum. Ribs 8—10 are attached indirectly to the sternum. For these ribs, the costal cartilage of each attaches to the cartilage of the next higher rib. The last false ribs (11-12) are also called floating (vertebral) ribs, because these ribs do not attach to the sternum at all. Instead, the ribs and their small costal cartilages terminate within the muscles of the lateral abdominal wall. Glossary angle of the rib portion of rib with greatest curvature; together, the rib angles form the most posterior extent of the thoracic cage body of the rib shaft portion of a rib clavicular notch paired notches located on the superior-lateral sides of the sternal manubrium, for articulation with the clavicle costal cartilage hyaline cartilage structure attached to the anterior end of each rib that provides for either direct or indirect attachment of most ribs to the sternum costal groove shallow groove along the inferior margin of a rib that provides passage for blood vessels and a nerve false ribs vertebrochondral ribs 8-12 whose costal cartilage either attaches indirectly to the sternum via the costal cartilage of the next higher rib or does not attach to the sternum at all floating ribs vertebral ribs 11—12 that do not attach to the sternum or to the costal cartilage of another rib head of the rib posterior end of a rib that articulates with the bodies of thoracic vertebrae jugular (suprasternal) notch shallow notch located on superior surface of sternal manubrium manubrium expanded, superior portion of the sternum neck of the rib narrowed region of a rib, next to the rib head sternal angle junction line between manubrium and body of the sternum and the site for attachment of the second rib to the sternum true ribs vertebrosternal ribs 1—7 that attach via their costal cartilage directly to the sternum tubercle of the rib small bump on the posterior side of a rib for articulation with the transverse process of a thoracic vertebra xiphoid process small process that forms the inferior tip of the sternum The Pectoral Girdle By the end of this section, you will be able to: ¢ Describe the bones that form the pectoral girdle e List the functions of the pectoral girdle The appendicular skeleton includes all of the limb bones, plus the bones that unite each limb with the axial skeleton ({link]). The bones that attach each upper limb to the axial skeleton form the pectoral girdle (shoulder girdle). This consists of two bones, the scapula and clavicle ([link]). The clavicle (collarbone) is an S-shaped bone located on the anterior side of the shoulder. It is attached on its medial end to the sternum of the thoracic cage, which is part of the axial skeleton. The lateral end of the clavicle articulates (joins) with the scapula just above the shoulder joint. You can easily palpate, or feel with your fingers, the entire length of your clavicle. Axial and Appendicular Skeletons Vertebral column Pelvic girdle (hip bones) w i Anterior view Skull Cranial portion Facial portion Pectoral (shoulder) girdle Clavicle Scapula Thoracic cage Sternum Ribs Upper limb Vertebral Humerus column vine Pelvic Radius girdle Carpals (hip bones) Metacarpals A Ny Phalanges Lower limb Femur Patella | Key Tee © Axial skeleton Fibula | Appendicular skeleton Tarsals Metatarsals | Phalanges Hy) Posterior view The axial skeleton forms the central axis of the body and consists of the skull, vertebral column, and thoracic cage. The appendicular skeleton consists of the pectoral and pelvic girdles, the limb bones, and the bones of the hands and feet. Pectoral Girdle Coracoclavicular Costoclavicular ligament ligament Clavicle Clavicle Acromioclavicular joint Anterior view of pectoral girdle Posterior view of pectoral girdle Posterior Lateral Anterior Superior view of clavicle Acromial end Sternal end Anterior Posterior Inferior view of clavicle The pectoral girdle consists of the clavicle and the scapula, which serve to attach the upper limb to the sternum of the axial skeleton. The scapula (shoulder blade) lies on the posterior aspect of the shoulder. It is supported by the clavicle, which also articulates with the humerus (arm bone) to form the shoulder joint. The scapula is a flat, triangular-shaped bone with a prominent ridge running across its posterior surface. This ridge extends out laterally, where it forms the bony tip of the shoulder and joins with the lateral end of the clavicle. By following along the clavicle, you can palpate out to the bony tip of the shoulder, and from there, you can move back across your posterior shoulder to follow the ridge of the scapula. Move your shoulder around and feel how the clavicle and scapula move together as a unit. Both of these bones serve as important attachment sites for muscles that aid with movements of the shoulder and arm. The right and left pectoral girdles are not joined to each other, allowing each to operate independently. In addition, the clavicle of each pectoral girdle is anchored to the axial skeleton by a single, highly mobile joint. This allows for the extensive mobility of the entire pectoral girdle, which in turn enhances movements of the shoulder and upper limb. Clavicle The clavicle is the only long bone that lies in a horizontal position in the body (see [link]). The clavicle has several important functions. First, anchored by muscles from above, it serves as a strut that extends laterally to support the scapula. This in turn holds the shoulder joint superiorly and laterally from the body trunk, allowing for maximal freedom of motion for the upper limb. The clavicle also transmits forces acting on the upper limb to the sternum and axial skeleton. Finally, it serves to protect the underlying nerves and blood vessels as they pass between the trunk of the body and the upper limb. The clavicle has three regions: the medial end, the lateral end, and the shaft. The medial end, known as the sternal end of the clavicle, has a triangular shape and articulates with the manubrium portion of the sternum. This forms the sternoclavicular joint, which is the only bony articulation between the pectoral girdle of the upper limb and the axial skeleton. This joint allows considerable mobility, enabling the clavicle and scapula to move in upward/downward and anterior/posterior directions during shoulder movements. The sternoclavicular joint is indirectly supported by the costoclavicular ligament (costo- = “rib”), which spans the sternal end of the clavicle and the underlying first rib. The lateral or acromial end of the clavicle articulates with the acromion of the scapula, the portion of the scapula that forms the bony tip of the shoulder. There are some sex differences in the morphology of the clavicle. In women, the clavicle tends to be shorter, thinner, and less curved. In men, the clavicle is heavier and longer, and has a greater curvature and rougher surfaces where muscles attach, features that are more pronounced in manual workers. The clavicle is the most commonly fractured bone in the body. Such breaks often occur because of the force exerted on the clavicle when a person falls onto his or her outstretched arms, or when the lateral shoulder receives a strong blow. Because the sternoclavicular joint is strong and rarely dislocated, excessive force results in the breaking of the clavicle, usually between the middle and lateral portions of the bone. If the fracture is complete, the shoulder and lateral clavicle fragment will drop due to the weight of the upper limb, causing the person to support the sagging limb with their other hand. Muscles acting across the shoulder will also pull the shoulder and lateral clavicle anteriorly and medially, causing the clavicle fragments to override. The clavicle overlies many important blood vessels and nerves for the upper limb, but fortunately, due to the anterior displacement of a broken clavicle, these structures are rarely affected when the clavicle is fractured. Scapula The scapula is also part of the pectoral girdle and thus plays an important role in anchoring the upper limb to the body. The scapula is located on the posterior side of the shoulder. It is surrounded by muscles on both its anterior (deep) and posterior (superficial) sides, and thus does not articulate with the ribs of the thoracic cage. The scapula has several important landmarks ([link]). The three margins or borders of the scapula, named for their positions within the body, are the superior border of the scapula, the medial border of the scapula, and the lateral border of the scapula. The suprascapular notch is located lateral to the midpoint of the superior border. The corners of the triangular scapula, at either end of the medial border, are the superior angle of the scapula, located between the medial and superior borders, and the inferior angle of the scapula, located between the medial and lateral borders. The inferior angle is the most inferior portion of the scapula, and is particularly important because it serves as the attachment point for several powerful muscles involved in shoulder and upper limb movements. The remaining corner of the scapula, between the superior and lateral borders, is the location of the glenoid cavity (glenoid fossa). This shallow depression articulates with the humerus bone of the arm to form the glenohumeral joint (shoulder joint). The small bony bumps located immediately above and below the glenoid cavity are the supraglenoid tubercle and the infraglenoid tubercle, respectively. These provide attachments for muscles of the arm. Scapula oS Pectoral girdle: Acromion Suprascapular Superior border Coracoid process Suprascapular notch Coracoid F process Acromion Glenoid . Glenoid cavity cavity Superior Sy ee angle ee - ¥ _~—=——_> Supraspinous l-—~\ b . \ fossa Te ; \ - ] fae ea la foss: \ ee fossa \ | ey Lateral border \7 Medial border =4_—_——_ inferior angle——— Lateral border Right scapula, anterior aspect Right scapula, posterior aspect The isolated scapula is shown here from its anterior (deep) side and its posterior (superficial) side. The scapula also has two prominent projections. Toward the lateral end of the superior border, between the suprascapular notch and glenoid cavity, is the hook-like coracoid process (coracoid = “shaped like a crow’s beak”). This process projects anteriorly and curves laterally. At the shoulder, the coracoid process is located inferior to the lateral end of the clavicle. It is anchored to the clavicle by a strong ligament, and serves as the attachment site for muscles of the anterior chest and arm. On the posterior aspect, the spine of the scapula is a long and prominent ridge that runs across its upper portion. Extending laterally from the spine is a flattened and expanded region called the acromion or acromial process. The acromion forms the bony tip of the superior shoulder region and articulates with the lateral end of the clavicle, forming the acromioclavicular joint (see [link]). Together, the clavicle, acromion, and spine of the scapula form a V-shaped bony line that provides for the attachment of neck and back muscles that act on the shoulder, as well as muscles that pass across the shoulder joint to act on the arm. The scapula has three depressions, each of which is called a fossa (plural = fossae). Two of these are found on the posterior scapula, above and below the scapular spine. Superior to the spine is the narrow supraspinous fossa, and inferior to the spine is the broad infraspinous fossa. The anterior (deep) surface of the scapula forms the broad subscapular fossa. All of these fossae provide large surface areas for the attachment of muscles that cross the shoulder joint to act on the humerus. The acromioclavicular joint transmits forces from the upper limb to the clavicle. The ligaments around this joint are relatively weak. A hard fall onto the elbow or outstretched hand can stretch or tear the acromioclavicular ligaments, resulting in a moderate injury to the joint. However, the primary support for the acromioclavicular joint comes from a very strong ligament called the coracoclavicular ligament (see [link]). This connective tissue band anchors the coracoid process of the scapula to the inferior surface of the acromial end of the clavicle and thus provides important indirect support for the acromioclavicular joint. Following a strong blow to the lateral shoulder, such as when a hockey player is driven into the boards, a complete dislocation of the acromioclavicular joint can result. In this case, the acromion is thrust under the acromial end of the clavicle, resulting in ruptures of both the acromioclavicular and coracoclavicular ligaments. The scapula then separates from the clavicle, with the weight of the upper limb pulling the shoulder downward. This dislocation injury of the acromioclavicular joint is known as a “shoulder separation” and is common in contact sports such as hockey, football, or martial arts. Chapter Review The pectoral girdle, consisting of the clavicle and the scapula, attaches each upper limb to the axial skeleton. The clavicle is an anterior bone whose sternal end articulates with the manubrium of the sternum at the sternoclavicular joint. The sternal end is also anchored to the first rib by the costoclavicular ligament. The acromial end of the clavicle articulates with the acromion of the scapula at the acromioclavicular joint. This end is also anchored to the coracoid process of the scapula by the coracoclavicular ligament, which provides indirect support for the acromioclavicular joint. The clavicle supports the scapula, transmits the weight and forces from the upper limb to the body trunk, and protects the underlying nerves and blood vessels. The scapula lies on the posterior aspect of the pectoral girdle. It mediates the attachment of the upper limb to the clavicle, and contributes to the formation of the glenohumeral (shoulder) joint. This triangular bone has three sides called the medial, lateral, and superior borders. The suprascapular notch is located on the superior border. The scapula also has three corners, two of which are the superior and inferior angles. The third corner is occupied by the glenoid cavity. Posteriorly, the spine separates the supraspinous and infraspinous fossae, and then extends laterally as the acromion. The subscapular fossa is located on the anterior surface of the scapula. The coracoid process projects anteriorly, passing inferior to the lateral end of the clavicle. Review Questions Exercise: Problem: Which part of the clavicle articulates with the manubrium? a. shaft b. sternal end c. acromial end d. coracoid process Solution: B Exercise: Problem:A shoulder separation results from injury to the a. glenohumeral joint b. costoclavicular joint c. acromioclavicular joint d. sternoclavicular joint Solution: C Exercise: Problem: Which feature lies between the spine and superior border of the scapula? a. suprascapular notch b. glenoid cavity c. superior angle d. supraspinous fossa Solution: D Exercise: Problem: What structure is an extension of the spine of the scapula? a. acromion b. coracoid process c. supraglenoid tubercle d. glenoid cavity Solution: A Exercise: Problem: Name the short, hook-like bony process of the scapula that projects anteriorly. a. acromial process b. clavicle c. coracoid process d. glenoid fossa Solution: C Critical Thinking Questions Exercise: Problem: Describe the shape and palpable line formed by the clavicle and scapula. Solution: The clavicle extends laterally across the anterior shoulder and can be palpated along its entire length. At its lateral end, the clavicle articulates with the acromion of the scapula, which forms the bony tip of the shoulder. The acromion is continuous with the spine of the scapula, which can be palpated medially and posteriorly along its length. Together, the clavicle, acromion, and spine of the scapula form a V-shaped line that serves as an important area for muscle attachment. Exercise: Problem: Discuss two possible injuries of the pectoral girdle that may occur following a strong blow to the shoulder or a hard fall onto an outstretched hand. Solution: A blow to the shoulder or falling onto an outstretched hand passes strong forces through the scapula to the clavicle and sternum. A hard fall may thus cause a fracture of the clavicle (broken collarbone) or may injure the ligaments of the acromioclavicular joint. In a severe case, the coracoclavicular ligament may also rupture, resulting in complete dislocation of the acromioclavicular joint (a “shoulder separation”). Glossary acromial end of the clavicle lateral end of the clavicle that articulates with the acromion of the scapula acromial process acromion of the scapula acromioclavicular joint articulation between the acromion of the scapula and the acromial end of the clavicle acromion flattened bony process that extends laterally from the scapular spine to form the bony tip of the shoulder clavicle collarbone; elongated bone that articulates with the manubrium of the sternum medially and the acromion of the scapula laterally coracoclavicular ligament strong band of connective tissue that anchors the coracoid process of the scapula to the lateral clavicle; provides important indirect support for the acromioclavicular joint coracoid process short, hook-like process that projects anteriorly and laterally from the superior margin of the scapula costoclavicular ligament band of connective tissue that unites the medial clavicle with the first rib fossa (plural = fossae) shallow depression on the surface of a bone glenohumeral joint shoulder joint; formed by the articulation between the glenoid cavity of the scapula and the head of the humerus glenoid cavity (also, glenoid fossa) shallow depression located on the lateral scapula, between the superior and lateral borders inferior angle of the scapula inferior corner of the scapula located where the medial and lateral borders meet infraglenoid tubercle small bump or roughened area located on the lateral border of the scapula, near the inferior margin of the glenoid cavity infraspinous fossa broad depression located on the posterior scapula, inferior to the spine lateral border of the scapula diagonally oriented lateral margin of the scapula medial border of the scapula elongated, medial margin of the scapula pectoral girdle shoulder girdle; the set of bones, consisting of the scapula and clavicle, which attaches each upper limb to the axial skeleton scapula shoulder blade bone located on the posterior side of the shoulder spine of the scapula prominent ridge passing mediolaterally across the upper portion of the posterior scapular surface sternal end of the clavicle medial end of the clavicle that articulates with the manubrium of the sternum sternoclavicular joint articulation between the manubrium of the sternum and the sternal end of the clavicle; forms the only bony attachment between the pectoral girdle of the upper limb and the axial skeleton subscapular fossa broad depression located on the anterior (deep) surface of the scapula superior angle of the scapula comer of the scapula between the superior and medial borders of the scapula superior border of the scapula superior margin of the scapula supraglenoid tubercle small bump located at the superior margin of the glenoid cavity suprascapular notch small notch located along the superior border of the scapula, medial to the coracoid process supraspinous fossa narrow depression located on the posterior scapula, superior to the spine Bones of the Upper Limb By the end of this section, you will be able to: e Identify the divisions of the upper limb and describe the bones in each region e List the bones and bony landmarks that articulate at each joint of the upper limb The upper limb is divided into three regions. These consist of the arm, located between the shoulder and elbow joints; the forearm, which is between the elbow and wrist joints; and the hand, which is located distal to the wrist. There are 30 bones in each upper limb (see [link]). The humerus is the single bone of the upper arm, and the ulna (medially) and the radius (laterally) are the paired bones of the forearm. The base of the hand contains eight bones, each called a carpal bone, and the palm of the hand is formed by five bones, each called a metacarpal bone. The fingers and thumb contain a total of 14 bones, each of which is a phalanx bone of the hand. Humerus The humerus is the single bone of the upper arm region ([link]). At its proximal end is the head of the humerus. This is the large, round, smooth region that faces medially. The head articulates with the glenoid cavity of the scapula to form the glenohumeral (shoulder) joint. The margin of the smooth area of the head is the anatomical neck of the humerus. Located on the lateral side of the proximal humerus is an expanded bony area called the greater tubercle. The smaller lesser tubercle of the humerus is found on the anterior aspect of the humerus. Both the greater and lesser tubercles serve as attachment sites for muscles that act across the shoulder joint. Passing between the greater and lesser tubercles is the narrow intertubercular groove (sulcus), which is also known as the bicipital groove because it provides passage for a tendon of the biceps brachii muscle. The surgical neck is located at the base of the expanded, proximal end of the humerus, where it joins the narrow shaft of the humerus. The surgical neck is a common site of arm fractures. The deltoid tuberosity is a roughened, V-shaped region located on the lateral side in the middle of the humerus shaft. As its name indicates, it is the site of attachment for the deltoid muscle. Humerus and Elbow Joint Humerus Anatomical Greater (Oy, Gin Greater tubercle 1 \ ) Head tubercle Lesser tubercle Intertubercular groove (sulcus) Deltoid | tuberosity | Body (shaft) Lateral supracondylar ridge Radial fossa Olecranon fossa Lateral Coronoid fossa epicondyle Olecranon of ulna Head of radius Capitulum —+~—— Medial Head of epicondyle radius Trochlea Coronoid process of ulna Anterior view Posterior view The humerus is the single bone of the upper arm region. It articulates with the radius and ulna bones of the forearm to form the elbow joint. Distally, the humerus becomes flattened. The prominent bony projection on the medial side is the medial epicondyle of the humerus. The much smaller lateral epicondyle of the humerus is found on the lateral side of the distal humerus. The roughened ridge of bone above the lateral epicondyle is the lateral supracondylar ridge. All of these areas are attachment points for muscles that act on the forearm, wrist, and hand. The powerful grasping muscles of the anterior forearm arise from the medial epicondyle, which is thus larger and more robust than the lateral epicondyle that gives rise to the weaker posterior forearm muscles. The distal end of the humerus has two articulation areas, which join the ulna and radius bones of the forearm to form the elbow joint. The more medial of these areas is the trochlea, a spindle- or pulley-shaped region (trochlea = “pulley”), which articulates with the ulna bone. Immediately lateral to the trochlea is the capitulum (“small head”), a knob-like structure located on the anterior surface of the distal humerus. The capitulum articulates with the radius bone of the forearm. Just above these bony areas are two small depressions. These spaces accommodate the forearm bones when the elbow is fully bent (flexed). Superior to the trochlea is the coronoid fossa, which receives the coronoid process of the ulna, and above the capitulum is the radial fossa, which receives the head of the radius when the elbow is flexed. Similarly, the posterior humerus has the olecranon fossa, a larger depression that receives the olecranon process of the ulna when the forearm is fully extended. Ulna The ulna is the medial bone of the forearm. It runs parallel to the radius, which is the lateral bone of the forearm ([link]). The proximal end of the ulna resembles a crescent wrench with its large, C-shaped trochlear notch. This region articulates with the trochlea of the humerus as part of the elbow joint. The inferior margin of the trochlear notch is formed by a prominent lip of bone called the coronoid process of the ulna. Just below this on the anterior ulna is a roughened area called the ulnar tuberosity. To the lateral side and slightly inferior to the trochlear notch is a small, smooth area called the radial notch of the ulna. This area is the site of articulation between the proximal radius and the ulna, forming the proximal radioulnar joint. The posterior and superior portions of the proximal ulna make up the olecranon process, which forms the bony tip of the elbow. Ulna and Radius er a ie Olecranon Radial notch on process of the ulna 1S Trochlear Head of Head of notch radius radius Neck of Coronoid di process Neck of radius radius Radial Proximal tuberosity radioulnar } joint | Interosseous \\ | membrane \ \ \ Uina Radius S Ulnar notch of the radius : Head of ulna Radius Distal radioulnar joint Styloid process of ulna Styloid process of radius The ulna is located on the medial side of the forearm, and the radius is on the lateral side. These bones are attached to each other by an interosseous membrane. More distal is the shaft of the ulna. The lateral side of the shaft forms a ridge called the interosseous border of the ulna. This is the line of attachment for the interosseous membrane of the forearm, a sheet of dense connective tissue that unites the ulna and radius bones. The small, rounded area that forms the distal end is the head of the ulna. Projecting from the posterior side of the ulnar head is the styloid process of the ulna, a short bony projection. This serves as an attachment point for a connective tissue structure that unites the distal ends of the ulna and radius. In the anatomical position, with the elbow fully extended and the palms facing forward, the arm and forearm do not form a straight line. Instead, the forearm deviates laterally by 5-15 degrees from the line of the arm. This deviation is called the carrying angle. It allows the forearm and hand to swing freely or to carry an object without hitting the hip. The carrying angle is larger in females to accommodate their wider pelvis. Radius The radius runs parallel to the ulna, on the lateral (thumb) side of the forearm (see [link]). The head of the radius is a disc-shaped structure that forms the proximal end. The small depression on the surface of the head articulates with the capitulum of the humerus as part of the elbow joint, whereas the smooth, outer margin of the head articulates with the radial notch of the ulna at the proximal radioulnar joint. The neck of the radius is the narrowed region immediately below the expanded head. Inferior to this point on the medial side is the radial tuberosity, an oval-shaped, bony protuberance that serves as a muscle attachment point. The shaft of the radius is slightly curved and has a small ridge along its medial side. This ridge forms the interosseous border of the radius, which, like the similar border of the ulna, is the line of attachment for the interosseous membrane that unites the two forearm bones. The distal end of the radius has a smooth surface for articulation with two carpal bones to form the radiocarpal joint or wrist joint ({link] and [link]). On the medial side of the distal radius is the ulnar notch of the radius. This shallow depression articulates with the head of the ulna, which together form the distal radioulnar joint. The lateral end of the radius has a pointed projection called the styloid process of the radius. This provides attachment for ligaments that support the lateral side of the wrist joint. Compared to the styloid process of the ulna, the styloid process of the radius projects more distally, thereby limiting the range of movement for lateral deviations of the hand at the wrist joint. Note: Watch this video to see how fractures of the distal radius bone can affect the wrist joint. Explain the problems that may occur if a fracture of the distal radius involves the joint surface of the radiocarpal joint of the wrist. Carpal Bones The wrist and base of the hand are formed by a series of eight small carpal bones (see [link]). The carpal bones are arranged in two rows, forming a proximal row of four carpal bones and a distal row of four carpal bones. The bones in the proximal row, running from the lateral (thumb) side to the medial side, are the scaphoid (“boat-shaped”), lunate (“moon-shaped”), triquetrum (“three-cornered”), and pisiform (“pea-shaped”) bones. The small, rounded pisiform bone articulates with the anterior surface of the triquetrum bone. The pisiform thus projects anteriorly, where it forms the bony bump that can be felt at the medial base of your hand. The distal bones (lateral to medial) are the trapezium (“table”), trapezoid (“resembles a table”), capitate (“head-shaped”), and hamate (“hooked bone”) bones. The hamate bone is characterized by a prominent bony extension on its anterior side called the hook of the hamate bone. A helpful mnemonic for remembering the arrangement of the carpal bones is “So Long To Pinky, Here Comes The Thumb.” This mnemonic starts on the lateral side and names the proximal bones from lateral to medial (scaphoid, lunate, triquetrum, pisiform), then makes a U-turn to name the distal bones from medial to lateral (hamate, capitate, trapezoid, trapezium). Thus, it starts and finishes on the lateral side. Bones of the Wrist and Hand | Carpals S i\\ \ ~;Metacarpals yyy ib Phalanges Middle finger Index finger Ring finger 4 Distal Little finger . Miceie Phalanges Thumb f (pollex) Phalanges: /f ie Distal Head —¥~q — Proximal a Shaft \ Base Head Metacarpals v) (1-5) J Shaft Carpals: \ y . : Trapezium - a on Va j Base Trapezoid t ) — ; _=—_ : f - Capitate : Carpals: Scaphoid x ‘< Pisiform ) : Trapezium VW y> Triquetrum p =: Trapeziod " \ f Lunate - Scaphoid 7 Ulna | Radius Anterior view Posterior view The eight carpal bones form the base of the hand. These are arranged into proximal and distal rows of four bones each. The metacarpal bones form the palm of the hand. The thumb and fingers consist of the phalanx bones. The carpal bones form the base of the hand. This can be seen in the radiograph (X-ray image) of the hand that shows the relationships of the hand bones to the skin creases of the hand (see [link]). Within the carpal bones, the four proximal bones are united to each other by ligaments to form a unit. Only three of these bones, the scaphoid, lunate, and triquetrum, contribute to the radiocarpal joint. The scaphoid and lunate bones articulate directly with the distal end of the radius, whereas the triquetrum bone articulates with a fibrocartilaginous pad that spans the radius and styloid process of the ulna. The distal end of the ulna thus does not directly articulate with any of the carpal bones. The four distal carpal bones are also held together as a group by ligaments. The proximal and distal rows of carpal bones articulate with each other to form the midcarpal joint (see [link]). Together, the radiocarpal and midcarpal joints are responsible for all movements of the hand at the wrist. The distal carpal bones also articulate with the metacarpal bones of the hand. Bones of the Hand Metacarpophalangeal joints Interphalangeal joints Carpometacarpal joints Midcarpal joint Radiocarpal (wrist) fs : This radiograph shows the position of the bones within the hand. Note the carpal bones that form the base of the hand. (credit: modification of work by Trace Meek) In the articulated hand, the carpal bones form a U-shaped grouping. A strong ligament called the flexor retinaculum spans the top of this U- shaped area to maintain this grouping of the carpal bones. The flexor retinaculum is attached laterally to the trapezium and scaphoid bones, and medially to the hamate and pisiform bones. Together, the carpal bones and the flexor retinaculum form a passageway called the carpal tunnel, with the carpal bones forming the walls and floor, and the flexor retinaculum forming the roof of this space ({link]). The tendons of nine muscles of the anterior forearm and an important nerve pass through this narrow tunnel to enter the hand. Overuse of the muscle tendons or wrist injury can produce inflammation and swelling within this space. This produces compression of the nerve, resulting in carpal tunnel syndrome, which is characterized by pain or numbness, and muscle weakness in those areas of the hand supplied by this nerve. Carpal Tunnel Carpal tunnel Muscle tendons — Flexor \ rpal bones: — i Carpal bones retinaculum Hamate Trapezoid Trapezium Capitate Nerve The carpal tunnel is the passageway by which nine muscle tendons and a major nerve enter the hand from the anterior forearm. The walls and floor of the carpal tunnel are formed by the U-shaped grouping of the carpal bones, and the roof is formed by the flexor retinaculum, a strong ligament that anteriorly unites the bones. Metacarpal Bones The palm of the hand contains five elongated metacarpal bones. These bones lie between the carpal bones of the wrist and the bones of the fingers and thumb (see [link]). The proximal end of each metacarpal bone articulates with one of the distal carpal bones. Each of these articulations is a Carpometacarpal joint (see [link]). The expanded distal end of each metacarpal bone articulates at the metacarpophalangeal joint with the proximal phalanx bone of the thumb or one of the fingers. The distal end also forms the knuckles of the hand, at the base of the fingers. The metacarpal bones are numbered 1-5, beginning at the thumb. The first metacarpal bone, at the base of the thumb, is separated from the other metacarpal bones. This allows it a freedom of motion that is independent of the other metacarpal bones, which is very important for thumb mobility. The remaining metacarpal bones are united together to form the palm of the hand. The second and third metacarpal bones are firmly anchored in place and are immobile. However, the fourth and fifth metacarpal bones have limited anterior-posterior mobility, a motion that is greater for the fifth bone. This mobility is important during power gripping with the hand ([link]). The anterior movement of these bones, particularly the fifth metacarpal bone, increases the strength of contact for the medial hand during gripping actions. Hand During Gripping (a) Loosely held (b) Firmly gripped During tight gripping—compare (b) to (a)—the fourth and, particularly, the fifth metatarsal bones are pulled anteriorly. This increases the contact between the object and the medial side of the hand, thus improving the firmness of the grip. Phalanx Bones The fingers and thumb contain 14 bones, each of which is called a phalanx bone (plural = phalanges), named after the ancient Greek phalanx (a rectangular block of soldiers). The thumb (pollex) is digit number 1 and has two phalanges, a proximal phalanx, and a distal phalanx bone (see [link]). Digits 2 (index finger) through 5 (little finger) have three phalanges each, called the proximal, middle, and distal phalanx bones. An interphalangeal joint is one of the articulations between adjacent phalanges of the digits (see [Link]). Note: ORs sao at ; — . =——: wm OPENSTAX COLLEGE Visit this site to explore the bones and joints of the hand. What are the three arches of the hand, and what is the importance of these during the sripping of an object? Note: Disorders of the... Appendicular System: Fractures of Upper Limb Bones Due to our constant use of the hands and the rest of our upper limbs, an injury to any of these areas will cause a significant loss of functional ability. Many fractures result from a hard fall onto an outstretched hand. The resulting transmission of force up the limb may result in a fracture of the humerus, radius, or scaphoid bones. These injuries are especially common in elderly people whose bones are weakened due to osteoporosis. Falls onto the hand or elbow, or direct blows to the arm, can result in fractures of the humerus ((link]). Following a fall, fractures at the surgical neck, the region at which the expanded proximal end of the humerus joins with the shaft, can result in an impacted fracture, in which the distal portion of the humerus is driven into the proximal portion. Falls or blows to the arm can also produce transverse or spiral fractures of the humeral shaft. In children, a fall onto the tip of the elbow frequently results in a distal humerus fracture. In these, the olecranon of the ulna is driven upward, resulting in a fracture across the distal humerus, above both epicondyles (supracondylar fracture), or a fracture between the epicondyles, thus separating one or both of the epicondyles from the body of the humerus (intercondylar fracture). With these injuries, the immediate concern is possible compression of the artery to the forearm due to swelling of the surrounding tissues. If compression occurs, the resulting ischemia (lack of oxygen) due to reduced blood flow can quickly produce irreparable damage to the forearm muscles. In addition, four major nerves for shoulder and upper limb muscles are closely associated with different regions of the humerus, and thus, humeral fractures may also damage these nerves. Another frequent injury following a fall onto an outstretched hand is a Colles fracture (“col-lees”’) of the distal radius (see [link]). This involves a complete transverse fracture across the distal radius that drives the separated distal fragment of the radius posteriorly and superiorly. This injury results in a characteristic “dinner fork” bend of the forearm just above the wrist due to the posterior displacement of the hand. This is the most frequent forearm fracture and is a common injury in persons over the age of 50, particularly in older women with osteoporosis. It also commonly occurs following a high-speed fall onto the hand during activities such as snowboarding or skating. The most commonly fractured carpal bone is the scaphoid, often resulting from a fall onto the hand. Deep pain at the lateral wrist may yield an initial diagnosis of a wrist sprain, but a radiograph taken several weeks after the injury, after tissue swelling has subsided, will reveal the fracture. Due to the poor blood supply to the scaphoid bone, healing will be slow and there is the danger of bone necrosis and subsequent degenerative joint disease of the wrist. Fractures of the Humerus and Radius Surgical neck fracture Transverse humeral fracture Fractures of the Humerus Supracondylar fracture KL i Eas Normal Normal Colles Fracture of the Distal Radius Falls or direct blows can result in fractures of the surgical neck or shaft of the humerus. Falls onto the elbow can fracture the distal humerus. A Colles fracture of the distal radius is the most common forearm fracture. Note: [=] [eae 1 openstax COLLEGE” Watch this video to learn about a Colles fracture, a break of the distal radius, usually caused by falling onto an outstretched hand. When would surgery be required and how would the fracture be repaired in this case? Chapter Review Each upper limb is divided into three regions and contains a total of 30 bones. The upper arm is the region located between the shoulder and elbow joints. This area contains the humerus. The proximal humerus consists of the head, which articulates with the scapula at the glenohumeral joint, the greater and lesser tubercles separated by the intertubercular (bicipital) groove, and the anatomical and surgical necks. The humeral shaft has the roughened area of the deltoid tuberosity on its lateral side. The distal humerus is flattened, forming a lateral supracondylar ridge that terminates at the small lateral epicondyle. The medial side of the distal humerus has the large, medial epicondyle. The articulating surfaces of the distal humerus consist of the trochlea medially and the capitulum laterally. Depressions on the humerus that accommodate the forearm bones during bending (flexing) and straightening (extending) of the elbow include the coronoid fossa, the radial fossa, and the olecranon fossa. The forearm is the region of the upper limb located between the elbow and wrist joints. This region contains two bones, the ulna medially and the radius on the lateral (thumb) side. The elbow joint is formed by the articulation between the trochlea of the humerus and the trochlear notch of the ulna, plus the articulation between the capitulum of the humerus and the head of the radius. The proximal radioulnar joint is the articulation between the head of the radius and the radial notch of the ulna. The proximal ulna also has the olecranon process, forming an expanded posterior region, and the coronoid process and ulnar tuberosity on its anterior aspect. On the proximal radius, the narrowed region below the head is the neck; distal to this is the radial tuberosity. The shaft portions of both the ulna and radius have an interosseous border, whereas the distal ends of each bone have a pointed styloid process. The distal radioulnar joint is found between the head of the ulna and the ulnar notch of the radius. The distal end of the radius articulates with the proximal carpal bones, but the ulna does not. The base of the hand is formed by eight carpal bones. The carpal bones are united into two rows of bones. The proximal row contains (from lateral to medial) the scaphoid, lunate, triquetrum, and pisiform bones. The scaphoid, lunate, and triquetrum bones contribute to the formation of the radiocarpal joint. The distal row of carpal bones contains (from medial to lateral) the hamate, capitate, trapezoid, and trapezium bones (“So Long To Pinky, Here Comes The Thumb”). The anterior hamate has a prominent bony hook. The proximal and distal carpal rows articulate with each other at the midcarpal joint. The carpal bones, together with the flexor retinaculum, also form the carpal tunnel of the wrist. The five metacarpal bones form the palm of the hand. The metacarpal bones are numbered 1-5, starting with the thumb side. The first metacarpal bone is freely mobile, but the other bones are united as a group. The digits are also numbered 1—5, with the thumb being number 1. The fingers and thumb contain a total of 14 phalanges (phalanx bones). The thumb contains a proximal and a distal phalanx, whereas the remaining digits each contain proximal, middle, and distal phalanges. Interactive Link Questions Exercise: Problem: Watch this video to see how fractures of the distal radius bone can affect the wrist joint. Explain the problems that may occur if a fracture of the distal radius involves the joint surface of the radiocarpal joint of the wrist. Solution: A fracture through the joint surface of the distal radius may make the articulating surface of the radius rough or jagged. This can then cause painful movements involving this joint and the early development of arthritis. Surgery can return the joint surface to its original smoothness, thus allowing for the return of normal function. Exercise: Problem: Visit this site to explore the bones and joints of the hand. What are the three arches of the hand, and what is the importance of these during the gripping of an object? Solution: The hand has a proximal transverse arch, a distal transverse arch, and a longitudinal arch. These allow the hand to conform to objects being held. These arches maximize the amount of surface contact between the hand and object, which enhances stability and increases sensory input. Exercise: Problem: Watch this video to learn about a Colles fracture, a break of the distal radius, usually caused by falling onto an outstretched hand. When would surgery be required and how would the fracture be repaired in this case? Solution: Surgery may be required if the fracture is unstable, meaning that the broken ends of the radius won’t stay in place to allow for proper healing. In this case, metal plates and screws can be used to stabilize the fractured bone. Review Questions Exercise: Problem:How many bones are there in the upper limbs combined? a. 20 b. 30 c. 40 d. 60 Solution: D Exercise: Problem: Which bony landmark is located on the lateral side of the proximal humerus? a. greater tubercle b. trochlea c. lateral epicondyle d. lesser tubercle Solution: A Exercise: Problem: Which region of the humerus articulates with the radius as part of the elbow joint? a. trochlea b. styloid process c. capitulum d. olecranon process Solution: C Exercise: Problem: Which is the lateral-most carpal bone of the proximal row? a. trapezium b. hamate c. pisiform d. scaphoid Solution: D Exercise: Problem: The radius bone a. is found on the medial side of the forearm b. has a head that articulates with the radial notch of the ulna c. does not articulate with any of the carpal bones d. has the radial tuberosity located near its distal end Solution: B Critical Thinking Questions Exercise: Problem: Your friend runs out of gas and you have to help push his car. Discuss the sequence of bones and joints that convey the forces passing from your hand, through your upper limb and your pectoral girdle, and to your axial skeleton. Solution: As you push against the car, forces will pass from the metacarpal bones of your hand into the carpal bones at the base of your hand. Forces will then pass through the midcarpal and radiocarpal joints into the radius and ulna bones of the forearm. These will pass the force through the elbow joint into the humerus of the arm, and then through the glenohumeral joint into the scapula. The force will travel through the acromioclavicular joint into the clavicle, and then through the sternoclavicular joint into the sternum, which is part of the axial skeleton. Exercise: Problem: Name the bones in the wrist and hand, and describe or sketch out their locations and articulations. Solution: The base of the hand is formed by the eight carpal bones arranged in two rows (distal and proximal) of four bones each. The proximal row contains (from lateral to medial) the scaphoid, lunate, triquetrum, and pisiform bones. The distal row contains (from medial to lateral) the hamate, capitate, trapezoid, and trapezium bones. (Use the mnemonic “So Long To Pinky, Here Comes The Thumb” to remember this sequence). The rows of the proximal and distal carpal bones articulate with each other at the midcarpal joint. The palm of the hand contains the five metacarpal bones, which are numbered 1-5 starting on the thumb side. The proximal ends of the metacarpal bones articulate with the distal row of the carpal bones. The distal ends of the metacarpal bones articulate with the proximal phalanx bones of the thumb and fingers. The thumb (digit 1) has both a proximal and distal phalanx bone. The fingers (digits 2—5) all contain proximal, middle, and distal phalanges. Glossary anatomical neck line on the humerus located around the outside margin of the humeral head arm region of the upper limb located between the shoulder and elbow joints; contains the humerus bone bicipital groove intertubercular groove; narrow groove located between the greater and lesser tubercles of the humerus capitate from the lateral side, the third of the four distal carpal bones; articulates with the scaphoid and lunate proximally, the trapezoid laterally, the hamate medially, and primarily with the third metacarpal distally capitulum knob-like bony structure located anteriorly on the lateral, distal end of the humerus carpal bone one of the eight small bones that form the wrist and base of the hand; these are grouped as a proximal row consisting of (from lateral to medial) the scaphoid, lunate, triquetrum, and pisiform bones, and a distal row containing (from lateral to medial) the trapezium, trapezoid, capitate, and hamate bones carpal tunnel passageway between the anterior forearm and hand formed by the carpal bones and flexor retinaculum Carpometacarpal joint articulation between one of the carpal bones in the distal row and a metacarpal bone of the hand coronoid fossa depression on the anterior surface of the humerus above the trochlea; this space receives the coronoid process of the ulna when the elbow is maximally flexed coronoid process of the ulna projecting bony lip located on the anterior, proximal ulna; forms the inferior margin of the trochlear notch deltoid tuberosity roughened, V-shaped region located laterally on the mid-shaft of the humerus distal radioulnar joint articulation between the head of the ulna and the ulnar notch of the radius elbow joint joint located between the upper arm and forearm regions of the upper limb; formed by the articulations between the trochlea of the humerus and the trochlear notch of the ulna, and the capitulum of the humerus and the head of the radius flexor retinaculum strong band of connective tissue at the anterior wrist that spans the top of the U-shaped grouping of the carpal bones to form the roof of the carpal tunnel forearm region of the upper limb located between the elbow and wrist joints; contains the radius and ulna bones greater tubercle enlarged prominence located on the lateral side of the proximal humerus hamate from the lateral side, the fourth of the four distal carpal bones; articulates with the lunate and triquetrum proximally, the fourth and fifth metacarpals distally, and the capitate laterally hand region of the upper limb distal to the wrist joint head of the humerus smooth, rounded region on the medial side of the proximal humerus; articulates with the glenoid fossa of the scapula to form the glenohumeral (shoulder) joint head of the radius disc-shaped structure that forms the proximal end of the radius; articulates with the capitulum of the humerus as part of the elbow joint, and with the radial notch of the ulna as part of the proximal radioulnar joint head of the ulna small, rounded distal end of the ulna; articulates with the ulnar notch of the distal radius, forming the distal radioulnar joint hook of the hamate bone bony extension located on the anterior side of the hamate carpal bone humerus single bone of the upper arm interosseous border of the radius narrow ridge located on the medial side of the radial shaft; for attachment of the interosseous membrane between the ulna and radius bones interosseous border of the ulna narrow ridge located on the lateral side of the ulnar shaft; for attachment of the interosseous membrane between the ulna and radius interosseous membrane of the forearm sheet of dense connective tissue that unites the radius and ulna bones interphalangeal joint articulation between adjacent phalanx bones of the hand or foot digits intertubercular groove (sulcus) bicipital groove; narrow groove located between the greater and lesser tubercles of the humerus lateral epicondyle of the humerus small projection located on the lateral side of the distal humerus lateral supracondylar ridge narrow, bony ridge located along the lateral side of the distal humerus, superior to the lateral epicondyle lesser tubercle small, bony prominence located on anterior side of the proximal humerus lunate from the lateral side, the second of the four proximal carpal bones; articulates with the radius proximally, the capitate and hamate distally, the scaphoid laterally, and the triquetrum medially medial epicondyle of the humerus enlarged projection located on the medial side of the distal humerus metacarpal bone one of the five long bones that form the palm of the hand; numbered 1—5, starting on the lateral (thumb) side of the hand metacarpophalangeal joint articulation between the distal end of a metacarpal bone of the hand and a proximal phalanx bone of the thumb or a finger midcarpal joint articulation between the proximal and distal rows of the carpal bones; contributes to movements of the hand at the wrist neck of the radius narrowed region immediately distal to the head of the radius olecranon fossa large depression located on the posterior side of the distal humerus; this space receives the olecranon process of the ulna when the elbow is fully extended olecranon process expanded posterior and superior portions of the proximal ulna; forms the bony tip of the elbow phalanx bone of the hand (plural = phalanges) one of the 14 bones that form the thumb and fingers; these include the proximal and distal phalanges of the thumb, and the proximal, middle, and distal phalanx bones of the fingers two through five pisiform from the lateral side, the fourth of the four proximal carpal bones; articulates with the anterior surface of the triquetrum pollex (also, thumb) digit 1 of the hand proximal radioulnar joint articulation formed by the radial notch of the ulna and the head of the radius radial fossa small depression located on the anterior humerus above the capitulum; this space receives the head of the radius when the elbow is maximally flexed radial notch of the ulna small, smooth area on the lateral side of the proximal ulna; articulates with the head of the radius as part of the proximal radioulnar joint radial tuberosity oval-shaped, roughened protuberance located on the medial side of the proximal radius radiocarpal joint wrist joint, located between the forearm and hand regions of the upper limb; articulation formed proximally by the distal end of the radius and the fibrocartilaginous pad that unites the distal radius and ulna bone, and distally by the scaphoid, lunate, and triquetrum carpal bones radius bone located on the lateral side of the forearm scaphoid from the lateral side, the first of the four proximal carpal bones; articulates with the radius proximally, the trapezoid, trapezium, and capitate distally, and the lunate medially shaft of the humerus narrow, elongated, central region of the humerus shaft of the radius narrow, elongated, central region of the radius shaft of the ulna narrow, elongated, central region of the ulna styloid process of the radius pointed projection located on the lateral end of the distal radius styloid process of the ulna short, bony projection located on the medial end of the distal ulna surgical neck region of the humerus where the expanded, proximal end joins with the narrower shaft trapezium from the lateral side, the first of the four distal carpal bones; articulates with the scaphoid proximally, the first and second metacarpals distally, and the trapezoid medially trapezoid from the lateral side, the second of the four distal carpal bones; articulates with the scaphoid proximally, the second metacarpal distally, the trapezium laterally, and the capitate medially triquetrum from the lateral side, the third of the four proximal carpal bones; articulates with the lunate laterally, the hamate distally, and has a facet for the pisiform trochlea pulley-shaped region located medially at the distal end of the humerus; articulates at the elbow with the trochlear notch of the ulna trochlear notch large, C-shaped depression located on the anterior side of the proximal ulna; articulates at the elbow with the trochlea of the humerus ulna bone located on the medial side of the forearm ulnar notch of the radius shallow, smooth area located on the medial side of the distal radius; articulates with the head of the ulna at the distal radioulnar joint ulnar tuberosity roughened area located on the anterior, proximal ulna inferior to the coronoid process The Pelvic Girdle and Pelvis By the end of this section, you will be able to: ¢ Define the pelvic girdle and describe the bones and ligaments of the pelvis e Explain the three regions of the hip bone and identify their bony landmarks e Describe the openings of the pelvis and the boundaries of the greater and lesser pelvis The pelvic girdle (hip girdle) is formed by a single bone, the hip bone or coxal bone (coxal = “hip”), which serves as the attachment point for each lower limb. Each hip bone, in turn, is firmly joined to the axial skeleton via its attachment to the sacrum of the vertebral column. The right and left hip bones also converge anteriorly to attach to each other. The bony pelvis is the entire structure formed by the two hip bones, the sacrum, and, attached inferiorly to the sacrum, the coccyx ([link]). Unlike the bones of the pectoral girdle, which are highly mobile to enhance the range of upper limb movements, the bones of the pelvis are strongly united to each other to form a largely immobile, weight-bearing structure. This is important for stability because it enables the weight of the body to be easily transferred laterally from the vertebral column, through the pelvic girdle and hip joints, and into either lower limb whenever the other limb is not bearing weight. Thus, the immobility of the pelvis provides a strong foundation for the upper body as it rests on top of the mobile lower limbs. Pelvis Sacroiliac joint Sacral promonitory Sacrum Pelvic brim Acetabulum Coccyx Obturator f " Eversion (g) Pronation (P) and supination (S) (h) Dorsiflexion and plantar flexion (i) Inversion and eversion A Retraction Protraction Elevation of of mandible of mandible ( | eae fs : Depression ‘4 of mandible (j) Protraction and retraction (k) Elevation and depression (I) Opposition (g) Supination of the forearm turns the hand to the palm forward position in which the radius and ulna are parallel, while forearm pronation turns the hand to the palm backward position in which the radius crosses over the ulna to form an "X." (h) Dorsiflexion of the foot at the ankle joint moves the top of the foot toward the leg, while plantar flexion lifts the heel and points the toes. (i) Eversion of the foot moves the bottom (sole) of the foot away from the midline of the body, while foot inversion faces the sole toward the midline. (j) Protraction of the mandible pushes the chin forward, and retraction pulls the chin back. (k) Depression of the mandible opens the mouth, while elevation closes it. (1) Opposition of the thumb brings the tip of the thumb into contact with the tip of the fingers of the same hand and reposition brings the thumb back next to the index finger. Flexion and Extension Flexion and extension are movements that take place within the sagittal plane and involve anterior or posterior movements of the body or limbs. For the vertebral column, flexion (anterior flexion) is an anterior (forward) bending of the neck or body, while extension involves a posterior-directed motion, such as straightening from a flexed position or bending backward. Lateral flexion is the bending of the neck or body toward the right or left side. These movements of the vertebral column involve both the symphysis joint formed by each intervertebral disc, as well as the plane type of synovial joint formed between the inferior articular processes of one vertebra and the superior articular processes of the next lower vertebra. In the limbs, flexion decreases the angle between the bones (bending of the joint), while extension increases the angle and straightens the joint. For the upper limb, all anterior-going motions are flexion and all posterior-going motions are extension. These include anterior-posterior movements of the arm at the shoulder, the forearm at the elbow, the hand at the wrist, and the fingers at the metacarpophalangeal and interphalangeal joints. For the thumb, extension moves the thumb away from the palm of the hand, within the same plane as the palm, while flexion brings the thumb back against the index finger or into the palm. These motions take place at the first carpometacarpal joint. In the lower limb, bringing the thigh forward and upward is flexion at the hip joint, while any posterior-going motion of the thigh is extension. Note that extension of the thigh beyond the anatomical (standing) position is greatly limited by the ligaments that support the hip joint. Knee flexion is the bending of the knee to bring the foot toward the posterior thigh, and extension is the straightening of the knee. Flexion and extension movements are seen at the hinge, condyloid, saddle, and ball-and- socket joints of the limbs (see [link ]a-d). Hyperextension is the abnormal or excessive extension of a joint beyond its normal range of motion, thus resulting in injury. Similarly, hyperflexion is excessive flexion at a joint. Hyperextension injuries are common at hinge joints such as the knee or elbow. In cases of “whiplash” in which the head is suddenly moved backward and then forward, a patient may experience both hyperextension and hyperflexion of the cervical region. Abduction and Adduction Abduction and adduction motions occur within the coronal plane and involve medial-lateral motions of the limbs, fingers, toes, or thumb. Abduction moves the limb laterally away from the midline of the body, while adduction is the opposing movement that brings the limb toward the body or across the midline. For example, abduction is raising the arm at the shoulder joint, moving it laterally away from the body, while adduction brings the arm down to the side of the body. Similarly, abduction and adduction at the wrist moves the hand away from or toward the midline of the body. Spreading the fingers or toes apart is also abduction, while bringing the fingers or toes together is adduction. For the thumb, abduction is the anterior movement that brings the thumb to a 90° perpendicular position, pointing straight out from the palm. Adduction moves the thumb back to the anatomical position, next to the index finger. Abduction and adduction movements are seen at condyloid, saddle, and ball-and-socket joints (see [link]e). Circumduction Circumduction is the movement of a body region in a circular manner, in which one end of the body region being moved stays relatively stationary while the other end describes a circle. It involves the sequential combination of flexion, adduction, extension, and abduction at a joint. This type of motion is found at biaxial condyloid and saddle joints, and at multiaxial ball-and-sockets joints (see [link]e). Rotation Rotation can occur within the vertebral column, at a pivot joint, or at a ball-and-socket joint. Rotation of the neck or body is the twisting movement produced by the summation of the small rotational movements available between adjacent vertebrae. At a pivot joint, one bone rotates in relation to another bone. This is a uniaxial joint, and thus rotation is the only motion allowed at a pivot joint. For example, at the atlantoaxial joint, the first cervical (C1) vertebra (atlas) rotates around the dens, the upward projection from the second cervical (C2) vertebra (axis). This allows the head to rotate from side to side as when shaking the head “no.” The proximal radioulnar joint is a pivot joint formed by the head of the radius and its articulation with the ulna. This joint allows for the radius to rotate along its length during pronation and supination movements of the forearm. Rotation can also occur at the ball-and-socket joints of the shoulder and hip. Here, the humerus and femur rotate around their long axis, which moves the anterior surface of the arm or thigh either toward or away from the midline of the body. Movement that brings the anterior surface of the limb toward the midline of the body is called medial (internal) rotation. Conversely, rotation of the limb so that the anterior surface moves away from the midline is lateral (external) rotation (see [link |f). Be sure to distinguish medial and lateral rotation, which can only occur at the multiaxial shoulder and hip joints, from circumduction, which can occur at either biaxial or multiaxial joints. Supination and Pronation Supination and pronation are movements of the forearm. In the anatomical position, the upper limb is held next to the body with the palm facing forward. This is the supinated position of the forearm. In this position, the radius and ulna are parallel to each other. When the palm of the hand faces backward, the forearm is in the pronated position, and the radius and ulna form an X-shape. Supination and pronation are the movements of the forearm that go between these two positions. Pronation is the motion that moves the forearm from the supinated (anatomical) position to the pronated (palm backward) position. This motion is produced by rotation of the radius at the proximal radioulnar joint, accompanied by movement of the radius at the distal radioulnar joint. The proximal radioulnar joint is a pivot joint that allows for rotation of the head of the radius. Because of the slight curvature of the shaft of the radius, this rotation causes the distal end of the radius to cross over the distal ulna at the distal radioulnar joint. This crossing over brings the radius and ulna into an X-shape position. Supination is the opposite motion, in which rotation of the radius returns the bones to their parallel positions and moves the palm to the anterior facing (supinated) position. It helps to remember that supination is the motion you use when scooping up soup with a spoon (see [link]g). Dorsiflexion and Plantar Flexion Dorsiflexion and plantar flexion are movements at the ankle joint, which is a hinge joint. Lifting the front of the foot, so that the top of the foot moves toward the anterior leg is dorsiflexion, while lifting the heel of the foot from the ground or pointing the toes downward is plantar flexion. These are the only movements available at the ankle joint (see [link]h). Inversion and Eversion Inversion and eversion are complex movements that involve the multiple plane joints among the tarsal bones of the posterior foot (intertarsal joints) and thus are not motions that take place at the ankle joint. Inversion is the turning of the foot to angle the bottom of the foot toward the midline, while eversion turns the bottom of the foot away from the midline. The foot has a greater range of inversion than eversion motion. These are important motions that help to stabilize the foot when walking or running on an uneven surface and aid in the quick side-to-side changes in direction used during active sports such as basketball, racquetball, or soccer (see [link ]i). Protraction and Retraction Protraction and retraction are anterior-posterior movements of the scapula or mandible. Protraction of the scapula occurs when the shoulder is moved forward, as when pushing against something or throwing a ball. Retraction is the opposite motion, with the scapula being pulled posteriorly and medially, toward the vertebral column. For the mandible, protraction occurs when the lower jaw is pushed forward, to stick out the chin, while retraction pulls the lower jaw backward. (See [link]j.) Depression and Elevation Depression and elevation are downward and upward movements of the scapula or mandible. The upward movement of the scapula and shoulder is elevation, while a downward movement is depression. These movements are used to shrug your shoulders. Similarly, elevation of the mandible is the upward movement of the lower jaw used to close the mouth or bite on something, and depression is the downward movement that produces opening of the mouth (see [link ]k). Excursion Excursion is the side to side movement of the mandible. Lateral excursion moves the mandible away from the midline, toward either the right or left side. Medial excursion returns the mandible to its resting position at the midline. Superior Rotation and Inferior Rotation Superior and inferior rotation are movements of the scapula and are defined by the direction of movement of the glenoid cavity. These motions involve rotation of the scapula around a point inferior to the scapular spine and are produced by combinations of muscles acting on the scapula. During superior rotation, the glenoid cavity moves upward as the medial end of the scapular spine moves downward. This is a very important motion that contributes to upper limb abduction. Without superior rotation of the scapula, the greater tubercle of the humerus would hit the acromion of the scapula, thus preventing any abduction of the arm above shoulder height. Superior rotation of the scapula is thus required for full abduction of the upper limb. Superior rotation is also used without arm abduction when carrying a heavy load with your hand or on your shoulder. You can feel this rotation when you pick up a load, such as a heavy book bag and carry it on only one shoulder. To increase its weight-bearing support for the bag, the shoulder lifts as the scapula superiorly rotates. Inferior rotation occurs during limb adduction and involves the downward motion of the glenoid cavity with upward movement of the medial end of the scapular spine. Opposition and Reposition Opposition is the thumb movement that brings the tip of the thumb in contact with the tip of a finger. This movement is produced at the first carpometacarpal joint, which is a saddle joint formed between the trapezium carpal bone and the first metacarpal bone. Thumb opposition is produced by a combination of flexion and abduction of the thumb at this joint. Returning the thumb to its anatomical position next to the index finger is called reposition (see [link]l). Movements of the Joints Type of Joint Movement Example Atlantoaxial joint (C1- Pivot Uniaxial joint; allows C2 vertebrae rotational movement articulation); proximal radioulnar joint Uniaxial joint; allows Knee; elbow; ankle; Hinge flexion/extension interphalangeal joints movements of fingers and toes Movements of the Joints Type of Joint Movement Biaxial joint; allows flexion/extension, Condyloid abduction/adduction, and circumduction movements Biaxial joint; allows flexion/extension, Saddle abduction/adduction, and circumduction movements Multiaxial joint; allows inversion and eversion of foot, or flexion, extension, and lateral flexion of the vertebral column Plane Multiaxial joint; allows flexion/extension, Ball-and- abduction/adduction, socket circumduction, and medial/lateral rotation movements Chapter Review The variety of movements provided by the different types of synovial joints Example Metacarpophalangeal (knuckle) joints of fingers; radiocarpal joint of wrist; metatarsophalangeal joints for toes First carpometacarpal joint of the thumb; stemoclavicular joint Intertarsal joints of foot; superior-inferior articular process articulations between vertebrae Shoulder and hip joints allows for a large range of body motions and gives you tremendous mobility. These movements allow you to flex or extend your body or limbs, medially rotate and adduct your arms and flex your elbows to hold a heavy object against your chest, raise your arms above your head, rotate or shake your head, and bend to touch the toes (with or without bending your knees). Each of the different structural types of synovial joints also allow for specific motions. The atlantoaxial pivot joint provides side-to-side rotation of the head, while the proximal radioulnar articulation allows for rotation of the radius during pronation and supination of the forearm. Hinge joints, such as at the knee and elbow, allow only for flexion and extension. Similarly, the hinge joint of the ankle only allows for dorsiflexion and plantar flexion of the foot. Condyloid and saddle joints are biaxial. These allow for flexion and extension, and abduction and adduction. The sequential combination of flexion, adduction, extension, and abduction produces circumduction. Multiaxial plane joints provide for only small motions, but these can add together over several adjacent joints to produce body movement, such as inversion and eversion of the foot. Similarly, plane joints allow for flexion, extension, and lateral flexion movements of the vertebral column. The multiaxial ball and socket joints allow for flexion-extension, abduction- adduction, and circumduction. In addition, these also allow for medial (internal) and lateral (external) rotation. Ball-and-socket joints have the greatest range of motion of all synovial joints. Interactive Link Questions Exercise: Problem: Watch this video to learn about anatomical motions. What motions involve increasing or decreasing the angle of the foot at the ankle? Solution: Dorsiflexion of the foot at the ankle decreases the angle of the ankle joint, while plantar flexion increases the angle of the ankle joint. Chapter Review Exercise: Problem: The joints between the articular processes of adjacent vertebrae can contribute to which movement? a. lateral flexion b. circumduction c. dorsiflexion d. abduction Solution: A Exercise: Problem: Which motion moves the bottom of the foot away from the midline of the body? a. elevation b. dorsiflexion c. eversion d. plantar flexion Solution: GC Exercise: Problem: Movement of a body region in a circular movement at a condyloid joint is what type of motion? a. rotation b. elevation c. abduction d. circumduction Solution: D Exercise: Problem: Supination is the motion that moves the a. hand from the palm backward position to the palm forward position b. foot so that the bottom of the foot faces the midline of the body c. hand from the palm forward position to the palm backward position d. scapula in an upward direction Solution: A Exercise: Problem: Movement at the shoulder joint that moves the upper limb laterally away from the body is called a. elevation b. eversion c. abduction d. lateral rotation Solution: ‘Ss Critical Thinking Questions Exercise: Problem: Briefly define the types of joint movements available at a ball-and- socket joint. Solution: Ball-and-socket joints are multiaxial joints that allow for flexion and extension, abduction and adduction, circumduction, and medial and lateral rotation. Exercise: Problem: Discuss the joints involved and movements required for you to cross your arms together in front of your chest. Solution: To cross your arms, you need to use both your shoulder and elbow joints. At the shoulder, the arm would need to flex and medially rotate. At the elbow, the forearm would need to be flexed. Glossary abduction movement in the coronal plane that moves a limb laterally away from the body; spreading of the fingers adduction movement in the coronal plane that moves a limb medially toward or across the midline of the body; bringing fingers together circumduction circular motion of the arm, thigh, hand, thumb, or finger that is produced by the sequential combination of flexion, abduction, extension, and adduction depression downward (inferior) motion of the scapula or mandible dorsiflexion movement at the ankle that brings the top of the foot toward the anterior leg elevation upward (superior) motion of the scapula or mandible eversion foot movement involving the intertarsal joints of the foot in which the bottom of the foot is turned laterally, away from the midline extension movement in the sagittal plane that increases the angle of a joint (straightens the joint); motion involving posterior bending of the vertebral column or returning to the upright position from a flexed position flexion movement in the sagittal plane that decreases the angle of a joint (bends the joint); motion involving anterior bending of the vertebral column hyperextension excessive extension of joint, beyond the normal range of movement hyperflexion excessive flexion of joint, beyond the normal range of movement inferior rotation movement of the scapula during upper limb adduction in which the glenoid cavity of the scapula moves in a downward direction as the medial end of the scapular spine moves in an upward direction inversion foot movement involving the intertarsal joints of the foot in which the bottom of the foot is turned toward the midline lateral excursion side-to-side movement of the mandible away from the midline, toward either the right or left side lateral flexion bending of the neck or body toward the right or left side lateral (external) rotation movement of the arm at the shoulder joint or the thigh at the hip joint that moves the anterior surface of the limb away from the midline of the body medial excursion side-to-side movement that returns the mandible to the midline medial (internal) rotation movement of the arm at the shoulder joint or the thigh at the hip joint that brings the anterior surface of the limb toward the midline of the body opposition thumb movement that brings the tip of the thumb in contact with the tip of a finger plantar flexion foot movement at the ankle in which the heel is lifted off of the ground pronated position forearm position in which the palm faces backward pronation forearm motion that moves the palm of the hand from the palm forward to the palm backward position protraction anterior motion of the scapula or mandible reposition movement of the thumb from opposition back to the anatomical position (next to index finger) retraction posterior motion of the scapula or mandible rotation movement of a bone around a central axis (atlantoaxial joint) or around its long axis (proximal radioulnar joint; shoulder or hip joint); twisting of the vertebral column resulting from the summation of small motions between adjacent vertebrae superior rotation movement of the scapula during upper limb abduction in which the glenoid cavity of the scapula moves in an upward direction as the medial end of the scapular spine moves in a downward direction supinated position forearm position in which the palm faces anteriorly (anatomical position) supination forearm motion that moves the palm of the hand from the palm backward to the palm forward position Anatomy of Selected Synovial Joints By the end of this section, you will be able to: e Describe the bones that articulate together to form selected synovial joints e Discuss the movements available at each joint e Describe the structures that support and prevent excess movements at each joint Each synovial joint of the body is specialized to perform certain movements. The movements that are allowed are determined by the structural classification for each joint. For example, a multiaxial ball-and- socket joint has much more mobility than a uniaxial hinge joint. However, the ligaments and muscles that support a joint may place restrictions on the total range of motion available. Thus, the ball-and-socket joint of the shoulder has little in the way of ligament support, which gives the shoulder a very large range of motion. In contrast, movements at the hip joint are restricted by strong ligaments, which reduce its range of motion but confer stability during standing and weight bearing. This section will examine the anatomy of selected synovial joints of the body. Anatomical names for most joints are derived from the names of the bones that articulate at that joint, although some joints, such as the elbow, hip, and knee joints are exceptions to this general naming scheme. Articulations of the Vertebral Column In addition to being held together by the intervertebral discs, adjacent vertebrae also articulate with each other at synovial joints formed between the superior and inferior articular processes called zygapophysial joints (facet joints) (see [link]). These are plane joints that provide for only limited motions between the vertebrae. The orientation of the articular processes at these joints varies in different regions of the vertebral column and serves to determine the types of motions available in each vertebral region. The cervical and lumbar regions have the greatest ranges of motions. In the neck, the articular processes of cervical vertebrae are flattened and generally face upward or downward. This orientation provides the cervical vertebral column with extensive ranges of motion for flexion, extension, lateral flexion, and rotation. In the thoracic region, the downward projecting and overlapping spinous processes, along with the attached thoracic cage, greatly limit flexion, extension, and lateral flexion. However, the flattened and vertically positioned thoracic articular processes allow for the greatest range of rotation within the vertebral column. The lumbar region allows for considerable extension, flexion, and lateral flexion, but the orientation of the articular processes largely prohibits rotation. The articulations formed between the skull, the atlas (C1 vertebra), and the axis (C2 vertebra) differ from the articulations in other vertebral areas and play important roles in movement of the head. The atlanto-occipital joint is formed by the articulations between the superior articular processes of the atlas and the occipital condyles on the base of the skull. This articulation has a pronounced U-shaped curvature, oriented along the anterior-posterior axis. This allows the skull to rock forward and backward, producing flexion and extension of the head. This moves the head up and down, as when shaking your head “yes.” The atlantoaxial joint, between the atlas and axis, consists of three articulations. The paired superior articular processes of the axis articulate with the inferior articular processes of the atlas. These articulating surfaces are relatively flat and oriented horizontally. The third articulation is the pivot joint formed between the dens, which projects upward from the body of the axis, and the inner aspect of the anterior arch of the atlas ({link]). A strong ligament passes posterior to the dens to hold it in position against the anterior arch. These articulations allow the atlas to rotate on top of the axis, moving the head toward the right or left, as when shaking your head “no.” Atlantoaxial Joint Dens of C2 (axis) Anterior arch Superior articular of C1 (atlas) facet Ligament Superior view of atlas The atlantoaxial joint is a pivot type of joint between the dens portion of the axis (C2 vertebra) and the anterior arch of the atlas (C1 vertebra), with the dens held in place by a ligament. Temporomandibular Joint The temporomandibular joint (TMJ) is the joint that allows for opening (mandibular depression) and closing (mandibular elevation) of the mouth, as well as side-to-side and protraction/retraction motions of the lower jaw. This joint involves the articulation between the mandibular fossa and articular tubercle of the temporal bone, with the condyle (head) of the mandible. Located between these bony structures, filling the gap between the skull and mandible, is a flexible articular disc ([link]). This disc serves to smooth the movements between the temporal bone and mandibular condyle. Movement at the TMJ during opening and closing of the mouth involves both gliding and hinge motions of the mandible. With the mouth closed, the mandibular condyle and articular disc are located within the mandibular fossa of the temporal bone. During opening of the mouth, the mandible hinges downward and at the same time is pulled anteriorly, causing both the condyle and the articular disc to glide forward from the mandibular fossa onto the downward projecting articular tubercle. The net result is a forward and downward motion of the condyle and mandibular depression. The temporomandibular joint is supported by an extrinsic ligament that anchors the mandible to the skull. This ligament spans the distance between the base of the skull and the lingula on the medial side of the mandibular ramus. Dislocation of the TMJ may occur when opening the mouth too wide (such as when taking a large bite) or following a blow to the jaw, resulting in the mandibular condyle moving beyond (anterior to) the articular tubercle. In this case, the individual would not be able to close his or her mouth. Temporomandibular joint disorder is a painful condition that may arise due to arthritis, wearing of the articular cartilage covering the bony surfaces of the joint, muscle fatigue from overuse or grinding of the teeth, damage to the articular disc within the joint, or jaw injury. Temporomandibular joint disorders can also cause headache, difficulty chewing, or even the inability to move the jaw (lock jaw). Pharmacologic agents for pain or other therapies, including bite guards, are used as treatments. Temporomandibular Joint Articular disc Mandibular fossa uperior SOT aay rticular joint cay tubercle a Interior ——_$ —————— joint cavity ; Articular capsule Mandibular condyle Ramus of mandible The temporomandibular joint is the articulation between the temporal bone of the skull and the condyle of the mandible, with an articular disc located between these bones. During depression of the mandible (opening of the mouth), the mandibular condyle moves both forward and hinges downward as it travels from the mandibular fossa onto the articular tubercle. a gC Watch this video to learn about TMJ. Opening of the mouth requires the combination of two motions at the temporomandibular joint, an anterior gliding motion of the articular disc and mandible and the downward hinging of the mandible. What is the initial movement of the mandible during opening and how much mouth opening does this produce? Shoulder Joint The shoulder joint is called the glenohumeral joint. This is a ball-and- socket joint formed by the articulation between the head of the humerus and the glenoid cavity of the scapula ([link]). This joint has the largest range of motion of any joint in the body. However, this freedom of movement is due to the lack of structural support and thus the enhanced mobility is offset by a loss of stability. Glenohumeral Joint Clavicle Acromioclavicular ligament Tendon of supraspinatus muscle —— Glenoid labrum Acromion of scapula = Coracoacromial FBT ligament ; eel Subacromial bursa Glenoid cavity SSRs Scapula ———__ “s= 5 ~~ Articular capsule Tendon sheath Tendon of biceps brachii muscles Articular cartilage Q\ . 7 (long heed) Articular capsule: Synovial membrane Fibrous membrane Head of humerus ml Humerus The glenohumeral (shoulder) joint is a ball-and- socket joint that provides the widest range of motions. It has a loose articular capsule and is supported by ligaments and the rotator cuff muscles. The large range of motions at the shoulder joint is provided by the articulation of the large, rounded humeral head with the small and shallow glenoid cavity, which is only about one third of the size of the humeral head. The socket formed by the glenoid cavity is deepened slightly by a small lip of fibrocartilage called the glenoid labrum, which extends around the outer margin of the cavity. The articular capsule that surrounds the glenohumeral joint is relatively thin and loose to allow for large motions of the upper limb. Some structural support for the joint is provided by thickenings of the articular capsule wall that form weak intrinsic ligaments. These include the coracohumeral ligament, running from the coracoid process of the scapula to the anterior humerus, and three ligaments, each called a glenohumeral ligament, located on the anterior side of the articular capsule. These ligaments help to strengthen the superior and anterior capsule walls. However, the primary support for the shoulder joint is provided by muscles crossing the joint, particularly the four rotator cuff muscles. These muscles (supraspinatus, infraspinatus, teres minor, and subscapularis) arise from the scapula and attach to the greater or lesser tubercles of the humerus. As these muscles cross the shoulder joint, their tendons encircle the head of the humerus and become fused to the anterior, superior, and posterior walls of the articular capsule. The thickening of the capsule formed by the fusion of these four muscle tendons is called the rotator cuff. Two bursae, the subacromial bursa and the subscapular bursa, help to prevent friction between the rotator cuff muscle tendons and the scapula as these tendons cross the glenohumeral joint. In addition to their individual actions of moving the upper limb, the rotator cuff muscles also serve to hold the head of the humerus in position within the glenoid cavity. By constantly adjusting their strength of contraction to resist forces acting on the shoulder, these muscles serve as “dynamic ligaments” and thus provide the primary structural support for the glenohumeral joint. Injuries to the shoulder joint are common. Repetitive use of the upper limb, particularly in abduction such as during throwing, swimming, or racquet sports, may lead to acute or chronic inflammation of the bursa or muscle tendons, a tear of the glenoid labrum, or degeneration or tears of the rotator cuff. Because the humeral head is strongly supported by muscles and ligaments around its anterior, superior, and posterior aspects, most dislocations of the humerus occur in an inferior direction. This can occur when force is applied to the humerus when the upper limb is fully abducted, as when diving to catch a baseball and landing on your hand or elbow. Inflammatory responses to any shoulder injury can lead to the formation of scar tissue between the articular capsule and surrounding structures, thus reducing shoulder mobility, a condition called adhesive capsulitis (“frozen shoulder”). Note: Dea a Pie — = Shenatax 6 COLLEGE Watch this video for a tutorial on the anatomy of the shoulder joint. What movements are available at the shoulder joint? Note: openstax COLLEGE Obs fal) Watch this video to learn more about the anatomy of the shoulder joint, including bones, joints, muscles, nerves, and blood vessels. What is the shape of the glenoid labrum in cross-section, and what is the importance of this shape? Elbow Joint The elbow joint is a uniaxial hinge joint formed by the humeroulnar joint, the articulation between the trochlea of the humerus and the trochlear notch of the ulna. Also associated with the elbow are the humeroradial joint and the proximal radioulnar joint. All three of these joints are enclosed within a single articular capsule ([link]). The articular capsule of the elbow is thin on its anterior and posterior aspects, but is thickened along its outside margins by strong intrinsic ligaments. These ligaments prevent side-to-side movements and hyperextension. On the medial side is the triangular ulnar collateral ligament. This arises from the medial epicondyle of the humerus and attaches to the medial side of the proximal ulna. The strongest part of this ligament is the anterior portion, which resists hyperextension of the elbow. The ulnar collateral ligament may be injured by frequent, forceful extensions of the forearm, as is seen in baseball pitchers. Reconstructive surgical repair of this ligament is referred to as Tommy John surgery, named for the former major league pitcher who was the first person to have this treatment. The lateral side of the elbow is supported by the radial collateral ligament. This arises from the lateral epicondyle of the humerus and then blends into the lateral side of the annular ligament. The annular ligament encircles the head of the radius. This ligament supports the head of the radius as it articulates with the radial notch of the ulna at the proximal radioulnar joint. This is a pivot joint that allows for rotation of the radius during supination and pronation of the forearm. Elbow Joint Humerus Articular capsule Fat pad Synovial membrane Tendon ; c Synovial of triceps 7 - - cavity muscle \F) Articular i. cartilage Bursa | \\ of trochlea i. \ } ) fj = Tendon of SY . + Trochlea wi branchialis By — . muscle Articular } cartilage of the trochlear notch Olecranon bursa | | eve ' . NX Una al Coronoid = process (a) Medial sagittal section through right elbow (lateral view) Articular capsule oF Anular ligament \ : a —— ee Radius » Ulnar \ = = aa collateral —Lil ; D, ~— ligament AW ) “aa k TU) \\\ __ Una , Cael a Coronoid process (c) Medial view of right elbow joint Humerus j | ie \ Lateral epicondyle \ a ae Ulna Articular | =a jn ular capsule Min : ade p / tT Radial collateral \ Radius \ ligament —_ aw Olecranon ye = process (b) Lateral view of right elbow joint (a) The elbow is a hinge joint that allows only for flexion and extension of the forearm. (b) It is supported by the ulnar and radial collateral ligaments. (c) The annular ligament supports the head of the radius at the proximal radioulnar joint, the pivot joint that allows for rotation of the radius. Note: openstax COLLEGE” = “A 1 eye Watch this animation to learn more about the anatomy of the elbow joint. Which structures provide the main stability for the elbow? Note: ORR pao a r —s ——. meee OPENStAX COLLEGE “ 1 eal Watch this video to learn more about the anatomy of the elbow joint, including bones, joints, muscles, nerves, and blood vessels. What are the functions of the articular cartilage? Hip Joint The hip joint is a multiaxial ball-and-socket joint between the head of the femur and the acetabulum of the hip bone ({link]). The hip carries the weight of the body and thus requires strength and stability during standing and walking. For these reasons, its range of motion is more limited than at the shoulder joint. The acetabulum is the socket portion of the hip joint. This space is deep and has a large articulation area for the femoral head, thus giving stability and weight bearing ability to the joint. The acetabulum is further deepened by the acetabular labrum, a fibrocartilage lip attached to the outer margin of the acetabulum. The surrounding articular capsule is strong, with several thickened areas forming intrinsic ligaments. These ligaments arise from the hip bone, at the margins of the acetabulum, and attach to the femur at the base of the neck. The ligaments are the iliofemoral ligament, pubofemoral ligament, and ischiofemoral ligament, all of which spiral around the head and neck of the femur. The ligaments are tightened by extension at the hip, thus pulling the head of the femur tightly into the acetabulum when in the upright, standing position. Very little additional extension of the thigh is permitted beyond this vertical position. These ligaments thus stabilize the hip joint and allow you to maintain an upright standing position with only minimal muscle contraction. Inside of the articular capsule, the ligament of the head of the femur (ligamentum teres) spans between the acetabulum and femoral head. This intracapsular ligament is normally slack and does not provide any significant joint support, but it does provide a pathway for an important artery that supplies the head of the femur. The hip is prone to osteoarthritis, and thus was the first joint for which a replacement prosthesis was developed. A common injury in elderly individuals, particularly those with weakened bones due to osteoporosis, is a “broken hip,” which is actually a fracture of the femoral neck. This may result from a fall, or it may cause the fall. This can happen as one lower limb is taking a step and all of the body weight is placed on the other limb, causing the femoral neck to break and producing a fall. Any accompanying disruption of the blood supply to the femoral neck or head can lead to necrosis of these areas, resulting in bone and cartilage death. Femoral fractures usually require surgical treatment, after which the patient will need mobility assistance for a prolonged period, either from family members or in a long-term care facility. Consequentially, the associated health care costs of “broken hips” are substantial. In addition, hip fractures are associated with increased rates of morbidity (incidences of disease) and mortality (death). Surgery for a hip fracture followed by prolonged bed rest may lead to life-threatening complications, including pneumonia, infection of pressure ulcers (bedsores), and thrombophlebitis (deep vein thrombosis; blood clot formation) that can result in a pulmonary embolism (blood clot within the lung). Hip Joint Articular cartilage Coxal (hip) bone Acetabular labrum Ligament of the head of the femur Synovial cavity Articular capsule (a) Frontal section through the right hip joint Anterior inferior A ) lliofemoral iliac spine ligament Greater tronchanter » “~% = \Os Pubofemoral . ligament (b) Anterior view of right hip joint, capsule in place Ischium v lliofemoral / ean ligament _ Greater trochanter of femur Ischiofemoral ligament (c) Posterior view of right hip joint, capsule in place (a) The ball-and-socket joint of the hip is a multiaxial joint that provides both stability and a wide range of motion. (b—c) When standing, the supporting ligaments are tight, pulling the head of the femur into the acetabulum. . Loy wees Openstax COLLEGE = “ a= [=] fu Watch this video for a tutorial on the anatomy of the hip joint. What is a possible consequence following a fracture of the femoral neck within the capsule of the hip joint? Note: al: [=] 7 r. — meee OPENStAX COLLEGE 1 bese Watch this video to learn more about the anatomy of the hip joint, including bones, joints, muscles, nerves, and blood vessels. Where is the articular cartilage thickest within the hip joint? Knee Joint The knee joint is the largest joint of the body ([link]). It actually consists of three articulations. The femoropatellar joint is found between the patella and the distal femur. The medial tibiofemoral joint and lateral tibiofemoral joint are located between the medial and lateral condyles of the femur and the medial and lateral condyles of the tibia. All of these articulations are enclosed within a single articular capsule. The knee functions as a hinge joint, allowing flexion and extension of the leg. This action is generated by both rolling and gliding motions of the femur on the tibia. In addition, some rotation of the leg is available when the knee is flexed, but not when extended. The knee is well constructed for weight bearing in its extended position, but is vulnerable to injuries associated with hyperextension, twisting, or blows to the medial or lateral side of the joint, particularly while weight bearing. At the femoropatellar joint, the patella slides vertically within a groove on the distal femur. The patella is a sesamoid bone incorporated into the tendon of the quadriceps femoris muscle, the large muscle of the anterior thigh. The patella serves to protect the quadriceps tendon from friction against the distal femur. Continuing from the patella to the anterior tibia just below the knee is the patellar ligament. Acting via the patella and patellar ligament, the quadriceps femoris is a powerful muscle that acts to extend the leg at the knee. It also serves as a “dynamic ligament” to provide very important support and stabilization for the knee joint. The medial and lateral tibiofemoral joints are the articulations between the rounded condyles of the femur and the relatively flat condyles of the tibia. During flexion and extension motions, the condyles of the femur both roll and glide over the surfaces of the tibia. The rolling action produces flexion or extension, while the gliding action serves to maintain the femoral condyles centered over the tibial condyles, thus ensuring maximal bony, weight-bearing support for the femur in all knee positions. As the knee comes into full extension, the femur undergoes a slight medial rotation in relation to tibia. The rotation results because the lateral condyle of the femur is slightly smaller than the medial condyle. Thus, the lateral condyle finishes its rolling motion first, followed by the medial condyle. The resulting small medial rotation of the femur serves to “lock” the knee into its fully extended and most stable position. Flexion of the knee is initiated by a slight lateral rotation of the femur on the tibia, which “unlocks” the knee. This lateral rotation motion is produced by the popliteus muscle of the posterior leg. Located between the articulating surfaces of the femur and tibia are two articular discs, the medial meniscus and lateral meniscus (see [link]b). Each is a C-shaped fibrocartilage structure that is thin along its inside margin and thick along the outer margin. They are attached to their tibial condyles, but do not attach to the femur. While both menisci are free to move during knee motions, the medial meniscus shows less movement because it is anchored at its outer margin to the articular capsule and tibial collateral ligament. The menisci provide padding between the bones and help to fill the gap between the round femoral condyles and flattened tibial condyles. Some areas of each meniscus lack an arterial blood supply and thus these areas heal poorly if damaged. The knee joint has multiple ligaments that provide support, particularly in the extended position (see [link]c). Outside of the articular capsule, located at the sides of the knee, are two extrinsic ligaments. The fibular collateral ligament (lateral collateral ligament) is on the lateral side and spans from the lateral epicondyle of the femur to the head of the fibula. The tibial collateral ligament (medial collateral ligament) of the medial knee runs from the medial epicondyle of the femur to the medial tibia. As it crosses the knee, the tibial collateral ligament is firmly attached on its deep side to the articular capsule and to the medial meniscus, an important factor when considering knee injuries. In the fully extended knee position, both collateral ligaments are taut (tight), thus serving to stabilize and support the extended knee and preventing side-to-side or rotational motions between the femur and tibia. The articular capsule of the posterior knee is thickened by intrinsic ligaments that help to resist knee hyperextension. Inside the knee are two intracapsular ligaments, the anterior cruciate ligament and posterior cruciate ligament. These ligaments are anchored inferiorly to the tibia at the intercondylar eminence, the roughened area between the tibial condyles. The cruciate ligaments are named for whether they are attached anteriorly or posteriorly to this tibial region. Each ligament runs diagonally upward to attach to the inner aspect of a femoral condyle. The cruciate ligaments are named for the X-shape formed as they pass each other (cruciate means “cross”). The posterior cruciate ligament is the stronger ligament. It serves to support the knee when it is flexed and weight bearing, as when walking downhill. In this position, the posterior cruciate ligament prevents the femur from sliding anteriorly off the top of the tibia. The anterior cruciate ligament becomes tight when the knee is extended, and thus resists hyperextension. Knee Joint Femur Tendon of quadriceps femoris Articular Suprapatellar bursa capsule Patella Prepatellar bursa Posterior cruciate ligament Synovial cavity ; Lateral meniscus meniscus Infrapatellar Anterior fat pad cruciate ligament Tibia Infrapatellar bursa Patellar ligament (a) Sagittal section through the right knee joint Quadriceps femoris muscle Tendon of —————————_ quadriceps femoris muscle a Medial patellar Lateral rosie |e Ay LX retinaculum retinaculum Z Tibial collateral Fibular collateral ligament ligament W! Patellar ligament Fibula = Tibia (c) Anterior view of right knee Anterior cruciate : ligament prienoe —_—— Articular cartilage on lateral tibial condyle Articular cartilage yo on medial F tibial condyle c Medial meniscus <<) Sf — Lateral meniscus \ ) in the metabolic pathway, occurs in the mitochondria. The SO fibers possess a large number of mitochondria and are capable of contracting for longer periods because of the large amount of ATP they can produce, but they have a relatively small diameter and do not produce a large amount of tension. SO fibers are extensively supplied with blood capillaries to supply Oz from the red blood cells in the bloodstream. The SO fibers also possess myoglobin, an O»-carrying molecule similar to Oj-carrying hemoglobin in the red blood cells. The myoglobin stores some of the needed O> within the fibers themselves (and gives SO fibers their red color). All of these features allow SO fibers to produce large quantities of ATP, which can sustain muscle activity without fatiguing for long periods of time. The fact that SO fibers can function for long periods without fatiguing makes them useful in maintaining posture, producing isometric contractions, stabilizing bones and joints, and making small movements that happen often but do not require large amounts of energy. They do not produce high tension, and thus they are not used for powerful, fast movements that require high amounts of energy and rapid cross-bridge cycling. FO fibers are sometimes called intermediate fibers because they possess characteristics that are intermediate between fast fibers and slow fibers. They produce ATP relatively quickly, more quickly than SO fibers, and thus can produce relatively high amounts of tension. They are oxidative because they produce ATP aerobically, possess high amounts of mitochondria, and do not fatigue quickly. However, FO fibers do not possess significant myoglobin, giving them a lighter color than the red SO fibers. FO fibers are used primarily for movements, such as walking, that require more energy than postural control but less energy than an explosive movement, such as sprinting. FO fibers are useful for this type of movement because they produce more tension than SO fibers but they are more fatigue-resistant than FG fibers. FG fibers primarily use anaerobic glycolysis as their ATP source. They have a large diameter and possess high amounts of glycogen, which is used in glycolysis to generate ATP quickly to produce high levels of tension. Because they do not primarily use aerobic metabolism, they do not possess substantial numbers of mitochondria or significant amounts of myoglobin and therefore have a white color. FG fibers are used to produce rapid, forceful contractions to make quick, powerful movements. These fibers fatigue quickly, permitting them to only be used for short periods. Most muscles possess a mixture of each fiber type. The predominant fiber type in a muscle is determined by the primary function of the muscle. Chapter Review ATP provides the energy for muscle contraction. The three mechanisms for ATP regeneration are creatine phosphate, anaerobic glycolysis, and aerobic metabolism. Creatine phosphate provides about the first 15 seconds of ATP at the beginning of muscle contraction. Anaerobic glycolysis produces small amounts of ATP in the absence of oxygen for a short period. Aerobic metabolism utilizes oxygen to produce much more ATP, allowing a muscle to work for longer periods. Muscle fatigue, which has many contributing factors, occurs when muscle can no longer contract. An oxygen debt is created as a result of muscle use. The three types of muscle fiber are slow oxidative (SO), fast oxidative (FO) and fast glycolytic (FG). SO fibers use aerobic metabolism to produce low power contractions over long periods and are slow to fatigue. FO fibers use aerobic metabolism to produce ATP but produce higher tension contractions than SO fibers. FG fibers use anaerobic metabolism to produce powerful, high-tension contractions but fatigue quickly. Review Questions Exercise: Problem: Muscle fatigue is caused by a. buildup of ATP and lactic acid levels b. exhaustion of energy reserves and buildup of lactic acid levels c. buildup of ATP and pyruvic acid levels d. exhaustion of energy reserves and buildup of pyruvic acid levels Solution: B Exercise: Problem: A sprinter would experience muscle fatigue sooner than a marathon runner due to a. anaerobic metabolism in the muscles of the sprinter b. anaerobic metabolism in the muscles of the marathon runner c. aerobic metabolism in the muscles of the sprinter d. glycolysis in the muscles of the marathon runner Solution: A Exercise: Problem: What aspect of creatine phosphate allows it to supply energy to muscles? a. ATPase activity b. phosphate bonds c. carbon bonds d. hydrogen bonds Solution: B Exercise: Problem: Drug X blocks ATP regeneration from ADP and phosphate. How will muscle cells respond to this drug? a. by absorbing ATP from the bloodstream b. by using ADP as an energy source c. by using glycogen as an energy source d. none of the above Solution: D Critical Thinking Questions Exercise: Problem: Why do muscle cells use creatine phosphate instead of glycolysis to supply ATP for the first few seconds of muscle contraction? Solution: Creatine phosphate is used because creatine phosphate and ADP are converted very quickly into ATP by creatine kinase. Glycolysis cannot generate ATP as quickly as creatine phosphate. Exercise: Problem: Is aerobic respiration more or less efficient than glycolysis? Explain your answer. Solution: Aerobic respiration is much more efficient than anaerobic glycolysis, yielding 36 ATP per molecule of glucose, as opposed to two ATP produced by glycolysis. Glossary fast glycolytic (FG) muscle fiber that primarily uses anaerobic glycolysis fast oxidative (FO) intermediate muscle fiber that is between slow oxidative and fast glycolytic fibers slow oxidative (SO) muscle fiber that primarily uses aerobic respiration Exercise and Muscle Performance By the end of this section, you will be able to: e Describe hypertrophy and atrophy e Explain how resistance exercise builds muscle e Explain how performance-enhancing substances affect muscle Physical training alters the appearance of skeletal muscles and can produce changes in muscle performance. Conversely, a lack of use can result in decreased performance and muscle appearance. Although muscle cells can change in size, new cells are not formed when muscles grow. Instead, structural proteins are added to muscle fibers in a process called hypertrophy, so cell diameter increases. The reverse, when structural proteins are lost and muscle mass decreases, is called atrophy. Age-related muscle atrophy is called sarcopenia. Cellular components of muscles can also undergo changes in response to changes in muscle use. Endurance Exercise Slow fibers are predominantly used in endurance exercises that require little force but involve numerous repetitions. The aerobic metabolism used by slow-twitch fibers allows them to maintain contractions over long periods. Endurance training modifies these slow fibers to make them even more efficient by producing more mitochondria to enable more aerobic metabolism and more ATP production. Endurance exercise can also increase the amount of myoglobin in a cell, as increased aerobic respiration increases the need for oxygen. Myoglobin is found in the sarcoplasm and acts as an oxygen storage supply for the mitochondria. The training can trigger the formation of more extensive capillary networks around the fiber, a process called angiogenesis, to supply oxygen and remove metabolic waste. To allow these capillary networks to supply the deep portions of the muscle, muscle mass does not greatly increase in order to maintain a smaller area for the diffusion of nutrients and gases. All of these cellular changes result in the ability to sustain low levels of muscle contractions for greater periods without fatiguing. The proportion of SO muscle fibers in muscle determines the suitability of that muscle for endurance, and may benefit those participating in endurance activities. Postural muscles have a large number of SO fibers and relatively few FO and FG fibers, to keep the back straight ({link]). Endurance athletes, like marathon-runners also would benefit from a larger proportion of SO fibers, but it is unclear if the most-successful marathoners are those with naturally high numbers of SO fibers, or whether the most successful marathon runners develop high numbers of SO fibers with repetitive training. Endurance training can result in overuse injuries such as stress fractures and joint and tendon inflammation. Marathoners a Long-distance runners have a large number of SO fibers and relatively few FO and FG fibers. (credit: “Tseo2”/Wikimedia Commons) Resistance Exercise Resistance exercises, as opposed to endurance exercise, require large amounts of FG fibers to produce short, powerful movements that are not repeated over long periods. The high rates of ATP hydrolysis and cross- bridge formation in FG fibers result in powerful muscle contractions. Muscles used for power have a higher ratio of FG to SO/FO fibers, and trained athletes possess even higher levels of FG fibers in their muscles. Resistance exercise affects muscles by increasing the formation of myofibrils, thereby increasing the thickness of muscle fibers. This added structure causes hypertrophy, or the enlargement of muscles, exemplified by the large skeletal muscles seen in body builders and other athletes ([link]). Because this muscular enlargement is achieved by the addition of structural proteins, athletes trying to build muscle mass often ingest large amounts of protein. Hypertrophy Body builders have a large number of FG fibers and relatively few FO and SO fibers. (credit: Lin Mei/flickr) Except for the hypertrophy that follows an increase in the number of sarcomeres and myofibrils in a skeletal muscle, the cellular changes observed during endurance training do not usually occur with resistance training. There is usually no significant increase in mitochondria or capillary density. However, resistance training does increase the development of connective tissue, which adds to the overall mass of the muscle and helps to contain muscles as they produce increasingly powerful contractions. Tendons also become stronger to prevent tendon damage, as the force produced by muscles is transferred to tendons that attach the muscle to bone. For effective strength training, the intensity of the exercise must continually be increased. For instance, continued weight lifting without increasing the weight of the load does not increase muscle size. To produce ever-greater results, the weights lifted must become increasingly heavier, making it more difficult for muscles to move the load. The muscle then adapts to this heavier load, and an even heavier load must be used if even greater muscle mass is desired. If done improperly, resistance training can lead to overuse injuries of the muscle, tendon, or bone. These injuries can occur if the load is too heavy or if the muscles are not given sufficient time between workouts to recover or if joints are not aligned properly during the exercises. Cellular damage to muscle fibers that occurs after intense exercise includes damage to the sarcolemma and myofibrils. This muscle damage contributes to the feeling of soreness after strenuous exercise, but muscles gain mass as this damage is repaired, and additional structural proteins are added to replace the damaged ones. Overworking skeletal muscles can also lead to tendon damage and even skeletal damage if the load is too great for the muscles to bear. Performance-Enhancing Substances Some athletes attempt to boost their performance by using various agents that may enhance muscle performance. Anabolic steroids are one of the more widely known agents used to boost muscle mass and increase power output. Anabolic steroids are a form of testosterone, a male sex hormone that stimulates muscle formation, leading to increased muscle mass. Endurance athletes may also try to boost the availability of oxygen to muscles to increase aerobic respiration by using substances such as erythropoietin (EPO), a hormone normally produced in the kidneys, which triggers the production of red blood cells. The extra oxygen carried by these blood cells can then be used by muscles for aerobic respiration. Human growth hormone (hGH) is another supplement, and although it can facilitate building muscle mass, its main role is to promote the healing of muscle and other tissues after strenuous exercise. Increased hGH may allow for faster recovery after muscle damage, reducing the rest required after exercise, and allowing for more sustained high-level performance. Although performance-enhancing substances often do improve performance, most are banned by governing bodies in sports and are illegal for nonmedical purposes. Their use to enhance performance raises ethical issues of cheating because they give users an unfair advantage over nonusers. A greater concern, however, is that their use carries serious health risks. The side effects of these substances are often significant, nonreversible, and in some cases fatal. The physiological strain caused by these substances is often greater than what the body can handle, leading to effects that are unpredictable and dangerous. Anabolic steroid use has been linked to infertility, aggressive behavior, cardiovascular disease, and brain cancer. Similarly, some athletes have used creatine to increase power output. Creatine phosphate provides quick bursts of ATP to muscles in the initial stages of contraction. Increasing the amount of creatine available to cells is thought to produce more ATP and therefore increase explosive power output, although its effectiveness as a supplement has been questioned. Note: Everyday Connection Aging and Muscle Tissue Although atrophy due to disuse can often be reversed with exercise, muscle atrophy with age, referred to as sarcopenia, is irreversible. This is a primary reason why even highly trained athletes succumb to declining performance with age. This decline is noticeable in athletes whose sports require strength and powerful movements, such as sprinting, whereas the effects of age are less noticeable in endurance athletes such as marathon runners or long-distance cyclists. As muscles age, muscle fibers die, and they are replaced by connective tissue and adipose tissue ([link]). Because those tissues cannot contract and generate force as muscle can, muscles lose the ability to produce powerful contractions. The decline in muscle mass causes a loss of strength, including the strength required for posture and mobility. This may be caused by a reduction in FG fibers that hydrolyze ATP quickly to produce short, powerful contractions. Muscles in older people sometimes possess greater numbers of SO fibers, which are responsible for longer contractions and do not produce powerful movements. There may also be a reduction in the size of motor units, resulting in fewer fibers being stimulated and less muscle tension being produced. Atrophy { ) Atrophied Muscle Normal Muscle Muscle mass is reduced as muscles atrophy with disuse. Sarcopenia can be delayed to some extent by exercise, as training adds structural proteins and causes cellular changes that can offset the effects of atrophy. Increased exercise can produce greater numbers of cellular mitochondria, increase capillary density, and increase the mass and strength of connective tissue. The effects of age-related atrophy are especially pronounced in people who are sedentary, as the loss of muscle cells is displayed as functional impairments such as trouble with locomotion, balance, and posture. This can lead to a decrease in quality of life and medical problems, such as joint problems because the muscles that stabilize bones and joints are weakened. Problems with locomotion and balance can also cause various injuries due to falls. Chapter Review Hypertrophy is an increase in muscle mass due to the addition of structural proteins. The opposite of hypertrophy is atrophy, the loss of muscle mass due to the breakdown of structural proteins. Endurance exercise causes an increase in cellular mitochondria, myoglobin, and capillary networks in SO fibers. Endurance athletes have a high level of SO fibers relative to the other fiber types. Resistance exercise causes hypertrophy. Power-producing muscles have a higher number of FG fibers than of slow fibers. Strenuous exercise causes muscle cell damage that requires time to heal. Some athletes use performance-enhancing substances to enhance muscle performance. Muscle atrophy due to age is called sarcopenia and occurs as muscle fibers die and are replaced by connective and adipose tissue. Review Questions Exercise: Problem: The muscles of a professional sprinter are most likely to have a. 80 percent fast-twitch muscle fibers and 20 percent slow-twitch muscle fibers b. 20 percent fast-twitch muscle fibers and 80 percent slow-twitch muscle fibers c. 50 percent fast-twitch muscle fibers and 50 percent slow-twitch muscle fibers d. 40 percent fast-twitch muscle fibers and 60 percent slow-twitch muscle fibers Solution: A Exercise: Problem: The muscles of a professional marathon runner are most likely to have a. 80 percent fast-twitch muscle fibers and 20 percent slow-twitch muscle fibers b. 20 percent fast-twitch muscle fibers and 80 percent slow-twitch muscle fibers c. 50 percent fast-twitch muscle fibers and 50 percent slow-twitch muscle fibers d. 40 percent fast-twitch muscle fibers and 60 percent slow-twitch muscle fibers Solution: B Exercise: Problem: Which of the following statements is true? a. Fast fibers have a small diameter. b. Fast fibers contain loosely packed myofibrils. c. Fast fibers have large glycogen reserves. d. Fast fibers have many mitochondria. Solution: Cc Exercise: Problem: Which of the following statements is false? a. Slow fibers have a small network of capillaries. b. Slow fibers contain the pigment myoglobin. c. Slow fibers contain a large number of mitochondria. d. Slow fibers contract for extended periods. Solution: A Critical Thinking Questions Exercise: Problem: What changes occur at the cellular level in response to endurance training? Solution: Endurance training modifies slow fibers to make them more efficient by producing more mitochondria to enable more aerobic metabolism and more ATP production. Endurance exercise can also increase the amount of myoglobin in a cell and formation of more extensive capillary networks around the fiber. Exercise: Problem: What changes occur at the cellular level in response to resistance training? Solution: Resistance exercises affect muscles by causing the formation of more actin and myosin, increasing the structure of muscle fibers. Glossary angiogenesis formation of blood capillary networks atrophy loss of structural proteins from muscle fibers hypertrophy addition of structural proteins to muscle fibers Sarcopenia age-related muscle atrophy Cardiac Muscle Tissue By the end of this section, you will be able to: ¢ Describe intercalated discs and gap junctions e Describe a desmosome Cardiac muscle tissue is only found in the heart. Highly coordinated contractions of cardiac muscle pump blood into the vessels of the circulatory system. Similar to skeletal muscle, cardiac muscle is striated and organized into sarcomeres, possessing the same banding organization as skeletal muscle ({link]). However, cardiac muscle fibers are shorter than skeletal muscle fibers and usually contain only one nucleus, which is located in the central region of the cell. Cardiac muscle fibers also possess many mitochondria and myoglobin, as ATP is produced primarily through aerobic metabolism. Cardiac muscle fibers cells also are extensively branched and are connected to one another at their ends by intercalated discs. An intercalated disc allows the cardiac muscle cells to contract in a wave-like pattern so that the heart can work as a pump. Cardiac Muscle Tissue Cardiac muscle tissue is only found in the heart. LM x 1600. (Micrograph provided by the Regents of University of Michigan Medical School © 2012) Note: == mmm ~OPENStaX COLLEGE’ — — . a . Dy) r | View the University of Michigan WebScope to explore the tissue sample in greater detail. Intercalated discs are part of the sarcolemma and contain two structures important in cardiac muscle contraction: gap junctions and desmosomes. A gap junction forms channels between adjacent cardiac muscle fibers that allow the depolarizing current produced by cations to flow from one cardiac muscle cell to the next. This joining is called electric coupling, and in cardiac muscle it allows the quick transmission of action potentials and the coordinated contraction of the entire heart. This network of electrically connected cardiac muscle cells creates a functional unit of contraction called a syncytium. The remainder of the intercalated disc is composed of desmosomes. A desmosome is a cell structure that anchors the ends of cardiac muscle fibers together so the cells do not pull apart during the stress of individual fibers contracting ([link]). Cardiac Muscle Capillary Intercalated discs Desmosome Gap junction Nucleus Cardiac muscle fiber Intercalated discs are part of the cardiac muscle sarcolemma and they contain gap junctions and desmosomes. Contractions of the heart (heartbeats) are controlled by specialized cardiac muscle cells called pacemaker cells that directly control heart rate. Although cardiac muscle cannot be consciously controlled, the pacemaker cells respond to signals from the autonomic nervous system (ANS) to speed up or slow down the heart rate. The pacemaker cells can also respond to various hormones that modulate heart rate to control blood pressure. The wave of contraction that allows the heart to work as a unit, called a functional syncytium, begins with the pacemaker cells. This group of cells is self-excitable and able to depolarize to threshold and fire action potentials on their own, a feature called autorhythmicity; they do this at set intervals which determine heart rate. Because they are connected with gap junctions to surrounding muscle fibers and the specialized fibers of the heart’s conduction system, the pacemaker cells are able to transfer the depolarization to the other cardiac muscle fibers in a manner that allows the heart to contract in a coordinated manner. Another feature of cardiac muscle is its relatively long action potentials in its fibers, having a sustained depolarization “plateau.” The plateau is produced by Ca** entry though voltage-gated calcium channels in the sarcolemma of cardiac muscle fibers. This sustained depolarization (and Ca** entry) provides for a longer contraction than is produced by an action potential in skeletal muscle. Unlike skeletal muscle, a large percentage of the Ca™* that initiates contraction in cardiac muscles comes from outside the cell rather than from the SR. Chapter Review Cardiac muscle is striated muscle that is present only in the heart. Cardiac muscle fibers have a single nucleus, are branched, and joined to one another by intercalated discs that contain gap junctions for depolarization between cells and desmosomes to hold the fibers together when the heart contracts. Contraction in each cardiac muscle fiber is triggered by Ca™ ions in a similar manner as skeletal muscle, but here the Ca** ions come from SR and through voltage-gated calcium channels in the sarcolemma. Pacemaker cells stimulate the spontaneous contraction of cardiac muscle as a functional unit, called a syncytium. Review Questions Exercise: Problem: Cardiac muscles differ from skeletal muscles in that they a. are striated b. utilize aerobic metabolism c. contain myofibrils d. contain intercalated discs Solution: D Exercise: Problem: If cardiac muscle cells were prevented from undergoing aerobic metabolism, they ultimately would a. undergo glycolysis b. synthesize ATP c. stop contracting d. start contracting Solution: Critical Thinking Questions Exercise: Problem: What would be the drawback of cardiac contractions being the same duration as skeletal muscle contractions? Solution: An action potential could reach a cardiac muscle cell before it has entered the relaxation phase, resulting in the sustained contractions of tetanus. If this happened, the heart would not beat regularly. Exercise: Problem: How are cardiac muscle cells similar to and different from skeletal muscle cells? Solution: Cardiac and skeletal muscle cells both contain ordered myofibrils and are striated. Cardiac muscle cells are branched and contain intercalated discs, which skeletal muscles do not have. Glossary autorhythmicity heart’s ability to control its own contractions desmosome cell structure that anchors the ends of cardiac muscle fibers to allow contraction to occur intercalated disc part of the sarcolemma that connects cardiac tissue, and contains gap junctions and desmosomes Smooth Muscle By the end of this section, you will be able to: e Describe a dense body e Explain how smooth muscle works with internal organs and passageways through the body e Explain how smooth muscles differ from skeletal and cardiac muscles e Explain the difference between single-unit and multi-unit smooth muscle Smooth muscle (so-named because the cells do not have striations) is present in the walls of hollow organs like the urinary bladder, uterus, stomach, intestines, and in the walls of passageways, such as the arteries and veins of the circulatory system, and the tracts of the respiratory, urinary, and reproductive systems ({link]ab). Smooth muscle is also present in the eyes, where it functions to change the size of the iris and alter the shape of the lens; and in the skin where it causes hair to stand erect in response to cold temperature or fear. Smooth Muscle Tissue Autonomic neurons Nucleus Muscle fibers (a) (b) Smooth muscle tissue is found around organs in the digestive, respiratory, reproductive tracts and the iris of the eye. LM x 1600. (Micrograph provided by the Regents of University of Michigan Medical School © 2012) Note: — 7 mess Openstax COLLEGE View the University of Michigan WebScope to explore the tissue sample in greater detail. Smooth muscle fibers are spindle-shaped (wide in the middle and tapered at both ends, somewhat like a football) and have a single nucleus; they range from about 30 to 200 pm (thousands of times shorter than skeletal muscle fibers), and they produce their own connective tissue, endomysium. Although they do not have striations and sarcomeres, smooth muscle fibers do have actin and myosin contractile proteins, and thick and thin filaments. These thin filaments are anchored by dense bodies. A dense body is analogous to the Z-discs of skeletal and cardiac muscle fibers and is fastened to the sarcolemma. Calcium ions are supplied by the SR in the fibers and by sequestration from the extracellular fluid through membrane indentations called calveoli. Because smooth muscle cells do not contain troponin, cross-bridge formation is not regulated by the troponin-tropomyosin complex but instead by the regulatory protein calmodulin. In a smooth muscle fiber, external Ca™* ions passing through opened calcium channels in the sarcolemma, and additional Ca** released from SR, bind to calmodulin. The Ca**- calmodulin complex then activates an enzyme called myosin (light chain) kinase, which, in turn, activates the myosin heads by phosphorylating them (converting ATP to ADP and P;, with the P; attaching to the head). The heads can then attach to actin-binding sites and pull on the thin filaments. The thin filaments also are anchored to the dense bodies; the structures invested in the inner membrane of the sarcolemma (at adherens junctions) that also have cord-like intermediate filaments attached to them. When the thin filaments slide past the thick filaments, they pull on the dense bodies, structures tethered to the sarcolemma, which then pull on the intermediate filaments networks throughout the sarcoplasm. This arrangement causes the entire muscle fiber to contract in a manner whereby the ends are pulled toward the center, causing the midsection to bulge in a corkscrew motion ([link]). Muscle Contraction Relaxed muscle cell Contracted muscle cell —> = Intermediate filaments Dense bodies The dense bodies and intermediate filaments are networked through the sarcoplasm, which cause the muscle fiber to contract. Although smooth muscle contraction relies on the presence of Ca** ions, smooth muscle fibers have a much smaller diameter than skeletal muscle cells. T-tubules are not required to reach the interior of the cell and therefore not necessary to transmit an action potential deep into the fiber. Smooth muscle fibers have a limited calcium-storing SR but have calcium channels in the sarcolemma (similar to cardiac muscle fibers) that open during the action potential along the sarcolemma. The influx of extracellular Ca** ions, which diffuse into the sarcoplasm to reach the calmodulin, accounts for most of the Ca** that triggers contraction of a smooth muscle cell. Muscle contraction continues until ATP-dependent calcium pumps actively transport Ca** ions back into the SR and out of the cell. However, a low concentration of calcium remains in the sarcoplasm to maintain muscle tone. This remaining calcium keeps the muscle slightly contracted, which is important in certain tracts and around blood vessels. Because most smooth muscles must function for long periods without rest, their power output is relatively low, but contractions can continue without using large amounts of energy. Some smooth muscle can also maintain contractions even as Ca** is removed and myosin kinase is inactivated/dephosphorylated. This can happen as a subset of cross-bridges between myosin heads and actin, called latch-bridges, keep the thick and thin filaments linked together for a prolonged period, and without the need for ATP. This allows for the maintaining of muscle “tone” in smooth muscle that lines arterioles and other visceral organs with very little energy expenditure. Smooth muscle is not under voluntary control; thus, it is called involuntary muscle. The triggers for smooth muscle contraction include hormones, neural stimulation by the ANS, and local factors. In certain locations, such as the walls of visceral organs, stretching the muscle can trigger its contraction (the stress-relaxation response). Axons of neurons in the ANS do not form the highly organized NMJs with smooth muscle, as seen between motor neurons and skeletal muscle fibers. Instead, there is a series of neurotransmitter-filled bulges called varicosities as an axon courses through smooth muscle, loosely forming motor units ({link]). A varicosity releases neurotransmitters into the synaptic cleft. Also, visceral muscle in the walls of the hollow organs (except the heart) contains pacesetter cells. A pacesetter cell can spontaneously trigger action potentials and contractions in the muscle. Motor Units Varicosity Vesicles with neurotransmitters . Autonomic neuron —— Smooth muscle cells A series of axon-like swelling, called varicosities or “boutons,” from autonomic neurons form motor units through the smooth muscle. Smooth muscle is organized in two ways: as single-unit smooth muscle, which is much more common; and as multiunit smooth muscle. The two types have different locations in the body and have different characteristics. Single-unit muscle has its muscle fibers joined by gap junctions so that the muscle contracts as a single unit. This type of smooth muscle is found in the walls of all visceral organs except the heart (which has cardiac muscle in its walls), and so it is commonly called visceral muscle. Because the muscle fibers are not constrained by the organization and stretchability limits of sarcomeres, visceral smooth muscle has a stress-relaxation response. This means that as the muscle of a hollow organ is stretched when it fills, the mechanical stress of the stretching will trigger contraction, but this is immediately followed by relaxation so that the organ does not empty its contents prematurely. This is important for hollow organs, such as the stomach or urinary bladder, which continuously expand as they fill. The smooth muscle around these organs also can maintain a muscle tone when the organ empties and shrinks, a feature that prevents “flabbiness” in the empty organ. In general, visceral smooth muscle produces slow, steady contractions that allow substances, such as food in the digestive tract, to move through the body. Multiunit smooth muscle cells rarely possess gap junctions, and thus are not electrically coupled. As a result, contraction does not spread from one cell to the next, but is instead confined to the cell that was originally stimulated. Stimuli for multiunit smooth muscles come from autonomic nerves or hormones but not from stretching. This type of tissue is found around large blood vessels, in the respiratory airways, and in the eyes. Hyperplasia in Smooth Muscle Similar to skeletal and cardiac muscle cells, smooth muscle can undergo hypertrophy to increase in size. Unlike other muscle, smooth muscle can also divide to produce more cells, a process called hyperplasia. This can most evidently be observed in the uterus at puberty, which responds to increased estrogen levels by producing more uterine smooth muscle fibers, and greatly increases the size of the myometrium. Sections Summary Smooth muscle is found throughout the body around various organs and tracts. Smooth muscle cells have a single nucleus, and are spindle-shaped. Smooth muscle cells can undergo hyperplasia, mitotically dividing to produce new cells. The smooth cells are nonstriated, but their sarcoplasm is filled with actin and myosin, along with dense bodies in the sarcolemma to anchor the thin filaments and a network of intermediate filaments involved in pulling the sarcolemma toward the fiber’s middle, shortening it in the process. Ca** ions trigger contraction when they are released from SR and enter through opened voltage-gated calcium channels. Smooth muscle contraction is initiated when the Ca‘™ binds to intracellular calmodulin, which then activates an enzyme called myosin kinase that phosphorylates myosin heads so they can form the cross-bridges with actin and then pull on the thin filaments. Smooth muscle can be stimulated by pacesetter cells, by the autonomic nervous system, by hormones, spontaneously, or by stretching. The fibers in some smooth muscle have latch-bridges, cross- bridges that cycle slowly without the need for ATP; these muscles can maintain low-level contractions for long periods. Single-unit smooth muscle tissue contains gap junctions to synchronize membrane depolarization and contractions so that the muscle contracts as a single unit. Single-unit smooth muscle in the walls of the viscera, called visceral muscle, has a stress-relaxation response that permits muscle to stretch, contract, and relax as the organ expands. Multiunit smooth muscle cells do not possess gap junctions, and contraction does not spread from one cell to the next. Multiple Choice Exercise: Problem: Smooth muscles differ from skeletal and cardiac muscles in that they a. lack myofibrils b. are under voluntary control c. lack myosin d. lack actin Solution: A Exercise: Problem: Which of the following statements describes smooth muscle cells? a. They are resistant to fatigue. b. They have a rapid onset of contractions. c. They cannot exhibit tetanus. d. They primarily use anaerobic metabolism. Solution: A Critical Thinking Questions Exercise: Problem: Why can smooth muscles contract over a wider range of resting lengths than skeletal and cardiac muscle? Solution: Smooth muscles can contract over a wider range of resting lengths because the actin and myosin filaments in smooth muscle are not as rigidly organized as those in skeletal and cardiac muscle. Exercise: Problem: Describe the differences between single-unit smooth muscle and multiunit smooth muscle. Solution: Single-unit smooth muscle is found in the walls of hollow organs; multiunit smooth muscle is found in airways to the lungs and large arteries. Single-unit smooth muscle cells contract synchronously, they are coupled by gap junctions, and they exhibit spontaneous action potential. Multiunit smooth cells lack gap junctions, and their contractions are not synchronous. Glossary calmodulin regulatory protein that facilitates contraction in smooth muscles dense body sarcoplasmic structure that attaches to the sarcolemma and shortens the muscle as thin filaments slide past thick filaments hyperplasia process in which one cell splits to produce new cells latch-bridges subset of a cross-bridge in which actin and myosin remain locked together pacesetter cell cell that triggers action potentials in smooth muscle stress-relaxation response relaxation of smooth muscle tissue after being stretched varicosity enlargement of neurons that release neurotransmitters into synaptic clefts visceral muscle smooth muscle found in the walls of visceral organs Development and Regeneration of Muscle Tissue By the end of this section, you will be able to: e Describe the function of satellite cells e Define fibrosis e Explain which muscle has the greatest regeneration ability Most muscle tissue of the body arises from embryonic mesoderm. Paraxial mesodermal cells adjacent to the neural tube form blocks of cells called somites. Skeletal muscles, excluding those of the head and limbs, develop from mesodermal somites, whereas skeletal muscle in the head and limbs develop from general mesoderm. Somites give rise to myoblasts. A myoblast is a muscle-forming stem cell that migrates to different regions in the body and then fuse(s) to form a syncytium, or myotube. As a myotube is formed from many different myoblast cells, it contains many nuclei, but has a continuous cytoplasm. This is why skeletal muscle cells are multinucleate, as the nucleus of each contributing myoblast remains intact in the mature skeletal muscle cell. However, cardiac and smooth muscle cells are not multinucleate because the myoblasts that form their cells do not fuse. Gap junctions develop in the cardiac and single-unit smooth muscle in the early stages of development. In skeletal muscles, ACh receptors are initially present along most of the surface of the myoblasts, but spinal nerve innervation causes the release of growth factors that stimulate the formation of motor end-plates and NMJs. As neurons become active, electrical signals that are sent through the muscle influence the distribution of slow and fast fibers in the muscle. Although the number of muscle cells is set during development, satellite cells help to repair skeletal muscle cells. A satellite cell is similar to a myoblast because it is a type of stem cell; however, satellite cells are incorporated into muscle cells and facilitate the protein synthesis required for repair and growth. These cells are located outside the sarcolemma and are stimulated to grow and fuse with muscle cells by growth factors that are released by muscle fibers under certain forms of stress. Satellite cells can regenerate muscle fibers to a very limited extent, but they primarily help to repair damage in living cells. If a cell is damaged to a greater extent than can be repaired by satellite cells, the muscle fibers are replaced by scar tissue in a process called fibrosis. Because scar tissue cannot contract, muscle that has sustained significant damage loses strength and cannot produce the same amount of power or endurance as it could before being damaged. Smooth muscle tissue can regenerate from a type of stem cell called a pericyte, which is found in some small blood vessels. Pericytes allow smooth muscle cells to regenerate and repair much more readily than skeletal and cardiac muscle tissue. Similar to skeletal muscle tissue, cardiac muscle does not regenerate to a great extent. Dead cardiac muscle tissue is replaced by scar tissue, which cannot contract. As scar tissue accumulates, the heart loses its ability to pump because of the loss of contractile power. However, some minor regeneration may occur due to stem cells found in the blood that occasionally enter cardiac tissue. Note: Career Connections Physical Therapist As muscle cells die, they are not regenerated but instead are replaced by connective tissue and adipose tissue, which do not possess the contractile abilities of muscle tissue. Muscles atrophy when they are not used, and over time if atrophy is prolonged, muscle cells die. It is therefore important that those who are susceptible to muscle atrophy exercise to maintain muscle function and prevent the complete loss of muscle tissue. In extreme cases, when movement is not possible, electrical stimulation can be introduced to a muscle from an external source. This acts as a substitute for endogenous neural stimulation, stimulating the muscle to contract and preventing the loss of proteins that occurs with a lack of use. Physiotherapists work with patients to maintain muscles. They are trained to target muscles susceptible to atrophy, and to prescribe and monitor exercises designed to stimulate those muscles. There are various causes of atrophy, including mechanical injury, disease, and age. After breaking a limb or undergoing surgery, muscle use is impaired and can lead to disuse atrophy. If the muscles are not exercised, this atrophy can lead to long-term muscle weakness. A stroke can also cause muscle impairment by interrupting neural stimulation to certain muscles. Without neural inputs, these muscles do not contract and thus begin to lose structural proteins. Exercising these muscles can help to restore muscle function and minimize functional impairments. Age-related muscle loss is also a target of physical therapy, as exercise can reduce the effects of age-related atrophy and improve muscle function. The goal of a physiotherapist is to improve physical functioning and reduce functional impairments; this is achieved by understanding the cause of muscle impairment and assessing the capabilities of a patient, after which a program to enhance these capabilities is designed. Some factors that are assessed include strength, balance, and endurance, which are continually monitored as exercises are introduced to track improvements in muscle function. Physiotherapists can also instruct patients on the proper use of equipment, such as crutches, and assess whether someone has sufficient strength to use the equipment and when they can function without it. Chapter Review Muscle tissue arises from embryonic mesoderm. Somites give rise to myoblasts and fuse to form a myotube. The nucleus of each contributing myoblast remains intact in the mature skeletal muscle cell, resulting in a mature, multinucleate cell. Satellite cells help to repair skeletal muscle cells. Smooth muscle tissue can regenerate from stem cells called pericytes, whereas dead cardiac muscle tissue is replaced by scar tissue. Aging causes muscle mass to decrease and be replaced by noncontractile connective tissue and adipose tissue. Review Questions Exercise: Problem: From which embryonic cell type does muscle tissue develop? a. ganglion cells b. myotube cells c. myoblast cells d. satellite cells Solution: C Exercise: Problem: Which cell type helps to repair injured muscle fibers? a. ganglion cells b. myotube cells c. myoblast cells d. satellite cells Solution: D Critical Thinking Questions Exercise: Problem: Why is muscle that has sustained significant damage unable to produce the same amount of power as it could before being damaged? Solution: If the damage exceeds what can be repaired by satellite cells, the damaged tissue is replaced by scar tissue, which cannot contract. Exercise: Problem: Which muscle type(s) (skeletal, smooth, or cardiac) can regenerate new muscle cells/fibers? Explain your answer. Solution: Smooth muscle tissue can regenerate from stem cells called pericytes, cells found in some small blood vessels. These allow smooth muscle cells to regenerate and repair much more readily than skeletal and cardiac muscle tissue. Glossary fibrosis replacement of muscle fibers by scar tissue myoblast muscle-forming stem cell myotube fusion of many myoblast cells pericyte stem cell that regenerates smooth muscle cells satellite cell stem cell that helps to repair muscle cells somites blocks of paraxial mesoderm cells Interactions of Skeletal Muscles, Their Fascicle Arrangement, and Their Lever Systems By the end of this section, you will be able to: e Compare and contrast agonist and antagonist muscles e Describe how fascicles are arranged within a skeletal muscle e Explain the major events of a skeletal muscle contraction within a muscle in generating force To move the skeleton, the tension created by the contraction of the fibers in most skeletal muscles is transferred to the tendons. The tendons are strong bands of dense, regular connective tissue that connect muscles to bones. The bone connection is why this muscle tissue is called skeletal muscle. Interactions of Skeletal Muscles in the Body To pull on a bone, that is, to change the angle at its synovial joint, which essentially moves the skeleton, a skeletal muscle must also be attached to a fixed part of the skeleton. The moveable end of the muscle that attaches to the bone being pulled is called the muscle’s insertion, and the end of the muscle attached to a fixed (stabilized) bone is called the origin. During forearm flexion—bending the elbow—the brachioradialis assists the brachialis. Although a number of muscles may be involved in an action, the principal muscle involved is called the prime mover, or agonist. To lift a cup, a muscle called the biceps brachii is actually the prime mover; however, because it can be assisted by the brachialis, the brachialis is called a synergist in this action ([link]). A synergist can also be a fixator that stabilizes the bone that is the attachment for the prime mover’s origin. Prime Movers and Synergists Biceps brachii (prime mover) Brachioradialis (synergist) Biceps brachii (dissected) Brachialis (synergist) Brachioradialis The biceps brachii flex the lower arm. The brachoradialis, in the forearm, and brachialis, located deep to the biceps in the upper arm, are both synergists that aid in this motion. A muscle with the opposite action of the prime mover is called an antagonist. Antagonists play two important roles in muscle function: (1) they maintain body or limb position, such as holding the arm out or standing erect; and (2) they control rapid movement, as in shadow boxing without landing a punch or the ability to check the motion of a limb. For example, to extend the knee, a group of four muscles called the quadriceps femoris in the anterior compartment of the thigh are activated (and would be called the agonists of knee extension). However, to flex the knee joint, an opposite or antagonistic set of muscles called the hamstrings is activated. As you can see, these terms would also be reversed for the opposing action. If you consider the first action as the knee bending, the hamstrings would be called the agonists and the quadriceps femoris would then be called the antagonists. See [link] for a list of some agonists and antagonists. Agonist and Antagonist Skeletal Muscle Pairs Agonist Biceps brachii: in the anterior compartment of the arm Hamstrings: group of three muscles in the posterior compartment of the thigh Flexor digitorum superficialis and flexor digitorum profundus: in the anterior compartment of the forearm Antagonist Triceps brachii: in the posterior compartment of the arm Quadriceps femoris: group of four muscles in the anterior compartment of the thigh Extensor digitorum: in the posterior compartment of the forearm Movement The biceps brachii flexes the forearm, whereas the triceps brachii extends it. The hamstrings flex the leg, whereas the quadriceps femoris extend it. The flexor digitorum superficialis and flexor digitorum profundus flex the fingers and the hand at the wrist, whereas the extensor digitorum extends the fingers and the hand at the wrist. There are also skeletal muscles that do not pull against the skeleton for movements. For example, there are the muscles that produce facial expressions. The insertions and origins of facial muscles are in the skin, so that certain individual muscles contract to form a smile or frown, form sounds or words, and raise the eyebrows. There also are skeletal muscles in the tongue, and the external urinary and anal sphincters that allow for voluntary regulation of urination and defecation, respectively. In addition, the diaphragm contracts and relaxes to change the volume of the pleural cavities but it does not move the skeleton to do this. Note: Everyday Connections Exercise and Stretching When exercising, it is important to first warm up the muscles. Stretching pulls on the muscle fibers and it also results in an increased blood flow to the muscles being worked. Without a proper warm-up, it is possible that you may either damage some of the muscle fibers or pull a tendon. A pulled tendon, regardless of location, results in pain, swelling, and diminished function; if it is moderate to severe, the injury could immobilize you for an extended period. Recall the discussion about muscles crossing joints to create movement. Most of the joints you use during exercise are synovial joints, which have synovial fluid in the joint space between two bones. Exercise and stretching may also have a beneficial effect on synovial joints. Synovial fluid is a thin, but viscous film with the consistency of egg whites. When you first get up and start moving, your joints feel stiff for a number of reasons. After proper stretching and warm-up, the synovial fluid may become less viscous, allowing for better joint function. Patterns of Fascicle Organization Skeletal muscle is enclosed in connective tissue scaffolding at three levels. Each muscle fiber (cell) is covered by endomysium and the entire muscle is covered by epimysium. When a group of muscle fibers is “bundled” as a unit within the whole muscle by an additional covering of a connective tissue called perimysium, that bundled group of muscle fibers is called a fascicle. Fascicle arrangement by perimysia is correlated to the force generated by a muscle; it also affects the range of motion of the muscle. Based on the patterns of fascicle arrangement, skeletal muscles can be classified in several ways. What follows are the most common fascicle arrangements. Parallel muscles have fascicles that are arranged in the same direction as the long axis of the muscle ([link]). The majority of skeletal muscles in the body have this type of organization. Some parallel muscles are flat sheets that expand at the ends to make broad attachments. Other parallel muscles are rotund with tendons at one or both ends. Muscles that seem to be plump have a large mass of tissue located in the middle of the muscle, between the insertion and the origin, which is known as the central body. A more common name for this muscle is belly. When a muscle contracts, the contractile fibers shorten it to an even larger bulge. For example, extend and then flex your biceps brachii muscle; the large, middle section is the belly ({link]). When a parallel muscle has a central, large belly that is spindle- shaped, meaning it tapers as it extends to its origin and insertion, it sometimes is called fusiform. Muscle Shapes and Fiber Alignment Orbicularis oris Circular Deltoid Multipennate Convergent Extensor digitorum Biceps brachii (posterior view) To origin Parallel- Belly fusiform Unipennate To insertion —— Rectus femoris Bipennate \\ Parallel ( (non-fusiform) ma { The skeletal muscles of the body typically come in seven different general shapes. Biceps Brachii Muscle Contraction The large mass at the center of a muscle is called the belly. Tendons emerge from both ends of the belly and connect the muscle to the bones, allowing the skeleton to move. The tendons of the bicep connect to the upper arm and the forearm. (credit: Victoria Garcia) Circular muscles are also called sphincters (see [link]). When they relax, the sphincters’ concentrically arranged bundles of muscle fibers increase the size of the opening, and when they contract, the size of the opening shrinks to the point of closure. The orbicularis oris muscle is a circular muscle that goes around the mouth. When it contracts, the oral opening becomes smaller, as when puckering the lips for whistling. Another example is the orbicularis oculi, one of which surrounds each eye. Consider, for example, the names of the two orbicularis muscles (orbicularis oris and oribicularis oculi), where part of the first name of both muscles is the same. The first part of orbicularis, orb (orb = “circular”), is a reference to a round or circular structure; it may also make one think of orbit, such as the moon’s path around the earth. The word oris (oris = “oral”) refers to the oral cavity, or the mouth. The word oculi (ocular = “eye’”) refers to the eye. There are other muscles throughout the body named by their shape or location. The deltoid is a large, triangular-shaped muscle that covers the shoulder. It is so-named because the Greek letter delta looks like a triangle. The rectus abdomis (rector = “straight”) is the straight muscle in the anterior wall of the abdomen, while the rectus femoris is the straight muscle in the anterior compartment of the thigh. When a muscle has a widespread expansion over a sizable area, but then the fascicles come to a single, common attachment point, the muscle is called convergent. The attachment point for a convergent muscle could be a tendon, an aponeurosis (a flat, broad tendon), or a raphe (a very slender tendon). The large muscle on the chest, the pectoralis major, is an example of a convergent muscle because it converges on the greater tubercle of the humerus via a tendon. The temporalis muscle of the cranium is another. Pennate muscles (penna = “feathers”) blend into a tendon that runs through the central region of the muscle for its whole length, somewhat like the quill of a feather with the muscle arranged similar to the feathers. Due to this design, the muscle fibers in a pennate muscle can only pull at an angle, and as a result, contracting pennate muscles do not move their tendons very far. However, because a pennate muscle generally can hold more muscle fibers within it, it can produce relatively more tension for its size. There are three subtypes of pennate muscles. In a unipennate muscle, the fascicles are located on one side of the tendon. The extensor digitorum of the forearm is an example of a unipennate muscle. A bipennate muscle has fascicles on both sides of the tendon. In some pennate muscles, the muscle fibers wrap around the tendon, sometimes forming individual fascicles in the process. This arrangement is referred to as multipennate. A common example is the deltoid muscle of the shoulder, which covers the shoulder but has a single tendon that inserts on the deltoid tuberosity of the humerus. Because of fascicles, a portion of a multipennate muscle like the deltoid can be stimulated by the nervous system to change the direction of the pull. For example, when the deltoid muscle contracts, the arm abducts (moves away from midline in the sagittal plane), but when only the anterior fascicle is stimulated, the arm will abduct and flex (move anteriorly at the shoulder joint). The Lever System of Muscle and Bone Interactions Skeletal muscles do not work by themselves. Muscles are arranged in pairs based on their functions. For muscles attached to the bones of the skeleton, the connection determines the force, speed, and range of movement. These characteristics depend on each other and can explain the general organization of the muscular and skeletal systems. The skeleton and muscles act together to move the body. Have you ever used the back of a hammer to remove a nail from wood? The handle acts as a lever and the head of the hammer acts as a fulcrum, the fixed point that the force is applied to when you pull back or push down on the handle. The effort applied to this system is the pulling or pushing on the handle to remove the nail, which is the load, or “resistance” to the movement of the handle in the system. Our musculoskeletal system works in a similar manner, with bones being stiff levers and the articular endings of the bones —encased in synovial joints—acting as fulcrums. The load would be an object being lifted or any resistance to a movement (your head is a load when you are lifting it), and the effort, or applied force, comes from contracting skeletal muscle. Chapter Review Skeletal muscles each have an origin and an insertion. The end of the muscle that attaches to the bone being pulled is called the muscle’s insertion and the end of the muscle attached to a fixed, or stabilized, bone is called the origin. The muscle primarily responsible for a movement is called the prime mover, and muscles that assist in this action are called synergists. A synergist that makes the insertion site more stable is called a fixator. Meanwhile, a muscle with the opposite action of the prime mover is called an antagonist. Several factors contribute to the force generated by a skeletal muscle. One is the arrangement of the fascicles in the skeletal muscle. Fascicles can be parallel, circular, convergent, pennate, fusiform, or triangular. Each arrangement has its own range of motion and ability to do work. Review Questions Exercise: Problem: Which of the following is unique to the muscles of facial expression? a. They all originate from the scalp musculature. b. They insert onto the cartilage found around the face. c. They only insert onto the facial bones. d. They insert into the skin. Solution: D Exercise: Problem: Which of the following helps an agonist work? a. a Synergist b. a fixator c. an insertion d. an antagonist Solution: A Exercise: Problem: Which of the following statements is correct about what happens during flexion? a. The angle between bones is increased. b. The angle between bones is decreased. c. The bone moves away from the body. d. The bone moves toward the center of the body. Solution: B Exercise: Problem: Which is moved the /east during muscle contraction? a. the origin b. the insertion c. the ligaments d. the joints Solution: A Exercise: Problem: Which muscle has a convergent pattern of fascicles? a. biceps brachii b. gluteus maximus c. pectoralis major d. rectus femoris Solution: C Exercise: Problem: A muscle that has a pattern of fascicles running along the long axis of the muscle has which of the following fascicle arrangements? a. Circular b. pennate c. parallel d. rectus Solution: C Exercise: Problem: Which arrangement best describes a bipennate muscle? a. The muscle fibers feed in on an angle to a long tendon from both sides. b. The muscle fibers feed in on an angle to a long tendon from all directions. c. The muscle fibers feed in on an angle to a long tendon from one side. d. The muscle fibers on one side of a tendon feed into it at a certain angle and muscle fibers on the other side of the tendon feed into it at the opposite angle. Solution: A Critical Thinking Questions Exercise: Problem: What effect does fascicle arrangement have on a muscle’s action? Solution: Fascicle arrangements determine what type of movement a muscle can make. For instance, circular muscles act as sphincters, closing orifices. Exercise: Problem: Movements of the body occur at joints. Describe how muscles are arranged around the joints of the body. Solution: Muscles work in pairs to facilitate movement of the bones around the joints. Agonists are the prime movers while antagonists oppose or resist the movements of the agonists. Synergists assist the agonists, and fixators stabilize a muscle’s origin. Exercise: Problem:Explain how a synergist assists an agonist by being a fixator. Solution: Agonists are the prime movers while antagonists oppose or resist the movements of the agonists. Synergists assist the agonists, and fixators stabilize a muscle’s origin. Glossary abduct move away from midline in the sagittal plane agonist (also, prime mover) muscle whose contraction is responsible for producing a particular motion antagonist muscle that opposes the action of an agonist belly bulky central body of a muscle bipennate pennate muscle that has fascicles that are located on both sides of the tendon circular (also, sphincter) fascicles that are concentrically arranged around an opening convergent fascicles that extend over a broad area and converge on a common attachment site fascicle muscle fibers bundled by perimysium into a unit fixator synergist that assists an agonist by preventing or reducing movement at another joint, thereby stabilizing the origin of the agonist flexion movement that decreases the angle of a joint fusiform muscle that has fascicles that are spindle-shaped to create large bellies insertion end of a skeletal muscle that is attached to the structure (usually a bone) that is moved when the muscle contracts multipennate pennate muscle that has a tendon branching within it origin end of a skeletal muscle that is attached to another structure (usually a bone) in a fixed position parallel fascicles that extend in the same direction as the long axis of the muscle pennate fascicles that are arranged differently based on their angles to the tendon prime mover (also, agonist) principle muscle involved in an action synergist muscle whose contraction helps a prime mover in an action unipennate pennate muscle that has fascicles located on one side of the tendon Naming Skeletal Muscles By the end of this section, you will be able to: e Describe the criteria used to name skeletal muscles e Explain how understanding the muscle names helps describe shapes, location, and actions of various muscles The Greeks and Romans conducted the first studies done on the human body in Western culture. The educated class of subsequent societies studied Latin and Greek, and therefore the early pioneers of anatomy continued to apply Latin and Greek terminology or roots when they named the skeletal muscles. The large number of muscles in the body and unfamiliar words can make learning the names of the muscles in the body seem daunting, but understanding the etymology can help. Etymology is the study of how the root of a particular word entered a language and how the use of the word evolved over time. Taking the time to learn the root of the words is crucial to understanding the vocabulary of anatomy and physiology. When you understand the names of muscles it will help you remember where the muscles are located and what they do ({link], [link], and [link]). Pronunciation of words and terms will take a bit of time to master, but after you have some basic information; the correct names and pronunciations will become easier. Overview of the Muscular System ‘ Occipitofrontalis Sternocleidomastoid i ———— (frontal belly) Deltoid Trapezius Pectoralis major Pectoralis minor ae Serratus anterior Rectus abdominis Biceps brachii Abdominal Brachialis external oblique Brachioradialis : Pronator teres Pectineus Flexor carpi radialis Adductor Tensor fasciae latae longus Sartorius llliopsoas Rectus femoris Js Gracilis Vastus lateralis Vastus medialis Soleus and Fibularis longus : gastrocnemius Tibialis anterior a) ip Major muscles of the body. Right side: superficial; left side: deep (anterior view) Occipitofrontalis (occipital belly) Epicranial aponeurosis Splenius capitis Levator scapulae Supraspinatus Rhomboids i Trapezius Teres minor Deltoid Infraspinatus f Latissimus dorsi Teres major : ri Brachioradialis Triceps brachii ee : Extensor carpi radialis Serratus posterior inferior Extensor digitorum External oblique Extensor carpi ulnaris Gluteus medius Flexor carpi ulnaris (dissected) Gluteus maximus 77] (dissected) Semimembranosus “> Gluteus minimus i \ + Gemellus muscles “YN Biceps femoris Semitendinosus Gracilis Gastrocnemius (dissected) Peroneus longus Tibialis posterior Soleus Major muscles of the body. Right side: superficial; left side: deep (posterior view) On the anterior and posterior views of the muscular system above, superficial muscles (those at the surface) are shown on the right side of the body while deep muscles (those underneath the superficial muscles) are shown on the left half of the body. For the legs, superficial muscles are shown in the anterior view while the posterior view shows both superficial and deep muscles. Understanding a Muscle Name from the Latin abductor ab = away from duct = to move a Ieee moves away from A muscle that abductor F diciti diaiti digitus = digit refers to a finger moves the gmt 9 9 9 or toe little finger or minimi oh: toe away pa minimus = : minimi ase little mini, tiny adductor ad = to, toward duct = to move enindscle te moves towards Sdduictor A muscle that diciti diaiti diaitus = diait refers to a finger moves the ont g 9 9 or toe little finger or minimi aE toe toward 8 cent minimus = \ minimi ae little mini, tiny Mnemonic Device for Latin Roots Latin or Greek Example Translation Mnemonic Device ad to; toward ADvance toward your goal ab away from n/a sub under SUBmarines move under water. something A conDUCTOR makes a train ductor that moves move. If you are antisocial, you are anti against against engaging in social activities. epi on top of n/a apo to the side of n/a Mnemonic Device for Latin Roots Example longissimus longus brevis maximus medius minimus rectus multi uni bi/di tri quad Latin or Greek Translation longest long short large medium tiny; little straight many one two three four Mnemonic Device “Longissimus” is longer than the word “long.” long brief max “Medius” and “medium” both begin with “med.” mini To RECTify a situation is to straighten it out. If something is MULTIcolored, it has many colors. A UNIcorn has one horn. If a ring is DIcast, it is made of two metals. TRIple the amount of money is three times as much. QUADruplets are four children born at one birth. Mnemonic Device for Latin Roots Latin or Greek Example Translation Mnemonic Device externus outside EXternal internus inside INternal Anatomists name the skeletal muscles according to a number of criteria, each of which describes the muscle in some way. These include naming the muscle after its shape, its size compared to other muscles in the area, its location in the body or the location of its attachments to the skeleton, how many origins it has, or its action. The skeletal muscle’s anatomical location or its relationship to a particular bone often determines its name. For example, the frontalis muscle is located on top of the frontal bone of the skull. Similarly, the shapes of some muscles are very distinctive and the names, such as orbicularis, reflect the shape. For the buttocks, the size of the muscles influences the names: gluteus maximus (largest), gluteus medius (medium), and the gluteus minimus (smallest). Names were given to indicate length—brevis (short), longus (long)—and to identify position relative to the midline: lateralis (to the outside away from the midline), and medialis (toward the midline). The direction of the muscle fibers and fascicles are used to describe muscles relative to the midline, such as the rectus (straight) abdominis, or the oblique (at an angle) muscles of the abdomen. Some muscle names indicate the number of muscles in a group. One example of this is the quadriceps, a group of four muscles located on the anterior (front) thigh. Other muscle names can provide information as to how many origins a particular muscle has, such as the biceps brachii. The prefix bi indicates that the muscle has two origins and tri indicates three origins. The location of a muscle’s attachment can also appear in its name. When the name of a muscle is based on the attachments, the origin is always named first. For instance, the sternocleidomastoid muscle of the neck has a dual origin on the sternum (sterno) and clavicle (cleido), and it inserts on the mastoid process of the temporal bone. The last feature by which to name a muscle is its action. When muscles are named for the movement they produce, one can find action words in their name. Some examples are flexor (decreases the angle at the joint), extensor (increases the angle at the joint), abductor (moves the bone away from the midline), or adductor (moves the bone toward the midline). Chapter Review Muscle names are based on many characteristics. The location of a muscle in the body is important. Some muscles are named based on their size and location, such as the gluteal muscles of the buttocks. Other muscle names can indicate the location in the body or bones with which the muscle is associated, such as the tibialis anterior. The shapes of some muscles are distinctive; for example, the direction of the muscle fibers is used to describe muscles of the body midline. The origin and/or insertion can also be features used to name a muscle; examples are the biceps brachii, triceps brachii, and the pectoralis major. Review Questions Exercise: Problem: The location of a muscle’s insertion and origin can determine a. action b. the force of contraction c. muscle name d. the load a muscle can carry Solution: A Exercise: Problem: Where is the temporalis muscle located? a. on the forehead b. in the neck c. on the side of the head d. on the chin Solution: c Exercise: Problem:Which muscle name does not make sense? a. extensor digitorum b. gluteus minimus c. biceps femoris d. extensor minimus longus Solution: D Exercise: Problem: Which of the following terms would be used in the name of a muscle that moves the leg away from the body? a. flexor b. adductor c. extensor d. abductor Solution: D Critical Thinking Questions Exercise: Problem: Describe the different criteria that contribute to how skeletal muscles are named. Solution: In anatomy and physiology, many word roots are Latin or Greek. Portions, or roots, of the word give us clues about the function, shape, action, or location of a muscle. Glossary abductor moves the bone away from the midline adductor moves the bone toward the midline bi two brevis short extensor muscle that increases the angle at the joint flexor muscle that decreases the angle at the joint lateralis to the outside longus long maximus largest medialis to the inside medius medium minimus smallest oblique at an angle rectus straight tri three Muscles of the Head, Neck, and Back By the end of this section, you will be able to: e Identify the axial muscles of the face, head, and neck e Identify the movement and function of the face, head, and neck muscles The skeletal muscles are divided into axial (muscles of the trunk and head) and appendicular (muscles of the arms and legs) categories. This system reflects the bones of the skeleton system, which are also arranged in this manner. The axial muscles are grouped based on location, function, or both. Some of the axial muscles may seem to blur the boundaries because they cross over to the appendicular skeleton. The first grouping of the axial muscles you will review includes the muscles of the head and neck, then you will review the muscles of the vertebral column, and finally you will review the oblique and rectus muscles. Muscles That Create Facial Expression The origins of the muscles of facial expression are on the surface of the skull (remember, the origin of a muscle does not move). The insertions of these muscles have fibers intertwined with connective tissue and the dermis of the skin. Because the muscles insert in the skin rather than on bone, when they contract, the skin moves to create facial expression ([link]). Muscles of Facial Expression Epicranial aponeurosis Occipitofrontalis (frontal belly) Corrugator supercilii Orbicularis oculi Occipitofrontalis (occipital belly) Orbicularis oris Facial muscles (anterior view) Facial muscles (lateral view) Many of the muscles of facial expression insert into the skin surrounding the eyelids, nose and mouth, producing facial expressions by moving the skin rather than bones. The orbicularis oris is a circular muscle that moves the lips, and the orbicularis oculi is a circular muscle that closes the eye. The occipitofrontalis muscle moves up the scalp and eyebrows. The muscle has a frontal belly and an occipital (near the occipital bone on the posterior part of the skull) belly. In other words, there is a muscle on the forehead (frontalis) and one on the back of the head (occipitalis), but there is no muscle across the top of the head. Instead, the two bellies are connected by a broad tendon called the epicranial aponeurosis, or galea aponeurosis (galea = “apple”). The physicians originally studying human anatomy thought the skull looked like an apple. A large portion of the face is composed of the buccinator muscle, which compresses the cheek. This muscle allows you to whistle, blow, and suck; and it contributes to the action of chewing. There are several small facial muscles, one of which is the corrugator supercilii, which is the prime mover of the eyebrows. Place your finger on your eyebrows at the point of the bridge of the nose. Raise your eyebrows as if you were surprised and lower your eyebrows as if you were frowning. With these movements, you can feel the action of the corrugator supercilli. Additional muscles of facial expression are presented in [link]. Muscles in Facial Expression Target motion direction Brow Furrowing Skin of scalp Anterior Occipito- Epicraneal Underneath brow frontalis, aponeurosis skin of frontal belly forehead Unfurrowing Skin of scalp Posterior Occipito- Occipital bone; Epicraneal brow frontalis, mastoid process aponeurosis occipital belly (temporal bone) Lowering Skin Inferior Corrugator Frontal bone Skin eyebrows underneath supercilii underneath (e.g., scowling, eyebrows eyebrow frowning) Nose Movement Target Prime mover Origin Insertion Flaring nostrils Nasal cartilage Inferior Nasal bone (pushes nostrils | compression; open when posterior cartilage is compression compressed) Raising Upper lip Elevation Levator labii Maxilla Underneath upper lip superioris skin at corners of the mouth; orbicularis oris Lowering Lower lip Depression Depressor Mandible Underneath lower lip labii inferioris skin of lower lip Opening mouth | Lower jaw Depression, Depressor Mandible Underneath and sliding lateral angulus oris skin at corners lower jaw left of mouth and right Smiling Corners of Lateral Zygomaticus Zygomatic bone Underneath skin at corners of mouth (dimple area); orbicularis oris Shaping of lips Lips Orbicularis Tissue Underneath (as during oris surrounding lips skin at corners speech) of the mouth Lateral Cheeks Lateral Buccinator Maxilla, mandible; | Orbicularis movement of sphenoid bone (via | oris cheeks (e.g., pterygomandibular sucking on a raphae) straw; also used to compress air in mouth while blowing) Pursing of lips Corners of Lateral Risorius Fascia of parotid Underneath by straightening | mouth salivary gland skin at corners them laterally of the mouth Mandible Underneath skin of chin mouth elevation major Protrusion of Lower lip and Protraction Mentalis lower lip (e.g., skin of chin pouting expression) Muscles That Move the Eyes The movement of the eyeball is under the control of the extrinsic eye muscles, which originate outside the eye and insert onto the outer surface of the white of the eye. These muscles are located inside the eye socket and cannot be seen on any part of the visible eyeball ({link] and [link]). If you have ever been to a doctor who held up a finger and asked you to follow it up, down, and to both sides, he or she is checking to make sure your eye muscles are acting in a coordinated pattern. Muscles of the Eyes Superior oblique Levator palpebrae superioris Superior oblique Superior 4 rectus ] Lateral rectus Sphenoid bone ~ eS S33 » Medial : oe ge % we Lateral \ ey Ee “= Ye, rectus i rectus Ss aeet Inferior rectus & 3 ge Sa = Medial rectus Inferior oblique Inferior oblique Inferior rectus (a) Right eye (lateral view) (b) Right eye (anterior view) (a) The extrinsic eye muscles originate outside of the eye on the skull. (b) Each muscle inserts onto the eyeball. Muscles of the Eyes Target motion Prime Movement Target direction mover Moves eyes up and toward Superior nose; rotates Eyeballs (elevates); Superior eyes from 1 medial rectus o’clock to 3 (adducts) o’clock Moves eyes down and Inferior toward nose; Eyeballs (depresses); Inferior rotates eyes medial rectus from 6 o’clock (adducts) to 3 o’clock mea Lateral Lateral away from Eyeballs ‘gee (abducts) rectus Origin Common tendinous ring (ring attaches to optic foramen) Common tendinous ring (ring attaches to optic foramen) Common tendinous ring (ring attaches to optic foramen) Insertion Superior surface of eyeball Inferior surface of eyeball Lateral surface of eyeball Muscles of the Eyes Movement Moves eyes toward nose Moves eyes up and away from nose; rotates eyeball from 12 o’clock to 9 o’clock Moves eyes down and away from nose; rotates eyeball from 6 o’clock to 9 o’clock Opens eyes Closes eyelids Target Eyeballs Eyeballs Eyeballs Upper eyelid Eyelid skin Target motion direction Medial (adducts) Superior (elevates); lateral (abducts) Superior (elevates); lateral (abducts) Superior (elevates) Compression along superior— inferior axis Muscles That Move the Lower Jaw Prime mover Medial rectus Inferior oblique Superior oblique Levator palpabrae superioris Orbicularis oculi Origin Common tendinous ring (ring attaches to optic foramen) Floor of orbit (maxilla) Sphenoid bone Roof of orbit (sphenoid bone) Medial bones composing the orbit Insertion Medial surface of eyeball Surface of eyeball between inferior rectus and lateral rectus Suface of eyeball between superior rectus and lateral rectus Skin of upper eyelids Circumference of orbit In anatomical terminology, chewing is called mastication. Muscles involved in chewing must be able to exert enough pressure to bite through and then chew food before it is swallowed ([link] and [link]). The masseter muscle is the main muscle used for chewing because it elevates the mandible (lower jaw) to close the mouth, and it is assisted by the temporalis muscle, which retracts the mandible. You can feel the temporalis move by putting your fingers to your temple as you chew. Muscles That Move the Lower Jaw Lateral pterygoid Area of superficial muscle dissection Medial pterygoid Chewing muscles (superficial) Chewing muscles (deep) The muscles that move the lower jaw are typically located within the cheek and originate from processes in the skull. This provides the jaw muscles with the large amount of leverage needed for chewing. Muscles of the Lower Jaw Target motion Prime Movement Target direction mover Origin Insertion Maxilla . : arch; ea ou Mandible supenor Masseter zygomatic Mandible aids chewing (elevates) arch (for masseter) Closes mouth; Superior pulls Lower jaw Mandible (elevates); Temporalis Jemporal Mandible in under upper posterior bone jaw (retracts) Inferior Opens mouth; (depresses); pushes lower posterior Pterygoid jaw out under Mandible (protracts); Lateral process of Mandible upper jaw; lateral pterygoid sphenoid moves lower (abducts); bone jaw side-to-side medial (adducts) Muscles of the Lower Jaw Target motion Prime Movement Target direction mover Origin Insertion Superior Closes mouth; (elevates); pushes lower posterior . Mandible; : . Sphenoid jaw out under : (protracts); Medial : temporo- : Mandible : bone; : upper jaw; lateral pterygoid faawilla mandibular moves lower (abducts); joint jaw side-to-side medial (adducts) Although the masseter and temporalis are responsible for elevating and closing the jaw to break food into digestible pieces, the medial pterygoid and lateral pterygoid muscles provide assistance in chewing and moving food within the mouth. Muscles That Move the Tongue Although the tongue is obviously important for tasting food, it is also necessary for mastication, deglutition (swallowing), and speech ({link] and [link]). Because it is so moveable, the tongue facilitates complex speech patterns and sounds. Muscles that Move the Tongue Styloglossus ~~ Pharyngopalatine arch Dorsal surface Palatine tonsil of tongue Palatoglossus Buccinator Hyoglossus i Fungiform Valate Z ; i papilla papilla Fe Mandible bone Genioglossus (a) Extrinsic tongue muscles (b) Palatoglossus and surface of tongue Muscles for Tongue Movement, Swallowing, and Speech Target motion Prime ene ae | Tgecnaren | Rome | onan | insertion | Tongue Moves tongue down; sticks Tongue Inferior (depresses); | Genioglossus | Mandible Tongue tongue out of mouth anterior (protracts) undersurface; hyoid bone Moves tongue up; retracts Tongue Superior (elevates); | Styloglossus Temporal Tongue tongue back into mouth posterior (retracts) bone (styloid | undersurface process) and sides Flattens tongue Tongue Inferior (depresses) | Hyoglossus Hyoid bone Sides of tongue Bulges tongue Tongue Superior (elevation) | Palatoglossus | Soft palate tongue Swallowing and speaking Raises the hyoid bone in a way Hyoid bone; | Superior (elevates) Digastric Mandible; Hyoid bone that also raises the larynx, larynx temporal allowing the epiglottis to cover bone the glottis during deglutition; also assists in opening the mouth by depressing the mandible Raises and retracts the hyoid Hyoid bone | Superior (elevates); | Stylohyoid Temporal Hyoid bone bone in a way that elongates posterior (retracts) bone (styloid the oral cavity during deglutition process) Mylohyoid Mandible Hyoid bone; Raises hyoid bone in a way Hyoid bone | Superior (elevates) that presses tongue against the roof of the mouth, pushing food back into the pharynx during deglutition Raises and moves hyoid bone Hyoid bone | Superior (elevates); | Geniohyoid Mandible Hyoid bone forward, widening pharynx anterior (protracts) during deglutition Retracts hyoid bone and Hyoid bone | Inferior (depresses); | Omohyoid Scapula Hyoid bone moves it down during later posterior (retracts) phases of deglutition Depresses the hyoid bone Hyoid bone | Inferior (depresses) | Sternohyoid Clavicle Hyoid bone during swallowing and speaking Shrinks distance between Hyoid bone; | Hyoid bone: inferior | Thyrohyoid Thyroid Hyoid bone thyroid cartilage and hyoid thyroid (depresses); thyroid cartilage bone, allowing production of cartilage cartilage: superior high-pitch vocalizations (elevates) Depresses larynx, thyroid Larynx; Inferior (depresses) | Sternothyroid | Sternum Thyroid cartilage, and hyoid bone to thyroid cartilage create different vocal tones cartilage; hyoid bone median raphe Rotates and tilts head to he Skull; Individually: medial Sternocleid- Sternum; Temporal bone side; tilts head forward cervical rotation; lateral omastoid; clavicle (mastoid vertebrae flexion; bilaterally: semispinalis Process); anterior (flexes) capitis occipital bone Rotates and tilts head to the Skull; Individually: lateral Splenius side; tilts head backwards cervical rotation; lateral capitis; vertebrae flexion; bilaterally: longissimus anterior (flexes) capitis Tongue muscles can be extrinsic or intrinsic. Extrinsic tongue muscles insert into the tongue from outside origins, and the intrinsic tongue muscles insert into the tongue from origins within it. The extrinsic muscles move the whole tongue in different directions, whereas the intrinsic muscles allow the tongue to change its shape (such as, curling the tongue in a loop or flattening it). The extrinsic muscles all include the word root glossus (glossus = “tongue’”’), and the muscle names are derived from where the muscle originates. The genioglossus (genio = “chin”) originates on the mandible and allows the tongue to move downward and forward. The styloglossus originates on the styloid bone, and allows upward and backward motion. The palatoglossus originates on the soft palate to elevate the back of the tongue, and the hyoglossus originates on the hyoid bone to move the tongue downward and flatten it. Note: Everyday Connections Anesthesia and the Tongue Muscles Before surgery, a patient must be made ready for general anesthesia. The normal homeostatic controls of the body are put “on hold” so that the patient can be prepped for surgery. Control of respiration must be switched from the patient’s homeostatic control to the control of the anesthesiologist. The drugs used for anesthesia relax a majority of the body’s muscles. Among the muscles affected during general anesthesia are those that are necessary for breathing and moving the tongue. Under anesthesia, the tongue can relax and partially or fully block the airway, and the muscles of respiration may not move the diaphragm or chest wall. To avoid possible complications, the safest procedure to use on a patient is called endotracheal intubation. Placing a tube into the trachea allows the doctors to maintain a patient’s (open) airway to the lungs and seal the airway off from the oropharynx. Post-surgery, the anesthesiologist gradually changes the mixture of the gases that keep the patient unconscious, and when the muscles of respiration begin to function, the tube is removed. It still takes about 30 minutes for a patient to wake up, and for breathing muscles to regain control of respiration. After surgery, most people have a sore or scratchy throat for a few days. Muscles of the Anterior Neck The muscles of the anterior neck assist in deglutition (swallowing) and speech by controlling the positions of the larynx (voice box), and the hyoid bone, a horseshoe-shaped bone that functions as a solid foundation on which the tongue can move. The muscles of the neck are categorized according to their position relative to the hyoid bone ([{link]). Suprahyoid muscles are superior to it, and the infrahyoid muscles are located inferiorly. Muscles of the Anterior Neck Suprahyoid muscles: Geniohyoid Digastric Mylohyoid Stylohyoid Inferior edge of mandible Styloglossus Hyoid bone Infrahyoid muscles: Thyroid cartilage Thyrohyoid of larynx Omohyoid Thyroid gland J Sternohyoid Sternothyroid Trachea Right and left —e_cclavicles L— ~ Sternum The anterior muscles of the neck facilitate swallowing and speech. The suprahyoid muscles originate from above the hyoid bone in the chin region. The infrahyoid muscles originate below the hyoid bone in the lower neck. Scapula The suprahyoid muscles raise the hyoid bone, the floor of the mouth, and the larynx during deglutition. These include the digastric muscle, which has anterior and posterior bellies that work to elevate the hyoid bone and larynx when one swallows; it also depresses the mandible. The stylohyoid muscle moves the hyoid bone posteriorly, elevating the larynx, and the mylohyoid muscle lifts it and helps press the tongue to the top of the mouth. The geniohyoid depresses the mandible in addition to raising and pulling the hyoid bone anteriorly. The strap-like infrahyoid muscles generally depress the hyoid bone and control the position of the larynx. The omohyoid muscle, which has superior and inferior bellies, depresses the hyoid bone in conjunction with the sternohyoid and thyrohyoid muscles. The thyrohyoid muscle also elevates the larynx’s thyroid cartilage, whereas the sternothyroid depresses it to create different tones of voice. Muscles That Move the Head The head, attached to the top of the vertebral column, is balanced, moved, and rotated by the neck muscles ([link]). When these muscles act unilaterally, the head rotates. When they contract bilaterally, the head flexes or extends. The major muscle that laterally flexes and rotates the head is the sternocleidomastoid. In addition, both muscles working together are the flexors of the head. Place your fingers on both sides of the neck and turn your head to the left and to the right. You will feel the movement originate there. This muscle divides the neck into anterior and posterior triangles when viewed from the side ([link]). Posterior and Lateral Views of the Neck (> = = ; Suboccipital muscles = < ” Splenius a \S capitis (cut) Sternocleidomastoid Levator Levator scapulae | . Longissimus Multifidus . i capitis muscles ‘ \ Acromion process of Semispinalis capitis 1st thoracic vertebrae Sma Scalenes Neck muscles Superficial neck muscles: Deep neck muscles: left (left lateral view) tight side trapezius removed side semispinalis capitis (posterior view) removed (posterior view) The superficial and deep muscles of the neck are responsible for moving the head, cervical vertebrae, and scapulas. Muscles That Move the Head Target motion Movement Target direction Prime mover Origin Insertion Rotates Individually: Temporal and tilts rotates head bone head to the Skull; to opposite Grcmuleidemacuid Sternum; (mastoid side; tilts vertebrae side; clavicle process); head bilaterally: occipital forward flexion bone Muscles That Move the Head Target motion Movement Target direction Prime mover Origin Insertion Individually: Transverse laterally and Rotates flexes and articular and tilts Skull; rotates head Semispinalis capitis processes Occipital head vertebrae to same of cervical bone backward side; and bilaterally: thoracic extension vertebra Individually: Rotates laterally Spinous Temporal and tilts flexes and processes bone head to the Skull; rotates head Sian niccanits of cervical (mastoid side; tilts vertebrae to same P P and process); head side; thoracic occipital backward bilaterally: vertebra bone extension Individually: Transverse Rotates laterally and and tilts flexes and articular Temporal head to the Skull; rotates head Longissimus capitis processes bone side; tilts vertebrae to same of cervical (mastoid head side; and process) backward bilaterally: thoracic extension vertebra Muscles of the Posterior Neck and the Back The posterior muscles of the neck are primarily concerned with head movements, like extension. The back muscles stabilize and move the vertebral column, and are grouped according to the lengths and direction of the fascicles. The splenius muscles originate at the midline and run laterally and superiorly to their insertions. From the sides and the back of the neck, the splenius capitis inserts onto the head region, and the splenius cervicis extends onto the cervical region. These muscles can extend the head, laterally flex it, and rotate it ([link]). Muscles of the Neck and Back Sternocleidomastoid Trapezius Splenius capitis Splenius Splenius cervicis Levator scapulae P Rhomboides minor Rhomboides major Trapezius Medial scalene Anterior scalene Muscles of the neck (left lateral view) Superficial (left side) and deep (right side) muscles of the neck and upper back (posterior view) Semispinalis capitis 4 Longissimus capitis (joined with deep spinalis capitis) ql lliocostalis cervicis Semispinalis S lliocostalis thoracis cervicis , ee Longissimus thoracis Longissimus cervicis lliocostalis lumborum Spinalis thoracis Transverse processes of vertebrae Semispinalis i / Rotator thoracis H brevis Rotator longus Interspinales Short rotator Intertransversarii Deep muscles of the back Deep spinal muscles (posterior view) (multifidus removed) The large, complex muscles of the neck and back move the head, shoulders, and vertebral column. The erector spinae group forms the majority of the muscle mass of the back and it is the primary extensor of the vertebral column. It controls flexion, lateral flexion, and rotation of the vertebral column, and maintains the lumbar curve. The erector spinae comprises the iliocostalis (laterally placed) group, the longissimus (intermediately placed) group, and the spinalis (medially placed) group. The iliocostalis group includes the iliocostalis cervicis, associated with the cervical region; the iliocostalis thoracis, associated with the thoracic region; and the iliocostalis lumborum, associated with the lumbar region. The three muscles of the longissimus group are the longissimus capitis, associated with the head region; the longissimus cervicis, associated with the cervical region; and the longissimus thoracis, associated with the thoracic region. The third group, the spinalis group, comprises the spinalis capitis (head region), the spinalis cervicis (cervical region), and the spinalis thoracis (thoracic region). The transversospinales muscles run from the transverse processes to the spinous processes of the vertebrae. Similar to the erector spinae muscles, the semispinalis muscles in this group are named for the areas of the body with which they are associated. The semispinalis muscles include the semispinalis capitis, the semispinalis cervicis, and the semispinalis thoracis. The multifidus muscle of the lumbar region helps extend and laterally flex the vertebral column. Important in the stabilization of the vertebral column is the segmental muscle group, which includes the interspinales and intertransversarii muscles. These muscles bring together the spinous and transverse processes of each consecutive vertebra. Finally, the scalene muscles work together to flex, laterally flex, and rotate the head. They also contribute to deep inhalation. The scalene muscles include the anterior scalene muscle (anterior to the middle scalene), the middle scalene muscle (the longest, intermediate between the anterior and posterior scalenes), and the posterior scalene muscle (the smallest, posterior to the middle scalene). Chapter Review Muscles are either axial muscles or appendicular. The axial muscles are grouped based on location, function, or both. Some axial muscles cross over to the appendicular skeleton. The muscles of the head and neck are all axial. The muscles in the face create facial expression by inserting into the skin rather than onto bone. Muscles that move the eyeballs are extrinsic, meaning they originate outside of the eye and insert onto it. Tongue muscles are both extrinsic and intrinsic. The genioglossus depresses the tongue and moves it anteriorly; the styloglossus lifts the tongue and retracts it; the palatoglossus elevates the back of the tongue; and the hyoglossus depresses and flattens it. The muscles of the anterior neck facilitate swallowing and speech, stabilize the hyoid bone and position the larynx. The muscles of the neck stabilize and move the head. The sternocleidomastoid divides the neck into anterior and posterior triangles. The muscles of the back and neck that move the vertebral column are complex, overlapping, and can be divided into five groups. The splenius group includes the splenius capitis and the splenius cervicis. The erector spinae has three subgroups. The iliocostalis group includes the iliocostalis cervicis, the iliocostalis thoracis, and the iliocostalis lumborum. The longissimus group includes the longissimus capitis, the longissimus cervicis, and the longissimus thoracis. The spinalis group includes the spinalis capitis, the spinalis cervicis, and the spinalis thoracis. The transversospinales include the semispinalis capitis, semispinalis cervicis, semispinalis thoracis, multifidus, and rotatores. The segmental muscles include the interspinales and intertransversarii. Finally, the scalenes include the anterior scalene, middle scalene, and posterior scalene. Review Questions Exercise: Problem: Which of the following is a prime mover in head flexion? a. occipitofrontalis b. corrugator supercilii c. sternocleidomastoid d. masseter Solution: C Exercise: Problem: Where is the inferior oblique muscle located? a. in the abdomen b. in the eye socket c. in the anterior neck d. in the face Solution: B Exercise: Problem: What is the action of the masseter? a. swallowing b. chewing c. moving the lips d. closing the eye Solution: B Exercise: Problem:The names of the extrinsic tongue muscles commonly end in a. -glottis b. -glossus c. -gluteus d. -hyoid Solution: B Exercise: Problem: What is the function of the erector spinae? a. movement of the arms b. stabilization of the pelvic girdle c. postural support d. rotating of the vertebral column Solution: C Critical Thinking Questions Exercise: Problem:Explain the difference between axial and appendicular muscles. Solution: Axial muscles originate on the axial skeleton (the bones in the head, neck, and core of the body), whereas appendicular muscles originate on the bones that make up the body’s limbs. Exercise: Problem: Describe the muscles of the anterior neck. Solution: The muscles of the anterior neck are arranged to facilitate swallowing and speech. They work on the hyoid bone, with the suprahyoid muscles pulling up and the infrahyoid muscles pulling down. Exercise: Problem: Why are the muscles of the face different from typical skeletal muscle? Solution: Most skeletal muscles create movement by actions on the skeleton. Facial muscles are different in that they create facial movements and expressions by pulling on the skin—no bone movements are involved. Glossary anterior scalene a muscle anterior to the middle scalene appendicular of the arms and legs axial of the trunk and head buccinator muscle that compresses the cheek corrugator supercilii prime mover of the eyebrows deglutition swallowing digastric muscle that has anterior and posterior bellies and elevates the hyoid bone and larynx when one swallows; it also depresses the mandible epicranial aponeurosis (also, galea aponeurosis) flat broad tendon that connects the frontalis and occipitalis erector spinae group large muscle mass of the back; primary extensor of the vertebral column extrinsic eye muscles originate outside the eye and insert onto the outer surface of the white of the eye, and create eyeball movement frontalis front part of the occipitofrontalis muscle genioglossus muscle that originates on the mandible and allows the tongue to move downward and forward geniohyoid muscle that depresses the mandible, and raises and pulls the hyoid bone anteriorly hyoglossus muscle that originates on the hyoid bone to move the tongue downward and flatten it iliocostalis cervicis muscle of the iliocostalis group associated with the cervical region iliocostalis group laterally placed muscles of the erector spinae iliocostalis lumborum muscle of the iliocostalis group associated with the lumbar region iliocostalis thoracis muscle of the iliocostalis group associated with the thoracic region infrahyoid muscles anterior neck muscles that are attached to, and inferior to the hyoid bone lateral pterygoid muscle that moves the mandible from side to side longissimus capitis muscle of the longissimus group associated with the head region longissimus cervicis muscle of the longissimus group associated with the cervical region longissimus group intermediately placed muscles of the erector spinae longissimus thoracis muscle of the longissimus group associated with the thoracic region masseter main muscle for chewing that elevates the mandible to close the mouth mastication chewing medial pterygoid muscle that moves the mandible from side to side middle scalene longest scalene muscle, located between the anterior and posterior scalenes multifidus muscle of the lumbar region that helps extend and laterally flex the vertebral column mylohyoid muscle that lifts the hyoid bone and helps press the tongue to the top of the mouth occipitalis posterior part of the occipitofrontalis muscle occipitofrontalis muscle that makes up the scalp with a frontal belly and an occipital belly omohyoid muscle that has superior and inferior bellies and depresses the hyoid bone orbicularis oculi circular muscle that closes the eye orbicularis oris circular muscle that moves the lips palatoglossus muscle that originates on the soft palate to elevate the back of the tongue posterior scalene smallest scalene muscle, located posterior to the middle scalene scalene muscles flex, laterally flex, and rotate the head; contribute to deep inhalation segmental muscle group interspinales and intertransversarii muscles that bring together the spinous and transverse processes of each consecutive vertebra semispinalis capitis transversospinales muscle associated with the head region semispinalis cervicis transversospinales muscle associated with the cervical region semispinalis thoracis transversospinales muscle associated with the thoracic region spinalis capitis muscle of the spinalis group associated with the head region spinalis cervicis muscle of the spinalis group associated with the cervical region spinalis group medially placed muscles of the erector spinae spinalis thoracis muscle of the spinalis group associated with the thoracic region splenius posterior neck muscles; includes the splenius capitis and splenius cervicis splenius capitis neck muscle that inserts into the head region splenius cervicis neck muscle that inserts into the cervical region sternocleidomastoid major muscle that laterally flexes and rotates the head sternohyoid muscle that depresses the hyoid bone sternothyroid muscle that depresses the larynx’s thyroid cartilage styloglossus muscle that originates on the styloid bone, and allows upward and backward motion of the tongue stylohyoid muscle that elevates the hyoid bone posteriorly suprahyoid muscles neck muscles that are superior to the hyoid bone temporalis muscle that retracts the mandible thyrohyoid muscle that depresses the hyoid bone and elevates the larynx’s thyroid cartilage transversospinales muscles that originate at the transverse processes and insert at the spinous processes of the vertebrae Muscles of the Abdominal Wall and Thorax By the end of this section, you will be able to: e Identify the intrinsic skeletal muscles of the back and neck, and the skeletal muscles of the abdominal wall and thorax e Identify the movement and function of the intrinsic skeletal muscles of the back and neck, and the skeletal muscles of the abdominal wall and thorax It is a complex job to balance the body on two feet and walk upright. The muscles of the vertebral column, thorax, and abdominal wall extend, flex, and stabilize different parts of the body’s trunk. The deep muscles of the core of the body help maintain posture as well as carry out other functions. The brain sends out electrical impulses to these various muscle groups to control posture by alternate contraction and relaxation. This is necessary so that no single muscle group becomes fatigued too quickly. If any one group fails to function, body posture will be compromised. Muscles of the Abdomen There are four pairs of abdominal muscles that cover the anterior and lateral abdominal region and meet at the anterior midline. These muscles of the anterolateral abdominal wall can be divided into four groups: the external obliques, the internal obliques, the transversus abdominis, and the rectus abdominis ([link] and (link). Muscles of the Abdomen Pectoralis major External oblique Latissimus dorsi Rectus Transversus sheath abdominis Anterior serratus muscles External oblique Linea alba (of the rectus sheath) Rectus abdominis (enclosed within rectus sheath) Tendinous intersections (between the anterior \ ea segments of the rectus } Rectus abdominis ; abdominis) Aponeurosis of internal oblique Internal oblique Quadratus lumborum llia of hip bones lliacus Sacrum Psoas major youy (a) The anterior abdominal muscles include the medially located rectus abdominis, which is covered by a sheet of connective tissue called the rectus sheath. On the flanks of the body, medial to the rectus abdominis, the abdominal wall is composed of three layers. The external oblique muscles form the superficial layer, while the internal oblique muscles form the middle layer, and the transverses abdominus forms the deepest layer. (b) The muscles of the lower back move the lumbar spine but also assist in femur movements. Muscles of the Abdomen Target motion Prime Movement Target direction mover Origin Insertion Satan External Ribs Ribs 7— Twisting at waist; also Vertebral P . obliques; ; 10; linea : ; lateral ; 5-12; bending to the side column ; internal Ba alba; flexion ° ilium a obliques ilium Squeezing abdomen nea Sternum; ; F Tlium; : during forceful Abdominal ; Transversus . linea : ; : Compression ; ribs 5— exhalations, defecation, cavity abdominus 10 alba; urination, and childbirth pubis Sternum; Ea ] é R F f ; Sitting up vee Flexion oe ; Pubis ribs 5 column abdominis and 7 . . Vertebral Lateral Quadratus lium; Ri Bending to the side : ribs 5— vertebrae column flexion lumborum 10 LLL4 There are three flat skeletal muscles in the antero-lateral wall of the abdomen. The external oblique, closest to the surface, extend inferiorly and medially, in the direction of sliding one’s four fingers into pants pockets. Perpendicular to it is the intermediate internal oblique, extending superiorly and medially, the direction the thumbs usually go when the other fingers are in the pants pocket. The deep muscle, the transversus abdominis, is arranged transversely around the abdomen, similar to the front of a belt on a pair of pants. This arrangement of three bands of muscles in different orientations allows various movements and rotations of the trunk. The three layers of muscle also help to protect the internal abdominal organs in an area where there is no bone. The linea alba is a white, fibrous band that is made of the bilateral rectus sheaths that join at the anterior midline of the body. These enclose the rectus abdominis muscles (a pair of long, linear muscles, commonly called the “sit-up” muscles) that originate at the pubic crest and symphysis, and extend the length of the body’s trunk. Each muscle is segmented by three transverse bands of collagen fibers called the tendinous intersections. This results in the look of “six-pack abs,” as each segment hypertrophies on individuals at the gym who do many sit-ups. The posterior abdominal wall is formed by the lumbar vertebrae, parts of the ilia of the hip bones, psoas major and iliacus muscles, and quadratus lumborum muscle. This part of the core plays a key role in stabilizing the rest of the body and maintaining posture. Note: Career Connections Physical Therapists Those who have a muscle or joint injury will most likely be sent to a physical therapist (PT) after seeing their regular doctor. PTs have a master’s degree or doctorate, and are highly trained experts in the mechanics of body movements. Many PTs also specialize in sports injuries. If you injured your shoulder while you were kayaking, the first thing a physical therapist would do during your first visit is to assess the functionality of the joint. The range of motion of a particular joint refers to the normal movements the joint performs. The PT will ask you to abduct and adduct, circumduct, and flex and extend the arm. The PT will note the shoulder’s degree of function, and based on the assessment of the injury, will create an appropriate physical therapy plan. The first step in physical therapy will probably be applying a heat pack to the injured site, which acts much like a warm-up to draw blood to the area, to enhance healing. You will be instructed to do a series of exercises to continue the therapy at home, followed by icing, to decrease inflammation and swelling, which will continue for several weeks. When physical therapy is complete, the PT will do an exit exam and send a detailed report on the improved range of motion and return of normal limb function to your doctor. Gradually, as the injury heals, the shoulder will begin to function correctly. A PT works closely with patients to help them get back to their normal level of physical activity. Muscles of the Thorax The muscles of the chest serve to facilitate breathing by changing the size of the thoracic cavity ([link]). When you inhale, your chest rises because the cavity expands. Alternately, when you exhale, your chest falls because the thoracic cavity decreases in size. Muscles of the Thorax Target motion Prime Movement Target direction mover Origin Insertion Sternum; Inhalation; Thoracic Compression; . ribs 6-12; Central : : ; Diaphragm exhalation cavity expansion lumbar tendon vertebrae Muscles of the Thorax Target motion Prime Movement Target direction mover Origin Insertion Rib Rib Elevation superior inferior to F : d External Inhalation;exhalation Ribs (expands : to each each : : intercostals é ; thoracic cavity) intercostal intercostal muscle muscle Movement Rib Rib along inferi : superior/inferior Internal cna cia eaieg Forced exhalation Ribs : : : each to each axis to bring intercostals : : . intercostal intercostal ribs closer muscle muscle together The Diaphragm The change in volume of the thoracic cavity during breathing is due to the alternate contraction and relaxation of the diaphragm ((link]). It separates the thoracic and abdominal cavities, and is dome-shaped at rest. The superior surface of the diaphragm is convex, creating the elevated floor of the thoracic cavity. The inferior surface is concave, creating the curved roof of the abdominal cavity. Muscles of the Diaphragm Central tendon of diaphragm - —— Sternum Vena cava passing through caval opening Esophagus passing through esophageal hiatus Aorta passing through aortic hiatus 12th (floating) ribs Left psoas major Left quadratus lumborum Vertebrae Diaphragm (inferior view) The diaphragm separates the thoracic and abdominal cavities. Defecating, urination, and even childbirth involve cooperation between the diaphragm and abdominal muscles (this cooperation is referred to as the “Valsalva maneuver”). You hold your breath by a steady contraction of the diaphragm; this stabilizes the volume and pressure of the peritoneal cavity. When the abdominal muscles contract, the pressure cannot push the diaphragm up, so it increases pressure on the intestinal tract (defecation), urinary tract (urination), or reproductive tract (childbirth). The inferior surface of the pericardial sac and the inferior surfaces of the pleural membranes (parietal pleura) fuse onto the central tendon of the diaphragm. To the sides of the tendon are the skeletal muscle portions of the diaphragm, which insert into the tendon while having a number of origins including the xiphoid process of the sternum anteriorly, the inferior six ribs and their cartilages laterally, and the lumbar vertebrae and 12th ribs posteriorly. The diaphragm also includes three openings for the passage of structures between the thorax and the abdomen. The inferior vena cava passes through the caval opening, and the esophagus and attached nerves pass through the esophageal hiatus. The aorta, thoracic duct, and azygous vein pass through the aortic hiatus of the posterior diaphragm. The Intercostal Muscles There are three sets of muscles, called intercostal muscles, which span each of the intercostal spaces. The principal role of the intercostal muscles is to assist in breathing by changing the dimensions of the rib cage (link). Intercostal Muscles Clavicle Gi - af Ribs = Pectoralis minor Innermost intercostal Pectoralis major (dissected) Internal intercostal Sternum Serratus anterior External —_ oe intercostals — alt intercostal Internal intercostal The external intercostals are located laterally on the sides of the body. The internal intercostals are located medially near the sternum. The innermost intercostals are located deep to both the internal and external intercostals. The 11 pairs of superficial external intercostal muscles aid in inspiration of air during breathing because when they contract, they raise the rib cage, which expands it. The 11 pairs of internal intercostal muscles, just under the externals, are used for expiration because they draw the ribs together to constrict the rib cage. The innermost intercostal muscles are the deepest, and they act as synergists for the action of the internal intercostals. Muscles of the Pelvic Floor and Perineum The pelvic floor is a muscular sheet that defines the inferior portion of the pelvic cavity. The pelvic diaphragm, spanning anteriorly to posteriorly from the pubis to the coccyx, comprises the levator ani and the ischiococcygeus. Its openings include the anal canal and urethra, and the vagina in women. The large levator ani consists of two skeletal muscles, the pubococcygeus and the iliococcygeus ((link]). The levator ani is considered the most important muscle of the pelvic floor because it supports the pelvic viscera. It resists the pressure produced by contraction of the abdominal muscles so that the pressure is applied to the colon to aid in defecation and to the uterus to aid in childbirth (assisted by the ischiococcygeus, which pulls the coccyx anteriorly). This muscle also creates skeletal muscle sphincters at the urethra and anus. Muscles of the Pelvic Floor Pubic crest Vaginal canal (females only) Rectal canal Sacrum lliac crests Pelvic diaphragm (superior view) The pelvic floor muscles support the pelvic organs, resist intra-abdominal pressure, and work as sphincters for the urethra, rectum, and vagina. The perineum is the diamond-shaped space between the pubic symphysis (anteriorly), the coccyx (posteriorly), and the ischial tuberosities (laterally), lying just inferior to the pelvic diaphragm (levator ani and coccygeus). Divided transversely into triangles, the anterior is the urogenital triangle, which includes the external genitals. The posterior is the anal triangle, which contains the anus ([link]). The perineum is also divided into superficial and deep layers with some of the muscles common to men and women ([(link]). Women also have the compressor urethrae and the sphincter urethrovaginalis, which function to close the vagina. In men, there is the deep transverse perineal muscle that plays a role in ejaculation. Muscles of the Perineum Penis Clitoris Ischiocavernosus Bulbospongiosus (aka bulbocavernosus) Urethra Vagina Transverse perineal muscles Anus External anal sphincter Levator ani Coccyx Gluteus maximus Male perineal muscles: inferior view Female perineal muscles: inferior view The perineum muscles play roles in urination in both sexes, ejaculation in men, and vaginal contraction in women. Muscles of the Perineum Common to Men and Women Target motion ; [tarot [Taman | Primemover | ova | insertion | Movement Defecation; Abdominal Superior Levator ani Pubis; ischium Urethra; anal urination; birth; cavity (resists pubococcygeus; canal; perineal coughing pressure levator ani body; coccyx during iliococcygeus abdominal compression) Superficial muscles None— Perineal body Superficial Ischium Perineal body supports transverse perineal body perineal maintaining anus at center of perineum Involuntary Urethra Compression Bulbospongiosus | Perineal body Perineal response that membrane; compresses corpus urethra when spongiosum excreting urine of penis; deep in both sexes or fascia of penis; while ejaculating clitoris in in males; also female aids in erection of penis in males Compresses Veins of penis Compression Ischiocavernosus | Ischium; ischial | Pubic veins to maintain | and clitoris rami; pubic rami | symphysis; erection of penis corpus in males; erection cavernosum of of clitoris in penis in male; females clitoris of female Deep muscles Voluntarily Urethra Compression External urethral Ischial rami; Male: median compresses sphincter pubic rami raphe; female: urethra during vaginal wall urination External anal sphincter Closes anus Sphincter Anoccoccygeal | Perineal body ligament Chapter Review Made of skin, fascia, and four pairs of muscle, the anterior abdominal wall protects the organs located in the abdomen and moves the vertebral column. These muscles include the rectus abdominis, which extends through the entire length of the trunk, the external oblique, the internal oblique, and the transversus abdominus. The quadratus lumborum forms the posterior abdominal wall. The muscles of the thorax play a large role in breathing, especially the dome-shaped diaphragm. When it contracts and flattens, the volume inside the pleural cavities increases, which decreases the pressure within them. As a result, air will flow into the lungs. The external and internal intercostal muscles span the space between the ribs and help change the shape of the rib cage and the volume-pressure ratio inside the pleural cavities during inspiration and expiration. The perineum muscles play roles in urination in both sexes, ejaculation in men, and vaginal contraction in women. The pelvic floor muscles support the pelvic organs, resist intra-abdominal pressure, and work as sphincters for the urethra, rectum, and vagina. Review Questions Exercise: Problem: Which of the following abdominal muscles is not a part of the anterior abdominal wall? a. quadratus lumborum b. rectus abdominis c. interior oblique d. exterior oblique Solution: A Exercise: Problem: Which muscle pair plays a role in respiration? a. intertransversarii, interspinales b. semispinalis cervicis, semispinalis thoracis c. trapezius, rhomboids d. diaphragm, scalene Solution: D Exercise: Problem: What is the linea alba? a. a small muscle that helps with compression of the abdominal organs b. a long tendon that runs down the middle of the rectus abdominis c. a long band of collagen fibers that connects the hip to the knee d. another name for the tendinous inscription Solution: B Critical Thinking Questions Exercise: Problem: Describe the fascicle arrangement in the muscles of the abdominal wall. How do they relate to each other? Solution: Arranged into layers, the muscles of the abdominal wall are the internal and external obliques, which run on diagonals, the rectus abdominis, which runs straight down the midline of the body, and the transversus abdominis, which wraps across the trunk of the body. Exercise: Problem: What are some similarities and differences between the diaphragm and the pelvic diaphragm? Solution: Both diaphragms are thin sheets of skeletal muscle that horizontally span areas of the trunk. The diaphragm separating the thoracic and abdominal cavities is the primary muscle of breathing. The pelvic diaphragm, consisting of two paired muscles, the coccygeus and the levator ani, forms the pelvic floor at the inferior end of the trunk. Glossary anal triangle posterior triangle of the perineum that includes the anus caval opening opening in the diaphragm that allows the inferior vena cava to pass through; foramen for the vena cava compressor urethrae deep perineal muscle in women deep transverse perineal deep perineal muscle in men diaphragm skeletal muscle that separates the thoracic and abdominal cavities and is dome-shaped at rest external intercostal superficial intercostal muscles that raise the rib cage external oblique superficial abdominal muscle with fascicles that extend inferiorly and medially iliococcygeus muscle that makes up the levator ani along with the pubococcygeus innermost intercostal the deepest intercostal muscles that draw the ribs together intercostal muscles muscles that span the spaces between the ribs internal intercostal muscles the intermediate intercostal muscles that draw the ribs together internal oblique flat, intermediate abdominal muscle with fascicles that run perpendicular to those of the external oblique ischiococcygeus muscle that assists the levator ani and pulls the coccyx anteriorly levator ani pelvic muscle that resists intra-abdominal pressure and supports the pelvic viscera linea alba white, fibrous band that runs along the midline of the trunk pelvic diaphragm muscular sheet that comprises the levator ani and the ischiococcygeus perineum diamond-shaped region between the pubic symphysis, coccyx, and ischial tuberosities pubococcygeus muscle that makes up the levator ani along with the iliococcygeus quadratus lumborum posterior part of the abdominal wall that helps with posture and stabilization of the body rectus abdominis long, linear muscle that extends along the middle of the trunk rectus sheaths tissue that makes up the linea alba sphincter urethrovaginalis deep perineal muscle in women tendinous intersections three transverse bands of collagen fibers that divide the rectus abdominis into segments transversus abdominis deep layer of the abdomen that has fascicles arranged transversely around the abdomen urogenital triangle anterior triangle of the perineum that includes the external genitals Muscles of the Pectoral Girdle and Upper Limbs By the end of this section, you will be able to: e Identify the muscles of the pectoral girdle and upper limbs e Identify the movement and function of the pectoral girdle and upper limbs Muscles of the shoulder and upper limb can be divided into four groups: muscles that stabilize and position the pectoral girdle, muscles that move the arm, muscles that move the forearm, and muscles that move the wrists, hands, and fingers. The pectoral girdle, or shoulder girdle, consists of the lateral ends of the clavicle and scapula, along with the proximal end of the humerus, and the muscles covering these three bones to stabilize the shoulder joint. The girdle creates a base from which the head of the humerus, in its ball-and-socket joint with the glenoid fossa of the scapula, can move the arm in multiple directions. Muscles That Position the Pectoral Girdle Muscles that position the pectoral girdle are located either on the anterior thorax or on the posterior thorax ([link] and [link]). The anterior muscles include the subclavius, pectoralis minor, and serratus anterior. The posterior muscles include the trapezius, rhomboid major, and rhomboid minor. When the rhomboids are contracted, your scapula moves medially, which can pull the shoulder and upper limb posteriorly. Muscles That Position the Pectoral Girdle Deltoid (cut) Coracoid process of scapula Pectoralis major (cut) Acromion process Rhomboid Scapula of scapula minor Subclavius Pectoralis minor Rhomboid major Sternum Serratus : anterior Deltoid Trapezius Pectoral girdle muscle (left anterior lateral view) Pectoral girdle muscles (posterior view) The muscles that stabilize the pectoral girdle make it a steady base on which other muscles can move the arm. Note that the pectoralis major and deltoid, which move the humerus, are cut here to show the deeper positioning muscles. Muscles that Position the Pectoral Girdle Position Target in the motion Prime thorax Movement Target direction mover Origin Insertion Stabilizes ; Anterior clavicle durin ia 6 Clavicle Depression Subclavius First rib surface of thorax movement by Ears depressing it Muscles that Position the Pectoral Girdle Position in the thorax Anterior thorax Anterior thorax Posterior thorax Posterior thorax Posterior thorax Movement Rotates shoulder anteriorly (throwing motion); assists with inhalation Moves arm from side of body to front of body; assists with inhalation Elevates shoulders (shrugging); pulls shoulder blades together; tilts head backwards Stabilizes scapula during pectoral girdle movement Stabilizes scapula during pectoral girdle movement Target Scapula; ribs Scapula; ribs Scapula; cervical spine Scapula Scapula Muscles That Move the Humerus Similar to the muscles that position the pectoral girdle, muscles that cross the shoulder joint and move the humerus Target motion direction Scapula: depresses; ribs: elevates Scapula: protracts; ribs: elevates Scapula: rotests inferiorly, retracts, elevates, and depresses; spine: extends Retracts; rotates inferiorly Retracts; rotates inferiorly Prime mover Pectoralis minor Serratus anterior Trapezius Rhomboid major Rhomboid minor Origin Anterior surfaces of certain ribs (2-4 or 3-5) Muscle slips from certain ribs (1-8 or 1-9) Skull; vertebral column Thoracic vertebrae (T2-T5) Cervical and thoracic vertebrae (C7 and T1) Insertion Coracoid process of scapula Anterior surface of vertebral border of scapula Acromion and spine of scapula; clavicle Medial border of scapula Medial border of scapula bone of the arm include both axial and scapular muscles ((link] and [link]). The two axial muscles are the pectoralis major and the latissimus dorsi. The pectoralis major is thick and fan-shaped, covering much of the superior portion of the anterior thorax. The broad, triangular latissimus dorsi is located on the inferior part of the back, where it inserts into a thick connective tissue shealth called an aponeurosis. Muscles That Move the Humerus Pectoralis major Latissimus dorsi AM (a) Pectoralis major and latissimus dorsi (left anterior lateral view) (b) Left deltoid and left latissimus dorsi (posterior view) Teres minor Supraspinatus Deltoid (cut) Spine of NN scapula | Deltoid (cut) Coracoid process of scapula Infraspinatus Pectoralis Humerus major (cut) Subscapularis Teres major Teres major Latissimus dorsi (near its origin) Serratus Triceps brachii: long head anterior Triceps brachii: lateral head B Ya (d) Deep muscles of the left shoulder (posterior view) (c) Deep muscles of the left shoulder (anterior lateral view) (a, c) The muscles that move the humerus anteriorly are generally located on the anterior side of the body and originate from the sternum (e.g., pectoralis major) or the anterior side of the scapula (e.g., subscapularis). (b) The muscles that move the humerus superiorly generally originate from the superior surfaces of the scapula and/or the clavicle (e.g., deltoids). The muscles that move the humerus inferiorly generally originate from middle or lower back (e.g., latissiumus dorsi). (d) The muscles that move the humerus posteriorly are generally located on the posterior side of the body and insert into the scapula (e.g., infraspinatus). Muscles That Move the Humerus Target motion direction Movement Target Prime mover Origin Insertion Axial muscles Brings elbows Humerus Flexion; Pectoralis Clavicle; sternum; | Greater together; moves adduction; major cartilage of certain | tubercle of elbow up (as medial ribs (1-6 or 1-7); | humerus during an uppercut rotation aponeurosis of punch) external oblique muscle Moves elbow back Humerus; Humerus: Latissimus Thoracic Intertubercular (as in elbowing scapula extension, dorsi vertebrae sulcus of someone standing adduction, and (T7-T12); lumbar | humerus behind you); medial rotation; vertebrae; lower spreads elbows scapula: ribs (9-12); apart depression iliac crest Scapular muscles Lifts arms at Humerus Abduction; Deltoid Trapezius; Deltoid shoulder flexion; clavicle; tuberosity extension; acromion; of humerus medial and spine of scapula lateral rotation Assists pectoralis major in bringing elbows together Humerus Medial Subscapularis Subscapular Lesser rotation fossa of tubercle of scapula humerus shoulder joint during movement of the pectoral girdle and stabilizes Rotates elbow Humerus Abduction Supraspinatus Supraspinous Greater outwards, as during fossa of scapula | tubercle of a tennis swing humerus Rotates elbow Humerus Extension; Infraspinatus Infraspinous fossa | Greater outwards, as during adduction of scapula tubercle of a tennis swing humerus Assists infraspinatus | Humerus Extension; Teres major Posterior surface | Intertubercular in rotating elbow adduction of scapula sulcus of outwards humerus Assists infraspinatus | Humerus Extension; Teres minor Lateral border of Greater in rotating elbow adduction dorsal scapular tubercle of outwards surface humerus Moves elbow up Humerus Flexion; Coracobra Coracoid process | Medial surface and across body, adduction chialis of scapula of humerus, as when putting shaft hand on chest The rest of the shoulder muscles originate on the scapula. The anatomical and ligamental structure of the shoulder joint and the arrangements of the muscles covering it, allows the arm to carry out different types of movements. The deltoid, the thick muscle that creates the rounded lines of the shoulder is the major abductor of the arm, but it also facilitates flexing and medial rotation, as well as extension and lateral rotation. The subscapularis originates on the anterior scapula and medially rotates the arm. Named for their locations, the supraspinatus (superior to the spine of the scapula) and the infraspinatus (inferior to the spine of the scapula) abduct the arm, and laterally rotate the arm, respectively. The thick and flat teres major is inferior to the teres minor and extends the arm, and assists in adduction and medial rotation of it. The long teres minor laterally rotates and extends the arm. Finally, the coracobrachialis flexes and adducts the arm. The tendons of the deep subscapularis, supraspinatus, infraspinatus, and teres minor connect the scapula to the humerus, forming the rotator cuff (musculotendinous cuff), the circle of tendons around the shoulder joint. When baseball pitchers undergo shoulder surgery it is usually on the rotator cuff, which becomes pinched and inflamed, and may tear away from the bone due to the repetitive motion of bring the arm overhead to throw a fast pitch. Muscles That Move the Forearm The forearm, made of the radius and ulna bones, has four main types of action at the hinge of the elbow joint: flexion, extension, pronation, and supination. The forearm flexors include the biceps brachii, brachialis, and brachioradialis. The extensors are the triceps brachii and anconeus. The pronators are the pronator teres and the pronator quadratus, and the supinator is the only one that turns the forearm anteriorly. When the forearm faces anteriorly, it is supinated. When the forearm faces posteriorly, it is pronated. The biceps brachii, brachialis, and brachioradialis flex the forearm. The two-headed biceps brachii crosses the shoulder and elbow joints to flex the forearm, also taking part in supinating the forearm at the radioulnar joints and flexing the arm at the shoulder joint. Deep to the biceps brachii, the brachialis provides additional power in flexing the forearm. Finally, the brachioradialis can flex the forearm quickly or help lift a load slowly. These muscles and their associated blood vessels and nerves form the anterior compartment of the arm (anterior flexor compartment of the arm) ([link] and [link]). Muscles That Move the Forearm Biceps brachii (short head) Triceps brachii . ¥ (lateral head) Biceps brachii (long head) Triceps brachii (long head) Left upper arm muscles (anterior lateral view) Left upper arm muscles (posterior view) Triceps brachii Brachioradialis Extensor carpi radialis longus Extensor carpi radialis brevis Lateral epicondyle of humerus Pronator teres Abductor pollicis longus Extensor pollicis brevis Extensor pollicis Flexor carpi radialis . Anconeus Palmaris longus longus Extensor carpi Flexor carpi ulnaris ulnaris Extensor digitorum Flexor digitorum superficialis Extensor digiti i minimi g Left forearm superficial muscles (palmar view) Left forearm superficial muscles (dorsal view) Lateral epicondyle of humerus Medial epicondyle of humerus Supinator Flexor pollicis longus Brachialis Abductor pollicis longus . (cut) \ Medial epicondyle Pronator quadratus of humerus Flexor digitorum Extensor pollicis longus profundus Fl 4 4 jexor Extensor pollicis Be digitorum brevis é : profundus Flexor carpi ulnaris Flexor retinaculum (cut) Extensor indicis y Extensor rz - retinaculum Left forearm deep muscles (palmar view) Left forearm deep muscles (dorsal view) The muscles originating in the upper arm flex, extend, pronate, and supinate the forearm. The muscles originating in the forearm move the wrists, hands, and fingers. Muscles That Move the Forearm Target motion ‘ aie | taroet | Tagetcnane | Primemover | ovgin | insertion | Anterior muscles (flexion) Performs a bicep Forearm Flexion; Biceps brachii Coracoid process; | Radial curl; also allows supination tubercle above tuberosity palm of hand to glenoid cavity point toward body while flexing Forearm Flexion Brachialis Front of distal Coronoid humerus process of ulna Assists and Forearm Flexion Brachioradialis Lateral Base of styloid stabilizes elbow supracondylar process of during bicep-curl tidge at distal end | radius motion of humerus Posterior muscles (extension) Extends forearm, Forearm Extension Triceps brachii Infraglenoid Olecranon as during a punch tubercle of process of ulna scapula; posterior shaft of humerus; posterior humeral shaft distal to radial groove Assists in extending | Forearm Extension; Anconeus Lateral epicondyle | Lateral aspect forearm; also allows abduction of humerus of olecranon forearm to extend process of ulna away from body Anterior muscles (pronation) Turns hand Forearm Pronation Pronator teres palm-down Assists in Forearm Pronation Pronator Distal portion Distal surface turning hand quadratus of anterior ulnar of anterior palm-down shaft radius Posterior muscles (supination) Medial epicondyle | Lateral radius of humerus; coronoid process of ulna Turns hand Forearm Supination Supinator Lateral epicondyle | Proximal end palm-up of humerus; of radius proximal ulna Muscles That Move the Wrist, Hand, and Fingers Wrist, hand, and finger movements are facilitated by two groups of muscles. The forearm is the origin of the extrinsic muscles of the hand. The palm is the origin of the intrinsic muscles of the hand. Muscles of the Arm That Move the Wrists, Hands, and Fingers The muscles in the anterior compartment of the forearm (anterior flexor compartment of the forearm) originate on the humerus and insert onto different parts of the hand. These make up the bulk of the forearm. From lateral to medial, the superficial anterior compartment of the forearm includes the flexor carpi radialis, palmaris longus, flexor carpi ulnaris, and flexor digitorum superficialis. The flexor digitorum superficialis flexes the hand as well as the digits at the knuckles, which allows for rapid finger movements, as in typing or playing a musical instrument (see [link] and [link]). However, poor ergonomics can irritate the tendons of these muscles as they slide back and forth with the carpal tunnel of the anterior wrist and pinch the median nerve, which also travels through the tunnel, causing Carpal Tunnel Syndrome. The deep anterior compartment produces flexion and bends fingers to make a fist. These are the flexor pollicis longus and the flexor digitorum profundus. The muscles in the superficial posterior compartment of the forearm (superficial posterior extensor compartment of the forearm) originate on the humerus. These are the extensor radialis longus, extensor carpi radialis brevis, extensor digitorum, extensor digiti minimi, and the extensor carpi ulnaris. The muscles of the deep posterior compartment of the forearm (deep posterior extensor compartment of the forearm) originate on the radius and ulna. These include the abductor pollicis longus, extensor pollicis brevis, extensor pollicis longus, and extensor indicis (see [link]). Muscles That Move the Wrist, Hands, and Forearm Target motion Target direction Superficial anterior compartment of forearm Eine mene | ori Insertion Bends wrist toward body; tilts | Wrist; hand to side away from body | hand Flexion; abduction Assists in bending hand up Flexion toward shoulder Flexor carpi radialis Medial epicondyle of humerus Palmaris longus Medial epicondyle of humerus Base of second and third metacarpals Palmar aponeurosis; skin and fascia of palm Assists in bending hand up toward shoulder; tilts hand to side away from body; stabilizes wrist Flexion, abduction also bends wrist toward body | fingers Bends fingers to make a fist | Wrist; Flexion fingers Deep anterior compartment of forearm Bends tip of Thumb Flexion thumb Bends fingers to make a fist; | Wrist; Flexion Superficial posterior compartment of forearm Wrist Extension; abduction Straightens wrist away from body; tilts hand to side away from body Assists extensor radialis Extension, longus in extending and abduction Flexor carpi ulnaris Medial epicondyle of humerus; olecranon process; posterior surface of ulna Flexor digitorum superficialis Medial epicondyle of humerus; coronoid process of ulna; shaft of radius Anterior surface of radius; interosseous membrane Flexor pollicis longus Flexor digitorum | Coronoid process; profundus anteromedial surface of ulna; interosseous membrane Extensor Lateral supracondylar radialis longus ridge of humerus Extensor carpi radialis brevis Lateral epicondyle of humerus abducting wrist; also stabilizes hand during finger flexion. Opens fingers and moves Wrist; Extension; them sideways away from fingers abduction the body Extends little finger Little Extension finger Straightens wrist away from | Wrist Extension; body; tilts hand to side adduction toward body Deep posterior compartment of forearm Thumb: abduction, Moves thumb sideways toward body; extends thumb; moves hand sideways toward body extension; wrist: abduction Extends thumb Thumb Extension Extends thumb Thumb Extension Extends index finger; Wrist; Extension straightens wrist away from index body finger Extensor Lateral epicondyle digitorum of humerus Extensor Lateral epicondyle digiti minimi of humerus Extensor carpi ulnaris Lateral epicondyle of humerus; posterior border of ulna Abductor Posterior surface pollicis longus of radius and ulna; interosseous membrane Extensor Dorsal shaft of radius pollicis brevis and ulna; interosseous membrane Extensor Dorsal shaft of radius pollicis longus and ulna; interosseous membrane Posterior surface of distal ulna; interosseous membrane Extensor indicis Pisiform, hamate bones, and base of fifth metacarpal Middle phalanges of fingers 2-5 Distal phalanx of thumb Distal phalanges of fingers 2-5 Base of second metacarpal Base of third metacarpal Extensor expansions; distal phalanges of fingers Extensor expansion; distal phalanx of finger 5 Base of fifth metacarpal Base of first metacarpal; trapezium Base of proximal phalanx of thumb Base of distal phalanx of thumb Tendon of extensor digitorum of index finger The tendons of the forearm muscles attach to the wrist and extend into the hand. Fibrous bands called retinacula sheath the tendons at the wrist. The flexor retinaculum extends over the palmar surface of the hand while the extensor retinaculum extends over the dorsal surface of the hand. Intrinsic Muscles of the Hand The intrinsic muscles of the hand both originate and insert within it ([link]). These muscles allow your fingers to also make precise movements for actions, such as typing or writing. These muscles are divided into three groups. The thenar muscles are on the radial aspect of the palm. The hypothenar muscles are on the medial aspect of the palm, and the intermediate muscles are midpalmar. The thenar muscles include the abductor pollicis brevis, opponens pollicis, flexor pollicis brevis, and the adductor pollicis. These muscles form the thenar eminence, the rounded contour of the base of the thumb, and all act on the thumb. The movements of the thumb play an integral role in most precise movements of the hand. The hypothenar muscles include the abductor digiti minimi, flexor digiti minimi brevis, and the opponens digiti minimi. These muscles form the hypothenar eminence, the rounded contour of the little finger, and as such, they all act on the little finger. Finally, the intermediate muscles act on all the fingers and include the lumbrical, the palmar interossei, and the dorsal interossei. Intrinsic Muscles of the Hand Vee Opponens pollicis Abductor digiti minimi Abductor pollicis brevl Flexor digiti ie minimi brevis Pisometacarpal Flexor pollicis ligament brevis FLX ‘I : Opponens digiti ; 2 minimi ! Adductor pollicis oe f y { | Lumbricalis muscles U Superficial muscles of left hand (palmar) Dorsal interossei muscles Palmar interossei muscles Interossei muscles of left hand (palmar view) Interossei muscles of left hand (dorsal view) The intrinsic muscles of the hand both originate and insert within the hand. These muscles provide the fine motor control of the fingers by flexing, extending, abducting, and adducting the more distal finger and thumb segments. Intrinsic Muscles of the Hand Target motion Prime Muscle Movement Target direction mover Thenar Moves thumb . eeu Thumb Abduction pollicis muscles toward body . brevis Moves thumb Thenar across palm to ve Opponens Th ice muscles touch other nae Pepesiien pollicis fingers Origin Flexor retinaculum; and nearby carpals Flexor retinaculum; trapezium Insertion Lateral base of proximal phalanx o thumb Anterior ¢ first metacarpe Intrinsic Muscles of the Hand Muscle Thenar muscles Thenar muscles Hypothenar muscles Hypothenar muscles Hypothenar muscles Intermediate muscles Movement Flexes thumb Moves thumb away from body Moves little finger toward body Flexes little finger Moves little finger across palm to touch thumb Flexes each finger at metacarpo- phalangeal joints; extends each finger at interphalangeal joints Target Thumb Thumb Little finger Little finger Little finger Fingers Target motion direction Flexion Adduction Abduction Flexion Opposition Flexion Prime mover Flexor pollicis brevis Adductor pollicis Abductor digiti minimi Flexor digiti minimi brevis Opponens digiti minimi Lumbricals Origin Flexor retinaculum; trapezium Capitate bone; bases of metacarpals 2-4; front of metacarpal 3 Pisiform bone Hamate bone; flexor retinaculum Hamate bone; flexor retinaculum Palm (lateral sides of tendons in flexor digitorum profundus) Insertion Lateral base of proximal phalanx o thumb Medial base of proximal phalanx o thumb Medial side of proximal phalanx o little finge Medial side of proximal phalanx o little finge Medial side of fifth metacarpé Fingers 2- 5 (lateral edges of extension: expansion on first phalanges Intrinsic Muscles of the Hand Target motion Prime Muscle Movement Target direction mover Origin Insertion Adducts and : Peters! Side of each expansion flexes each : ; metacarpal on first Bngenat that faces hal oe phalanx o ‘ metacarpo- Adduction; f Intermediate : an Palmar metacarpal each finge phalangeal Fingers flexion; . ; muscles eeeeen : interossei 3 (absent (except joints; extends extension ; é from finger 3) each finger at : ; metacarpal on side interphalangeal 3) facing join : JoMts finger 3 Abducts and ee ae flexes the three foi eee i middle fingers Sian at metacarpo- finger 5 halangeal Ab ion; : : Intermediate pe nge ‘ dnctic Dorsal Sides of extensor joints; extends Fingers flexion; . . : muscles : interossei metacarpals expansion the three extension sen tict middle fingers phalanx o a ; a side interphalangeal opposite join 3 jeans finger 3 Chapter Review The clavicle and scapula make up the pectoral girdle, which provides a stable origin for the muscles that move the humerus. The muscles that position and stabilize the pectoral girdle are located on the thorax. The anterior thoracic muscles are the subclavius, pectoralis minor, and the serratus anterior. The posterior thoracic muscles are the trapezius, levator scapulae, rhomboid major, and rhomboid minor. Nine muscles cross the shoulder joint to move the humerus. The ones that originate on the axial skeleton are the pectoralis major and the latissimus dorsi. The deltoid, subscapularis, supraspinatus, infraspinatus, teres major, teres minor, and coracobrachialis originate on the scapula. The forearm flexors include the biceps brachii, brachialis, and brachioradialis. The extensors are the triceps brachii and anconeus. The pronators are the pronator teres and the pronator quadratus. The supinator is the only one that turns the forearm anteriorly. The extrinsic muscles of the hands originate along the forearm and insert into the hand in order to facilitate crude movements of the wrists, hands, and fingers. The superficial anterior compartment of the forearm produces flexion. These muscles are the flexor carpi radialis, palmaris longus, flexor carpi ulnaris, and the flexor digitorum superficialis. The deep anterior compartment produces flexion as well. These are the flexor pollicis longus and the flexor digitorum profundus. The rest of the compartments produce extension. The extensor carpi radialis longus, extensor carpi radialis brevis, extensor digitorum, extensor digiti minimi, and extensor carpi ulnaris are the muscles found in the superficial posterior compartment. The deep posterior compartment includes the abductor longus, extensor pollicis brevis, extensor pollicis longus, and the extensor indicis. Finally, the intrinsic muscles of the hands allow our fingers to make precise movements, such as typing and writing. They both originate and insert within the hand. The thenar muscles, which are located on the lateral part of the palm, are the abductor pollicis brevis, opponens pollicis, flexor pollicis brevis, and adductor pollicis. The hypothenar muscles, which are located on the medial part of the palm, are the abductor digiti minimi, flexor digiti minimi brevis, and opponens digiti minimi. The intermediate muscles, located in the middle of the palm, are the lumbricals, palmar interossei, and dorsal interossei. Review Questions Exercise: Problem:The rhomboid major and minor muscles are deep to the a. rectus abdominis b. scalene muscles c. trapezius d. ligamentum nuchae Solution: C Exercise: Problem:Which muscle extends the forearm? a. biceps brachii b. triceps brachii c. brachialis d. deltoid Solution: B Exercise: Problem: What is the origin of the wrist flexors? a. the lateral epicondyle of the humerus b. the medial epicondyle of the humerus c. the carpal bones of the wrist d. the deltoid tuberosity of the humerus Solution: B Exercise: Problem:Which muscles stabilize the pectoral girdle? a. axial and scapular b. axial c. appendicular d. axial and appendicular Solution: A Critical Thinking Questions Exercise: Problem:The tendons of which muscles form the rotator cuff? Why is the rotator cuff important? Solution: Tendons of the infraspinatus, supraspinatus, teres minor, and the subscapularis form the rotator cuff, which forms a foundation on which the arms and shoulders can be stabilized and move. Exercise: Problem: List the general muscle groups of the shoulders and upper limbs as well as their subgroups. Solution: The muscles that make up the shoulders and upper limbs include the muscles that position the pelvic girdle, the muscles that move the humerus, the muscles that move the forearm, and the muscles that move the wrists, hands, and fingers. Glossary abductor digiti minimi muscle that abducts the little finger adductor pollicis muscle that adducts the thumb abductor pollicis brevis muscle that abducts the thumb abductor pollicis longus muscle that inserts into the first metacarpal anconeus small muscle on the lateral posterior elbow that extends the forearm anterior compartment of the arm (anterior flexor compartment of the arm) the biceps brachii, brachialis, brachioradialis, and their associated blood vessels and nerves anterior compartment of the forearm (anterior flexor compartment of the forearm) deep and superficial muscles that originate on the humerus and insert into the hand biceps brachii two-headed muscle that crosses the shoulder and elbow joints to flex the forearm while assisting in supinating it and flexing the arm at the shoulder brachialis muscle deep to the biceps brachii that provides power in flexing the forearm. brachioradialis muscle that can flex the forearm quickly or help lift a load slowly coracobrachialis muscle that flexes and adducts the arm deep anterior compartment flexor pollicis longus, flexor digitorum profundus, and their associated blood vessels and nerves deep posterior compartment of the forearm (deep posterior extensor compartment of the forearm) the abductor pollicis longus, extensor pollicis brevis, extensor pollicis longus, extensor indicis, and their associated blood vessels and nerves deltoid shoulder muscle that abducts the arm as well as flexes and medially rotates it, and extends and laterally rotates it dorsal interossei muscles that abduct and flex the three middle fingers at the metacarpophalangeal joints and extend them at the interphalangeal joints extensor carpi radialis brevis muscle that extends and abducts the hand at the wrist extensor carpi ulnaris muscle that extends and adducts the hand extensor digiti minimi muscle that extends the little finger extensor digitorum muscle that extends the hand at the wrist and the phalanges extensor indicis muscle that inserts onto the tendon of the extensor digitorum of the index finger extensor pollicis brevis muscle that inserts onto the base of the proximal phalanx of the thumb extensor pollicis longus muscle that inserts onto the base of the distal phalanx of the thumb extensor radialis longus muscle that extends and abducts the hand at the wrist extensor retinaculum band of connective tissue that extends over the dorsal surface of the hand extrinsic muscles of the hand muscles that move the wrists, hands, and fingers and originate on the arm flexor carpi radialis muscle that flexes and abducts the hand at the wrist flexor carpi ulnaris muscle that flexes and adducts the hand at the wrist flexor digiti minimi brevis muscle that flexes the little finger flexor digitorum profundus muscle that flexes the phalanges of the fingers and the hand at the wrist flexor digitorum superficialis muscle that flexes the hand and the digits flexor pollicis brevis muscle that flexes the thumb flexor pollicis longus muscle that flexes the distal phalanx of the thumb flexor retinaculum band of connective tissue that extends over the palmar surface of the hand hypothenar group of muscles on the medial aspect of the palm hypothenar eminence rounded contour of muscle at the base of the little finger infraspinatus muscle that laterally rotates the arm intermediate group of midpalmar muscles intrinsic muscles of the hand muscles that move the wrists, hands, and fingers and originate in the palm latissimus dorsi broad, triangular axial muscle located on the inferior part of the back lumbrical muscle that flexes each finger at the metacarpophalangeal joints and extend each finger at the interphalangeal joints opponens digiti minimi muscle that brings the little finger across the palm to meet the thumb opponens pollicis muscle that moves the thumb across the palm to meet another finger palmar interossei muscles that abduct and flex each finger at the metacarpophalangeal joints and extend each finger at the interphalangeal joints palmaris longus muscle that provides weak flexion of the hand at the wrist pectoral girdle shoulder girdle, made up of the clavicle and scapula pectoralis major thick, fan-shaped axial muscle that covers much of the superior thorax pectoralis minor muscle that moves the scapula and assists in inhalation pronator quadratus pronator that originates on the ulna and inserts on the radius pronator teres pronator that originates on the humerus and inserts on the radius retinacula fibrous bands that sheath the tendons at the wrist rhomboid major muscle that attaches the vertebral border of the scapula to the spinous process of the thoracic vertebrae rhomboid minor muscle that attaches the vertebral border of the scapula to the spinous process of the thoracic vertebrae rotator cuff (also, musculotendinous cuff) the circle of tendons around the shoulder joint serratus anterior large and flat muscle that originates on the ribs and inserts onto the scapula subclavius muscle that stabilizes the clavicle during movement subscapularis muscle that originates on the anterior scapula and medially rotates the arm superficial anterior compartment of the forearm flexor carpi radialis, palmaris longus, flexor carpi ulnaris, flexor digitorum superficialis, and their associated blood vessels and nerves superficial posterior compartment of the forearm extensor radialis longus, extensor carpi radialis brevis, extensor digitorum, extensor digiti minimi, extensor carpi ulnaris, and their associated blood vessels and nerves supinator muscle that moves the palm and forearm anteriorly supraspinatus muscle that abducts the arm teres major muscle that extends the arm and assists in adduction and medial rotation of it teres minor muscle that laterally rotates and extends the arm thenar group of muscles on the lateral aspect of the palm thenar eminence rounded contour of muscle at the base of the thumb trapezius muscle that stabilizes the upper part of the back triceps brachii three-headed muscle that extends the forearm Muscles of the Pelvic Girdle and Lower Limbs By the end of this section, you will be able to: e Identify the appendicular muscles of the pelvic girdle and lower limb e Identify the movement and function of the pelvic girdle and lower limb The appendicular muscles of the lower body position and stabilize the pelvic girdle, which serves as a foundation for the lower limbs. Comparatively, there is much more movement at the pectoral girdle than at the pelvic girdle. There is very little movement of the pelvic girdle because of its connection with the sacrum at the base of the axial skeleton. The pelvic girdle is less range of motion because it was designed to stabilize and support the body. Muscles of the Thigh What would happen if the pelvic girdle, which attaches the lower limbs to the torso, were capable of the same range of motion as the pectoral girdle? For one thing, walking would expend more energy if the heads of the femurs were not secured in the acetabula of the pelvis. The body’s center of gravity is in the area of the pelvis. If the center of gravity were not to remain fixed, standing up would be difficult as well. Therefore, what the leg muscles lack in range of motion and versatility, they make up for in size and power, facilitating the body’s stabilization, posture, and movement. Gluteal Region Muscles That Move the Femur Most muscles that insert on the femur (the thigh bone) and move it, originate on the pelvic girdle. The psoas major and iliacus make up the iliopsoas group. Some of the largest and most powerful muscles in the body are the gluteal muscles or gluteal group. The gluteus maximus is the largest; deep to the gluteus maximus is the gluteus medius, and deep to the gluteus medius is the gluteus minimus, the smallest of the trio ((link] and [link]). Hip and Thigh Muscles Quadratus lumborum Psoas major lliacus ; Pectineus ihn Sacrum Tensor Adductor longus fascia latae Gracilis Adductor Rectus magnus femoris Sartorius Vastus Vastus medialis lateralis eer tendon (or patellar tendon) i Patellar ligament Superficial pelvic and Bee muscles of right leg (anterior view) Crest of ilium Gluteus medius (cut) i Sacrum lliac ; Gluteus crest Pectineus roe minimus : urator o : Pubis interns Piriformis Cbturator Glluteus Superior externus rasanlis gemellus Inferior gemellus Obturator externus Adductor (cut) brevis Adductor Adductor longus group Adductor Gracilis magnus Quadratus ‘ femoris Semimembranosus Biceps Semitendinosus femoris Deep pelvic and thigh muscles Pelvic and thigh muscles of of right leg (anterior view) right leg (posterior view) The large and powerful muscles of the hip that move the femur generally originate on the pelvic girdle and insert into the femur. The muscles that move the lower leg typically originate on the femur and insert into the bones of the knee joint. The anterior muscles of the femur extend the lower leg but also aid in flexing the thigh. The posterior muscles of the femur flex the lower leg but also aid in extending the thigh. A combination of gluteal and thigh muscles also adduct, abduct, and rotate the thigh and lower leg. Gluteal Region Muscles That Move the Femur Target motion A 7 | __ Movement | Tarot | T™gescgne” | Primemover | origin | serton | lliopsoas group Raises knee at hip, as if performing a knee attack; assists lateral rotators in twisting thigh (and lower leg) outward; assists with bending over, maintaining posture Raises knee at hip, as if performing a knee attack; assists lateral rotators in twisting thigh (and lower leg) outward; assists with bending over, maintaining posture Gluteal group Lowers knee and moves thigh back, as when getting ready to kick a ball Opens thighs, as when doing a split Brings the thighs back together Assists with raising knee at hip and opening thighs; maintains posture by stabilizing the iliotibial track, which connects to the knee Lateral rotators Twists thigh (and lower leg) outward; maintains posture by stabilizing hip joint Twists thigh (and lower leg) outward; maintains posture by stabilizing hip joint Twists thigh (and lower leg) outward; maintains posture by stabilizing hip joint Twists thigh (and lower leg) outward; maintains posture by stabilizing hip joint Twists thigh (and lower leg) outward; maintains posture by stabilizing hip joint Twists thigh (and lower leg) outward; maintains posture by stabilizing hip joint Adductors Brings the thighs back together; assists with raising the knee Brings the thighs back together; assists with raising the knee Brings the thighs back together; assists with raising the knee and moving the thigh back Opens thighs; assists with raising the knee and turning the thigh (and lower leg) inward Lumbar vertebrae (L1-L5); thoracic vertebra (T12) Psoas major lliacus Gluteus maximus Thigh: flexion and lateral rotation; torso: flexion lliac fossa; iliac crest; lateral sacrum Thigh: flexion and lateral rotation; torso: flexion Extension Dorsal ilium; sacrum; coccyx Femur Femur Gluteus minimus Femur Flexion; abduction Tensor fascia lata Obutrator internus Femur Anterior aspect of iliac crest; anterior superior iliac spine Anterolateral surface of sacrum Lateral rotation Inner surface of obturator membrane; greater sciatic notch; margins of obturator foramen Lateral rotation Outer surfaces of obturator membrane, pubic, and ischium; margins of obturator foramen Obturator externus Lateral rotation Lateral rotation Superior gemellus Ischial spine Inferior gemellus Lateral rotation Ischial tuberosity Quadratus femoris Lateral rotation Ischial tuberosity Adduction; flexion Adductor longus | Pubis near pubic symphysis Adduction; Adductor brevis flexion Body of pubis; inferior ramus of pubis Adductor magnus Adduction; flexion; extension Ischial rami; pubic rami; ischial tuberosity Adduction; flexion; | Pectineus Pectineal line of pubis medial rotation Lesser trochanter of femur Lesser trochanter of femur Gluteal tuberosity of femur; iliotibial tract Abduction Gluteus Lateral surface of illum | Greater trochanter medius of femur External surface of Greater trochanter ilium of femur lliotibial tract Greater trochanter of femur Greater trochanter in front of piriformis Trochanteric fossa of posterior femur Greater trochanter of femur Greater trochanter of femur Trochanteric crest of femur Linea aspera Linea aspera above adductor longus Linea aspera; adductor tubercle of femur Lesser trochanter to linea aspera of posterior aspect of femur The tensor fascia latae is a thick, squarish muscle in the superior aspect of the lateral thigh. It acts as a synergist of the gluteus medius and iliopsoas in flexing and abducting the thigh. It also helps stabilize the lateral aspect of the knee by pulling on the iliotibial tract (band), making it taut. Deep to the gluteus maximus, the piriformis, obturator internus, obturator externus, superior gemellus, inferior gemellus, and quadratus femoris laterally rotate the femur at the hip. The adductor longus, adductor brevis, and adductor magnus can both medially and laterally rotate the thigh depending on the placement of the foot. The adductor longus flexes the thigh, whereas the adductor magnus extends it. The pectineus adducts and flexes the femur at the hip as well. The pectineus is located in the femoral triangle, which is formed at the junction between the hip and the leg and also includes the femoral nerve, the femoral artery, the femoral vein, and the deep inguinal lymph nodes. Thigh Muscles That Move the Femur, Tibia, and Fibula Deep fascia in the thigh separates it into medial, anterior, and posterior compartments (see [link] and [link]). The muscles in the medial compartment of the thigh are responsible for adducting the femur at the hip. Along with the adductor longus, adductor brevis, adductor magnus, and pectineus, the strap-like gracilis adducts the thigh in addition to flexing the leg at the knee. Thigh Muscles That Move the Femur, Tibia, and Fibula Target Target metion Prime mover direction Medial compartment of thigh Moves back of lower legs up toward buttocks, as when kneeling; assists in opening thighs Femur; tibia/fibula Tibia/fibula: flexion; thigh: adduction Gracilis Anterior compartment of thigh: Quadriceps femoris group Moves lower leg out in front of body, as when kicking; assists in raising the knee Moves lower leg out in front of body, as when kicking Moves lower leg out in front of body, as when kicking Moves lower leg out in front of body, as when kicking Moves back of lower legs up and back toward the buttocks, as when kneeling; assists in moving thigh diagonally upward and outward as when mounting a bike Posterior compartment of thigh: Hamstring group Moves back of lower legs up and back oward the buttocks, as when kneeling; moves thigh down and back; twists the high (and lower leg) outward Moves back of lower legs up toward buttocks, as when kneeling; moves high down and back; twists the thigh (and lower leg) inward Moves back of lower legs up and back toward the buttocks as when kneeling; moves thigh down and back; twists the thigh (and lower leg) inward Femur; tibia/fibula Tibia/fibula Tibia/fibula Tibia/fibula Femur; tibia/fibula Femur; tibia/fibula Femur; tibia/fibula Femur; tibia/fibula Tibia/fibula: extension; thigh: flexion Extension Extension Extension Tibia: flexion; thigh: flexion, abduction, lateral rotation Tibia/fibula: flexion; thigh: extension, lateral rotation Tibia/fibula: flexion; thigh: extension, medial rotation Tibia/fibula: flexion; thigh: extension, medial rotation Rectus femoris Vastus lateralis Vastus medialis Vastus intermedius Sartorius Biceps femoris Semitendinosus Semi- membranosus Inferior ramus; body of pubis; ischial ramus Anterior inferior iliac spine; superior margin of acetabulum Greater trochanter; intertrochanteric line; linea aspera Linea aspera; intertrochanteric line Proximal femur shaft Anterior superior iliac spine Ischial tuberosity; linea aspera; distal femur Ischial tuberosity Ischial tuberosity Medial surface of tibia Patella; tibial tuberosity Patella; tibial tuberosity Patella; tibial tuberosity Patella; tibial tuberosity Medial aspect of proximal tibia Head of fibula; lateral condyle of tibia Upper tibial shaft Medial condyle of tibia; lateral condyle of femur The muscles of the anterior compartment of the thigh flex the thigh and extend the leg. This compartment contains the quadriceps femoris group, which actually comprises four muscles that extend and stabilize the knee. The rectus femoris is on the anterior aspect of the thigh, the vastus lateralis is on the lateral aspect of the thigh, the vastus medialis is on the medial aspect of the thigh, and the vastus intermedius is between the vastus lateralis and vastus medialis and deep to the rectus femoris. The tendon common to all four is the quadriceps tendon (patellar tendon), which inserts into the patella and continues below it as the patellar ligament. The patellar ligament attaches to the tibial tuberosity. In addition to the quadriceps femoris, the sartorius is a band-like muscle that extends from the anterior superior iliac spine to the medial side of the proximal tibia. This versatile muscle flexes the leg at the knee and flexes, abducts, and laterally rotates the leg at the hip. This muscle allows us to sit cross- legged. The posterior compartment of the thigh includes muscles that flex the leg and extend the thigh. The three long muscles on the back of the knee are the hamstring group, which flexes the knee. These are the biceps femoris, semitendinosus, and semimembranosus. The tendons of these muscles form the popliteal fossa, the diamond-shaped space at the back of the knee. Muscles That Move the Feet and Toes Similar to the thigh muscles, the muscles of the leg are divided by deep fascia into compartments, although the leg has three: anterior, lateral, and posterior ([link] and [link]). Muscles of the i Leg Gastrocnemius (lateral head) Superior extensor" retinaculum Inferior edeneare—— retinaculum Superficial muscles of the right lower leg (anterior view) Tibialis anterior Fibularis longus Extensor digitorum longus Fibularis brevis Extensor hallucis longus Fibularis tertius Gastrocnemius (medial head) Plantaris Soleus Calcaneal (Achilles) | tendon } | > Superficial muscles of the right lower leg (posterior view) Calcaneus (heel) Popliteus Soleus (cut) Fibularis longus Tibialis posterior Flexor digitorum longus Flexor hallucis longus Fibularis brevis Deep muscles of the right lower leg (posterior view) The muscles of the anterior compartment of the lower leg are generally responsible for dorsiflexion, and the muscles of the posterior compartment of the lower leg are generally responsible for plantar flexion. The lateral and medial muscles in both compartments invert, evert, and rotate the foot. Muscles That Move the Feet and Toes Target motion ‘ Movement | Target | Taegeamenon | Primemover | origin] serton | Anterior compartment of leg Raises the sole of the foot off the ground, as when preparing to foot-tap; bends the inside of the foot upwards, as when catching your balance while falling laterally toward the opposite side as the balancing foot Raises the sole of the foot off the ground, as when preparing to foot-tap; extends the big toe Raises the sole of the foot off the ground, as when preparing to foot-tap; extends toes Lateral compartment of leg Lowers the sole of the foot to the ground, as when foot-tapping or jumping; bends the inside of the foot downwards, as when catching your balance while falling laterally toward the same side as the balancing foot Lowers the sole of the foot to the ground, as when foot-tapping or jumping; bends the inside of the foot downward, as when catching your balance while falling laterally toward the same side as the balancing foot Foot; big toe Foot; toes 2-5 Dorsiflexion; inversion Foot: dorsiflexion; big toe: extension Foot: dorsiflexion; toes: extension Plantar flexion and eversion Plantar flexion and eversion Posterior compartment of leg: Superficial muscles Lowers the sole of the foot to the ground, as when foot-tapping or jumping; assists in moving the back of the lower legs up and back toward the buttocks Lowers the sole of the foot to the ground, as when foot-tapping or jumping; maintains posture while walking Lowers the sole of the foot to the ground, as when foot-tapping or jumping; assists in moving the back of the lower legs up and back toward the buttocks Lowers the sole of the foot to the ground, as when foot-tapping or jumping Posterior compartment of leg: Deep muscles Moves the back of the lower legs up and back toward the buttocks; assists in rotation of the leg at the knee and thigh Lowers the sole of the foot to the ground, as when foot-tapping or jumping; bends the inside of the foot upward and flexes toes Flexes the big toe Foot; tibia/ fibula Foot; tibia/ fibula Tibia/ fibula Foot; toes 2-5 Big toe; foot Foot: plantar flexion; tibia/fibula: flexion Plantar flexion Foot: plantar flexion; tibia/fibula: flexion Plantar flexion Tibialis anterior Extensor hallucis longus Extensor digitorum longus Fibularis longus Fibularis (peroneus) brevis Gastrocnemius Plantaris Tibialis posterior Tibia/fibula: flexion} Popliteus thigh and lower leg: medial and lateral rotation Foot: plantar flexion and inversion toes: flexion Big toe: flexion foot: plantar flexion Flexor digitorum longus Flexor hallucis longus Lateral condyle and upper tibial shaft; interosseous membrane Anteromedial fibula shaft; interosseous membrane Lateral condyle of tibia; proximal portion of fibula; interosseous membrane Upper portion of lateral fibula Distal fibula shaft Medial and lateral condyles of femur Superior tibia; fibula; interosseous membrane Posterior femur above lateral condyle Superior tibia and fibula; interosseous membrane Lateral condyle of femur; lateral meniscus Posterior tibia Midshaft of fibula; interosseous membrane Interior surface of medial cuneiform; First metatarsal bone Distal phalanx of big toe Middle and distal phalanges of toes 2-5 First metatarsal; medial cuneiform Proximal end of fifth metatarsal Posterior calcaneus Posterior calcaneus Calcaneus or calcaneus tendon Several tarsals and metatarsals 2-4 Proximal tibia Distal phalanges of toes 2-5 Distal phalanx of big toe The muscles in the anterior compartment of the leg: the tibialis anterior, a long and thick muscle on the lateral surface of the tibia, the extensor hallucis longus, deep under it, and the extensor digitorum longus, lateral to it, all contribute to raising the front of the foot when they contract. The fibularis tertius, a small muscle that originates on the anterior surface of the fibula, is associated with the extensor digitorum longus and sometimes fused to it, but is not present in all people. Thick bands of connective tissue called the superior extensor retinaculum (transverse ligament of the ankle) and the inferior extensor retinaculum, hold the tendons of these muscles in place during dorsiflexion. The lateral compartment of the leg includes two muscles: the fibularis longus (peroneus longus) and the fibularis brevis (peroneus brevis). The superficial muscles in the posterior compartment of the leg all insert onto the calcaneal tendon (Achilles tendon), a strong tendon that inserts into the calcaneal bone of the ankle. The muscles in this compartment are large and strong and keep humans upright. The most superficial and visible muscle of the calf is the gastrocnemius. Deep to the gastrocnemius is the wide, flat soleus. The plantaris runs obliquely between the two; some people may have two of these muscles, whereas no plantaris is observed in about seven percent of other cadaver dissections. The plantaris tendon is a desirable substitute for the fascia lata in hernia repair, tendon transplants, and repair of ligaments. There are four deep muscles in the posterior compartment of the leg as well: the popliteus, flexor digitorum longus, flexor hallucis longus, and tibialis posterior. The foot also has intrinsic muscles, which originate and insert within it (similar to the intrinsic muscles of the hand). These muscles primarily provide support for the foot and its arch, and contribute to movements of the toes ([link] and [link]). The principal support for the longitudinal arch of the foot is a deep fascia called plantar aponeurosis, which runs from the calcaneus bone to the toes (inflammation of this tissue is the cause of “plantar fasciitis,” which can affect runners. The intrinsic muscles of the foot consist of two groups. The dorsal group includes only one muscle, the extensor digitorum brevis. The second group is the plantar group, which consists of four layers, starting with the most superficial. Intrinsic Muscles of the Foot Tendocalcaneus Fibularis longus Extensor digitorum brevis ——_____ Tibialis anterior Extensor digitorum longus -. Extensor hallucis longus Y Fibularis brevis Fibularis tertius (a) Dorsal superficial muscles of the right foot (lateral view) Abductor digiti minimi Plantar aponeurosis Quadratus Flexor digiti a plantae minimi brevis allucis Flexor Flexor digitorum hallucis brevis ae brevis y @ ti) i (Vy . ; Cl} Wh \\\/ \ Wi Lumbricals v dH W (b) Superficial muscles of the (c) Intermediate muscles of (d) Deep muscles of the left sole (plantar view) the left sole (plantar view) left sole (plantar view) The muscles along the dorsal side of the foot (a) generally extend the toes while the muscles of the plantar side of the foot (b, c, d) generally flex the toes. The plantar muscles exist in three layers, providing the foot the strength to counterbalance the weight of the body. In this diagram, these three layers are shown from a plantar view beginning with the bottom-most layer just under the plantar skin of the foot (b) and ending with the top-most layer (d) located just inferior to the foot and toe bones. Intrinsic Muscles in the Foot Target motion Prime Dorsal group Extends toes 2-5 Toes 2-5 | Extension Extensor Calcaneus; Base of proximal digitorum extensor phalanx of big toe; brevis retinaculum extensor expansions on toes 2-5 Plantar group (layer 1) Abducts and flexes Adduction; Abductor Calcaneal Proximal phalanx of big toe flexion hallucis tuberosity; flexor big toe retinaculum Flexes toes 2-4 Middle Flexion Flexor Calcaneal Middle phalanx of toes digitorum tuberosity toes 2-4 brevis Abducts and flexes Toe 5 Abduction; Abductor Calcaneal tuberosity | Proximal phalanx small toe flexion digiti minimi of little toe Plantar group (layer 2) Assists in flexing Toes 2-5 | Flexion Quadratus Medial and lateral Tendon of flexor toes 2-5 plantae sides of calcaneus digitorum longus Extends toes 2-5 at | Toes 2-5 | Extension; Lumbricals Tendons of flexor Medial side of the interphalangeal flexion digitorum longus proximal phalanx of joints; flexes the toes 2-5 small toes at the metatarsophalangeal joints Plantar group (layer 3) Flexes big toe Big toe Flexion Flexor Lateral cuneiform; Base of proximal hallucis cuboid bones phalanx of big toe brevis Adducts and flexes Adduction; Adductor Bases of Base of proximal big toe flexion hallucis metatarsals 2—4; phalanx of big toe fibularis longus tendon sheath; ligament across metatarsophalangeal joints Flexes small toe Little toe | Flexion Flexor digiti Base of metatarsal 5; | Base of proximal minimi brevis | tendon sheath of phalanx of little toe fibularis longus Plantar group (layer 4) Abducts and flexes Middle Abduction; Dorsal Sides of metatarsals | Both sides of toe 2; middle toes at toes flexion; interossei for each other toe, metatarsophalangeal extension extensor expansion joints; extends over first phalanx on middle toes at side opposite toe 2 interphalangeal joints Abducts toes 3-5; Small Abduction; Plantar Side of each Extensor expansion flexes proximal toes flexion; interossei metatarsal that faces | on first phalanx of phalanges and extension metatarsal 2 (absent | each toe (except to extends distal from metatarsal 2) 2) on side facing toe 2 phalanges Chapter Review The pelvic girdle attaches the legs to the axial skeleton. The hip joint is where the pelvic girdle and the leg come together. The hip is joined to the pelvic girdle by many muscles. In the gluteal region, the psoas major and iliacus form the iliopsoas. The large and strong gluteus maximus, gluteus medius, and gluteus minimus extend and abduct the femur. Along with the gluteus maximus, the tensor fascia lata muscle forms the iliotibial tract. The lateral rotators of the femur at the hip are the piriformis, obturator internus, obturator externus, superior gemellus, inferior gemellus, and quadratus femoris. On the medial part of the thigh, the adductor longus, adductor brevis, and adductor magnus adduct the thigh and medially rotate it. The pectineus muscle adducts and flexes the femur at the hip. The thigh muscles that move the femur, tibia, and fibula are divided into medial, anterior, and posterior compartments. The medial compartment includes the adductors, pectineus, and the gracilis. The anterior compartment comprises the quadriceps femoris, quadriceps tendon, patellar ligament, and the sartorius. The quadriceps femoris is made of four muscles: the rectus femoris, the vastus lateralis, the vastus medius, and the vastus intermedius, which together extend the knee. The posterior compartment of the thigh includes the hamstrings: the biceps femoris, semitendinosus, and the semimembranosus, which all flex the knee. The muscles of the leg that move the foot and toes are divided into anterior, lateral, superficial- and deep-posterior compartments. The anterior compartment includes the tibialis anterior, the extensor hallucis longus, the extensor digitorum longus, and the fibularis (peroneus) tertius. The lateral compartment houses the fibularis (peroneus) longus and the fibularis (peroneus) brevis. The superficial posterior compartment has the gastrocnemius, soleus, and plantaris; and the deep posterior compartment has the popliteus, tibialis posterior, flexor digitorum longus, and flexor hallucis longus. Review Questions Exercise: Problem: The large muscle group that attaches the leg to the pelvic girdle and produces extension of the hip joint is the group. a. gluteal b. obturator c. adductor d. abductor Solution: A Exercise: Problem: Which muscle produces movement that allows you to cross your legs? a. the gluteus maximus b. the piriformis c. the gracilis d. the sartorius Solution: D Exercise: Problem: What is the largest muscle in the lower leg? a. soleus b. gastrocnemius c. tibialis anterior d. tibialis posterior Solution: B Exercise: Problem: The vastus intermedius muscle is deep to which of the following muscles? a. biceps femoris b. rectus femoris c. vastus medialis d. vastus lateralis Solution: B Critical Thinking Questions Exercise: Problem: Which muscles form the hamstrings? How do they function together? Solution: The biceps femoris, semimembranosus, and semitendinosus form the hamstrings. The hamstrings flex the leg at the knee joint. Exercise: Problem: Which muscles form the quadriceps? How do they function together? Solution: The rectus femoris, vastus medialis, vastus lateralis, and vastus intermedius form the quadriceps. The quadriceps muscles extend the leg at the knee joint. Glossary adductor brevis muscle that adducts and medially rotates the thigh adductor longus muscle that adducts, medially rotates, and flexes the thigh adductor magnus muscle with an anterior fascicle that adducts, medially rotates and flexes the thigh, and a posterior fascicle that assists in thigh extension anterior compartment of the leg region that includes muscles that dorsiflex the foot anterior compartment of the thigh region that includes muscles that flex the thigh and extend the leg biceps femoris hamstring muscle calcaneal tendon (also, Achilles tendon) strong tendon that inserts into the calcaneal bone of the ankle dorsal group region that includes the extensor digitorum brevis extensor digitorum brevis muscle that extends the toes extensor digitorum longus muscle that is lateral to the tibialis anterior extensor hallucis longus muscle that is partly deep to the tibialis anterior and extensor digitorum longus femoral triangle region formed at the junction between the hip and the leg and includes the pectineus, femoral nerve, femoral artery, femoral vein, and deep inguinal lymph nodes fibularis brevis (also, peroneus brevis) muscle that plantar flexes the foot at the ankle and everts it at the intertarsal joints fibularis longus (also, peroneus longus) muscle that plantar flexes the foot at the ankle and everts it at the intertarsal joints fibularis tertius small muscle that is associated with the extensor digitorum longus flexor digitorum longus muscle that flexes the four small toes flexor hallucis longus muscle that flexes the big toe gastrocnemius most superficial muscle of the calf gluteal group muscle group that extends, flexes, rotates, adducts, and abducts the femur gluteus maximus largest of the gluteus muscles that extends the femur gluteus medius muscle deep to the gluteus maximus that abducts the femur at the hip gluteus minimus smallest of the gluteal muscles and deep to the gluteus medius gracilis muscle that adducts the thigh and flexes the leg at the knee hamstring group three long muscles on the back of the leg iliacus muscle that, along with the psoas major, makes up the iliopsoas iliopsoas group muscle group consisting of iliacus and psoas major muscles, that flexes the thigh at the hip, rotates it laterally, and flexes the trunk of the body onto the hip iliotibial tract muscle that inserts onto the tibia; made up of the gluteus maximus and connective tissues of the tensor fasciae latae inferior extensor retinaculum cruciate ligament of the ankle inferior gemellus muscle deep to the gluteus maximus on the lateral surface of the thigh that laterally rotates the femur at the hip lateral compartment of the leg region that includes the fibularis (peroneus) longus and the fibularis (peroneus) brevis and their associated blood vessels and nerves medial compartment of the thigh a region that includes the adductor longus, adductor brevis, adductor magnus, pectineus, gracilis, and their associated blood vessels and nerves obturator externus muscle deep to the gluteus maximus on the lateral surface of the thigh that laterally rotates the femur at the hip obturator internus muscle deep to the gluteus maximus on the lateral surface of the thigh that laterally rotates the femur at the hip patellar ligament extension of the quadriceps tendon below the patella pectineus muscle that abducts and flexes the femur at the hip pelvic girdle hips, a foundation for the lower limb piriformis muscle deep to the gluteus maximus on the lateral surface of the thigh that laterally rotates the femur at the hip plantar aponeurosis muscle that supports the longitudinal arch of the foot plantar group four-layered group of intrinsic foot muscles plantaris muscle that runs obliquely between the gastrocnemius and the soleus popliteal fossa diamond-shaped space at the back of the knee popliteus muscle that flexes the leg at the knee and creates the floor of the popliteal fossa posterior compartment of the leg region that includes the superficial gastrocnemius, soleus, and plantaris, and the deep popliteus, flexor digitorum longus, flexor hallucis longus, and tibialis posterior posterior compartment of the thigh region that includes muscles that flex the leg and extend the thigh psoas major muscle that, along with the iliacus, makes up the iliopsoas quadratus femoris muscle deep to the gluteus maximus on the lateral surface of the thigh that laterally rotates the femur at the hip quadriceps femoris group four muscles, that extend and stabilize the knee quadriceps tendon (also, patellar tendon) tendon common to all four quadriceps muscles, inserts into the patella rectus femoris quadricep muscle on the anterior aspect of the thigh Sartorius band-like muscle that flexes, abducts, and laterally rotates the leg at the hip semimembranosus hamstring muscle semitendinosus hamstring muscle soleus wide, flat muscle deep to the gastrocnemius superior extensor retinaculum transverse ligament of the ankle superior gemellus muscle deep to the gluteus maximus on the lateral surface of the thigh that laterally rotates the femur at the hip tensor fascia lata muscle that flexes and abducts the thigh tibialis anterior muscle located on the lateral surface of the tibia tibialis posterior muscle that plantar flexes and inverts the foot vastus intermedius quadricep muscle that is between the vastus lateralis and vastus medialis and is deep to the rectus femoris vastus lateralis quadricep muscle on the lateral aspect of the thigh vastus medialis quadricep muscle on the medial aspect of the thigh Heart Anatomy By the end of this section, you will be able to: e Describe the location and position of the heart within the body cavity e Describe the internal and external anatomy of the heart e Identify the tissue layers of the heart e Relate the structure of the heart to its function as a pump e Compare systemic circulation to pulmonary circulation e Identify the veins and arteries of the coronary circulation system e Trace the pathway of oxygenated and deoxygenated blood thorough the chambers of the heart The vital importance of the heart is obvious. If one assumes an average rate of contraction of 75 contractions per minute, a human heart would contract approximately 108,000 times in one day, more than 39 million times in one year, and nearly 3 billion times during a 75-year lifespan. Each of the major pumping chambers of the heart ejects approximately 70 mL blood per contraction in a resting adult. This would be equal to 5.25 liters of fluid per minute and approximately 14,000 liters per day. Over one year, that would equal 10,000,000 liters or 2.6 million gallons of blood sent through roughly 60,000 miles of vessels. In order to understand how that happens, it is necessary to understand the anatomy and physiology of the heart. Location of the Heart The human heart is located within the thoracic cavity, medially between the lungs in the space known as the mediastinum. [link] shows the position of the heart within the thoracic cavity. Within the mediastinum, the heart is separated from the other mediastinal structures by a tough membrane known as the pericardium, or pericardial sac, and sits in its own space called the pericardial cavity. The dorsal surface of the heart lies near the bodies of the vertebrae, and its anterior surface sits deep to the sternum and costal cartilages. The great veins, the superior and inferior venae cavae, and the great arteries, the aorta and pulmonary trunk, are attached to the superior surface of the heart, called the base. The base of the heart is located at the level of the third costal cartilage, as seen in [link]. The inferior tip of the heart, the apex, lies just to the left of the sternum between the junction of the fourth and fifth ribs near their articulation with the costal cartilages. The right side of the heart is deflected anteriorly, and the left side is deflected posteriorly. It is important to remember the position and orientation of the heart when placing a stethoscope on the chest of a patient and listening for heart sounds, and also when looking at images taken from a midsagittal perspective. The slight deviation of the apex to the left is reflected in a depression in the medial surface of the inferior lobe of the left lung, called the cardiac notch. Position of the Heart in the Thorax Thoracic aorta Sagittal view Mediastinum ; Arch of aorta Superior vena cava Right lung Pulmonary trunk Right auricle Left auricle Right atrium Left lung Right ventricle Left ventricle Ribs (cut) Pericardial cavity Ditiimwiesien Apex of heart Edge of parietal Edge of parietal pleura (cut) pericardium (cut) The heart is located within the thoracic cavity, medially between the lungs in the mediastinum. It is about the size of a fist, is broad at the top, and tapers toward the base. Note: Everyday Connection CPR The position of the heart in the torso between the vertebrae and sternum (see [link] for the position of the heart within the thorax) allows for individuals to apply an emergency technique known as cardiopulmonary resuscitation (CPR) if the heart of a patient should stop. By applying pressure with the flat portion of one hand on the sternum in the area between the line at T4 and T9 ((link]), it is possible to manually compress the blood within the heart enough to push some of the blood within it into the pulmonary and systemic circuits. This is particularly critical for the brain, as irreversible damage and death of neurons occur within minutes of loss of blood flow. Current standards call for compression of the chest at least 5 cm deep and at a rate of 100 compressions per minute, a rate equal to the beat in “Staying Alive,” recorded in 1977 by the Bee Gees. If you are unfamiliar with this song, a version is available on www.youtube.com. At this stage, the emphasis is on performing high-quality chest compressions, rather than providing artificial respiration. CPR is generally performed until the patient regains spontaneous contraction or is declared dead by an experienced healthcare professional. When performed by untrained or overzealous individuals, CPR can result in broken ribs or a broken sternum, and can inflict additional severe damage on the patient. It is also possible, if the hands are placed too low on the sternum, to manually drive the xiphoid process into the liver, a consequence that may prove fatal for the patient. Proper training is essential. This proven life-sustaining technique is so valuable that virtually all medical personnel as well as concerned members of the public should be certified and routinely recertified in its application. CPR courses are offered at a variety of locations, including colleges, hospitals, the American Red Cross, and some commercial companies. They normally include practice of the compression technique on a mannequin. CPR Technique If the heart should stop, CPR can maintain the flow of blood until the heart resumes beating. By applying pressure to the sternum, the blood within the heart will be squeezed out of the heart and into the circulation. Proper positioning of the hands on the sternum to perform CPR would be between the lines at T4 and T9. Note: meee OPENStAX COLLEGE it ‘s . L OS Visit the American Heart Association website to help locate a course near your home in the United States. There are also many other national and regional heart associations that offer the same service, depending upon the location. Shape and Size of the Heart The shape of the heart is similar to a pinecone, rather broad at the superior surface and tapering to the apex (see [link]). A typical heart is approximately the size of your fist: 12 cm (5 in) in length, 8 cm (3.5 in) wide, and 6 cm (2.5 in) in thickness. Given the size difference between most members of the sexes, the weight of a female heart is approximately 250-300 grams (9 to 11 ounces), and the weight of a male heart is approximately 300-350 grams (11 to 12 ounces). The heart of a well- trained athlete, especially one specializing in aerobic sports, can be considerably larger than this. Cardiac muscle responds to exercise in a manner similar to that of skeletal muscle. That is, exercise results in the addition of protein myofilaments that increase the size of the individual cells without increasing their numbers, a concept called hypertrophy. Hearts of athletes can pump blood more effectively at lower rates than those of nonathletes. Enlarged hearts are not always a result of exercise; they can result from pathologies, such as hypertrophic cardiomyopathy. The cause of an abnormally enlarged heart muscle is unknown, but the condition is often undiagnosed and can cause sudden death in apparently otherwise healthy young people. Chambers and Circulation through the Heart The human heart consists of four chambers: The left side and the right side each have one atrium and one ventricle. Each of the upper chambers, the right atrium (plural = atria) and the left atrium, acts as a receiving chamber and contracts to push blood into the lower chambers, the right ventricle and the left ventricle. The ventricles serve as the primary pumping chambers of the heart, propelling blood to the lungs or to the rest of the body. There are two distinct but linked circuits in the human circulation called the pulmonary and systemic circuits. Although both circuits transport blood and everything it carries, we can initially view the circuits from the point of view of gases. The pulmonary circuit transports blood to and from the lungs, where it picks up oxygen and delivers carbon dioxide for exhalation. The systemic circuit transports oxygenated blood to virtually all of the tissues of the body and returns relatively deoxygenated blood and carbon dioxide to the heart to be sent back to the pulmonary circulation. The right ventricle pumps deoxygenated blood into the pulmonary trunk, which leads toward the lungs and bifurcates into the left and right pulmonary arteries. These vessels in turn branch many times before reaching the pulmonary capillaries, where gas exchange occurs: Carbon dioxide exits the blood and oxygen enters. The pulmonary trunk arteries and their branches are the only arteries in the post-natal body that carry relatively deoxygenated blood. Highly oxygenated blood returning from the pulmonary capillaries in the lungs passes through a series of vessels that join together to form the pulmonary veins—the only post-natal veins in the body that carry highly oxygenated blood. The pulmonary veins conduct blood into the left atrium, which pumps the blood into the left ventricle, which in turn pumps oxygenated blood into the aorta and on to the many branches of the systemic circuit. Eventually, these vessels will lead to the systemic capillaries, where exchange with the tissue fluid and cells of the body occurs. In this case, oxygen and nutrients exit the systemic capillaries to be used by the cells in their metabolic processes, and carbon dioxide and waste products will enter the blood. The blood exiting the systemic capillaries is lower in oxygen concentration than when it entered. The capillaries will ultimately unite to form venules, joining to form ever-larger veins, eventually flowing into the two major systemic veins, the superior vena cava and the inferior vena cava, which return blood to the right atrium. The blood in the superior and inferior venae cavae flows into the right atrium, which pumps blood into the right ventricle. This process of blood circulation continues as long as the individual remains alive. Understanding the flow of blood through the pulmonary and systemic circuits is critical to all health professions ({link]). Dual System of the Human Blood Circulation Aorta Left pulmonary Right pulmonary arteries arteries Pulmonary trunk Left atrium Right pulmonary veins Left pulmonary veins Pulmonary semilunar valve Aortic semilunar valve Mitral valve Right atrium Tricuspid valve Left ventricle Right ventricle Systemic Systemic veins from capillaries upper body of upper body Systemic arteries to Pulmona es upper body capillaries in lungs Pulmonary Right atrium trunk Left atrium Right ventricle Left \ ventricle Systemic veins from Systemic lower body arteries to lower body Systemic capillaries of lower body Blood flows from the right atrium to the right ventricle, where it is pumped into the pulmonary circuit. The blood in the pulmonary artery branches is low in oxygen but relatively high in carbon dioxide. Gas exchange occurs in the pulmonary capillaries (oxygen into the blood, carbon dioxide out), and blood high in oxygen and low in carbon dioxide is returned to the left atrium. From here, blood enters the left ventricle, which pumps it into the systemic circuit. Following exchange in the systemic capillaries (oxygen and nutrients out of the capillaries and carbon dioxide and wastes in), blood returns to the right atrium and the cycle is repeated. Membranes, Surface Features, and Layers Our exploration of more in-depth heart structures begins by examining the membrane that surrounds the heart, the prominent surface features of the heart, and the layers that form the wall of the heart. Each of these components plays its own unique role in terms of function. Membranes The membrane that directly surrounds the heart and defines the pericardial cavity is called the pericardium or pericardial sac. It also surrounds the “roots” of the major vessels, or the areas of closest proximity to the heart. The pericardium, which literally translates as “around the heart,” consists of two distinct sublayers: the sturdy outer fibrous pericardium and the inner serous pericardium. The fibrous pericardium is made of tough, dense connective tissue that protects the heart and maintains its position in the thorax. The more delicate serous pericardium consists of two layers: the parietal pericardium, which is fused to the fibrous pericardium, and an inner visceral pericardium, or epicardium, which is fused to the heart and is part of the heart wall. The pericardial cavity, filled with lubricating serous fluid, lies between the epicardium and the pericardium. In most organs within the body, visceral serous membranes such as the epicardium are microscopic. However, in the case of the heart, it is not a microscopic layer but rather a macroscopic layer, consisting of a simple squamous epithelium called a mesothelium, reinforced with loose, irregular, or areolar connective tissue that attaches to the pericardium. This mesothelium secretes the lubricating serous fluid that fills the pericardial cavity and reduces friction as the heart contracts. [link] illustrates the pericardial membrane and the layers of the heart. Pericardial Membranes and Layers of the Heart Wall Pericardial cavity Endocardium Fibrous pericardium Myocardium Parietal layer of serous pericardium Epicardium (viceral layer of serous pericardium) The pericardial membrane that surrounds the heart consists of three layers and the pericardial cavity. The heart wall also consists of three layers. The pericardial membrane and the heart wall share the epicardium. Note: Disorders of the... Heart: Cardiac Tamponade If excess fluid builds within the pericardial space, it can lead to a condition called cardiac tamponade, or pericardial tamponade. With each contraction of the heart, more fluid—in most instances, blood—accumulates within the pericardial cavity. In order to fill with blood for the next contraction, the heart must relax. However, the excess fluid in the pericardial cavity puts pressure on the heart and prevents full relaxation, so the chambers within the heart contain slightly less blood as they begin each heart cycle. Over time, less and less blood is ejected from the heart. If the fluid builds up slowly, as in hypothyroidism, the pericardial cavity may be able to expand gradually to accommodate this extra volume. Some cases of fluid in excess of one liter within the pericardial cavity have been reported. Rapid accumulation of as little as 100 mL of fluid following trauma may trigger cardiac tamponade. Other common causes include myocardial rupture, pericarditis, cancer, or even cardiac surgery. Removal of this excess fluid requires insertion of drainage tubes into the pericardial cavity. Premature removal of these drainage tubes, for example, following cardiac surgery, or clot formation within these tubes are causes of this condition. Untreated, cardiac tamponade can lead to death. Surface Features of the Heart Inside the pericardium, the surface features of the heart are visible, including the four chambers. There is a superficial leaf-like extension of the atria near the superior surface of the heart, one on each side, called an auricle—a name that means “ear like”—because its shape resembles the external ear of a human ([link]). Auricles are relatively thin-walled structures that can fill with blood and empty into the atria or upper chambers of the heart. You may also hear them referred to as atrial appendages. Also prominent is a series of fat-filled grooves, each of which is known as a sulcus (plural = sulci), along the superior surfaces of the heart. Major coronary blood vessels are located in these sulci. The deep coronary sulcus is located between the atria and ventricles. Located between the left and right ventricles are two additional sulci that are not as deep as the coronary sulcus. The anterior interventricular sulcus is visible on the anterior surface of the heart, whereas the posterior interventricular sulcus is visible on the posterior surface of the heart. [link] illustrates anterior and posterior views of the surface of the heart. External Anatomy of the Heart Brachiocephalic trunk Left common carotid artery Left subclavian artery Aortic arch Ligamentum arteriosum Left pulmonary artery Superior vena cava Right pulmonary artery Ascending aorta Pulmonary trunk F Right pulmonary << é veins Left pulmonary veins Auricle of left atrium Anterior view Right atrium Circumflex artery Right coronary artery Left coronary artery Anterior cardiac vein : ; Left ventricle Right ventricle Great cardiac vein Anterior interventricular artery Right marginal artery Small cardiac vein Inferior vena cava Apex Superior vena cava Right pul t Left pulmonary artery ignt pulmonary artery . Right pulmonary veins Left pulmonary veins Auricle of left atrium Right atrium Left atrium Inferior vena cava Circumflex branch Coronary sinus Posterior view of left coronary artery Small cardiac vein Great cardiac vein Right coronary artery Posterior vein of left ventricle Posterior interventricular artery Middle cardiac vein Left ventricle Right ventricle Inside the pericardium, the surface features of the heart are visible. Layers The wall of the heart is composed of three layers of unequal thickness. From superficial to deep, these are the epicardium, the myocardium, and the endocardium (see [link]). The outermost layer of the wall of the heart is also the innermost layer of the pericardium, the epicardium, or the visceral pericardium discussed earlier. The middle and thickest layer is the myocardium, made largely of cardiac muscle cells. It is built upon a framework of collagenous fibers, plus the blood vessels that supply the myocardium and the nerve fibers that help regulate the heart. It is the contraction of the myocardium that pumps blood through the heart and into the major arteries. The muscle pattern is elegant and complex, as the muscle cells swirl and spiral around the chambers of the heart. They form a figure 8 pattern around the atria and around the bases of the great vessels. Deeper ventricular muscles also form a figure 8 around the two ventricles and proceed toward the apex. More superficial layers of ventricular muscle wrap around both ventricles. This complex swirling pattern allows the heart to pump blood more effectively than a simple linear pattern would. [link] illustrates the arrangement of muscle cells. Heart Musculature Atrial musculature Ventricular musculature The swirling pattern of cardiac muscle tissue contributes significantly to the heart’s ability to pump blood effectively. Although the ventricles on the right and left sides pump the same amount of blood per contraction, the muscle of the left ventricle is much thicker and better developed than that of the right ventricle. In order to overcome the high resistance required to pump blood into the long systemic circuit, the left ventricle must generate a great amount of pressure. The right ventricle does not need to generate as much pressure, since the pulmonary circuit is shorter and provides less resistance. [link] illustrates the differences in muscular thickness needed for each of the ventricles. Differences in Ventricular Muscle Thickness Left ventricle Right ventricle Relaxed Contracted The myocardium in the left ventricle is significantly thicker than that of the right ventricle. Both ventricles pump the same amount of blood, but the left ventricle must generate a much greater pressure to overcome greater resistance in the systemic circuit. The ventricles are shown in both relaxed and contracting states. Note the differences in the relative size of the lumens, the region inside each ventricle where the blood is contained. The innermost layer of the heart wall, the endocardium, is joined to the myocardium with a thin layer of connective tissue. The endocardium lines the chambers where the blood circulates and covers the heart valves. It is made of simple squamous epithelium called endothelium, which is continuous with the endothelial lining of the blood vessels (see [link]). Once regarded as a simple lining layer, recent evidence indicates that the endothelium of the endocardium and the coronary capillaries may play active roles in regulating the contraction of the muscle within the myocardium. The endothelium may also regulate the growth patterns of the cardiac muscle cells throughout life, and the endothelins it secretes create an environment in the surrounding tissue fluids that regulates ionic concentrations and states of contractility. Endothelins are potent vasoconstrictors and, in a normal individual, establish a homeostatic balance with other vasoconstrictors and vasodilators. Internal Structure of the Heart Recall that the heart’s contraction cycle follows a dual pattern of circulation —the pulmonary and systemic circuits—because of the pairs of chambers that pump blood into the circulation. In order to develop a more precise understanding of cardiac function, it is first necessary to explore the internal anatomical structures in more detail. Septa of the Heart The word septum is derived from the Latin for “something that encloses;” in this case, a septum (plural = septa) refers to a wall or partition that divides the heart into chambers. The septa are physical extensions of the myocardium lined with endocardium. Located between the two atria is the interatrial septum. Normally in an adult heart, the interatrial septum bears an oval-shaped depression known as the fossa ovalis, a remnant of an opening in the fetal heart known as the foramen ovale. The foramen ovale allowed blood in the fetal heart to pass directly from the right atrium to the left atrium, allowing some blood to bypass the pulmonary circuit. Within seconds after birth, a flap of tissue known as the septum primum that previously acted as a valve closes the foramen ovale and establishes the typical cardiac circulation pattern. Between the two ventricles is a second septum known as the interventricular septum. Unlike the interatrial septum, the interventricular septum is normally intact after its formation during fetal development. It is substantially thicker than the interatrial septum, since the ventricles generate far greater pressure when they contract. The septum between the atria and ventricles is known as the atrioventricular septum. It is marked by the presence of four openings that allow blood to move from the atria into the ventricles and from the ventricles into the pulmonary trunk and aorta. Located in each of these openings between the atria and ventricles is a valve, a specialized structure that ensures one-way flow of blood. The valves between the atria and ventricles are known generically as atrioventricular valves. The valves at the openings that lead to the pulmonary trunk and aorta are known generically as semilunar valves. The interventricular septum is visible in [link]. In this figure, the atrioventricular septum has been removed to better show the bicupid and tricuspid valves; the interatrial septum is not visible, since its location is covered by the aorta and pulmonary trunk. Since these openings and valves structurally weaken the atrioventricular septum, the remaining tissue is heavily reinforced with dense connective tissue called the cardiac skeleton, or skeleton of the heart. It includes four rings that surround the openings between the atria and ventricles, and the openings to the pulmonary trunk and aorta, and serve as the point of attachment for the heart valves. The cardiac skeleton also provides an important boundary in the heart electrical conduction system. Internal Structures of the Heart Superior vena cava Left pulmonary artery Right pulmonary artery peiaidum Pulmonary trunk Left pulmonary veins Right pulmonary veins Mitral (bicuspid) valve Right atrium Aortic valve Fossa ovalis Tricuspid valve Pulmonary valve Right ventricle Left ventricle Chordae tendineae Papillary muscle Interventricular septum Epicardium Myocardium Endocardium Trabeculae carneae Moderator band Inferior vena cava Anterior view This anterior view of the heart shows the four chambers, the major vessels and their early branches, as well as the valves. The presence of the pulmonary trunk and aorta covers the interatrial septum, and the atrioventricular septum is cut away to show the atrioventricular valves. Note: Disorders of the... Heart: Heart Defects One very common form of interatrial septum pathology is patent foramen ovale, which occurs when the septum primum does not close at birth, and the fossa ovalis is unable to fuse. The word patent is from the Latin root patens for “open.” It may be benign or asymptomatic, perhaps never being diagnosed, or in extreme cases, it may require surgical repair to close the opening permanently. As much as 20—25 percent of the general population may have a patent foramen ovale, but fortunately, most have the benign, asymptomatic version. Patent foramen ovale is normally detected by auscultation of a heart murmur (an abnormal heart sound) and confirmed by imaging with an echocardiogram. Despite its prevalence in the general population, the causes of patent ovale are unknown, and there are no known risk factors. In nonlife-threatening cases, it is better to monitor the condition than to risk heart surgery to repair and seal the opening. Coarctation of the aorta is a congenital abnormal narrowing of the aorta that is normally located at the insertion of the ligamentum arteriosum, the remnant of the fetal shunt called the ductus arteriosus. If severe, this condition drastically restricts blood flow through the primary systemic artery, which is life threatening. In some individuals, the condition may be fairly benign and not detected until later in life. Detectable symptoms in an infant include difficulty breathing, poor appetite, trouble feeding, or failure to thrive. In older individuals, symptoms include dizziness, fainting, shortness of breath, chest pain, fatigue, headache, and nosebleeds. Treatment involves surgery to resect (remove) the affected region or angioplasty to open the abnormally narrow passageway. Studies have shown that the earlier the surgery is performed, the better the chance of survival. A patent ductus arteriosus is a congenital condition in which the ductus arteriosus fails to close. The condition may range from severe to benign. Failure of the ductus arteriosus to close results in blood flowing from the higher pressure aorta into the lower pressure pulmonary trunk. This additional fluid moving toward the lungs increases pulmonary pressure and makes respiration difficult. Symptoms include shortness of breath (dyspnea), tachycardia, enlarged heart, a widened pulse pressure, and poor weight gain in infants. Treatments include surgical closure (ligation), manual closure using platinum coils or specialized mesh inserted via the femoral artery or vein, or nonsteroidal anti-inflammatory drugs to block the synthesis of prostaglandin E2, which maintains the vessel in an open position. If untreated, the condition can result in congestive heart failure. Septal defects are not uncommon in individuals and may be congenital or caused by various disease processes. Tetralogy of Fallot is a congenital condition that may also occur from exposure to unknown environmental factors; it occurs when there is an opening in the interventricular septum caused by blockage of the pulmonary trunk, normally at the pulmonary semilunar valve. This allows blood that is relatively low in oxygen from the right ventricle to flow into the left ventricle and mix with the blood that is relatively high in oxygen. Symptoms include a distinct heart murmur, low blood oxygen percent saturation, dyspnea or difficulty in breathing, polycythemia, broadening (clubbing) of the fingers and toes, and in children, difficulty in feeding or failure to grow and develop. It is the most common cause of cyanosis following birth. The term “tetralogy” is derived from the four components of the condition, although only three may be present in an individual patient: pulmonary infundibular stenosis (rigidity of the pulmonary valve), overriding aorta (the aorta is shifted above both ventricles), ventricular septal defect (opening), and right ventricular hypertrophy (enlargement of the right ventricle). Other heart defects may also accompany this condition, which is typically confirmed by echocardiography imaging. Tetralogy of Fallot occurs in approximately AO0 out of one million live births. Normal treatment involves extensive surgical repair, including the use of stents to redirect blood flow and replacement of valves and patches to repair the septal defect, but the condition has a relatively high mortality. Survival rates are currently 75 percent during the first year of life; 60 percent by 4 years of age; 30 percent by 10 years; and 5 percent by 40 years. In the case of severe septal defects, including both tetralogy of Fallot and patent foramen ovale, failure of the heart to develop properly can lead to a condition commonly known as a “blue baby.” Regardless of normal skin pigmentation, individuals with this condition have an insufficient supply of oxygenated blood, which leads to cyanosis, a blue or purple coloration of the skin, especially when active. Septal defects are commonly first detected through auscultation, listening to the chest using a stethoscope. In this case, instead of hearing normal heart sounds attributed to the flow of blood and closing of heart valves, unusual heart sounds may be detected. This is often followed by medical imaging to confirm or rule out a diagnosis. In many cases, treatment may not be needed. Some common congenital heart defects are illustrated in [link]. Congenital Heart Defects Narrow segment Ha of aorta Foramen ovale fails to close (a) Patent foramen ovale (b) Coarctation of the aorta Ductus arteriosus remains open Stenosed pulmonary semilunar valve (c) Patent ductus arteriosus (d) Tetralogy of Fallot Aorta emerges from both ventricles Interventricular septal defect Enlarged right ventricle (a) A patent foramen ovale defect is an abnormal opening in the interatrial septum, or more commonly, a failure of the foramen ovale to close. (b) Coarctation of the aorta is an abnormal narrowing of the aorta. (c) A patent ductus arteriosus is the failure of the ductus arteriosus to close. (d) Tetralogy of Fallot includes an abnormal opening in the interventricular septum. Right Atrium The right atrium serves as the receiving chamber for blood returning to the heart from the systemic circulation. The two major systemic veins, the superior and inferior venae cavae, and the large coronary vein called the coronary sinus that drains the heart myocardium empty into the right atrium. The superior vena cava drains blood from regions superior to the diaphragm: the head, neck, upper limbs, and the thoracic region. It empties into the superior and posterior portions of the right atrium. The inferior vena cava drains blood from areas inferior to the diaphragm: the lower limbs and abdominopelvic region of the body. It, too, empties into the posterior portion of the atria, but inferior to the opening of the superior vena cava. Immediately superior and slightly medial to the opening of the inferior vena cava on the posterior surface of the atrium is the opening of the coronary sinus. This thin-walled vessel drains most of the coronary veins that return systemic blood from the heart. The majority of the internal heart structures discussed in this and subsequent sections are illustrated in [link]. While the bulk of the internal surface of the right atrium is smooth, the depression of the fossa ovalis is medial, and the anterior surface demonstrates prominent ridges of muscle called the pectinate muscles. The right auricle also has pectinate muscles. The left atrium does not have pectinate muscles except in the auricle. The atria receive venous blood on a nearly continuous basis, preventing venous flow from stopping while the ventricles are contracting. While most ventricular filling occurs while the atria are relaxed, they do demonstrate a contractile phase and actively pump blood into the ventricles just prior to ventricular contraction. The opening between the atrium and ventricle is guarded by the tricuspid valve. Right Ventricle The right ventricle receives blood from the right atrium through the tricuspid valve. Each flap of the valve is attached to strong strands of connective tissue, the chordae tendineae, literally “tendinous cords,” or sometimes more poetically referred to as “heart strings.” There are several chordae tendineae associated with each of the flaps. They are composed of approximately 80 percent collagenous fibers with the remainder consisting of elastic fibers and endothelium. They connect each of the flaps to a papillary muscle that extends from the inferior ventricular surface. There are three papillary muscles in the right ventricle, called the anterior, posterior, and septal muscles, which correspond to the three sections of the valves. When the myocardium of the ventricle contracts, pressure within the ventricular chamber rises. Blood, like any fluid, flows from higher pressure to lower pressure areas, in this case, toward the pulmonary trunk and the atrium. To prevent any potential backflow, the papillary muscles also contract, generating tension on the chordae tendineae. This prevents the flaps of the valves from being forced into the atria and regurgitation of the blood back into the atria during ventricular contraction. [link] shows papillary muscles and chordae tendineae attached to the tricuspid valve. Chordae Tendineae and Papillary Muscles Chordae tendineae Papillary muscles Trabeculae carneae In this frontal section, you can see papillary muscles attached to the tricuspid valve on the right as well as the mitral valve on the left via chordae tendineae. (credit: modification of work by “PV KS”/flickr.com) The walls of the ventricle are lined with trabeculae carneae, ridges of cardiac muscle covered by endocardium. In addition to these muscular ridges, a band of cardiac muscle, also covered by endocardium, known as the moderator band (see [link]) reinforces the thin walls of the right ventricle and plays a crucial role in cardiac conduction. It arises from the inferior portion of the interventricular septum and crosses the interior space of the right ventricle to connect with the inferior papillary muscle. When the right ventricle contracts, it ejects blood into the pulmonary trunk, which branches into the left and right pulmonary arteries that carry it to each lung. The superior surface of the right ventricle begins to taper as it approaches the pulmonary trunk. At the base of the pulmonary trunk is the pulmonary semilunar valve that prevents backflow from the pulmonary trunk. Left Atrium After exchange of gases in the pulmonary capillaries, blood returns to the left atrium high in oxygen via one of the four pulmonary veins. While the left atrium does not contain pectinate muscles, it does have an auricle that includes these pectinate ridges. Blood flows nearly continuously from the pulmonary veins back into the atrium, which acts as the receiving chamber, and from here through an opening into the left ventricle. Most blood flows passively into the heart while both the atria and ventricles are relaxed, but toward the end of the ventricular relaxation period, the left atrium will contract, pumping blood into the ventricle. This atrial contraction accounts for approximately 20 percent of ventricular filling. The opening between the left atrium and ventricle is guarded by the mitral valve. Left Ventricle Recall that, although both sides of the heart will pump the same amount of blood, the muscular layer is much thicker in the left ventricle compared to the right (see [link]). Like the right ventricle, the left also has trabeculae carneae, but there is no moderator band. The mitral valve is connected to papillary muscles via chordae tendineae. There are two papillary muscles on the left—the anterior and posterior—as opposed to three on the right. The left ventricle is the major pumping chamber for the systemic circuit; it ejects blood into the aorta through the aortic semilunar valve. Heart Valve Structure and Function A transverse section through the heart slightly above the level of the atrioventricular septum reveals all four heart valves along the same plane ({link]). The valves ensure unidirectional blood flow through the heart. Between the right atrium and the right ventricle is the right atrioventricular valve, or tricuspid valve. It typically consists of three flaps, or leaflets, made of endocardium reinforced with additional connective tissue. The flaps are connected by chordae tendineae to the papillary muscles, which control the opening and closing of the valves. Heart Valves Posterior Tricuspid valve Bicuspid (mitral) Aortic valve Pulmonary valve Anterior With the atria and major vessels removed, all four valves are clearly visible, although it is difficult to distinguish the three separate cusps of the tricuspid valve. Emerging from the right ventricle at the base of the pulmonary trunk is the pulmonary semilunar valve, or the pulmonary valve; it is also known as the pulmonic valve or the right semilunar valve. The pulmonary valve is comprised of three small flaps of endothelium reinforced with connective tissue. When the ventricle relaxes, the pressure differential causes blood to flow back into the ventricle from the pulmonary trunk. This flow of blood fills the pocket-like flaps of the pulmonary valve, causing the valve to close and producing an audible sound. Unlike the atrioventricular valves, there are no papillary muscles or chordae tendineae associated with the pulmonary valve. Located at the opening between the left atrium and left ventricle is the mitral valve, also called the bicuspid valve or the left atrioventricular valve. Structurally, this valve consists of two cusps, known as the anterior medial cusp and the posterior medial cusp, compared to the three cusps of the tricuspid valve. In a clinical setting, the valve is referred to as the mitral valve, rather than the bicuspid valve. The two cusps of the mitral valve are attached by chordae tendineae to two papillary muscles that project from the wall of the ventricle. At the base of the aorta is the aortic semilunar valve, or the aortic valve, which prevents backflow from the aorta. It normally is composed of three flaps. When the ventricle relaxes and blood attempts to flow back into the ventricle from the aorta, blood will fill the cusps of the valve, causing it to close and producing an audible sound. In [link]a, the two atrioventricular valves are open and the two semilunar valves are closed. This occurs when both atria and ventricles are relaxed and when the atria contract to pump blood into the ventricles. [link]b shows a frontal view. Although only the left side of the heart is illustrated, the process is virtually identical on the right. Blood Flow from the Left Atrium to the Left Ventricle Posterior Tricuspid Bicuspid (mitral) valve valve \ Left side of WA . < \ side of heart valve (closed) Anterior (closed) (a) Mitral valve (open) Aortic valve (Closed) Chordae tendineae (loose) Papillary muscle (relaxed) (b) (a) A transverse section through the heart illustrates the four heart valves. The two atrioventricular valves are open; the two semilunar valves are closed. The atria and vessels have been removed. (b) A frontal section through the heart illustrates blood flow through the mitral valve. When the mitral valve is open, it allows blood to move from the left atrium to the left ventricle. The aortic semilunar valve is closed to prevent backflow of blood from the aorta to the left ventricle. [link]a shows the atrioventricular valves closed while the two semilunar valves are open. This occurs when the ventricles contract to eject blood into the pulmonary trunk and aorta. Closure of the two atrioventricular valves prevents blood from being forced back into the atria. This stage can be seen from a frontal view in [link |b. Blood Flow from the Left Ventricle into the Great Vessels Posterior Tricuspid Bicuspid (mitral) valve valve - 5 ——— Jf (closed) \.. {~ Aortic valve Pulmonary (open) valve Anterior open) (a) Mitral valve (closed) Aortic valve (open) Chordae tendineae (tight) Papillary muscle (contracted) (b) (a) A transverse section through the heart illustrates the four heart valves during ventricular contraction. The two atrioventricular valves are closed, but the two semilunar valves are open. The atria and vessels have been removed. (b) A frontal view shows the closed mitral (bicuspid) valve that prevents backflow of blood into the left atrium. The aortic semilunar valve is open to allow blood to be ejected into the aorta. When the ventricles begin to contract, pressure within the ventricles rises and blood flows toward the area of lowest pressure, which is initially in the atria. This backflow causes the cusps of the tricuspid and mitral (bicuspid) valves to close. These valves are tied down to the papillary muscles by chordae tendineae. During the relaxation phase of the cardiac cycle, the papillary muscles are also relaxed and the tension on the chordae tendineae is slight (see [link]b). However, as the myocardium of the ventricle contracts, so do the papillary muscles. This creates tension on the chordae tendineae (see [link]b), helping to hold the cusps of the atrioventricular valves in place and preventing them from being blown back into the atria. The aortic and pulmonary semilunar valves lack the chordae tendineae and papillary muscles associated with the atrioventricular valves. Instead, they consist of pocket-like folds of endocardium reinforced with additional connective tissue. When the ventricles relax and the change in pressure forces the blood toward the ventricles, the blood presses against these cusps and seals the openings. Note: — meee OPENSTAX COLLEGE F Bear es Visit this site to observe an echocardiogram of actual heart valves opening and closing. Although much of the heart has been “removed” from this gif loop so the chordae tendineae are not visible, why is their presence more critical for the atrioventricular valves (tricuspid and mitral) than the semilunar (aortic and pulmonary) valves? Note: Disorders of the... Heart Valves When heart valves do not function properly, they are often described as incompetent and result in valvular heart disease, which can range from benign to lethal. Some of these conditions are congenital, that is, the individual was born with the defect, whereas others may be attributed to disease processes or trauma. Some malfunctions are treated with medications, others require surgery, and still others may be mild enough that the condition is merely monitored since treatment might trigger more serious Consequences. Valvular disorders are often caused by carditis, or inflammation of the heart. One common trigger for this inflammation is rheumatic fever, or scarlet fever, an autoimmune response to the presence of a bacterium, Streptococcus pyogenes, normally a disease of childhood. While any of the heart valves may be involved in valve disorders, mitral regurgitation is the most common, detected in approximately 2 percent of the population, and the pulmonary semilunar valve is the least frequently involved. When a valve malfunctions, the flow of blood to a region will often be disrupted. The resulting inadequate flow of blood to this region will be described in general terms as an insufficiency. The specific type of insufficiency is named for the valve involved: aortic insufficiency, mitral insufficiency, tricuspid insufficiency, or pulmonary insufficiency. If one of the cusps of the valve is forced backward by the force of the blood, the condition is referred to as a prolapsed valve. Prolapse may occur if the chordae tendineae are damaged or broken, causing the closure mechanism to fail. The failure of the valve to close properly disrupts the normal one-way flow of blood and results in regurgitation, when the blood flows backward from its normal path. Using a stethoscope, the disruption to the normal flow of blood produces a heart murmur. Stenosis is a condition in which the heart valves become rigid and may calcify over time. The loss of flexibility of the valve interferes with normal function and may cause the heart to work harder to propel blood through the valve, which eventually weakens the heart. Aortic stenosis affects approximately 2 percent of the population over 65 years of age, and the percentage increases to approximately 4 percent in individuals over 85 years. Occasionally, one or more of the chordae tendineae will tear or the papillary muscle itself may die as a component of a myocardial infarction (heart attack). In this case, the patient’s condition will deteriorate dramatically and rapidly, and immediate surgical intervention may be required. Auscultation, or listening to a patient’s heart sounds, is one of the most useful diagnostic tools, since it is proven, safe, and inexpensive. The term auscultation is derived from the Latin for “to listen,” and the technique has been used for diagnostic purposes as far back as the ancient Egyptians. Valve and septal disorders will trigger abnormal heart sounds. If a valvular disorder is detected or suspected, a test called an echocardiogram, or simply an “echo,” may be ordered. Echocardiograms are sonograms of the heart and can help in the diagnosis of valve disorders as well as a wide variety of heart pathologies. Note: [ml aC Visit this site for a free download, including excellent animations and audio of heart sounds. Note: Career Connection Cardiologist Cardiologists are medical doctors that specialize in the diagnosis and treatment of diseases of the heart. After completing 4 years of medical school, cardiologists complete a three-year residency in internal medicine followed by an additional three or more years in cardiology. Following this 10-year period of medical training and clinical experience, they qualify for a rigorous two-day examination administered by the Board of Internal Medicine that tests their academic training and clinical abilities, including diagnostics and treatment. After successful completion of this examination, a physician becomes a board-certified cardiologist. Some board-certified cardiologists may be invited to become a Fellow of the American College of Cardiology (FACC). This professional recognition is awarded to outstanding physicians based upon merit, including outstanding credentials, achievements, and community contributions to cardiovascular medicine. Note: — meee OPENStAX COLLEGE _——+ — . a . AR ee: r a Visit this site to learn more about cardiologists. Note: Career Connection Cardiovascular Technologist/Technician Cardiovascular technologists/technicians are trained professionals who perform a variety of imaging techniques, such as sonograms or echocardiograms, used by physicians to diagnose and treat diseases of the heart. Nearly all of these positions require an associate degree, and these technicians earn a median salary of $49,410 as of May 2010, according to the U.S. Bureau of Labor Statistics. Growth within the field is fast, projected at 29 percent from 2010 to 2020. There is a considerable overlap and complementary skills between cardiac technicians and vascular technicians, and so the term cardiovascular technician is often used. Special certifications within the field require documenting appropriate experience and completing additional and often expensive certification examinations. These subspecialties include Certified Rhythm Analysis Technician (CRAT), Certified Cardiographic Technician (CCT), Registered Congenital Cardiac Sonographer (RCCS), Registered Cardiac Electrophysiology Specialist (RCES), Registered Cardiovascular Invasive Specialist (RCIS), Registered Cardiac Sonographer (RCS), Registered Vascular Specialist (RVS), and Registered Phlebology Sonographer (RPhS). Note: [= [=] mh a Openstax COLLEGE =ve t eo Visit this site for more information on cardiovascular technologists/technicians. Coronary Circulation You will recall that the heart is a remarkable pump composed largely of cardiac muscle cells that are incredibly active throughout life. Like all other cells, a cardiomyocyte requires a reliable supply of oxygen and nutrients, and a way to remove wastes, so it needs a dedicated, complex, and extensive coronary circulation. And because of the critical and nearly ceaseless activity of the heart throughout life, this need for a blood supply is even greater than for a typical cell. However, coronary circulation is not continuous; rather, it cycles, reaching a peak when the heart muscle is relaxed and nearly ceasing while it is contracting. Coronary Arteries Coronary arteries supply blood to the myocardium and other components of the heart. The first portion of the aorta after it arises from the left ventricle gives rise to the coronary arteries. There are three dilations in the wall of the aorta just superior to the aortic semilunar valve. Two of these, the left posterior aortic sinus and anterior aortic sinus, give rise to the left and right coronary arteries, respectively. The third sinus, the right posterior aortic sinus, typically does not give rise to a vessel. Coronary vessel branches that remain on the surface of the artery and follow the sulci are called epicardial coronary arteries. The left coronary artery distributes blood to the left side of the heart, the left atrium and ventricle, and the interventricular septum. The circumflex artery arises from the left coronary artery and follows the coronary sulcus to the left. Eventually, it will fuse with the small branches of the right coronary artery. The larger anterior interventricular artery, also known as the left anterior descending artery (LAD), is the second major branch arising from the left coronary artery. It follows the anterior interventricular sulcus around the pulmonary trunk. Along the way it gives rise to numerous smaller branches that interconnect with the branches of the posterior interventricular artery, forming anastomoses. An anastomosis is an area where vessels unite to form interconnections that normally allow blood to circulate to a region even if there may be partial blockage in another branch. The anastomoses in the heart are very small. Therefore, this ability is somewhat restricted in the heart so a coronary artery blockage often results in death of the cells (myocardial infarction) supplied by the particular vessel. The right coronary artery proceeds along the coronary sulcus and distributes blood to the right atrium, portions of both ventricles, and the heart conduction system. Normally, one or more marginal arteries arise from the right coronary artery inferior to the right atrium. The marginal arteries supply blood to the superficial portions of the right ventricle. On the posterior surface of the heart, the right coronary artery gives rise to the posterior interventricular artery, also known as the posterior descending artery. It runs along the posterior portion of the interventricular sulcus toward the apex of the heart, giving rise to branches that supply the interventricular septum and portions of both ventricles. [link] presents views of the coronary circulation from both the anterior and posterior views. Coronary Circulation Aortic arch Left coronary z t Ascending aorta ately Pulmonary trunk Right coronary Circumflex artery artery Right atrium , ; Anterior Atrial arteries interventricular artery Anterior cardiac veins : Great cardiac : vein Small cardiac vein Marginal artery . . Coronary sinus Small cardiac vein Anterior view Circumflex artery Great cardiac vein Marginal artery Posterior interventricular artery Posterior cardiac vein Right coronary artery Middle cardiac Marginal vein artery Posterior view The anterior view of the heart shows the prominent coronary surface vessels. The posterior view of the heart shows the prominent coronary surface vessels. Note: Diseases of the... Heart: Myocardial Infarction Myocardial infarction (MI) is the formal term for what is commonly referred to as a heart attack. It normally results from a lack of blood flow (ischemia) and oxygen (hypoxia) to a region of the heart, resulting in death of the cardiac muscle cells. An MI often occurs when a coronary artery is blocked by the buildup of atherosclerotic plaque consisting of lipids, cholesterol and fatty acids, and white blood cells, primarily macrophages. It can also occur when a portion of an unstable atherosclerotic plaque travels through the coronary arterial system and lodges in one of the smaller vessels. The resulting blockage restricts the flow of blood and oxygen to the myocardium and causes death of the tissue. MIs may be triggered by excessive exercise, in which the partially occluded artery is no longer able to pump sufficient quantities of blood, or severe stress, which may induce spasm of the smooth muscle in the walls of the vessel. In the case of acute MI, there is often sudden pain beneath the sternum (retrosternal pain) called angina pectoris, often radiating down the left arm in males but not in female patients. Until this anomaly between the sexes was discovered, many female patients suffering MIs were misdiagnosed and sent home. In addition, patients typically present with difficulty breathing and shortness of breath (dyspnea), irregular heartbeat (palpations), nausea and vomiting, sweating (diaphoresis), anxiety, and fainting (syncope), although not all of these symptoms may be present. Many of the symptoms are shared with other medical conditions, including anxiety attacks and simple indigestion, so differential diagnosis is critical. It is estimated that between 22 and 64 percent of MIs present without any symptoms. An MI can be confirmed by examining the patient’s ECG, which frequently reveals alterations in the ST and Q components. Some classification schemes of MI are referred to as ST-elevated MI (STEMI) and non-elevated MI (non-STEMTI). In addition, echocardiography or cardiac magnetic resonance imaging may be employed. Common blood tests indicating an MI include elevated levels of creatine kinase MB (an enzyme that catalyzes the conversion of creatine to phosphocreatine, consuming ATP) and cardiac troponin (the regulatory protein for muscle contraction), both of which are released by damaged cardiac muscle cells. Immediate treatments for MI are essential and include administering supplemental oxygen, aspirin that helps to break up clots, and nitroglycerine administered sublingually (under the tongue) to facilitate its absorption. Despite its unquestioned success in treatments and use since the 1880s, the mechanism of nitroglycerine is still incompletely understood but is believed to involve the release of nitric oxide, a known vasodilator, and endothelium-derived releasing factor, which also relaxes the smooth muscle in the tunica media of coronary vessels. Longer-term treatments include injections of thrombolytic agents such as streptokinase that dissolve the clot, the anticoagulant heparin, balloon angioplasty and stents to open blocked vessels, and bypass surgery to allow blood to pass around the site of blockage. If the damage is extensive, coronary replacement with a donor heart or coronary assist device, a sophisticated mechanical device that supplements the pumping activity of the heart, may be employed. Despite the attention, development of artificial hearts to augment the severely limited supply of heart donors has proven less than satisfactory but will likely improve in the future. MIs may trigger cardiac arrest, but the two are not synonymous. Important risk factors for MI include cardiovascular disease, age, smoking, high blood levels of the low-density lipoprotein (LDL, often referred to as “bad” cholesterol), low levels of high-density lipoprotein (HDL, or “good” cholesterol), hypertension, diabetes mellitus, obesity, lack of physical exercise, chronic kidney disease, excessive alcohol consumption, and use of illegal drugs. Coronary Veins Coronary veins drain the heart and generally parallel the large surface arteries (see [link]). The great cardiac vein can be seen initially on the surface of the heart following the interventricular sulcus, but it eventually flows along the coronary sulcus into the coronary sinus on the posterior surface. The great cardiac vein initially parallels the anterior interventricular artery and drains the areas supplied by this vessel. It receives several major branches, including the posterior cardiac vein, the middle cardiac vein, and the small cardiac vein. The posterior cardiac vein parallels and drains the areas supplied by the marginal artery branch of the circumflex artery. The middle cardiac vein parallels and drains the areas supplied by the posterior interventricular artery. The small cardiac vein parallels the right coronary artery and drains the blood from the posterior surfaces of the right atrium and ventricle. The coronary sinus is a large, thin-walled vein on the posterior surface of the heart lying within the atrioventricular sulcus and emptying directly into the right atrium. The anterior cardiac veins parallel the small cardiac arteries and drain the anterior surface of the right ventricle. Unlike these other cardiac veins, it bypasses the coronary sinus and drains directly into the right atrium. Note: Diseases of the... Heart: Coronary Artery Disease Coronary artery disease is the leading cause of death worldwide. It occurs when the buildup of plaque—a fatty material including cholesterol, connective tissue, white blood cells, and some smooth muscle cells— within the walls of the arteries obstructs the flow of blood and decreases the flexibility or compliance of the vessels. This condition is called atherosclerosis, a hardening of the arteries that involves the accumulation of plaque. As the coronary blood vessels become occluded, the flow of blood to the tissues will be restricted, a condition called ischemia that causes the cells to receive insufficient amounts of oxygen, called hypoxia. [link] shows the blockage of coronary arteries highlighted by the injection of dye. Some individuals with coronary artery disease report pain radiating from the chest called angina pectoris, but others remain asymptomatic. If untreated, coronary artery disease can lead to MI or a heart attack. Atherosclerotic Coronary Arteries ik ol = Blockage of common trunk wy of left coronary artery In this coronary angiogram (X-ray), the dye makes visible two occluded coronary arteries. Such blockages can lead to decreased blood flow (ischemia) and insufficient oxygen (hypoxia) delivered to the cardiac tissues. If uncorrected, this can lead to cardiac muscle death (myocardial infarction). The disease progresses slowly and often begins in children and can be seen as fatty “streaks” in the vessels. It then gradually progresses throughout life. Well-documented risk factors include smoking, family history, hypertension, obesity, diabetes, high alcohol consumption, lack of exercise, stress, and hyperlipidemia or high circulating levels of lipids in the blood. Treatments may include medication, changes to diet and exercise, angioplasty with a balloon catheter, insertion of a stent, or coronary bypass procedure. Angioplasty is a procedure in which the occlusion is mechanically widened with a balloon. A specialized catheter with an expandable tip is inserted into a superficial vessel, normally in the leg, and then directed to the site of the occlusion. At this point, the balloon is inflated to compress the plaque material and to open the vessel to increase blood flow. Then, the balloon is deflated and retracted. A stent consisting of a specialized mesh is typically inserted at the site of occlusion to reinforce the weakened and damaged walls. Stent insertions have been routine in cardiology for more than 40 years. Coronary bypass surgery may also be performed. This surgical procedure grafts a replacement vessel obtained from another, less vital portion of the body to bypass the occluded area. This procedure is clearly effective in treating patients experiencing a MI, but overall does not increase longevity. Nor does it seem advisable in patients with stable although diminished cardiac capacity since frequently loss of mental acuity occurs following the procedure. Long-term changes to behavior, emphasizing diet and exercise plus a medicine regime tailored to lower blood pressure, lower cholesterol and lipids, and reduce clotting are equally as effective. Chapter Review The heart resides within the pericardial sac and is located in the mediastinal space within the thoracic cavity. The pericardial sac consists of two fused layers: an outer fibrous capsule and an inner parietal pericardium lined with a serous membrane. Between the pericardial sac and the heart is the pericardial cavity, which is filled with lubricating serous fluid. The walls of the heart are composed of an outer epicardium, a thick myocardium, and an inner lining layer of endocardium. The human heart consists of a pair of atria, which receive blood and pump it into a pair of ventricles, which pump blood into the vessels. The right atrium receives systemic blood relatively low in oxygen and pumps it into the right ventricle, which pumps it into the pulmonary circuit. Exchange of oxygen and carbon dioxide occurs in the lungs, and blood high in oxygen returns to the left atrium, which pumps blood into the left ventricle, which in turn pumps blood into the aorta and the remainder of the systemic circuit. The septa are the partitions that separate the chambers of the heart. They include the interatrial septum, the interventricular septum, and the atrioventricular septum. Two of these openings are guarded by the atrioventricular valves, the right tricuspid valve and the left mitral valve, which prevent the backflow of blood. Each is attached to chordae tendineae that extend to the papillary muscles, which are extensions of the myocardium, to prevent the valves from being blown back into the atria. The pulmonary valve is located at the base of the pulmonary trunk, and the left semilunar valve is located at the base of the aorta. The right and left coronary arteries are the first to branch off the aorta and arise from two of the three sinuses located near the base of the aorta and are generally located in the sulci. Cardiac veins parallel the small cardiac arteries and generally drain into the coronary sinus. Interactive Link Questions Exercise: Problem: Visit this site to observe an echocardiogram of actual heart valves opening and closing. Although much of the heart has been “removed” from this gif loop so the chordae tendineae are not visible, why is their presence more critical for the atrioventricular valves (tricuspid and mitral) than the semilunar (aortic and pulmonary) valves? Solution: The pressure gradient between the atria and the ventricles is much greater than that between the ventricles and the pulmonary trunk and aorta. Without the presence of the chordae tendineae and papillary muscles, the valves would be blown back (prolapsed) into the atria and blood would regurgitate. Review Questions Exercise: Problem: Which of the following is not important in preventing backflow of blood? a. chordae tendineae b. papillary muscles c. AV valves d. endocardium Solution: D Exercise: Problem: Which valve separates the left atrium from the left ventricle? a. mitral b. tricuspid c. pulmonary d. aortic Solution: A Exercise: Problem: Which of the following lists the valves in the order through which the blood flows from the vena cava through the heart? a. tricuspid, pulmonary semilunar, bicuspid, aortic semilunar b. mitral, pulmonary semilunar, bicuspid, aortic semilunar c. aortic semilunar, pulmonary semilunar, tricuspid, bicuspid d. bicuspid, aortic semilunar, tricuspid, pulmonary semilunar Solution: A Exercise: Problem: Which chamber initially receives blood from the systemic circuit? a. left atrium b. left ventricle c. right atrium d. right ventricle Solution: CG Exercise: Problem: The layer secretes chemicals that help to regulate ionic environments and strength of contraction and serve as powerful vasoconstrictors. a. pericardial sac b. endocardium c. myocardium d. epicardium Solution: B Exercise: Problem:The myocardium would be the thickest in the a. left atrium b. left ventricle c. right atrium d. right ventricle Solution: B Exercise: Problem:In which septum is it normal to find openings in the adult? a. interatrial septum b. interventricular septum c. atrioventricular septum d. all of the above Solution: C Critical Thinking Questions Exercise: Problem: Describe how the valves keep the blood moving in one direction. Solution: When the ventricles contract and pressure begins to rise in the ventricles, there is an initial tendency for blood to flow back (regurgitate) to the atria. However, the papillary muscles also contract, placing tension on the chordae tendineae and holding the atrioventricular valves (tricuspid and mitral) in place to prevent the valves from prolapsing and being forced back into the atria. The semilunar valves (pulmonary and aortic) lack chordae tendineae and papillary muscles, but do not face the same pressure gradients as do the atrioventricular valves. As the ventricles relax and pressure drops within the ventricles, there is a tendency for the blood to flow backward. However, the valves, consisting of reinforced endothelium and connective tissue, fill with blood and seal off the opening preventing the return of blood. Exercise: Problem: Why is the pressure in the pulmonary circulation lower than in the systemic circulation? Solution: The pulmonary circuit consists of blood flowing to and from the lungs, whereas the systemic circuit carries blood to and from the entire body. The systemic circuit is far more extensive, consisting of far more vessels and offers much greater resistance to the flow of blood, so the heart must generate a higher pressure to overcome this resistance. This can be seen in the thickness of the myocardium in the ventricles. Glossary anastomosis (plural = anastomoses) area where vessels unite to allow blood to circulate even if there may be partial blockage in another branch anterior cardiac veins vessels that parallel the small cardiac arteries and drain the anterior surface of the right ventricle; bypass the coronary sinus and drain directly into the right atrium anterior interventricular artery (also, left anterior descending artery or LAD) major branch of the left coronary artery that follows the anterior interventricular sulcus anterior interventricular sulcus sulcus located between the left and right ventricles on the anterior surface of the heart aortic valve (also, aortic semilunar valve) valve located at the base of the aorta atrioventricular septum cardiac septum located between the atria and ventricles; atrioventricular valves are located here atrioventricular valves one-way valves located between the atria and ventricles; the valve on the right is called the tricuspid valve, and the one on the left is the mitral or bicuspid valve atrium (plural = atria) upper or receiving chamber of the heart that pumps blood into the lower chambers just prior to their contraction; the right atrium receives blood from the systemic circuit that flows into the right ventricle; the left atrium receives blood from the pulmonary circuit that flows into the left ventricle auricle extension of an atrium visible on the superior surface of the heart bicuspid valve (also, mitral valve or left atrioventricular valve) valve located between the left atrium and ventricle; consists of two flaps of tissue cardiac notch depression in the medial surface of the inferior lobe of the left lung where the apex of the heart is located cardiac skeleton (also, skeleton of the heart) reinforced connective tissue located within the atrioventricular septum; includes four rings that surround the openings between the atria and ventricles, and the openings to the pulmonary trunk and aorta; the point of attachment for the heart valves cardiomyocyte muscle cell of the heart chordae tendineae string-like extensions of tough connective tissue that extend from the flaps of the atrioventricular valves to the papillary muscles circumflex artery branch of the left coronary artery that follows coronary sulcus coronary arteries branches of the ascending aorta that supply blood to the heart; the left coronary artery feeds the left side of the heart, the left atrium and ventricle, and the interventricular septum; the right coronary artery feeds the right atrium, portions of both ventricles, and the heart conduction system coronary sinus large, thin-walled vein on the posterior surface of the heart that lies within the atrioventricular sulcus and drains the heart myocardium directly into the right atrium coronary sulcus sulcus that marks the boundary between the atria and ventricles coronary veins vessels that drain the heart and generally parallel the large surface arteries endocardium innermost layer of the heart lining the heart chambers and heart valves; composed of endothelium reinforced with a thin layer of connective tissue that binds to the myocardium endothelium layer of smooth, simple squamous epithelium that lines the endocardium and blood vessels epicardial coronary arteries surface arteries of the heart that generally follow the sulci epicardium innermost layer of the serous pericardium and the outermost layer of the heart wall foramen ovale opening in the fetal heart that allows blood to flow directly from the right atrium to the left atrium, bypassing the fetal pulmonary circuit fossa ovalis oval-shaped depression in the interatrial septum that marks the former location of the foramen ovale great cardiac vein vessel that follows the interventricular sulcus on the anterior surface of the heart and flows along the coronary sulcus into the coronary sinus on the posterior surface; parallels the anterior interventricular artery and drains the areas supplied by this vessel hypertrophic cardiomyopathy pathological enlargement of the heart, generally for no known reason inferior vena cava large systemic vein that returns blood to the heart from the inferior portion of the body interatrial septum cardiac septum located between the two atria; contains the fossa ovalis after birth interventricular septum cardiac septum located between the two ventricles left atrioventricular valve (also, mitral valve or bicuspid valve) valve located between the left atrium and ventricle; consists of two flaps of tissue marginal arteries branches of the right coronary artery that supply blood to the superficial portions of the right ventricle mesothelium simple squamous epithelial portion of serous membranes, such as the superficial portion of the epicardium (the visceral pericardium) and the deepest portion of the pericardium (the parietal pericardium) middle cardiac vein vessel that parallels and drains the areas supplied by the posterior interventricular artery; drains into the great cardiac vein mitral valve (also, left atrioventricular valve or bicuspid valve) valve located between the left atrium and ventricle; consists of two flaps of tissue moderator band band of myocardium covered by endocardium that arises from the inferior portion of the interventricular septum in the right ventricle and crosses to the anterior papillary muscle; contains conductile fibers that carry electrical signals followed by contraction of the heart myocardium thickest layer of the heart composed of cardiac muscle cells built upon a framework of primarily collagenous fibers and blood vessels that supply it and the nervous fibers that help to regulate it papillary muscle extension of the myocardium in the ventricles to which the chordae tendineae attach pectinate muscles muscular ridges seen on the anterior surface of the right atrium pericardial cavity cavity surrounding the heart filled with a lubricating serous fluid that reduces friction as the heart contracts pericardial sac (also, pericardium) membrane that separates the heart from other mediastinal structures; consists of two distinct, fused sublayers: the fibrous pericardium and the parietal pericardium pericardium (also, pericardial sac) membrane that separates the heart from other mediastinal structures; consists of two distinct, fused sublayers: the fibrous pericardium and the parietal pericardium posterior cardiac vein vessel that parallels and drains the areas supplied by the marginal artery branch of the circumflex artery; drains into the great cardiac vein posterior interventricular artery (also, posterior descending artery) branch of the right coronary artery that runs along the posterior portion of the interventricular sulcus toward the apex of the heart and gives rise to branches that supply the interventricular septum and portions of both ventricles posterior interventricular sulcus sulcus located between the left and right ventricles on the anterior surface of the heart pulmonary arteries left and right branches of the pulmonary trunk that carry deoxygenated blood from the heart to each of the lungs pulmonary capillaries capillaries surrounding the alveoli of the lungs where gas exchange occurs: carbon dioxide exits the blood and oxygen enters pulmonary circuit blood flow to and from the lungs pulmonary trunk large arterial vessel that carries blood ejected from the right ventricle; divides into the left and right pulmonary arteries pulmonary valve (also, pulmonary semilunar valve, the pulmonic valve, or the right semilunar valve) valve at the base of the pulmonary trunk that prevents backflow of blood into the right ventricle; consists of three flaps pulmonary veins veins that carry highly oxygenated blood into the left atrium, which pumps the blood into the left ventricle, which in turn pumps oxygenated blood into the aorta and to the many branches of the systemic circuit right atrioventricular valve (also, tricuspid valve) valve located between the right atrium and ventricle; consists of three flaps of tissue semilunar valves valves located at the base of the pulmonary trunk and at the base of the aorta septum (plural = septa) walls or partitions that divide the heart into chambers septum primum flap of tissue in the fetus that covers the foramen ovale within a few seconds after birth small cardiac vein parallels the right coronary artery and drains blood from the posterior surfaces of the right atrium and ventricle; drains into the great cardiac vein sulcus (plural = sulci) fat-filled groove visible on the surface of the heart; coronary vessels are also located in these areas superior vena cava large systemic vein that returns blood to the heart from the superior portion of the body systemic circuit blood flow to and from virtually all of the tissues of the body trabeculae carneae ridges of muscle covered by endocardium located in the ventricles tricuspid valve term used most often in clinical settings for the right atrioventricular valve valve in the cardiovascular system, a specialized structure located within the heart or vessels that ensures one-way flow of blood ventricle one of the primary pumping chambers of the heart located in the lower portion of the heart; the left ventricle is the major pumping chamber on the lower left side of the heart that ejects blood into the systemic circuit via the aorta and receives blood from the left atrium; the right ventricle is the major pumping chamber on the lower right side of the heart that ejects blood into the pulmonary circuit via the pulmonary trunk and receives blood from the right atrium Cardiac Muscle and Electrical Activity By the end of this section, you will be able to: e Describe the structure of cardiac muscle e Identify and describe the components of the conducting system that distributes electrical impulses through the heart ¢ Compare the effect of ion movement on membrane potential of cardiac conductive and contractile cells e Relate characteristics of an electrocardiogram to events in the cardiac cycle e Identify blocks that can interrupt the cardiac cycle Recall that cardiac muscle shares a few characteristics with both skeletal muscle and smooth muscle, but it has some unique properties of its own. Not the least of these exceptional properties is its ability to initiate an electrical potential at a fixed rate that spreads rapidly from cell to cell to trigger the contractile mechanism. This property is known as autorhythmicity. Neither smooth nor skeletal muscle can do this. Even though cardiac muscle has autorhythmicity, heart rate is modulated by the endocrine and nervous systems. There are two major types of cardiac muscle cells: myocardial contractile cells and myocardial conducting cells. The myocardial contractile cells constitute the bulk (99 percent) of the cells in the atria and ventricles. Contractile cells conduct impulses and are responsible for contractions that pump blood through the body. The myocardial conducting cells (1 percent of the cells) form the conduction system of the heart. Except for Purkinje cells, they are generally much smaller than the contractile cells and have few of the myofibrils or filaments needed for contraction. Their function is similar in many respects to neurons, although they are specialized muscle cells. Myocardial conduction cells initiate and propagate the action potential (the electrical impulse) that travels throughout the heart and triggers the contractions that propel the blood. Structure of Cardiac Muscle Compared to the giant cylinders of skeletal muscle, cardiac muscle cells, or cardiomyocytes, are considerably shorter with much smaller diameters. Cardiac muscle also demonstrates striations, the alternating pattern of dark A bands and light I bands attributed to the precise arrangement of the myofilaments and fibrils that are organized in sarcomeres along the length of the cell ({link]a). These contractile elements are virtually identical to skeletal muscle. T (transverse) tubules penetrate from the surface plasma membrane, the sarcolemma, to the interior of the cell, allowing the electrical impulse to reach the interior. The T tubules are only found at the Z, discs, whereas in skeletal muscle, they are found at the junction of the A and I bands. Therefore, there are one-half as many T tubules in cardiac muscle as in skeletal muscle. In addition, the sarcoplasmic reticulum stores few calcium ions, so most of the calcium ions must come from outside the cells. The result is a slower onset of contraction. Mitochondria are plentiful, providing energy for the contractions of the heart. Typically, cardiomyocytes have a single, central nucleus, but two or more nuclei may be found in some cells. Cardiac muscle cells branch freely. A junction between two adjoining cells is marked by a critical structure called an intercalated disc, which helps support the synchronized contraction of the muscle ([link]b). The sarcolemmas from adjacent cells bind together at the intercalated discs. They consist of desmosomes, specialized linking proteoglycans, tight junctions, and large numbers of gap junctions that allow the passage of ions between the cells and help to synchronize the contraction ([link]c). Intercellular connective tissue also helps to bind the cells together. The importance of strongly binding these cells together is necessitated by the forces exerted by contraction. Cardiac Muscle Intercalated discs Intercalated discs Mitochondria Intercalated discs Gap junction Cardiac muscle fiber (a) Desmosome — A band | band (c) (a) Cardiac muscle cells have myofibrils composed of myofilaments arranged in sarcomeres, T tubules to transmit the impulse from the sarcolemma to the interior of the cell, numerous mitochondria for energy, and intercalated discs that are found at the junction of different cardiac muscle cells. (b) A photomicrograph of cardiac muscle cells shows the nuclei and intercalated discs. (c) An intercalated disc connects cardiac muscle cells and consists of desmosomes and gap junctions. LM x 1600. (Micrograph provided by the Regents of the University of Michigan Medical School © 2012) Cardiac muscle undergoes aerobic respiration patterns, primarily metabolizing lipids and carbohydrates. Myoglobin, lipids, and glycogen are all stored within the cytoplasm. Cardiac muscle cells undergo twitch-type contractions with long refractory periods followed by brief relaxation periods. The relaxation is essential so the heart can fill with blood for the next cycle. The refractory period is very long to prevent the possibility of tetany, a condition in which muscle remains involuntarily contracted. In the heart, tetany is not compatible with life, since it would prevent the heart from pumping blood. Note: Everyday Connection Repair and Replacement Damaged cardiac muscle cells have extremely limited abilities to repair themselves or to replace dead cells via mitosis. Recent evidence indicates that at least some stem cells remain within the heart that continue to divide and at least potentially replace these dead cells. However, newly formed or repaired cells are rarely as functional as the original cells, and cardiac function is reduced. In the event of a heart attack or MI, dead cells are often replaced by patches of scar tissue. Autopsies performed on individuals who had successfully received heart transplants show some proliferation of original cells. If researchers can unlock the mechanism that generates new cells and restore full mitotic capabilities to heart muscle, the prognosis for heart attack survivors will be greatly enhanced. To date, myocardial cells produced within the patient (in situ) by cardiac stem cells seem to be nonfunctional, although those grown in Petri dishes (in vitro) do beat. Perhaps soon this mystery will be solved, and new advances in treatment will be commonplace. Conduction System of the Heart If embryonic heart cells are separated into a Petri dish and kept alive, each is capable of generating its own electrical impulse followed by contraction. When two independently beating embryonic cardiac muscle cells are placed together, the cell with the higher inherent rate sets the pace, and the impulse spreads from the faster to the slower cell to trigger a contraction. As more cells are joined together, the fastest cell continues to assume control of the rate. A fully developed adult heart maintains the capability of generating its own electrical impulse, triggered by the fastest cells, as part of the cardiac conduction system. The components of the cardiac conduction system include the sinoatrial node, the atrioventricular node, the atrioventricular bundle, the atrioventricular bundle branches, and the Purkinje cells ([link]). Conduction System of the Heart ——™ Yd ~ ~. Frontal plane = 2 | through heart | Arch of aorta = ~~ (SS ———— Bachman’s bundle <= Sinoatrial LL Pe | (SA) node Anterior internodal Atrioventricular (AV) node Middle internodal Posterior internodal Left atrium Atrioventricular (AV) bundle (bundle of His) Left ventricle Right and left bundle branches Right atrium Right ventricle Purkinje fibers Anterior view of frontal section Specialized conducting components of the heart include the sinoatrial node, the internodal pathways, the atrioventricular node, the atrioventricular bundle, the right and left bundle branches, and the Purkinje fibers. Sinoatrial (SA) Node Normal cardiac rhythm is established by the sinoatrial (SA) node, a specialized clump of myocardial conducting cells located in the superior and posterior walls of the right atrium in close proximity to the orifice of the superior vena cava. The SA node has the highest inherent rate of depolarization and is known as the pacemaker of the heart. It initiates the sinus rhythm, or normal electrical pattern followed by contraction of the heart. This impulse spreads from its initiation in the SA node throughout the atria through specialized internodal pathways, to the atrial myocardial contractile cells and the atrioventricular node. The internodal pathways consist of three bands (anterior, middle, and posterior) that lead directly from the SA node to the next node in the conduction system, the atrioventricular node (see [link]). The impulse takes approximately 50 ms (milliseconds) to travel between these two nodes. The relative importance of this pathway has been debated since the impulse would reach the atrioventricular node simply following the cell-by-cell pathway through the contractile cells of the myocardium in the atria. In addition, there is a specialized pathway called Bachmann’s bundle or the interatrial band that conducts the impulse directly from the right atrium to the left atrium. Regardless of the pathway, as the impulse reaches the atrioventricular septum, the connective tissue of the cardiac skeleton prevents the impulse from spreading into the myocardial cells in the ventricles except at the atrioventricular node. [link] illustrates the initiation of the impulse in the SA node that then spreads the impulse throughout the atria to the atrioventricular node. Cardiac Conduction ACS =o BN ae “aN (. |. © \ 7 (1) The sinoatrial (SA) node and the remainder of the conduction system are at rest. (2) The SA node initiates the action potential, which sweeps across the atria. (3) After reaching the atrioventricular node, there is a delay of approximately 100 ms that allows the atria to complete pumping blood before the impulse is transmitted to the atrioventricular bundle. (4) Following the delay, the impulse travels through the atrioventricular bundle and bundle branches to the Purkinje fibers, and also reaches the right papillary muscle via the moderator band. (5) The impulse spreads to the contractile fibers of the ventricle. (6) Ventricular contraction begins. The electrical event, the wave of depolarization, is the trigger for muscular contraction. The wave of depolarization begins in the right atrium, and the impulse spreads across the superior portions of both atria and then down through the contractile cells. The contractile cells then begin contraction from the superior to the inferior portions of the atria, efficiently pumping blood into the ventricles. Atrioventricular (AV) Node The atrioventricular (AV) node is a second clump of specialized myocardial conductive cells, located in the inferior portion of the right atrium within the atrioventricular septum. The septum prevents the impulse from spreading directly to the ventricles without passing through the AV node. There is a critical pause before the AV node depolarizes and transmits the impulse to the atrioventricular bundle (see [link], step 3). This delay in transmission is partially attributable to the small diameter of the cells of the node, which slow the impulse. Also, conduction between nodal cells is less efficient than between conducting cells. These factors mean that it takes the impulse approximately 100 ms to pass through the node. This pause is critical to heart function, as it allows the atrial cardiomyocytes to complete their contraction that pumps blood into the ventricles before the impulse is transmitted to the cells of the ventricle itself. With extreme stimulation by the SA node, the AV node can transmit impulses maximally at 220 per minute. This establishes the typical maximum heart rate in a healthy young individual. Damaged hearts or those stimulated by drugs can contract at higher rates, but at these rates, the heart can no longer effectively pump blood. Atrioventricular Bundle (Bundle of His), Bundle Branches, and Purkinje Fibers Arising from the AV node, the atrioventricular bundle, or bundle of His, proceeds through the interventricular septum before dividing into two atrioventricular bundle branches, commonly called the left and right bundle branches. The left bundle branch has two fascicles. The left bundle branch supplies the left ventricle, and the right bundle branch the right ventricle. Since the left ventricle is much larger than the right, the left bundle branch is also considerably larger than the right. Portions of the right bundle branch are found in the moderator band and supply the right papillary muscles. Because of this connection, each papillary muscle receives the impulse at approximately the same time, so they begin to contract simultaneously just prior to the remainder of the myocardial contractile cells of the ventricles. This is believed to allow tension to develop on the chordae tendineae prior to right ventricular contraction. There is no corresponding moderator band on the left. Both bundle branches descend and reach the apex of the heart where they connect with the Purkinje fibers (see [link], step 4). This passage takes approximately 25 ms. The Purkinje fibers are additional myocardial conductive fibers that spread the impulse to the myocardial contractile cells in the ventricles. They extend throughout the myocardium from the apex of the heart toward the atrioventricular septum and the base of the heart. The Purkinje fibers have a fast inherent conduction rate, and the electrical impulse reaches all of the ventricular muscle cells in about 75 ms (see [link], step 5). Since the electrical stimulus begins at the apex, the contraction also begins at the apex and travels toward the base of the heart, similar to squeezing a tube of toothpaste from the bottom. This allows the blood to be pumped out of the ventricles and into the aorta and pulmonary trunk. The total time elapsed from the initiation of the impulse in the SA node until depolarization of the ventricles is approximately 225 ms. Membrane Potentials and Ion Movement in Cardiac Conductive Cells Action potentials are considerably different between cardiac conductive cells and cardiac contractive cells. While Na* and K* play essential roles, Ca** is also critical for both types of cells. Unlike skeletal muscles and neurons, cardiac conductive cells do not have a stable resting potential. Conductive cells contain a series of sodium ion channels that allow a normal and slow influx of sodium ions that causes the membrane potential to rise slowly from an initial value of -60 mV up to about —40 mV. The resulting movement of sodium ions creates spontaneous depolarization (or prepotential depolarization). At this point, calcium ion channels open and Ca** enters the cell, further depolarizing it at a more rapid rate until it reaches a value of approximately +5 mV. At this point, the calcium ion channels close and K* channels open, allowing outflux of K* and resulting in repolarization. When the membrane potential reaches approximately —60 mV, the K* channels close and Na™ channels open, and the prepotential phase begins again. This phenomenon explains the autorhythmicity properties of cardiac muscle ({link]). Action Potential at the SA Node lies Rapid influx of Ca2* ae Outflux of Kt Depolarization fe) Repolarization Slow influx of Nat Prepotential potential my) 40 \ Threshold -60 Membrane _90 —80 0.8 1.6 Time (s) The prepotential is due to a slow influx of sodium ions until the threshold is reached followed by a rapid depolarization and repolarization. The prepotential accounts for the membrane reaching threshold and initiates the spontaneous depolarization and contraction of the cell. Note the lack of a resting potential. Membrane Potentials and Ion Movement in Cardiac Contractile Cells There is a distinctly different electrical pattern involving the contractile cells. In this case, there is a rapid depolarization, followed by a plateau phase and then repolarization. This phenomenon accounts for the long refractory periods required for the cardiac muscle cells to pump blood effectively before they are capable of firing for a second time. These cardiac myocytes normally do not initiate their own electrical potential but rather wait for an impulse to reach them. Contractile cells demonstrate a much more stable resting phase than conductive cells at approximately —80 mV for cells in the atria and -90 mV for cells in the ventricles. Despite this initial difference, the other components of their action potentials are virtually identical. In both cases, when stimulated by an action potential, voltage-gated channels rapidly open, beginning the positive-feedback mechanism of depolarization. This rapid influx of positively charged ions raises the membrane potential to approximately +30 mV, at which point the sodium channels close. The rapid depolarization period typically lasts 3-5 ms. Depolarization is followed by the plateau phase, in which membrane potential declines relatively slowly. This is due in large part to the opening of the slow Ca** channels, allowing Ca** to enter the cell while few K* channels are open, allowing K* to exit the cell. The relatively long plateau phase lasts approximately 175 ms. Once the membrane potential reaches approximately zero, the Ca** channels close and K* channels open, allowing K* to exit the cell. The repolarization lasts approximately 75 ms. At this point, membrane potential drops until it reaches resting levels once more and the cycle repeats. The entire event lasts between 250 and 300 ms ([link]). The absolute refractory period for cardiac contractile muscle lasts approximately 200 ms, and the relative refractory period lasts approximately 50 ms, for a total of 250 ms. This extended period is critical, since the heart muscle must contract to pump blood effectively and the contraction must follow the electrical events. Without extended refractory periods, premature contractions would occur in the heart and would not be compatible with life. Action Potential in Cardiac Contractile Cells Na* channels close Slow Ca?* channels open oe Slow Ca2* channels close Repolarization The plateau Rapid depolarization K* channels close mV Voltage-gated Refractory period ion channels we open Absolute Relative Influx of Na* Time (ms) (a) Skeletal muscle Cardiac muscle Action potential Action potential Contraction Contraction Tension Tension Time (ms) Time (ms) (b) (a) Note the long plateau phase due to the influx of calcium ions. The extended refractory period allows the cell to fully contract before another electrical event can occur. (b) The action potential for heart muscle is compared to that of skeletal muscle. Calcium Ions Calcium ions play two critical roles in the physiology of cardiac muscle. Their influx through slow calcium channels accounts for the prolonged plateau phase and absolute refractory period that enable cardiac muscle to function properly. Calcium ions also combine with the regulatory protein troponin in the troponin-tropomyosin complex; this complex removes the inhibition that prevents the heads of the myosin molecules from forming cross bridges with the active sites on actin that provide the power stroke of contraction. This mechanism is virtually identical to that of skeletal muscle. Approximately 20 percent of the calcium required for contraction is supplied by the influx of Ca?* during the plateau phase. The remaining Ca?* for contraction is released from storage in the sarcoplasmic reticulum. Comparative Rates of Conduction System Firing The pattern of prepotential or spontaneous depolarization, followed by rapid depolarization and repolarization just described, are seen in the SA node and a few other conductive cells in the heart. Since the SA node is the pacemaker, it reaches threshold faster than any other component of the conduction system. It will initiate the impulses spreading to the other conducting cells. The SA node, without nervous or endocrine control, would initiate a heart impulse approximately 80—100 times per minute. Although each component of the conduction system is capable of generating its own impulse, the rate progressively slows as you proceed from the SA node to the Purkinje fibers. Without the SA node, the AV node would generate a heart rate of 40-60 beats per minute. If the AV node were blocked, the atrioventricular bundle would fire at a rate of approximately 30—40 impulses per minute. The bundle branches would have an inherent rate of 20—30 impulses per minute, and the Purkinje fibers would fire at 15— 20 impulses per minute. While a few exceptionally trained aerobic athletes demonstrate resting heart rates in the range of 30—40 beats per minute (the lowest recorded figure is 28 beats per minute for Miguel Indurain, a cyclist), for most individuals, rates lower than 50 beats per minute would indicate a condition called bradycardia. Depending upon the specific individual, as rates fall much below this level, the heart would be unable to maintain adequate flow of blood to vital tissues, initially resulting in decreasing loss of function across the systems, unconsciousness, and ultimately death. Electrocardiogram By careful placement of surface electrodes on the body, it is possible to record the complex, compound electrical signal of the heart. This tracing of the electrical signal is the electrocardiogram (ECG), also commonly abbreviated EKG (K coming kardiology, from the German term for cardiology). Careful analysis of the ECG reveals a detailed picture of both normal and abnormal heart function, and is an indispensable clinical diagnostic tool. The standard electrocardiograph (the instrument that generates an ECG) uses 3, 5, or 12 leads. The greater the number of leads an electrocardiograph uses, the more information the ECG provides. The term “lead” may be used to refer to the cable from the electrode to the electrical recorder, but it typically describes the voltage difference between two of the electrodes. The 12-lead electrocardiograph uses 10 electrodes placed in standard locations on the patient’s skin ([link]). In continuous ambulatory electrocardiographs, the patient wears a small, portable, battery- operated device known as a Holter monitor, or simply a Holter, that continuously monitors heart electrical activity, typically for a period of 24 hours during the patient’s normal routine. Standard Placement of ECG Leads In a 12-lead ECG, six electrodes are placed on the chest, and four electrodes are placed on the limbs. A normal ECG tracing is presented in [link]. Each component, segment, and interval is labeled and corresponds to important electrical events, demonstrating the relationship between these events and contraction in the heart. There are five prominent points on the ECG: the P wave, the QRS complex, and the T wave. The small P wave represents the depolarization of the atria. The atria begin contracting approximately 25 ms after the start of the P wave. The large QRS complex represents the depolarization of the ventricles, which requires a much stronger electrical signal because of the larger size of the ventricular cardiac muscle. The ventricles begin to contract as the QRS reaches the peak of the R wave. Lastly, the T wave represents the repolarization of the ventricles. The repolarization of the atria occurs during the QRS complex, which masks it on an ECG. The major segments and intervals of an ECG tracing are indicated in [link]. Segments are defined as the regions between two waves. Intervals include one segment plus one or more waves. For example, the PR segment begins at the end of the P wave and ends at the beginning of the QRS complex. The PR interval starts at the beginning of the P wave and ends with the beginning of the QRS complex. The PR interval is more clinically relevant, as it measures the duration from the beginning of atrial depolarization (the P wave) to the initiation of the QRS complex. Since the Q wave may be difficult to view in some tracings, the measurement is often extended to the R that is more easily visible. Should there be a delay in passage of the impulse from the SA node to the AV node, it would be visible in the PR interval. [link] correlates events of heart contraction to the corresponding segments and intervals of an ECG. Note: fe — meee, . S; is the sound created by the closing of the atrioventricular valves during ventricular contraction and is normally described as a “lub,” or first heart sound. The second heart sound, Sp, is the sound of the closing of the semilunar valves during ventricular diastole and is described as a “dub” ({link]). In both cases, as the valves close, the openings within the atrioventricular septum guarded by the valves will become reduced, and blood flow through the opening will become more turbulent until the valves are fully closed. There is a third heart sound, S3, but it is rarely heard in healthy individuals. It may be the sound of blood flowing into the atria, or blood sloshing back and forth in the ventricle, or even tensing of the chordae tendineae. S3 may be heard in youth, some athletes, and pregnant women. If the sound is heard later in life, it may indicate congestive heart failure, warranting further tests. Some cardiologists refer to the collective S1, Sz, and S3 sounds as the “Kentucky gallop,” because they mimic those produced by a galloping horse. The fourth heart sound, Sy, results from the contraction of the atria pushing blood into a stiff or hypertrophic ventricle, indicating failure of the left ventricle. S, occurs prior to S; and the collective sounds S,, S;, and S> are referred to by some cardiologists as the “Tennessee gallop,” because of their similarity to the sound produced by a galloping horse with a different gait. A few individuals may have both S3 and Sy, and this combined sound is referred to as Sv. Heart Sounds and the Cardiac Cycle Semilunar 120 valves close Semilunar 100 valves open aS Aortic pressure Ventricular pressure Pressure (mm Hg) (op) ro] AV valves open / AV valves close Atrial pressure 1st 2nd 3rd Heart sounds |), “| ub” “Dub” In this illustration, the x-axis reflects time with a recording of the heart sounds. The y-axis represents pressure. The term murmur is used to describe an unusual sound coming from the heart that is caused by the turbulent flow of blood. Murmurs are graded on a scale of 1 to 6, with 1 being the most common, the most difficult sound to detect, and the least serious. The most severe is a 6. Phonocardiograms or auscultograms can be used to record both normal and abnormal sounds using specialized electronic stethoscopes. During auscultation, it is common practice for the clinician to ask the patient to breathe deeply. This procedure not only allows for listening to airflow, but it may also amplify heart murmurs. Inhalation increases blood flow into the right side of the heart and may increase the amplitude of right- sided heart murmurs. Expiration partially restricts blood flow into the left side of the heart and may amplify left-sided heart murmurs. [link] indicates proper placement of the bell of the stethoscope to facilitate auscultation. Stethoscope Placement for Auscultation Aortic valve Pulmonary valve Tricuspid valve Mitral valve Proper placement of the bell of the stethoscope facilitates auscultation. At each of the four locations on the chest, a different valve can be heard. Chapter Review The cardiac cycle comprises a complete relaxation and contraction of both the atria and ventricles, and lasts approximately 0.8 seconds. Beginning with all chambers in diastole, blood flows passively from the veins into the atria and past the atrioventricular valves into the ventricles. The atria begin to contract (atrial systole), following depolarization of the atria, and pump blood into the ventricles. The ventricles begin to contract (ventricular systole), raising pressure within the ventricles. When ventricular pressure rises above the pressure in the atria, blood flows toward the atria, producing the first heart sound, S, or lub. As pressure in the ventricles rises above two major arteries, blood pushes open the two semilunar valves and moves into the pulmonary trunk and aorta in the ventricular ejection phase. Following ventricular repolarization, the ventricles begin to relax (ventricular diastole), and pressure within the ventricles drops. As ventricular pressure drops, there is a tendency for blood to flow back into the atria from the major arteries, producing the dicrotic notch in the ECG and closing the two semilunar valves. The second heart sound, S» or dub, occurs when the semilunar valves close. When the pressure falls below that of the atria, blood moves from the atria into the ventricles, opening the atrioventricular valves and marking one complete heart cycle. The valves prevent backflow of blood. Failure of the valves to operate properly produces turbulent blood flow within the heart; the resulting heart murmur can often be heard with a stethoscope. Review Questions Exercise: Problem: The cardiac cycle consists of a distinct relaxation and contraction phase. Which term is typically used to refer ventricular contraction while no blood is being ejected? a. systole b. diastole c. quiescent d. isovolumic contraction Solution: D Exercise: Problem: Most blood enters the ventricle during a. atrial systole b. atrial diastole c. ventricular systole d. isovolumic contraction Solution: B Exercise: Problem: The first heart sound represents which portion of the cardiac cycle? a. atrial systole b. ventricular systole c. closing of the atrioventricular valves d. closing of the semilunar valves Solution: ‘s Exercise: Problem: Ventricular relaxation immediately follows a. atrial depolarization b. ventricular repolarization c. ventricular depolarization d. atrial repolarization Solution: B Critical Thinking Questions Exercise: Problem: Describe one cardiac cycle, beginning with both atria and ventricles relaxed. Solution: The cardiac cycle comprises a complete relaxation and contraction of both the atria and ventricles, and lasts approximately 0.8 seconds. Beginning with all chambers in diastole, blood flows passively from the veins into the atria and past the atrioventricular valves into the ventricles. The atria begin to contract following depolarization of the atria and pump blood into the ventricles. The ventricles begin to contract, raising pressure within the ventricles. When ventricular pressure rises above the pressure in the two major arteries, blood pushes open the two semilunar valves and moves into the pulmonary trunk and aorta in the ventricular ejection phase. Following ventricular repolarization, the ventricles begin to relax, and pressure within the ventricles drops. When the pressure falls below that of the atria, blood moves from the atria into the ventricles, opening the atrioventricular valves and marking one complete heart cycle. Glossary cardiac cycle period of time between the onset of atrial contraction (atrial systole) and ventricular relaxation (ventricular diastole) diastole period of time when the heart muscle is relaxed and the chambers fill with blood end diastolic volume (EDV) (also, preload) the amount of blood in the ventricles at the end of atrial systole just prior to ventricular contraction end systolic volume (ESV) amount of blood remaining in each ventricle following systole heart sounds sounds heard via auscultation with a stethoscope of the closing of the atrioventricular valves (“lub”) and semilunar valves (“dub”’) isovolumic contraction (also, isovolumetric contraction) initial phase of ventricular contraction in which tension and pressure in the ventricle increase, but no blood is pumped or ejected from the heart isovolumic ventricular relaxation phase initial phase of the ventricular diastole when pressure in the ventricles drops below pressure in the two major arteries, the pulmonary trunk, and the aorta, and blood attempts to flow back into the ventricles, producing the dicrotic notch of the ECG and closing the two semilunar valves murmur unusual heart sound detected by auscultation; typically related to septal or valve defects preload (also, end diastolic volume) amount of blood in the ventricles at the end of atrial systole just prior to ventricular contraction systole period of time when the heart muscle is contracting ventricular ejection phase second phase of ventricular systole during which blood is pumped from the ventricle Structure and Function of Blood Vessels By the end of this section, you will be able to: e Compare and contrast the three tunics that make up the walls of most blood vessels e Distinguish between elastic arteries, muscular arteries, and arterioles on the basis of structure, location, and function e Describe the basic structure of a capillary bed, from the supplying metarteriole to the venule into which it drains e Explain the structure and function of venous valves in the large veins of the extremities Blood is carried through the body via blood vessels. An artery is a blood vessel that carries blood away from the heart, where it branches into ever- smaller vessels. Eventually, the smallest arteries, vessels called arterioles, further branch into tiny capillaries, where nutrients and wastes are exchanged, and then combine with other vessels that exit capillaries to form venules, small blood vessels that carry blood to a vein, a larger blood vessel that returns blood to the heart. Arteries and veins transport blood in two distinct circuits: the systemic circuit and the pulmonary circuit ([link]). Systemic arteries provide blood rich in oxygen to the body’s tissues. The blood returned to the heart through systemic veins has less oxygen, since much of the oxygen carried by the arteries has been delivered to the cells. In contrast, in the pulmonary circuit, arteries carry blood low in oxygen exclusively to the lungs for gas exchange. Pulmonary veins then return freshly oxygenated blood from the lungs to the heart to be pumped back out into systemic circulation. Although arteries and veins differ structurally and functionally, they share certain features. Cardiovascular Circulation Lungs 2c GO : 5 & Pulmonary Pulmonary vein =) §3 artery as Vena cava Aorta Upper body Liver Hepatic vein Hepatic artery Hepatic portal vein Systemic circulation Stomach, intestines Hi Vessels transporting oxygenated blood Renal artery BH Vessels transporting Renal vein deoxygenated blood Midneye Bi Vessels involved in gas excange Lower body The pulmonary circuit moves blood from the right side of the heart to the lungs and back to the heart. The systemic circuit moves blood from the left side of the heart to the head and body and returns it to the right side of the heart to repeat the cycle. The arrows indicate the direction of blood flow, and the colors show the relative levels of oxygen concentration. Shared Structures Different types of blood vessels vary slightly in their structures, but they share the same general features. Arteries and arterioles have thicker walls than veins and venules because they are closer to the heart and receive blood that is surging at a far greater pressure ({link]). Each type of vessel has a lumen—a hollow passageway through which blood flows. Arteries have smaller lumens than veins, a characteristic that helps to maintain the pressure of blood moving through the system. Together, their thicker walls and smaller diameters give arterial lumens a more rounded appearance in cross section than the lumens of veins. Structure of Blood Vessels Artery Vein = + Tunica externa LD i Tunica externa Ae Tunica media S _ Tunica intima Smooth muscle Internal elastic membrane Vasa vasorum External elastic membrane Nervi vasorum Endothelium Elastic fiber Endothelium (a) Arteries and (b) veins share the same general features, but the walls of arteries are much thicker because of the higher pressure of the blood that flows through them. (c) A micrograph shows the relative differences in thickness. LM x 160. (Micrograph provided by the Regents of the University of Michigan Medical School © 2012) By the time blood has passed through capillaries and entered venules, the pressure initially exerted upon it by heart contractions has diminished. In other words, in comparison to arteries, venules and veins withstand a much lower pressure from the blood that flows through them. Their walls are considerably thinner and their lumens are correspondingly larger in diameter, allowing more blood to flow with less vessel resistance. In addition, many veins of the body, particularly those of the limbs, contain valves that assist the unidirectional flow of blood toward the heart. This is critical because blood flow becomes sluggish in the extremities, as a result of the lower pressure and the effects of gravity. The walls of arteries and veins are largely composed of living cells and their products (including collagenous and elastic fibers); the cells require nourishment and produce waste. Since blood passes through the larger vessels relatively quickly, there is limited opportunity for blood in the lumen of the vessel to provide nourishment to or remove waste from the vessel’s cells. Further, the walls of the larger vessels are too thick for nutrients to diffuse through to all of the cells. Larger arteries and veins contain small blood vessels within their walls known as the vasa vasorum —literally “vessels of the vessel”—to provide them with this critical exchange. Since the pressure within arteries is relatively high, the vasa vasorum must function in the outer layers of the vessel (see [link]) or the pressure exerted by the blood passing through the vessel would collapse it, preventing any exchange from occurring. The lower pressure within veins allows the vasa vasorum to be located closer to the lumen. The restriction of the vasa vasorum to the outer layers of arteries is thought to be one reason that arterial diseases are more common than venous diseases, since its location makes it more difficult to nourish the cells of the arteries and remove waste products. There are also minute nerves within the walls of both types of vessels that control the contraction and dilation of smooth muscle. These minute nerves are known as the nervi vasorum. Both arteries and veins have the same three distinct tissue layers, called tunics (from the Latin term tunica), for the garments first worn by ancient Romans; the term tunic is also used for some modem garments. From the most interior layer to the outer, these tunics are the tunica intima, the tunica media, and the tunica externa (see [link]). [link] compares and contrasts the tunics of the arteries and veins. Comparison of Tunics in Arteries and Veins Arteries General Thick walls with small lumens appearance Generally appear rounded Endothelium usually appears wavy due to constriction of smooth muscle Internal elastic membrane present in larger vessels Tunica intima Veins Thin walls with large lumens Generally appear flattened Endothelium appears smooth Internal elastic membrane absent Comparison of Tunics in Arteries and Veins Tunica media Tunica externa Arteries Normally the thickest layer in arteries Smooth muscle cells and elastic fibers predominate (the proportions of these vary with distance from the heart) External elastic membrane present in larger vessels Normally thinner than the tunica media in all but the largest arteries Collagenous and elastic fibers Nervi vasorum and vasa vasorum present Veins Normally thinner than the tunica externa Smooth muscle cells and collagenous fibers predominate Nervi vasorum and vasa vasorum present External elastic membrane absent Normally the thickest layer in veins Collagenous and smooth fibers predominate Some smooth muscle fibers Nervi vasorum and vasa vasorum present Tunica Intima The tunica intima (also called the tunica interna) is composed of epithelial and connective tissue layers. Lining the tunica intima is the specialized simple squamous epithelium called the endothelium, which is continuous throughout the entire vascular system, including the lining of the chambers of the heart. Damage to this endothelial lining and exposure of blood to the collagenous fibers beneath is one of the primary causes of clot formation. Until recently, the endothelium was viewed simply as the boundary between the blood in the lumen and the walls of the vessels. Recent studies, however, have shown that it is physiologically critical to such activities as helping to regulate capillary exchange and altering blood flow. The endothelium releases local chemicals called endothelins that can constrict the smooth muscle within the walls of the vessel to increase blood pressure. Uncompensated overproduction of endothelins may contribute to hypertension (high blood pressure) and cardiovascular disease. Next to the endothelium is the basement membrane, or basal lamina, that effectively binds the endothelium to the connective tissue. The basement membrane provides strength while maintaining flexibility, and it is permeable, allowing materials to pass through it. The thin outer layer of the tunica intima contains a small amount of areolar connective tissue that consists primarily of elastic fibers to provide the vessel with additional flexibility; it also contains some collagenous fibers to provide additional strength. In larger arteries, there is also a thick, distinct layer of elastic fibers known as the internal elastic membrane (also called the internal elastic lamina) at the boundary with the tunica media. Like the other components of the tunica intima, the internal elastic membrane provides structure while allowing the vessel to stretch. It is permeated with small openings that allow exchange of materials between the tunics. The internal elastic membrane is not apparent in veins. In addition, many veins, particularly in the lower limbs, contain valves formed by sections of thickened endothelium that are reinforced with connective tissue, extending into the lumen. Under the microscope, the lumen and the entire tunica intima of a vein will appear smooth, whereas those of an artery will normally appear wavy because of the partial constriction of the smooth muscle in the tunica media, the next layer of blood vessel walls. Tunica Media The tunica media is the substantial middle layer of the vessel wall (see [link]). It is generally the thickest layer in arteries, and it is much thicker in arteries than it is in veins. The tunica media consists of layers of smooth muscle supported by connective tissue that is primarily made up of elastic fibers, most of which are arranged in circular sheets. Toward the outer portion of the tunic, there are also layers of longitudinal muscle. Contraction and relaxation of the circular muscles decrease and increase the diameter of the vessel lumen, respectively. Specifically in arteries, vasoconstriction decreases blood flow as the smooth muscle in the walls of the tunica media contracts, making the lumen narrower and increasing blood pressure. Similarly, vasodilation increases blood flow as the smooth muscle relaxes, allowing the lumen to widen and blood pressure to drop. Both vasoconstriction and vasodilation are regulated in part by small vascular nerves, known as nervi vasorum, or “nerves of the vessel,” that run within the walls of blood vessels. These are generally all sympathetic fibers, although some trigger vasodilation and others induce vasoconstriction, depending upon the nature of the neurotransmitter and receptors located on the target cell. Parasympathetic stimulation does trigger vasodilation as well as erection during sexual arousal in the external genitalia of both sexes. Nervous control over vessels tends to be more generalized than the specific targeting of individual blood vessels. Local controls, discussed later, account for this phenomenon. (Seek additional content for more information on these dynamic aspects of the autonomic nervous system.) Hormones and local chemicals also control blood vessels. Together, these neural and chemical mechanisms reduce or increase blood flow in response to changing body conditions, from exercise to hydration. Regulation of both blood flow and blood pressure is discussed in detail later in this chapter. The smooth muscle layers of the tunica media are supported by a framework of collagenous fibers that also binds the tunica media to the inner and outer tunics. Along with the collagenous fibers are large numbers of elastic fibers that appear as wavy lines in prepared slides. Separating the tunica media from the outer tunica externa in larger arteries is the external elastic membrane (also called the external elastic lamina), which also appears wavy in Slides. This structure is not usually seen in smaller arteries, nor is it seen in veins. Tunica Externa The outer tunic, the tunica externa (also called the tunica adventitia), is a substantial sheath of connective tissue composed primarily of collagenous fibers. Some bands of elastic fibers are found here as well. The tunica externa in veins also contains groups of smooth muscle fibers. This is normally the thickest tunic in veins and may be thicker than the tunica media in some larger arteries. The outer layers of the tunica externa are not distinct but rather blend with the surrounding connective tissue outside the vessel, helping to hold the vessel in relative position. If you are able to palpate some of the superficial veins on your upper limbs and try to move them, you will find that the tunica externa prevents this. If the tunica externa did not hold the vessel in place, any movement would likely result in disruption of blood flow. Arteries An artery is a blood vessel that conducts blood away from the heart. All arteries have relatively thick walls that can withstand the high pressure of blood ejected from the heart. However, those close to the heart have the thickest walls, containing a high percentage of elastic fibers in all three of their tunics. This type of artery is known as an elastic artery ((link)). Vessels larger than 10 mm in diameter are typically elastic. Their abundant elastic fibers allow them to expand, as blood pumped from the ventricles passes through them, and then to recoil after the surge has passed. If artery walls were rigid and unable to expand and recoil, their resistance to blood flow would greatly increase and blood pressure would rise to even higher levels, which would in turn require the heart to pump harder to increase the volume of blood expelled by each pump (the stroke volume) and maintain adequate pressure and flow. Artery walls would have to become even thicker in response to this increased pressure. The elastic recoil of the vascular wall helps to maintain the pressure gradient that drives the blood through the arterial system. An elastic artery is also known as a conducting artery, because the large diameter of the lumen enables it to accept a large volume of blood from the heart and conduct it to smaller branches. Types of Arteries and Arterioles Elastic Tunica Muscular Tunica Arteriole Tunica artery a— externa artery = = externa = externa unica Tunica media DP Tu nica “as intima Tunica media b Tunica intima media 5 ie Tunica intima Comparison of the walls of an elastic artery, a muscular artery, and an arteriole is shown. In terms of scale, the diameter of an arteriole is measured in micrometers compared to millimeters for elastic and muscular arteries. Farther from the heart, where the surge of blood has dampened, the percentage of elastic fibers in an artery’s tunica intima decreases and the amount of smooth muscle in its tunica media increases. The artery at this point is described as a muscular artery. The diameter of muscular arteries typically ranges from 0.1 mm to 10 mm. Their thick tunica media allows muscular arteries to play a leading role in vasoconstriction. In contrast, their decreased quantity of elastic fibers limits their ability to expand. Fortunately, because the blood pressure has eased by the time it reaches these more distant vessels, elasticity has become less important. Notice that although the distinctions between elastic and muscular arteries are important, there is no “line of demarcation” where an elastic artery suddenly becomes muscular. Rather, there is a gradual transition as the vascular tree repeatedly branches. In turn, muscular arteries branch to distribute blood to the vast network of arterioles. For this reason, a muscular artery is also known as a distributing artery. Arterioles An arteriole is a very small artery that leads to a capillary. Arterioles have the same three tunics as the larger vessels, but the thickness of each is greatly diminished. The critical endothelial lining of the tunica intima is intact. The tunica media is restricted to one or two smooth muscle cell layers in thickness. The tunica externa remains but is very thin (see [link]). With a lumen averaging 30 micrometers or less in diameter, arterioles are critical in slowing down—or resisting—blood flow and, thus, causing a substantial drop in blood pressure. Because of this, you may see them referred to as resistance vessels. The muscle fibers in arterioles are normally slightly contracted, causing arterioles to maintain a consistent muscle tone—in this case referred to as vascular tone—in a similar manner to the muscular tone of skeletal muscle. In reality, all blood vessels exhibit vascular tone due to the partial contraction of smooth muscle. The importance of the arterioles is that they will be the primary site of both resistance and regulation of blood pressure. The precise diameter of the lumen of an arteriole at any given moment is determined by neural and chemical controls, and vasoconstriction and vasodilation in the arterioles are the primary mechanisms for distribution of blood flow. Capillaries A capillary is a microscopic channel that supplies blood to the tissues themselves, a process called perfusion. Exchange of gases and other substances occurs in the capillaries between the blood and the surrounding cells and their tissue fluid (interstitial fluid). The diameter of a capillary lumen ranges from 5—10 micrometers; the smallest are just barely wide enough for an erythrocyte to squeeze through. Flow through capillaries is often described as microcirculation. The wall of a capillary consists of the endothelial layer surrounded by a basement membrane with occasional smooth muscle fibers. There is some variation in wall structure: In a large capillary, several endothelial cells bordering each other may line the lumen; in a small capillary, there may be only a single cell layer that wraps around to contact itself. For capillaries to function, their walls must be leaky, allowing substances to pass through. There are three major types of capillaries, which differ according to their degree of “leakiness:” continuous, fenestrated, and sinusoid capillaries ({Link]). Continuous Capillaries The most common type of capillary, the continuous capillary, is found in almost all vascularized tissues. Continuous capillaries are characterized by a complete endothelial lining with tight junctions between endothelial cells. Although a tight junction is usually impermeable and only allows for the passage of water and ions, they are often incomplete in capillaries, leaving intercellular clefts that allow for exchange of water and other very small molecules between the blood plasma and the interstitial fluid. Substances that can pass between cells include metabolic products, such as glucose, water, and small hydrophobic molecules like gases and hormones, as well as various leukocytes. Continuous capillaries not associated with the brain are rich in transport vesicles, contributing to either endocytosis or exocytosis. Those in the brain are part of the blood-brain barrier. Here, there are tight junctions and no intercellular clefts, plus a thick basement membrane and astrocyte extensions called end feet; these structures combine to prevent the movement of nearly all substances. Types of Capillaries Continuous Fenestrated Sinusoid Endothelial layer (tunica intima) Incomplete basement membrane Intercellular cleft Fenestrations Intercellular gap The three major types of capillaries: continuous, fenestrated, and sinusoid. Fenestrated Capillaries A fenestrated capillary is one that has pores (or fenestrations) in addition to tight junctions in the endothelial lining. These make the capillary permeable to larger molecules. The number of fenestrations and their degree of permeability vary, however, according to their location. Fenestrated capillaries are common in the small intestine, which is the primary site of nutrient absorption, as well as in the kidneys, which filter the blood. They are also found in the choroid plexus of the brain and many endocrine structures, including the hypothalamus, pituitary, pineal, and thyroid glands. Sinusoid Capillaries A sinusoid capillary (or sinusoid) is the least common type of capillary. Sinusoid capillaries are flattened, and they have extensive intercellular gaps and incomplete basement membranes, in addition to intercellular clefts and fenestrations. This gives them an appearance not unlike Swiss cheese. These very large openings allow for the passage of the largest molecules, including plasma proteins and even cells. Blood flow through sinusoids is very slow, allowing more time for exchange of gases, nutrients, and wastes. Sinusoids are found in the liver and spleen, bone marrow, lymph nodes (where they carry lymph, not blood), and many endocrine glands including the pituitary and adrenal glands. Without these specialized capillaries, these organs would not be able to provide their myriad of functions. For example, when bone marrow forms new blood cells, the cells must enter the blood supply and can only do so through the large openings of a sinusoid capillary; they cannot pass through the small openings of continuous or fenestrated capillaries. The liver also requires extensive specialized sinusoid capillaries in order to process the materials brought to it by the hepatic portal vein from both the digestive tract and spleen, and to release plasma proteins into circulation. Metarterioles and Capillary Beds A metarteriole is a type of vessel that has structural characteristics of both an arteriole and a capillary. Slightly larger than the typical capillary, the smooth muscle of the tunica media of the metarteriole is not continuous but forms rings of smooth muscle (sphincters) prior to the entrance to the capillaries. Each metarteriole arises from a terminal arteriole and branches to supply blood to a capillary bed that may consist of 10—100 capillaries. The precapillary sphincters, circular smooth muscle cells that surround the capillary at its origin with the metarteriole, tightly regulate the flow of blood from a metarteriole to the capillaries it supplies. Their function is critical: If all of the capillary beds in the body were to open simultaneously, they would collectively hold every drop of blood in the body and there would be none in the arteries, arterioles, venules, veins, or the heart itself. Normally, the precapillary sphincters are closed. When the surrounding tissues need oxygen and have excess waste products, the precapillary sphincters open, allowing blood to flow through and exchange to occur before closing once more ((link]). If all of the precapillary sphincters in a capillary bed are closed, blood will flow from the metarteriole directly into a thoroughfare channel and then into the venous circulation, bypassing the capillary bed entirely. This creates what is known as a vascular shunt. In addition, an arteriovenous anastomosis may bypass the capillary bed and lead directly to the venous system. Although you might expect blood flow through a capillary bed to be smooth, in reality, it moves with an irregular, pulsating flow. This pattern is called vasomotion and is regulated by chemical signals that are triggered in response to changes in internal conditions, such as oxygen, carbon dioxide, hydrogen ion, and lactic acid levels. For example, during strenuous exercise when oxygen levels decrease and carbon dioxide, hydrogen ion, and lactic acid levels all increase, the capillary beds in skeletal muscle are open, as they would be in the digestive system when nutrients are present in the digestive tract. During sleep or rest periods, vessels in both areas are largely closed; they open only occasionally to allow oxygen and nutrient supplies to travel to the tissues to maintain basic life processes. Capillary Bed Capillary bed eee Capillary Arteriole —————————_ Venule Precapillary ( ZB | sphincter Se SS —— Thoroughfare channel Metarteriole (serves as vascular shunt when precapillary sphincters are closed) Arteriovenous ————____4. i anastomosis In a capillary bed, arterioles give rise to metarterioles. Precapillary sphincters located at the junction of a metarteriole with a capillary regulate blood flow. A thoroughfare channel connects the metarteriole to a venule. An arteriovenous anastomosis, which directly connects the arteriole with the venule, is shown at the bottom. Venules A venule is an extremely small vein, generally 8-100 micrometers in diameter. Postcapillary venules join multiple capillaries exiting from a capillary bed. Multiple venules join to form veins. The walls of venules consist of endothelium, a thin middle layer with a few muscle cells and elastic fibers, plus an outer layer of connective tissue fibers that constitute a very thin tunica externa ({link]). Venules as well as capillaries are the primary sites of emigration or diapedesis, in which the white blood cells adhere to the endothelial lining of the vessels and then squeeze through adjacent cells to enter the tissue fluid. Veins A vein is a blood vessel that conducts blood toward the heart. Compared to arteries, veins are thin-walled vessels with large and irregular lumens (see [link]). Because they are low-pressure vessels, larger veins are commonly equipped with valves that promote the unidirectional flow of blood toward the heart and prevent backflow toward the capillaries caused by the inherent low blood pressure in veins as well as the pull of gravity. [link] compares the features of arteries and veins. Comparison of Veins and Venules Large vein Tunica externa +—— Tunica media Garin — ® Tunica intima @ yy Af Smooth muscle cell in tunica externa Vasa vasorum Nervi vasorum mate [tunica externa +—— Tunica media Tunica intima Valves (closed) Tunica externa Tunica media Tunica intima Venule Many veins have valves to prevent back flow of blood, whereas venules do not. In terms of scale, the diameter of a venule is measured in micrometers compared to millimeters for veins. Comparison of Arteries and Veins Arteries Veins Direction of Conducts Conducts blood toward the blood away blood flow heart from the heart General Rounded Irregular, often collapsed appearance Pressure High Low ye Il Thick Thin thickness Higher in Relative aaa . : : arteries Lower in systemic veins oxygen : : : ; : Lower in Higher in pulmonary veins concentration pulmonary arteries Present most commonly in Valves Not present limbs and in veins inferior to the heart Note: Disorders of the... Cardiovascular System: Edema and Varicose Veins Despite the presence of valves and the contributions of other anatomical and physiological adaptations we will cover shortly, over the course of a day, some blood will inevitably pool, especially in the lower limbs, due to the pull of gravity. Any blood that accumulates in a vein will increase the pressure within it, which can then be reflected back into the smaller veins, venules, and eventually even the capillaries. Increased pressure will promote the flow of fluids out of the capillaries and into the interstitial fluid. The presence of excess tissue fluid around the cells leads to a condition called edema. Most people experience a daily accumulation of tissue fluid, especially if they spend much of their work life on their feet (like most health professionals). However, clinical edema goes beyond normal swelling and requires medical treatment. Edema has many potential causes, including hypertension and heart failure, severe protein deficiency, renal failure, and many others. In order to treat edema, which is a sign rather than a discrete disorder, the underlying cause must be diagnosed and alleviated. Varicose Veins Varicose veins are commonly found in the lower limbs. (credit: Thomas Kriese) Edema may be accompanied by varicose veins, especially in the superficial veins of the legs ([link]). This disorder arises when defective valves allow blood to accumulate within the veins, causing them to distend, twist, and become visible on the surface of the integument. Varicose veins may occur in both sexes, but are more common in women and are often related to pregnancy. More than simple cosmetic blemishes, varicose veins are often painful and sometimes itchy or throbbing. Without treatment, they tend to grow worse over time. The use of support hose, as well as elevating the feet and legs whenever possible, may be helpful in alleviating this condition. Laser surgery and interventional radiologic procedures can reduce the size and severity of varicose veins. Severe cases may require conventional surgery to remove the damaged vessels. As there are typically redundant circulation patterns, that is, anastomoses, for the smaller and more superficial veins, removal does not typically impair the circulation. There is evidence that patients with varicose veins suffer a greater risk of developing a thrombus or clot. Veins as Blood Reservoirs In addition to their primary function of returning blood to the heart, veins may be considered blood reservoirs, since systemic veins contain approximately 64 percent of the blood volume at any given time ([link]). Their ability to hold this much blood is due to their high capacitance, that is, their capacity to distend (expand) readily to store a high volume of blood, even at a low pressure. The large lumens and relatively thin walls of veins make them far more distensible than arteries; thus, they are said to be Capacitance vessels. Distribution of Blood Flow Systemic circulation Systemic veins Large veins 84% 64% 18% Large venous networks (liver, bone marrow, and integument) 21% Venules and medium-sized veins 25% Systemic arteries Arterioles 13% 2% Muscular arteries 5% Elastic arteries 4% Aorta 2% Systemic capillaries Systemic capillaries 7% 7% Pulmonary circulation Pulmonary veins 9% 4% Pulmonary capillaries 2% se mens a 3% When blood flow needs to be redistributed to other portions of the body, the vasomotor center located in the medulla oblongata sends sympathetic stimulation to the smooth muscles in the walls of the veins, causing constriction—or in this case, venoconstriction. Less dramatic than the vasoconstriction seen in smaller arteries and arterioles, venoconstriction may be likened to a “stiffening” of the vessel wall. This increases pressure on the blood within the veins, speeding its return to the heart. As you will note in [link], approximately 21 percent of the venous blood is located in venous networks within the liver, bone marrow, and integument. This volume of blood is referred to as venous reserve. Through venoconstriction, this “reserve” volume of blood can get back to the heart more quickly for redistribution to other parts of the circulation. Note: Career Connection Vascular Surgeons and Technicians Vascular surgery is a specialty in which the physician deals primarily with diseases of the vascular portion of the cardiovascular system. This includes repair and replacement of diseased or damaged vessels, removal of plaque from vessels, minimally invasive procedures including the insertion of venous catheters, and traditional surgery. Following completion of medical school, the physician generally completes a 5-year surgical residency followed by an additional 1 to 2 years of vascular specialty training. In the United States, most vascular surgeons are members of the Society of Vascular Surgery. Vascular technicians are specialists in imaging technologies that provide information on the health of the vascular system. They may also assist physicians in treating disorders involving the arteries and veins. This profession often overlaps with cardiovascular technology, which would also include treatments involving the heart. Although recognized by the American Medical Association, there are currently no licensing requirements for vascular technicians, and licensing is voluntary. Vascular technicians typically have an Associate’s degree or certificate, involving 18 months to 2 years of training. The United States Bureau of Labor projects this profession to grow by 29 percent from 2010 to 2020. Note: OF: [=] F => enue COLLEGE” : q eee te Visit this site to learn more about vascular surgery. Note: Visit this site to learn more about vascular technicians. Chapter Review Blood pumped by the heart flows through a series of vessels known as arteries, arterioles, capillaries, venules, and veins before returning to the heart. Arteries transport blood away from the heart and branch into smaller vessels, forming arterioles. Arterioles distribute blood to capillary beds, the sites of exchange with the body tissues. Capillaries lead back to small vessels known as venules that flow into the larger veins and eventually back to the heart. The arterial system is a relatively high-pressure system, so arteries have thick walls that appear round in cross section. The venous system is a lower-pressure system, containing veins that have larger lumens and thinner walls. They often appear flattened. Arteries, arterioles, venules, and veins are composed of three tunics known as the tunica intima, tunica media, and tunica externa. Capillaries have only a tunica intima layer. The tunica intima is a thin layer composed of a simple squamous epithelium known as endothelium and a small amount of connective tissue. The tunica media is a thicker area composed of variable amounts of smooth muscle and connective tissue. It is the thickest layer in all but the largest arteries. The tunica externa is primarily a layer of connective tissue, although in veins, it also contains some smooth muscle. Blood flow through vessels can be dramatically influenced by vasoconstriction and vasodilation in their walls. Review Questions Exercise: Problem:The endothelium is found in the a. tunica intima b. tunica media c. tunica externa d. lumen Solution: A Exercise: Problem: Nervi vasorum control a. vasoconstriction b. vasodilation c. capillary permeability d. both vasoconstriction and vasodilation Solution: D Exercise: Problem: Closer to the heart, arteries would be expected to have a higher percentage of a. endothelium b. smooth muscle fibers c. elastic fibers d. collagenous fibers Solution: @ Exercise: Problem: Which of the following best describes veins? a. thick walled, small lumens, low pressure, lack valves b. thin walled, large lumens, low pressure, have valves c. thin walled, small lumens, high pressure, have valves d. thick walled, large lumens, high pressure, lack valves Solution: B Exercise: Problem: An especially leaky type of capillary found in the liver and certain other tissues is called a a. capillary bed b. fenestrated capillary c. sinusoid capillary d. metarteriole Solution: GC Critical Thinking Questions Exercise: Problem: Arterioles are often referred to as resistance vessels. Why? Solution: Arterioles receive blood from arteries, which are vessels with a much larger lumen. As their own lumen averages just 30 micrometers or less, arterioles are critical in slowing down—or resisting—blood flow. The arterioles can also constrict or dilate, which varies their resistance, to help distribute blood flow to the tissues. Exercise: Problem: Cocaine use causes vasoconstriction. Is this likely to increase or decrease blood pressure, and why? Solution: Vasoconstriction causes the lumens of blood vessels to narrow. This increases the pressure of the blood flowing within the vessel. Exercise: Problem: A blood vessel with a few smooth muscle fibers and connective tissue, and only a very thin tunica externa conducts blood toward the heart. What type of vessel is this? Solution: This is a venule. Glossary arteriole (also, resistance vessel) very small artery that leads to a capillary arteriovenous anastomosis short vessel connecting an arteriole directly to a venule and bypassing the capillary beds artery blood vessel that conducts blood away from the heart; may be a conducting or distributing vessel Capacitance ability of a vein to distend and store blood Capacitance vessels veins capillary smallest of blood vessels where physical exchange occurs between the blood and tissue cells surrounded by interstitial fluid capillary bed network of 10-100 capillaries connecting arterioles to venules continuous capillary most common type of capillary, found in virtually all tissues except epithelia and cartilage; contains very small gaps in the endothelial lining that permit exchange elastic artery (also, conducting artery) artery with abundant elastic fibers located closer to the heart, which maintains the pressure gradient and conducts blood to smaller branches external elastic membrane membrane composed of elastic fibers that separates the tunica media from the tunica externa; seen in larger arteries fenestrated capillary type of capillary with pores or fenestrations in the endothelium that allow for rapid passage of certain small materials internal elastic membrane membrane composed of elastic fibers that separates the tunica intima from the tunica media; seen in larger arteries lumen interior of a tubular structure such as a blood vessel or a portion of the alimentary canal through which blood, chyme, or other substances travel metarteriole short vessel arising from a terminal arteriole that branches to supply a capillary bed microcirculation blood flow through the capillaries muscular artery (also, distributing artery) artery with abundant smooth muscle in the tunica media that branches to distribute blood to the arteriole network nervi vasorum small nerve fibers found in arteries and veins that trigger contraction of the smooth muscle in their walls perfusion distribution of blood into the capillaries so the tissues can be supplied precapillary sphincters circular rings of smooth muscle that surround the entrance to a capillary and regulate blood flow into that capillary sinusoid capillary rarest type of capillary, which has extremely large intercellular gaps in the basement membrane in addition to clefts and fenestrations; found in areas such as the bone marrow and liver where passage of large molecules occurs thoroughfare channel continuation of the metarteriole that enables blood to bypass a capillary bed and flow directly into a venule, creating a vascular shunt tunica externa (also, tunica adventitia) outermost layer or tunic of a vessel (except capillaries) tunica intima (also, tunica interna) innermost lining or tunic of a vessel tunica media middle layer or tunic of a vessel (except capillaries) vasa vasorum small blood vessels located within the walls or tunics of larger vessels that supply nourishment to and remove wastes from the cells of the vessels vascular shunt continuation of the metarteriole and thoroughfare channel that allows blood to bypass the capillary beds to flow directly from the arterial to the venous circulation vasoconstriction constriction of the smooth muscle of a blood vessel, resulting in a decreased vascular diameter vasodilation relaxation of the smooth muscle in the wall of a blood vessel, resulting in an increased vascular diameter vasomotion irregular, pulsating flow of blood through capillaries and related structures vein blood vessel that conducts blood toward the heart venous reserve volume of blood contained within systemic veins in the integument, bone marrow, and liver that can be returned to the heart for circulation, if needed venule small vessel leading from the capillaries to veins Circulatory Pathways By the end of this section, you will be able to: e Identify the vessels through which blood travels within the pulmonary circuit, beginning from the right ventricle of the heart and ending at the left atrium e Create a flow chart showing the major systemic arteries through which blood travels from the aorta and its major branches, to the most significant arteries feeding into the right and left upper and lower limbs ¢ Create a flow chart showing the major systemic veins through which blood travels from the feet to the right atrium of the heart Virtually every cell, tissue, organ, and system in the body is impacted by the circulatory system. This includes the generalized and more specialized functions of transport of materials, capillary exchange, maintaining health by transporting white blood cells and various immunoglobulins (antibodies), hemostasis, regulation of body temperature, and helping to maintain acid-base balance. In addition to these shared functions, many systems enjoy a unique relationship with the circulatory system. [link] summarizes these relationships. Interaction of the Circulatory System with Other Body Systems Digestive Absorbs nutrients and water; delivers nutrients (except most lipids) to liver for processing by hepatic portal vein; provides nutrients essential for hematopoiesis and building hemoglobin Delivers hormones: atrial natriuretic hormone (peptide) secreted by the heart atrial cells to help regulate blood volumes and pressures; epinephrine, ANH, angiotensin Il, ADH, and thyroxine to help regulate blood pressure; estrogen to promote vascular health in women and men Carries clotting factors, platelets, and white blood cells for hemostasis, fighting infection, and repairing damage; regulates temperature by controlling blood flow to the surface, where heat can be dissipated; provides some coloration of integument; acts as a blood reservoir Lymphatic Pes Transports various white blood cells, including those produced by lymphatic tissue, and immunoglobulins (antibodies) throughout the body to maintain health; carries excess tissue fluid not able to be reabsorbed by the vascular capillaries back to the lymphatic system for processing Provides nutrients and oxygen for contraction; removes lactic acid and distributes heat generated by contraction; muscular pumps aid in venous return; exercise contributes to cardiovascular health and helps to prevent atherosclerosis Nervous Produces cerebrospinal fluid (CSF) within choroid plexuses; contributes to blood-brain barrier; cardiac and vasomotor centers regulate cardiac output and blood flow through vessels via autonomic system Reproductive : Aids in erection of genitalia in both sexes during sexual arousal; transports gonadotropic hormones that regulate reproductive functions Respiratory Provides blood for critical exchange of gases to carry oxygen needed for metabolic reactions and carbon dioxide generated as byproducts of these processes Skeletal a Provides calcium, phosphate, and other minerals critical for bone s matrix; transports hormones regulating buildup and absorption of matrix including growth hormone (somatotropin), thyroid hormone, calcitonins, and parathyroid hormone; erythropoietin stimulates myeloid cell hematopoiesis; some level of protection for select vessels by bony structures Delivers 20% of resting circulation to kidneys for filtering, reabsorption of useful products, and secretion of excesses; regulates blood volume and pressure by regulating fluid loss in the form of urine and by releasing the enzyme renin that is essential in the renin-angiotensin-aldosterone mechanism As you learn about the vessels of the systemic and pulmonary circuits, notice that many arteries and veins share the same names, parallel one another throughout the body, and are very similar on the right and left sides of the body. These pairs of vessels will be traced through only one side of the body. Where differences occur in branching patterns or when vessels are singular, this will be indicated. For example, you will find a pair of femoral arteries and a pair of femoral veins, with one vessel on each side of the body. In contrast, some vessels closer to the midline of the body, such as the aorta, are unique. Moreover, some superficial veins, such as the great saphenous vein in the femoral region, have no arterial counterpart. Another phenomenon that can make the study of vessels challenging is that names of vessels can change with location. Like a street that changes name as it passes through an intersection, an artery or vein can change names as it passes an anatomical landmark. For example, the left subclavian artery becomes the axillary artery as it passes through the body wall and into the axillary region, and then becomes the brachial artery as it flows from the axillary region into the upper arm (or brachium). You will also find examples of anastomoses where two blood vessels that previously branched reconnect. Anastomoses are especially common in veins, where they help maintain blood flow even when one vessel is blocked or narrowed, although there are some important ones in the arteries supplying the brain. As you read about circular pathways, notice that there is an occasional, very large artery referred to as a trunk, a term indicating that the vessel gives rise to several smaller arteries. For example, the celiac trunk gives rise to the left gastric, common hepatic, and splenic arteries. As you study this section, imagine you are on a “Voyage of Discovery” similar to Lewis and Clark’s expedition in 1804—1806, which followed rivers and streams through unfamiliar territory, seeking a water route from the Atlantic to the Pacific Ocean. You might envision being inside a miniature boat, exploring the various branches of the circulatory system. This simple approach has proven effective for many students in mastering these major circulatory patterns. Another approach that works well for many students is to create simple line drawings similar to the ones provided, labeling each of the major vessels. It is beyond the scope of this text to name every vessel in the body. However, we will attempt to discuss the major pathways for blood and acquaint you with the major named arteries and veins in the body. Also, please keep in mind that individual variations in circulation patterns are not uncommon. Note: a": aes mess’ OPENStAX COLLEGE” fa io rh Visit this site for a brief summary of the arteries. Pulmonary Circulation Recall that blood returning from the systemic circuit enters the right atrium ({link]) via the superior and inferior venae cavae and the coronary sinus, which drains the blood supply of the heart muscle. These vessels will be described more fully later in this section. This blood is relatively low in oxygen and relatively high in carbon dioxide, since much of the oxygen has been extracted for use by the tissues and the waste gas carbon dioxide was picked up to be transported to the lungs for elimination. From the right atrium, blood moves into the right ventricle, which pumps it to the lungs for gas exchange. This system of vessels is referred to as the pulmonary circuit. The single vessel exiting the right ventricle is the pulmonary trunk. At the base of the pulmonary trunk is the pulmonary semilunar valve, which prevents backflow of blood into the right ventricle during ventricular diastole. As the pulmonary trunk reaches the superior surface of the heart, it curves posteriorly and rapidly bifurcates (divides) into two branches, a left and a right pulmonary artery. To prevent confusion between these vessels, it is important to refer to the vessel exiting the heart as the pulmonary trunk, rather than also calling it a pulmonary artery. The pulmonary arteries in turn branch many times within the lung, forming a series of smaller arteries and arterioles that eventually lead to the pulmonary capillaries. The pulmonary capillaries surround lung structures known as alveoli that are the sites of oxygen and carbon dioxide exchange. Once gas exchange is completed, oxygenated blood flows from the pulmonary capillaries into a series of pulmonary venules that eventually lead to a series of larger pulmonary veins. Four pulmonary veins, two on the left and two on the right, return blood to the left atrium. At this point, the pulmonary circuit is complete. [link] defines the major arteries and veins of the pulmonary circuit discussed in the text. Pulmonary Circuit Ascending aorta Aortic arch Superior vena cava Pulmonary trunk Right lung Left lung Left pulmonary arteries Left pulmonary Right pulmonary wink arteries Right pulmonary veins ~ Pulmonary capillaries Inferior vena cava Descending aorta Blood exiting from the right ventricle flows into the pulmonary trunk, which bifurcates into the two pulmonary arteries. These vessels branch to supply blood to the pulmonary capillaries, where gas exchange occurs within the lung alveoli. Blood returns via the pulmonary veins to the left atrium. Pulmonary Arteries and Veins Vessel Description Pulmonary Arteries and Veins Vessel Description Pulmonary Single large vessel exiting the right ventricle that trunk divides to form the right and left pulmonary arteries Peionay Left and right vessels that form from the pulmonary , a trunk and lead to smaller arterioles and eventually to arteries eae fi the pulmonary capillaries Two sets of paired vessels—one pair on each side— Pulmonary that are formed from the small venules, leading away veins from the pulmonary capillaries to flow into the left atrium Overview of Systemic Arteries Blood relatively high in oxygen concentration is returned from the pulmonary circuit to the left atrium via the four pulmonary veins. From the left atrium, blood moves into the left ventricle, which pumps blood into the aorta. The aorta and its branches—the systemic arteries—send blood to virtually every organ of the body ([link]). Systemic Arteries Vertebral es Right common carotid Left common carotid Left subclavian Axillary il Pulmonary trunk Right subclavian Brachiocephalic trunk Aortic arch Ascending aorta Br VT . Descending aorta SN \) Diaphragm Celiac trunk = ) — Esophageal thoracic aorta L =~ D> L Mediastinal = - 4 Ba __ > —__ aw A intercostal a , Sa Saewa es Pericardial Superior phrenic af | = XN fl Aortic hiatus Inferior phrenic > = y Diaoh SS Celiac trunk iaphragm —__ 7 x a Yh 7 Left gastric Adrenal me Splenic ; = . aS \ ‘: Renal i SJ — Common hepatic Gonadal : ; Superior mesenteric Lumbar ————__4#*_———————— SS / A Abdominal aorta Median sacral ————_——______*._ =a Common iliac [ = Inferior mesenteric Internal iliac en iliac The thoracic aorta gives rise to the arteries of the visceral and parietal branches. Arteries of the Thoracic Region Vessel Visceral branches Bronchial artery Pericardial artery Esophageal artery Mediastinal artery Parietal branches Intercostal artery Superior phrenic artery Description A group of arterial branches of the thoracic aorta; supplies blood to the viscera (i.e., organs) of the thorax Systemic branch from the aorta that provides oxygenated blood to the lungs; this blood supply is in addition to the pulmonary circuit that brings blood for oxygenation Branch of the thoracic aorta; supplies blood to the pericardium Branch of the thoracic aorta; supplies blood to the esophagus Branch of the thoracic aorta; supplies blood to the mediastinum Also called somatic branches, a group of arterial branches of the thoracic aorta; include those that supply blood to the thoracic wall, vertebral column, and the superior surface of the diaphragm Branch of the thoracic aorta; supplies blood to the muscles of the thoracic cavity and vertebral column Branch of the thoracic aorta; supplies blood to the superior surface of the diaphragm Abdominal Aorta and Major Branches After crossing through the diaphragm at the aortic hiatus, the thoracic aorta is called the abdominal aorta (see [link]). This vessel remains to the left of the vertebral column and is embedded in adipose tissue behind the peritoneal cavity. It formally ends at approximately the level of vertebra L4, where it bifurcates to form the common iliac arteries. Before this division, the abdominal aorta gives rise to several important branches. A single celiac trunk (artery) emerges and divides into the left gastric artery to supply blood to the stomach and esophagus, the splenic artery to supply blood to the spleen, and the common hepatic artery, which in turn gives rise to the hepatic artery proper to supply blood to the liver, the right gastric artery to supply blood to the stomach, the cystic artery to supply blood to the gall bladder, and several branches, one to supply blood to the duodenum and another to supply blood to the pancreas. Two additional single vessels arise from the abdominal aorta. These are the superior and inferior mesenteric arteries. The superior mesenteric artery arises approximately 2.5 cm after the celiac trunk and branches into several major vessels that supply blood to the small intestine (duodenum, jejunum, and ileum), the pancreas, and a majority of the large intestine. The inferior mesenteric artery supplies blood to the distal segment of the large intestine, including the rectum. It arises approximately 5 cm superior to the common iliac arteries. In addition to these single branches, the abdominal aorta gives rise to several significant paired arteries along the way. These include the inferior phrenic arteries, the adrenal arteries, the renal arteries, the gonadal arteries, and the lumbar arteries. Each inferior phrenic artery is a counterpart of a superior phrenic artery and supplies blood to the inferior surface of the diaphragm. The adrenal artery supplies blood to the adrenal (suprarenal) glands and arises near the superior mesenteric artery. Each renal artery branches approximately 2.5 cm inferior to the superior mesenteric arteries and supplies a kidney. The right renal artery is longer than the left since the aorta lies to the left of the vertebral column and the vessel must travel a greater distance to reach its target. Renal arteries branch repeatedly to supply blood to the kidneys. Each gonadal artery supplies blood to the gonads, or reproductive organs, and is also described as either an ovarian artery or a testicular artery (internal spermatic), depending upon the sex of the individual. An ovarian artery supplies blood to an ovary, uterine (Fallopian) tube, and the uterus, and is located within the suspensory ligament of the uterus. It is considerably shorter than a testicular artery, which ultimately travels outside the body cavity to the testes, forming one component of the spermatic cord. The gonadal arteries arise inferior to the renal arteries and are generally retroperitoneal. The ovarian artery continues to the uterus where it forms an anastomosis with the uterine artery that supplies blood to the uterus. Both the uterine arteries and vaginal arteries, which distribute blood to the vagina, are branches of the internal iliac artery. The four paired lumbar arteries are the counterparts of the intercostal arteries and supply blood to the lumbar region, the abdominal wall, and the spinal cord. In some instances, a fifth pair of lumbar arteries emerges from the median sacral artery. The aorta divides at approximately the level of vertebra L4 into a left and a right common iliac artery but continues as a small vessel, the median sacral artery, into the sacrum. The common iliac arteries provide blood to the pelvic region and ultimately to the lower limbs. They split into external and internal iliac arteries approximately at the level of the lumbar-sacral articulation. Each internal iliac artery sends branches to the urinary bladder, the walls of the pelvis, the external genitalia, and the medial portion of the femoral region. In females, they also provide blood to the uterus and vagina. The much larger external iliac artery supplies blood to each of the lower limbs. [link] shows the distribution of the major branches of the aorta into the thoracic and abdominal regions. [link] shows the distribution of the major branches of the common iliac arteries. [link] summarizes the major branches of the abdominal aorta. Major Branches of the Aorta UNPAIRED E Thoracic aorta Air passages of Bronchial respiratory tract, lung arteries tissue PAIRED Vertebrae, spinal Pericardial Pericardium Intercostal cord, back muscles, arteries arteries body wall, skin Vertebrae, spinal Esophageal Esophagus I'elilelmdictiiem! Cord, back muscles, arteries arteries body wall, skin, diaphragm Mediastinal Mediastinal structures arteries Abdominal aorta Stomach, SiMe r-T ce adjacent ities lalg-tltemg) Diaphragm, inferior artery portion of arteries portion of esophagus esophagus Spleen, Splenic stomach, Adrenal Adrenal glands artery pancreas arteries Liver, stomach, gallbladder, Renal Kidneys duodenum, arteries pancreas Common hepatic artery Pancreas, small Superior intestine, appendix, Gonadal Testes or ovaries Hitt Cdlerlaciall first two-thirds of arteries large intestine Vetebrae, spinal cord, abdominal wall, lumbar region Inferior Last third of large mesenteric artery Bintsun-) Common iliac arteries Median sacral artery The flow chart summarizes the distribution of the major branches of the aorta into the thoracic and abdominal regions. Major Branches of the Iliac Arteries Abdominal aorta Left common iliac (follows pattern similar to right common iliac) Right common Pelvis and right lower iliac limb Pelvic muscles, skin, Right lower (Uaifeln ata) tel aiiiceacciam viscera of pelvis, iliac perineum, gluteal region, medial thigh Superior gluteal Hip muscles, hip joint Rectum, anus, Internal perineal muscles, pudendal external genitalia, lateral rotators of hip Ilium, hip and thigh Obturator muscles, hip joint, femoral head Lateral Skin and muscles of sacral sacrum The flow chart summarizes the distribution of the major branches of the common iliac arteries into the pelvis and lower limbs. The left side follows a similar pattern to the right. Vessels of the Abdominal Aorta Vessel Description Vessels of the Abdominal Aorta Vessel Celiac trunk Left gastric artery Splenic artery Common hepatic artery Hepatic artery proper Right gastric artery Cystic artery Superior mesenteric artery Description Also called the celiac artery; a major branch of the abdominal aorta; gives rise to the left gastric artery, the splenic artery, and the common hepatic artery that forms the hepatic artery to the liver, the right gastric artery to the stomach, and the cystic artery to the gall bladder Branch of the celiac trunk; supplies blood to the stomach Branch of the celiac trunk; supplies blood to the spleen Branch of the celiac trunk that forms the hepatic artery, the right gastric artery, and the cystic artery Branch of the common hepatic artery; supplies systemic blood to the liver Branch of the common hepatic artery; supplies blood to the stomach Branch of the common hepatic artery; supplies blood to the gall bladder Branch of the abdominal aorta; supplies blood to the small intestine (duodenum, jejunum, and ileum), the pancreas, and a majority of the large intestine Vessels of the Abdominal Aorta Vessel Inferior mesenteric artery Inferior phrenic arteries Adrenal artery Renal artery Gonadal artery Ovarian artery Testicular artery Lumbar arteries Common iliac artery Description Branch of the abdominal aorta; supplies blood to the distal segment of the large intestine and rectum Branches of the abdominal aorta; supply blood to the inferior surface of the diaphragm Branch of the abdominal aorta; supplies blood to the adrenal (suprarenal) glands Branch of the abdominal aorta; supplies each kidney Branch of the abdominal aorta; supplies blood to the gonads or reproductive organs; also described as Ovarian arteries or testicular arteries, depending upon the sex of the individual Branch of the abdominal aorta; supplies blood to ovary, uterine (Fallopian) tube, and uterus Branch of the abdominal aorta; ultimately travels outside the body cavity to the testes and forms one component of the spermatic cord Branches of the abdominal aorta; supply blood to the lumbar region, the abdominal wall, and spinal cord Branch of the aorta that leads to the internal and external iliac arteries Vessels of the Abdominal Aorta Vessel Description Median sacral Continuation of the aorta into the sacrum artery Branch from the common iliac arteries; supplies blood to the urinary bladder, walls of the pelvis, extemal genitalia, and the medial portion of the femoral region; in females, also provides blood to the uterus and vagina Internal iliac artery Branch of the common iliac artery that leaves the body cavity and becomes a femoral artery; supplies blood to the lower limbs External iliac artery Arteries Serving the Upper Limbs As the subclavian artery exits the thorax into the axillary region, it is renamed the axillary artery. Although it does branch and supply blood to the region near the head of the humerus (via the humeral circumflex arteries), the majority of the vessel continues into the upper arm, or brachium, and becomes the brachial artery ({link]). The brachial artery supplies blood to much of the brachial region and divides at the elbow into several smaller branches, including the deep brachial arteries, which provide blood to the posterior surface of the arm, and the ulnar collateral arteries, which supply blood to the region of the elbow. As the brachial artery approaches the coronoid fossa, it bifurcates into the radial and ulnar arteries, which continue into the forearm, or antebrachium. The radial artery and ulnar artery parallel their namesake bones, giving off smaller branches until they reach the wrist, or carpal region. At this level, they fuse to form the superficial and deep palmar arches that supply blood to the hand, as well as the digital arteries that supply blood to the digits. [link] shows the distribution of systemic arteries from the heart into the upper limb. [link] summarizes the arteries serving the upper limbs. Major Arteries Serving the Thorax and Upper Limb Right subclavian Axillary i —— Y ” ea Humeral circumflex —~L Ae be | i —— Deep brachial > TF Brachial || Ulnar collateral Tf] PS ll WA Radial ny ee Anterior crural interosseous Ulnar Deep palmar arch Superficial palmar [~ arch | [; Digital The arteries that supply blood to the arms and hands are extensions of the subclavian arteries. Major Arteries of the Upper Limb Spinal cord, cervical F vertebrae, fuses with left ipifelnis ight Het vertebral to form basilar EWS sé] artery in cranium Left vertebral common common carotid carotid Muscles, tissues A . 7 ween Right - 5 ; Left skin of neck, thyroid = Right Brachiocephalic Left A gland, shoulders, diesen ASS ETEY anal mUbclavian pial upper back Right Skin and muscles of pectoral Right chest and abdomen, ae Ap Aortic eile al and axilla JEYCIETS mammary gland, thoracic arch craareetete muscles pericardium Left axillary Structures of the arm (isttliis Left and elbow MverHie|| brachial Thoracic Left aorta ventricle Radial Ulnar side of side of forearm forearm Digital Digital arteries arteries The flow chart summarizes the distribution of the major arteries from the heart into the upper limb. Arteries Serving the Upper Limbs Description Arteries Serving the Upper Limbs Vessel Axillary artery Brachial artery Radial artery Ulnar artery Palmar arches (superficial and deep) Description Continuation of the subclavian artery as it penetrates the body wall and enters the axillary region; supplies blood to the region near the head of the humerus (humeral circumflex arteries); the majority of the vessel continues into the brachium and becomes the brachial artery Continuation of the axillary artery in the brachium; supplies blood to much of the brachial region; gives off several smaller branches that provide blood to the posterior surface of the arm in the region of the elbow; bifurcates into the radial and ulnar arteries at the coronoid fossa Formed at the bifurcation of the brachial artery; parallels the radius; gives off smaller branches until it reaches the carpal region where it fuses with the ulnar artery to form the superficial and deep palmar arches; supplies blood to the lower arm and carpal region Formed at the bifurcation of the brachial artery; parallels the ulna; gives off smaller branches until it reaches the carpal region where it fuses with the radial artery to form the superficial and deep palmar arches; supplies blood to the lower arm and carpal region Formed from anastomosis of the radial and ulnar arteries; supply blood to the hand and digital arteries Arteries Serving the Upper Limbs Vessel Description Digital Formed from the superficial and deep palmar arches; arteries supply blood to the digits Arteries Serving the Lower Limbs The external iliac artery exits the body cavity and enters the femoral region of the lower leg ({link]). As it passes through the body wall, it is renamed the femoral artery. It gives off several smaller branches as well as the lateral deep femoral artery that in turn gives rise to a lateral circumflex artery. These arteries supply blood to the deep muscles of the thigh as well as ventral and lateral regions of the integument. The femoral artery also gives rise to the genicular artery, which provides blood to the region of the knee. As the femoral artery passes posterior to the knee near the popliteal fossa, it is called the popliteal artery. The popliteal artery branches into the anterior and posterior tibial arteries. The anterior tibial artery is located between the tibia and fibula, and supplies blood to the muscles and integument of the anterior tibial region. Upon reaching the tarsal region, it becomes the dorsalis pedis artery, which branches repeatedly and provides blood to the tarsal and dorsal regions of the foot. The posterior tibial artery provides blood to the muscles and integument on the posterior surface of the tibial region. The fibular or peroneal artery branches from the posterior tibial artery. It bifurcates and becomes the medial plantar artery and lateral plantar artery, providing blood to the plantar surfaces. There is an anastomosis with the dorsalis pedis artery, and the medial and lateral plantar arteries form two arches called the dorsal arch (also called the arcuate arch) and the plantar arch, which provide blood to the remainder of the foot and toes. [link] shows the distribution of the major systemic arteries in the lower limb. [link] summarizes the major systemic arteries discussed in the text. Major Arteries Serving the Lower Limb External iliac ld = Common iliac 1 Internal iliac Inguinal ligament Lateral sacral Internal pudendal Deep femoral Suiaise Lateral femoral circumflex Femoral Genicular Popliteal Dorsalis pedis Dorsal arch Anterior view Right external iliac Deep femoral Lateral femoral circumflex Femoral Genicular Popliteal Peroneal Anterior tibial Posterior tibial Fibular Lateral plantar Medial plantar Plantar arch Posterior view Major arteries serving the lower limb are shown in anterior and posterior views. Systemic Arteries of the Lower Limb External iliac Hip joint, femoral head, Deep Femoral Thigh deep thigh femoral muscles Decending Knee joint, genicular skin of leg Adductor Median Lateral . muscles, femoral femoral Quadriceps Popliteal Leg and obturator circumflex circumflex muscles foot muscles, hip joint Anterior Posterior f tibial tibial AlSOEL Dorsalis pedis, dorsal arch, Peroneal plantar arch Dorsal ‘ metatarsal, Distal foot, dorsal digital [imsaed The flow chart summarizes the distribution of the systemic arteries from the external iliac artery into the lower limb. Arteries Serving the Lower Limbs Vessel Description Arteries Serving the Lower Limbs Vessel Femoral artery Deep femoral artery Lateral circumflex artery Genicular artery Popliteal artery Anterior tibial artery Dorsalis pedis artery Posterior tibial artery Description Continuation of the external iliac artery after it passes through the body cavity; divides into several smaller branches, the lateral deep femoral artery, and the genicular artery; becomes the popliteal artery as it passes posterior to the knee Branch of the femoral artery; gives rise to the lateral circumflex arteries Branch of the deep femoral artery; supplies blood to the deep muscles of the thigh and the ventral and lateral regions of the integument Branch of the femoral artery; supplies blood to the region of the knee Continuation of the femoral artery posterior to the knee; branches into the anterior and posterior tibial arteries Branches from the popliteal artery; supplies blood to the anterior tibial region; becomes the dorsalis pedis artery Forms from the anterior tibial artery; branches repeatedly to supply blood to the tarsal and dorsal regions of the foot Branches from the popliteal artery and gives rise to the fibular or peroneal artery; supplies blood to the posterior tibial region Arteries Serving the Lower Limbs Vessel Description Medial Arises from the bifurcation of the posterior tibial plantar arteries; supplies blood to the medial plantar surfaces artery of the foot Lateral Arises from the bifurcation of the posterior tibial plantar arteries; supplies blood to the lateral plantar surfaces artery of the foot Dorsal or Formed from the anastomosis of the dorsalis pedis arcuate artery and the medial and plantar arteries; branches arch supply the distal portions of the foot and digits Formed from the anastomosis of the dorsalis pedis Plantar : ; aah artery and the medial and plantar arteries; branches supply the distal portions of the foot and digits Overview of Systemic Veins Systemic veins return blood to the right atrium. Since the blood has already passed through the systemic capillaries, it will be relatively low in oxygen concentration. In many cases, there will be veins draining organs and regions of the body with the same name as the arteries that supplied these regions and the two often parallel one another. This is often described as a “complementary” pattern. However, there is a great deal more variability in the venous circulation than normally occurs in the arteries. For the sake of brevity and clarity, this text will discuss only the most commonly encountered patterns. However, keep this variation in mind when you move from the classroom to clinical practice. In both the neck and limb regions, there are often both superficial and deeper levels of veins. The deeper veins generally correspond to the complementary arteries. The superficial veins do not normally have direct arterial counterparts, but in addition to returning blood, they also make contributions to the maintenance of body temperature. When the ambient temperature is warm, more blood is diverted to the superficial veins where heat can be more easily dissipated to the environment. In colder weather, there is more constriction of the superficial veins and blood is diverted deeper where the body can retain more of the heat. The “Voyage of Discovery” analogy and stick drawings mentioned earlier remain valid techniques for the study of systemic veins, but veins present a more difficult challenge because there are numerous anastomoses and multiple branches. It is like following a river with many tributaries and channels, several of which interconnect. Tracing blood flow through arteries follows the current in the direction of blood flow, so that we move from the heart through the large arteries and into the smaller arteries to the capillaries. From the capillaries, we move into the smallest veins and follow the direction of blood flow into larger veins and back to the heart. [link] outlines the path of the major systemic veins. Note: Visit this site for a brief online summary of the veins. Major Systemic Veins of the Body External jugular Subclavian <= fi ay > Axillary ee } Cephalic ——————_f/_// | Brachial T \ / J Basilic I) ST Hepatic if | A Median cubital Radial Median antebrachial Ulnar Palmar venous arches Digital vA y AK Popliteal Small saphenous ——y ||) Fibular Plantar venous arch Dorsal venous arch Internal jugular Brachiocephalic Superior vena cava Intercostal Inferior vena cava Saas . imvy Renal , aa. Gonadal Tt ..— Lumbar Right and left common iliac External iliac Internal iliac Posterior tibial Anterior tibial The major systemic veins of the body are shown here in an anterior view. The right atrium receives all of the systemic venous return. Most of the blood flows into either the superior vena cava or inferior vena cava. If you draw an imaginary line at the level of the diaphragm, systemic venous circulation from above that line will generally flow into the superior vena cava; this includes blood from the head, neck, chest, shoulders, and upper limbs. The exception to this is that most venous blood flow from the coronary veins flows directly into the coronary sinus and from there directly into the right atrium. Beneath the diaphragm, systemic venous flow enters the inferior vena cava, that is, blood from the abdominal and pelvic regions and the lower limbs. The Superior Vena Cava The superior vena cava drains most of the body superior to the diaphragm ({link]). On both the left and right sides, the subclavian vein forms when the axillary vein passes through the body wall from the axillary region. It fuses with the external and internal jugular veins from the head and neck to form the brachiocephalic vein. Each vertebral vein also flows into the brachiocephalic vein close to this fusion. These veins arise from the base of the brain and the cervical region of the spinal cord, and flow largely through the intervertebral foramina in the cervical vertebrae. They are the counterparts of the vertebral arteries. Each internal thoracic vein, also known as an internal mammary vein, drains the anterior surface of the chest wall and flows into the brachiocephalic vein. The remainder of the blood supply from the thorax drains into the azygos vein. Each intercostal vein drains muscles of the thoracic wall, each esophageal vein delivers blood from the inferior portions of the esophagus, each bronchial vein drains the systemic circulation from the lungs, and several smaller veins drain the mediastinal region. Bronchial veins carry approximately 13 percent of the blood that flows into the bronchial arteries; the remainder intermingles with the pulmonary circulation and returns to the heart via the pulmonary veins. These veins flow into the azygos vein, and with the smaller hemiazygos vein (hemi- = “half”) on the left of the vertebral column, drain blood from the thoracic region. The hemiazygos vein does not drain directly into the superior vena cava but enters the brachiocephalic vein via the superior intercostal vein. The azygos vein passes through the diaphragm from the thoracic cavity on the right side of the vertebral column and begins in the lumbar region of the thoracic cavity. It flows into the superior vena cava at approximately the level of T2, making a significant contribution to the flow of blood. It combines with the two large left and right brachiocephalic veins to form the superior vena Cava. [link] summarizes the veins of the thoracic region that flow into the superior vena Cava. Veins of the Thoracic and Abdominal Regions Vertebral Internal jugular Superior . vena cava External jugular Subclavian iad Brachiocephalic Axillary Esophageal Bsonaie Internal werdes Hemiazygos Azygos Intercostal Hepatic Inferior vena cava Renal Phrenic Gonadal Adrenal Lumbar Common iliac Internal iliac External iliac Veins of the thoracic and abdominal regions drain blood from the area above the diaphragm, returning it to the right atrium via the superior vena cava. Veins of the Thoracic Region Vessel Superior vena cava Subclavian vein Brachiocephalic veins Vertebral vein Description Large systemic vein; drains blood from most areas superior to the diaphragm; empties into the right atrium Located deep in the thoracic cavity; formed by the axillary vein as it enters the thoracic cavity from the axillary region; drains the axillary and smaller local veins near the scapular region and leads to the brachiocephalic vein Pair of veins that form from a fusion of the external and internal jugular veins and the subclavian vein; subclavian, external and internal jugulars, vertebral, and internal thoracic veins flow into it; drain the upper thoracic region and lead to the superior vena cava Arises from the base of the brain and the cervical region of the spinal cord; passes through the intervertebral foramina in the cervical vertebrae; drains smaller veins from the cranium, spinal cord, and vertebrae, and leads to the brachiocephalic vein; counterpart of the vertebral artery Veins of the Thoracic Region Vessel Description Also called internal mammary veins; drain the anterior surface of the chest wall and lead to the brachiocephalic vein Internal thoracic veins Drains the muscles of the thoracic wall and Intercostal vein leads to the azygos vein Esophageal Drains the inferior portions of the esophagus vein and leads to the azygos vein Drains the systemic circulation from the lungs Bronchial vein . and leads to the azygos vein Originates in the lumbar region and passes through the diaphragm into the thoracic cavity on the right side of the vertebral column; drains AZzygos vein blood from the intercostal veins, esophageal veins, bronchial veins, and other veins draining the mediastinal region, and leads to the superior vena cava Smaller vein complementary to the azygos vein; drains the esophageal veins from the esophagus and the left intercostal veins, and leads to the brachiocephalic vein via the superior intercostal vein Hemiazygos vein Veins of the Head and Neck Blood from the brain and the superficial facial vein flow into each internal jugular vein ({link]). Blood from the more superficial portions of the head, scalp, and cranial regions, including the temporal vein and maxillary vein, flow into each external jugular vein. Although the external and internal jugular veins are separate vessels, there are anastomoses between them close to the thoracic region. Blood from the external jugular vein empties into the subclavian vein. [link] summarizes the major veins of the head and neck. Major Veins of the Head and Neck Vessel Description Parallel to the common carotid artery, which is more or Internal less its counterpart, and passes through the jugular jugular foramen and canal; primarily drains blood from the vein brain, receives the superficial facial vein, and empties into the subclavian vein Temporal Drains blood from the temporal region and flows into vein the external jugular vein Maxillary Drains blood from the maxillary region and flows into vein the external jugular vein External Drains blood from the more superficial portions of the jugular head, scalp, and cranial regions, and leads to the vein subclavian vein Venous Drainage of the Brain Circulation to the brain is both critical and complex (see [link]). Many smaller veins of the brain stem and the superficial veins of the cerebrum lead to larger vessels referred to as intracranial sinuses. These include the superior and inferior sagittal sinuses, straight sinus, cavernous sinuses, left and right sinuses, the petrosal sinuses, and the occipital sinuses. Ultimately, sinuses will lead back to either the inferior jugular vein or vertebral vein. Most of the veins on the superior surface of the cerebrum flow into the largest of the sinuses, the superior sagittal sinus. It is located midsagittally between the meningeal and periosteal layers of the dura mater within the falx cerebri and, at first glance in images or models, can be mistaken for the subarachnoid space. Most reabsorption of cerebrospinal fluid occurs via the chorionic villi (arachnoid granulations) into the superior sagittal sinus. Blood from most of the smaller vessels originating from the inferior cerebral veins flows into the great cerebral vein and into the straight sinus. Other cerebral veins and those from the eye socket flow into the cavernous sinus, which flows into the petrosal sinus and then into the internal jugular vein. The occipital sinus, sagittal sinus, and straight sinuses all flow into the left and right transverse sinuses near the lambdoid suture. The transverse sinuses in turn flow into the sigmoid sinuses that pass through the jugular foramen and into the internal jugular vein. The internal jugular vein flows parallel to the common carotid artery and is more or less its counterpart. It empties into the brachiocephalic vein. The veins draining the cervical vertebrae and the posterior surface of the skull, including some blood from the occipital sinus, flow into the vertebral veins. These parallel the vertebral arteries and travel through the transverse foramina of the cervical vertebrae. The vertebral veins also flow into the brachiocephalic veins. [link] summarizes the major veins of the brain. Veins of the Head and Neck Superior sagittal sinus Inferior sagittal sinus Straight sinus Occipital Temporal Cavernous sinus Right transverse sinus Maxillary Occipital sinus Facial Sigmoid sinus Petrosal sinus External jugular Internal jugular Vertebral Right subclavian Axillary Superior vena cava This left lateral view shows the veins of the head and neck, including the intercranial sinuses. Major Veins of the Brain Vessel Description Major Veins of the Brain Vessel Superior Sagittal sinus Great cerebral vein Straight sinus Cavernous sinus Petrosal sinus Occipital sinus Transverse sinuses Description Enlarged vein located midsagittally between the meningeal and periosteal layers of the dura mater within the falx cerebri; receives most of the blood drained from the superior surface of the cerebrum and leads to the inferior jugular vein and the vertebral vein Receives most of the smaller vessels from the inferior cerebral veins and leads to the straight sinus Enlarged vein that drains blood from the brain; receives most of the blood from the great cerebral vein and leads to the left or right transverse sinus Enlarged vein that receives blood from most of the other cerebral veins and the eye socket, and leads to the petrosal sinus Enlarged vein that receives blood from the cavernous sinus and leads into the internal jugular veins Enlarged vein that drains the occipital region near the falx cerebelli and leads to the left and right transverse sinuses, and also the vertebral veins Pair of enlarged veins near the lambdoid suture that drains the occipital, sagittal, and straight sinuses, and leads to the sigmoid sinuses Major Veins of the Brain Vessel Description Enlarged vein that receives blood from the transverse sinuses and leads through the jugular foramen to the internal jugular vein Sigmoid sinuses Veins Draining the Upper Limbs The digital veins in the fingers come together in the hand to form the palmar venous arches ((link]). From here, the veins come together to form the radial vein, the ulnar vein, and the median antebrachial vein. The radial vein and the ulnar vein parallel the bones of the forearm and join together at the antebrachium to form the brachial vein, a deep vein that flows into the axillary vein in the brachium. The median antebrachial vein parallels the ulnar vein, is more medial in location, and joins the basilic vein in the forearm. As the basilic vein reaches the antecubital region, it gives off a branch called the median cubital vein that crosses at an angle to join the cephalic vein. The median cubital vein is the most common site for drawing venous blood in humans. The basilic vein continues through the arm medially and superficially to the axillary vein. The cephalic vein begins in the antebrachium and drains blood from the superficial surface of the arm into the axillary vein. It is extremely superficial and easily seen along the surface of the biceps brachii muscle in individuals with good muscle tone and in those without excessive subcutaneous adipose tissue in the arms. The subscapular vein drains blood from the subscapular region and joins the cephalic vein to form the axillary vein. As it passes through the body wall and enters the thorax, the axillary vein becomes the subclavian vein. Many of the larger veins of the thoracic and abdominal region and upper limb are further represented in the flow chart in [link]. [link] summarizes the veins of the upper limbs. Veins of the Upper Limb Subclavian Axillary Cephalic Subscapular Brachial Basilic Median cubital Cephalic Radial Median antebrachial Why) “i ik Cae LAA! Al } ay Palmar venous arches YW pr) j Deep veins Uy rd / 7 [i Superficial veins Digital This anterior view shows the veins that drain the upper limb. KEY Veins Flowing into the Superior Vena Cava Collects blood from cranium, |§gile]n3 spinal cord, vertebral vertebrae Collects blood ; A Right Right Collects blood atl ie bath external cule from cranium, se Se jugular jugular face, neck Right Right Left Left subclavian brachiocephalic brachiocephalic subclavian F Collects ; Right upper Right Left limb RUE internal internal Lee roe axillary brachial (see left limb) Ia peaalens thoracic thoracic thoracic wall Collects Collects wacti Suporte blood from (ULES FS) blood from Mediaeunal Le arm medial ESS (sdiElla) arm lateral vena cava surface surface Right Median cubital, atrium median antebrachial Left radial Hemiazygos Esophageal Left intercostal | Digital The flow chart summarizes the distribution of the veins flowing into the superior vena cava. Veins of the Upper Limbs Veiselof the UppBekdrmbdon Vessel Digital veins Palmar venous arches Radial vein Ulnar vein Brachial vein Median antebrachial vein Basilic vein Description Drain the digits and lead to the palmar arches of the hand and dorsal venous arch of the foot Drain the hand and digits, and lead to the radial vein, ulnar veins, and the median antebrachial vein Vein that parallels the radius and radial artery; arises from the palmar venous arches and leads to the brachial vein Vein that parallels the ulna and ulnar artery; arises from the palmar venous arches and leads to the brachial vein Deeper vein of the arm that forms from the radial and ulnar veins in the lower arm; leads to the axillary vein Vein that parallels the ulnar vein but is more medial in location; intertwines with the palmar venous arches; leads to the basilic vein Superficial vein of the arm that arises from the median antebrachial vein, intersects with the median cubital vein, parallels the ulnar vein, and continues into the upper arm; along with the brachial vein, it leads to the axillary vein Veins of the Upper Limbs Vessel Description Superficial vessel located in the antecubital region Median that links the cephalic vein to the basilic vein in the cubital vein form of a v; a frequent site from which to draw blood Cephalic Superficial vessel in the upper arm; leads to the vein axillary vein Subscapular Drains blood from the subscapular region and leads vein to the axillary vein Axillary The major vein in the axillary region; drains the vein upper limb and becomes the subclavian vein The Inferior Vena Cava Other than the small amount of blood drained by the azygos and hemiazygos veins, most of the blood inferior to the diaphragm drains into the inferior vena cava before it is returned to the heart (see [link]). Lying just beneath the parietal peritoneum in the abdominal cavity, the inferior vena Cava parallels the abdominal aorta, where it can receive blood from abdominal veins. The lumbar portions of the abdominal wall and spinal cord are drained by a series of lumbar veins, usually four on each side. The ascending lumbar veins drain into either the azygos vein on the right or the hemiazygos vein on the left, and return to the superior vena cava. The remaining lumbar veins drain directly into the inferior vena cava. Blood supply from the kidneys flows into each renal vein, normally the largest veins entering the inferior vena cava. A number of other, smaller veins empty into the left renal vein. Each adrenal vein drains the adrenal or suprarenal glands located immediately superior to the kidneys. The right adrenal vein enters the inferior vena cava directly, whereas the left adrenal vein enters the left renal vein. From the male reproductive organs, each testicular vein flows from the scrotum, forming a portion of the spermatic cord. Each ovarian vein drains an ovary in females. Each of these veins is generically called a gonadal vein. The right gonadal vein empties directly into the inferior vena cava, and the left gonadal vein empties into the left renal vein. Each side of the diaphragm drains into a phrenic vein; the right phrenic vein empties directly into the inferior vena cava, whereas the left phrenic vein empties into the left renal vein. Blood supply from the liver drains into each hepatic vein and directly into the inferior vena cava. Since the inferior vena cava lies primarily to the right of the vertebral column and aorta, the left renal vein is longer, as are the left phrenic, adrenal, and gonadal veins. The longer length of the left renal vein makes the left kidney the primary target of surgeons removing this organ for donation. [link] provides a flow chart of the veins flowing into the inferior vena cava. [link] summarizes the major veins of the abdominal region. Venous Flow into Inferior Vena Cava Inferior vena cava Hepatic veins Phrenic wena Diaphragm Gonads (testes or Gonadal ovaries) veins Adrenal F Adrenal glands veins Spinal cord and body wall Kidneys Right common Left common iliac iliac Pelvic muscles, skin, viscera of pelvis, perineum, gluteal region Left internal Left external Rater iliac iliac limb Right lower figile aon Cine! Right internal limb iliac iliac Lateral sacral veins Superior Internal gluteal pudendal veins veins Obturator veins The flow chart summarizes veins that deliver blood to the inferior vena cava. Major Veins of the Abdominal Region Vessel Inferior vena cava Lumbar veins Renal vein Adrenal vein Testicular vein Ovarian vein Gonadal vein Description Large systemic vein that drains blood from areas largely inferior to the diaphragm; empties into the right atrium Series of veins that drain the lumbar portion of the abdominal wall and spinal cord; the ascending lumbar veins drain into the azygos vein on the right or the hemiazygos vein on the left; the remaining lumbar veins drain directly into the inferior vena cava Largest vein entering the inferior vena cava; drains the kidneys and flows into the inferior vena cava Drains the adrenal or suprarenal; the right adrenal vein enters the inferior vena cava directly and the left adrenal vein enters the left renal vein Drains the testes and forms part of the spermatic cord; the right testicular vein empties directly into the inferior vena cava and the left testicular vein empties into the left renal vein Drains the ovary; the right ovarian vein empties directly into the inferior vena cava and the left ovarian vein empties into the left renal vein Generic term for a vein draining a reproductive organ; may be either an ovarian vein or a testicular vein, depending on the sex of the individual Major Veins of the Abdominal Region Vessel Description Drains the diaphragm; the right phrenic vein flows into Phrenic ener ane oh the inferior vena cava and the left phrenic vein empties into the left renal vein Hepatic Drains systemic blood from the liver and flows into vein the inferior vena cava Veins Draining the Lower Limbs The superior surface of the foot drains into the digital veins, and the inferior surface drains into the plantar veins, which flow into a complex series of anastomoses in the feet and ankles, including the dorsal venous arch and the plantar venous arch ((link]). From the dorsal venous arch, blood supply drains into the anterior and posterior tibial veins. The anterior tibial vein drains the area near the tibialis anterior muscle and combines with the posterior tibial vein and the fibular vein to form the popliteal vein. The posterior tibial vein drains the posterior surface of the tibia and joins the popliteal vein. The fibular vein drains the muscles and integument in proximity to the fibula and also joins the popliteal vein. The small saphenous vein located on the lateral surface of the leg drains blood from the superficial regions of the lower leg and foot, and flows into to the popliteal vein. As the popliteal vein passes behind the knee in the popliteal region, it becomes the femoral vein. It is palpable in patients without excessive adipose tissue. Close to the body wall, the great saphenous vein, the deep femoral vein, and the femoral circumflex vein drain into the femoral vein. The great saphenous vein is a prominent surface vessel located on the medial surface of the leg and thigh that collects blood from the superficial portions of these areas. The deep femoral vein, as the name suggests, drains blood from the deeper portions of the thigh. The femoral circumflex vein forms a loop around the femur just inferior to the trochanters and drains blood from the areas in proximity to the head and neck of the femur. As the femoral vein penetrates the body wall from the femoral portion of the upper limb, it becomes the external iliac vein, a large vein that drains blood from the leg to the common iliac vein. The pelvic organs and integument drain into the internal iliac vein, which forms from several smaller veins in the region, including the umbilical veins that run on either side of the bladder. The external and internal iliac veins combine near the inferior portion of the sacroiliac joint to form the common iliac vein. In addition to blood supply from the external and internal iliac veins, the middle sacral vein drains the sacral region into the common iliac vein. Similar to the common iliac arteries, the common iliac veins come together at the level of L5 to form the inferior vena cava. [link] is a flow chart of veins flowing into the lower limb. [link] summarizes the major veins of the lower limbs. Major Veins Serving the Lower Limbs Common iliac External iliac | Internal iliac External iliac = 42 Gluteal Internal ; Lateral sacral pudendal Gluteal Internal pudendal Obturator Obturator Femoral Femoral Deep femoral Deep femoral Femoral Femoral circumflex circumflex Femoral Femoral Great saphenous Great saphenous Popliteal Popliteal Small saphenous Small Anterior tibial eppnenGys Posterior tibial Anterior tibial Posterior Fibular tibial Fibular Lateral plantar Medial plantar Plantar venous Dorsal venous arch arch Digital Anterior view Posterior view Anterior and posterior views show the major veins that drain the lower limb into the inferior vena cava. Major Veins of the Lower Limb External iliac Collects blood from Collects blood Deep Eemoral Great the superficial from the thigh femoral EY-)9)il-JaleltESa) veins of the lower limb Collects blood from superficial Small veins of the leg saphenous and foot Popliteal Anterior Posterior tibial tibial AES Lateral and medial plantar, dorsal arch, plantar arch Metatarsal, Distal foot, digital toes The flow chart summarizes venous flow from the lower limb. Veins of the Lower Limbs Vessel Plantar veins Dorsal venous arch Plantar venous arch Anterior tibial vein Posterior tibial vein Fibular vein Small saphenous vein Popliteal vein Description Drain the foot and flow into the plantar venous arch Drains blood from digital veins and vessels on the superior surface of the foot Formed from the plantar veins; flows into the anterior and posterior tibial veins through anastomoses Formed from the dorsal venous arch; drains the area near the tibialis anterior muscle and flows into the popliteal vein Formed from the dorsal venous arch; drains the area near the posterior surface of the tibia and flows into the popliteal vein Drains the muscles and integument near the fibula and flows into the popliteal vein Located on the lateral surface of the leg; drains blood from the superficial regions of the lower leg and foot, and flows into the popliteal vein Drains the region behind the knee and forms from the fusion of the fibular, anterior, and posterior tibial veins; flows into the femoral vein Veins of the Lower Limbs Vessel Great saphenous vein Deep femoral vein Femoral circumflex vein Femoral vein External iliac vein Internal iliac vein Middle sacral vein Description Prominent surface vessel located on the medial surface of the leg and thigh; drains the superficial portions of these areas and flows into the femoral vein Drains blood from the deeper portions of the thigh and flows into the femoral vein Forms a loop around the femur just inferior to the trochanters; drains blood from the areas around the head and neck of the femur; flows into the femoral vein Drains the upper leg; receives blood from the great saphenous vein, the deep femoral vein, and the femoral circumflex vein; becomes the external iliac vein when it crosses the body wall Formed when the femoral vein passes into the body cavity; drains the legs and flows into the common iliac vein Drains the pelvic organs and integument; formed from several smaller veins in the region; flows into the common iliac vein Drains the sacral region and flows into the left common iliac vein Veins of the Lower Limbs Vessel Description Flows into the inferior vena cava at the level of L5; the left common iliac vein drains the sacral region; formed from the union of the external and internal iliac veins near the inferior portion of the sacroiliac joint Common iliac vein Hepatic Portal System The liver is a complex biochemical processing plant. It packages nutrients absorbed by the digestive system; produces plasma proteins, clotting factors, and bile; and disposes of worn-out cell components and waste products. Instead of entering the circulation directly, absorbed nutrients and certain wastes (for example, materials produced by the spleen) travel to the liver for processing. They do so via the hepatic portal system ((Link)). Portal systems begin and end in capillaries. In this case, the initial capillaries from the stomach, small intestine, large intestine, and spleen lead to the hepatic portal vein and end in specialized capillaries within the liver, the hepatic sinusoids. You saw the only other portal system with the hypothalamic-hypophyseal portal vessel in the endocrine chapter. The hepatic portal system consists of the hepatic portal vein and the veins that drain into it. The hepatic portal vein itself is relatively short, beginning at the level of L2 with the confluence of the superior mesenteric and splenic veins. It also receives branches from the inferior mesenteric vein, plus the splenic veins and all their tributaries. The superior mesenteric vein receives blood from the small intestine, two-thirds of the large intestine, and the stomach. The inferior mesenteric vein drains the distal third of the large intestine, including the descending colon, the sigmoid colon, and the rectum. The splenic vein is formed from branches from the spleen, pancreas, and portions of the stomach, and the inferior mesenteric vein. After its formation, the hepatic portal vein also receives branches from the gastric veins of the stomach and cystic veins from the gall bladder. The hepatic portal vein delivers materials from these digestive and circulatory organs directly to the liver for processing. Because of the hepatic portal system, the liver receives its blood supply from two different sources: from normal systemic circulation via the hepatic artery and from the hepatic portal vein. The liver processes the blood from the portal system to remove certain wastes and excess nutrients, which are stored for later use. This processed blood, as well as the systemic blood that came from the hepatic artery, exits the liver via the right, left, and middle hepatic veins, and flows into the inferior vena cava. Overall systemic blood composition remains relatively stable, since the liver is able to metabolize the absorbed digestive components. Hepatic Portal System Hepatic portal Cystic ey Gall bladder Superior mesenteric Gastro-omental () Right gastric Splenic Gastroepiploic . Pancreatic Pancreaticoduodenal Middle colic Right colic cis ~~ Inferior mesenteric i X Left colic Ileocolic J > A Intestinal ( on s xeeyl The liver receives blood from the normal systemic circulation via the hepatic artery. It also receives and processes blood from other organs, delivered via the veins of the hepatic portal system. All blood exits the liver via the hepatic vein, which delivers the blood to the inferior vena cava. (Different colors are used to help distinguish among the different vessels in the system.) Sigmoid Superior rectal Chapter Review The right ventricle pumps oxygen-depleted blood into the pulmonary trunk and right and left pulmonary arteries, which carry it to the right and left lungs for gas exchange. Oxygen-rich blood is transported by pulmonary veins to the left atrium. The left ventricle pumps this blood into the aorta. The main regions of the aorta are the ascending aorta, aortic arch, and descending aorta, which is further divided into the thoracic and abdominal aorta. The coronary arteries branch from the ascending aorta. After oxygenating tissues in the capillaries, systemic blood is returned to the right atrium from the venous system via the superior vena cava, which drains most of the veins superior to the diaphragm, the inferior vena cava, which drains most of the veins inferior to the diaphragm, and the coronary veins via the coronary sinus. The hepatic portal system carries blood to the liver for processing before it enters circulation. Review the figures provided in this section for circulation of blood through the blood vessels. Review Questions Exercise: Problem:The coronary arteries branch off of the a. aortic valve b. ascending aorta c. aortic arch d. thoracic aorta Solution: B Exercise: Problem: Which of the following statements is true? a. The left and right common carotid arteries both branch off of the brachiocephalic trunk. b. The brachial artery is the distal branch of the axillary artery. c. The radial and ulnar arteries join to form the palmar arch. d. All of the above are true. Solution: C Exercise: Problem: Arteries serving the stomach, pancreas, and liver all branch from the a. Superior mesenteric artery b. inferior mesenteric artery c. celiac trunk d. splenic artery Solution: C Exercise: Problem:The right and left brachiocephalic veins a. drain blood from the right and left internal jugular veins b. drain blood from the right and left subclavian veins c. drain into the superior vena cava d. all of the above are true Solution: D Exercise: Problem: The hepatic portal system delivers blood from the digestive organs to the a. liver b. hypothalamus c. spleen d. left atrium Solution: A Critical Thinking Questions Exercise: Problem: Identify the ventricle of the heart that pumps oxygen-depleted blood and the arteries of the body that carry oxygen-depleted blood. Solution: The right ventricle of the heart pumps oxygen-depleted blood to the pulmonary arteries. Exercise: Problem:What organs do the gonadal veins drain? Solution: The gonadal veins drain the testes in males and the ovaries in females. Exercise: Problem: What arteries play the leading roles in supplying blood to the brain? Solution: The internal carotid arteries and the vertebral arteries provide most of the brain’s blood supply. Glossary abdominal aorta portion of the aorta inferior to the aortic hiatus and superior to the common iliac arteries adrenal artery branch of the abdominal aorta; supplies blood to the adrenal (suprarenal) glands adrenal vein drains the adrenal or suprarenal glands that are immediately superior to the kidneys; the right adrenal vein enters the inferior vena cava directly and the left adrenal vein enters the left renal vein anterior cerebral artery arises from the internal carotid artery; supplies the frontal lobe of the cerebrum anterior communicating artery anastomosis of the right and left internal carotid arteries; supplies blood to the brain anterior tibial artery branches from the popliteal artery; supplies blood to the anterior tibial region; becomes the dorsalis pedis artery anterior tibial vein forms from the dorsal venous arch; drains the area near the tibialis anterior muscle and leads to the popliteal vein aorta largest artery in the body, originating from the left ventricle and descending to the abdominal region where it bifurcates into the common iliac arteries at the level of the fourth lumbar vertebra; arteries originating from the aorta distribute blood to virtually all tissues of the body aortic arch arc that connects the ascending aorta to the descending aorta; ends at the intervertebral disk between the fourth and fifth thoracic vertebrae aortic hiatus opening in the diaphragm that allows passage of the thoracic aorta into the abdominal region where it becomes the abdominal aorta arterial circle (also, circle of Willis) anastomosis located at the base of the brain that ensures continual blood supply; formed from branches of the internal carotid and vertebral arteries; supplies blood to the brain ascending aorta initial portion of the aorta, rising from the left ventricle for a distance of approximately 5 cm axillary artery continuation of the subclavian artery as it penetrates the body wall and enters the axillary region; supplies blood to the region near the head of the humerus (humeral circumflex arteries); the majority of the vessel continues into the brachium and becomes the brachial artery axillary vein major vein in the axillary region; drains the upper limb and becomes the subclavian vein azygos vein originates in the lumbar region and passes through the diaphragm into the thoracic cavity on the right side of the vertebral column; drains blood from the intercostal veins, esophageal veins, bronchial veins, and other veins draining the mediastinal region; leads to the superior vena cava basilar artery formed from the fusion of the two vertebral arteries; sends branches to the cerebellum, brain stem, and the posterior cerebral arteries; the main blood supply to the brain stem basilic vein superficial vein of the arm that arises from the palmar venous arches, intersects with the median cubital vein, parallels the ulnar vein, and continues into the upper arm; along with the brachial vein, it leads to the axillary vein brachial artery continuation of the axillary artery in the brachium; supplies blood to much of the brachial region; gives off several smaller branches that provide blood to the posterior surface of the arm in the region of the elbow; bifurcates into the radial and ulnar arteries at the coronoid fossa brachial vein deeper vein of the arm that forms from the radial and ulnar veins in the lower arm; leads to the axillary vein brachiocephalic artery single vessel located on the right side of the body; the first vessel branching from the aortic arch; gives rise to the right subclavian artery and the right common carotid artery; supplies blood to the head, neck, upper limb, and wall of the thoracic region brachiocephalic vein one of a pair of veins that form from a fusion of the external and internal jugular veins and the subclavian vein; subclavian, external and internal jugulars, vertebral, and internal thoracic veins lead to it; drains the upper thoracic region and flows into the superior vena cava bronchial artery systemic branch from the aorta that provides oxygenated blood to the lungs in addition to the pulmonary circuit bronchial vein drains the systemic circulation from the lungs and leads to the azygos vein cavernous sinus enlarged vein that receives blood from most of the other cerebral veins and the eye socket, and leads to the petrosal sinus celiac trunk (also, celiac artery) major branch of the abdominal aorta; gives rise to the left gastric artery, the splenic artery, and the common hepatic artery that forms the hepatic artery to the liver, the right gastric artery to the stomach, and the cystic artery to the gall bladder cephalic vein superficial vessel in the upper arm; leads to the axillary vein cerebrovascular accident (CVA) blockage of blood flow to the brain; also called a stroke circle of Willis (also, arterial circle) anastomosis located at the base of the brain that ensures continual blood supply; formed from branches of the internal carotid and vertebral arteries; supplies blood to the brain common carotid artery right common carotid artery arises from the brachiocephalic artery, and the left common carotid arises from the aortic arch; gives rise to the external and internal carotid arteries; supplies the respective sides of the head and neck common hepatic artery branch of the celiac trunk that forms the hepatic artery, the right gastric artery, and the cystic artery common iliac artery branch of the aorta that leads to the internal and external iliac arteries common iliac vein one of a pair of veins that flows into the inferior vena cava at the level of L5; the left common iliac vein drains the sacral region; divides into external and internal iliac veins near the inferior portion of the sacroiliac joint cystic artery branch of the common hepatic artery; supplies blood to the gall bladder deep femoral artery branch of the femoral artery; gives rise to the lateral circumflex arteries deep femoral vein drains blood from the deeper portions of the thigh and leads to the femoral vein descending aorta portion of the aorta that continues downward past the end of the aortic arch; subdivided into the thoracic aorta and the abdominal aorta digital arteries formed from the superficial and deep palmar arches; supply blood to the digits digital veins drain the digits and feed into the palmar arches of the hand and dorsal venous arch of the foot dorsal arch (also, arcuate arch) formed from the anastomosis of the dorsalis pedis artery and medial and plantar arteries; branches supply the distal portions of the foot and digits dorsal venous arch drains blood from digital veins and vessels on the superior surface of the foot dorsalis pedis artery forms from the anterior tibial artery; branches repeatedly to supply blood to the tarsal and dorsal regions of the foot esophageal artery branch of the thoracic aorta; supplies blood to the esophagus esophageal vein drains the inferior portions of the esophagus and leads to the azygos vein external carotid artery arises from the common carotid artery; supplies blood to numerous structures within the face, lower jaw, neck, esophagus, and larynx external iliac artery branch of the common iliac artery that leaves the body cavity and becomes a femoral artery; supplies blood to the lower limbs external iliac vein formed when the femoral vein passes into the body cavity; drains the legs and leads to the common iliac vein external jugular vein one of a pair of major veins located in the superficial neck region that drains blood from the more superficial portions of the head, scalp, and cranial regions, and leads to the subclavian vein femoral artery continuation of the external iliac artery after it passes through the body cavity; divides into several smaller branches, the lateral deep femoral artery, and the genicular artery; becomes the popliteal artery as it passes posterior to the knee femoral circumflex vein forms a loop around the femur just inferior to the trochanters; drains blood from the areas around the head and neck of the femur; leads to the femoral vein femoral vein drains the upper leg; receives blood from the great saphenous vein, the deep femoral vein, and the femoral circumflex vein; becomes the external iliac vein when it crosses the body wall fibular vein drains the muscles and integument near the fibula and leads to the popliteal vein genicular artery branch of the femoral artery; supplies blood to the region of the knee gonadal artery branch of the abdominal aorta; supplies blood to the gonads or reproductive organs; also described as ovarian arteries or testicular arteries, depending upon the sex of the individual gonadal vein generic term for a vein draining a reproductive organ; may be either an ovarian vein or a testicular vein, depending on the sex of the individual great cerebral vein receives most of the smaller vessels from the inferior cerebral veins and leads to the straight sinus great saphenous vein prominent surface vessel located on the medial surface of the leg and thigh; drains the superficial portions of these areas and leads to the femoral vein hemiazygos vein smaller vein complementary to the azygos vein; drains the esophageal veins from the esophagus and the left intercostal veins, and leads to the brachiocephalic vein via the superior intercostal vein hepatic artery proper branch of the common hepatic artery; supplies systemic blood to the liver hepatic portal system specialized circulatory pathway that carries blood from digestive organs to the liver for processing before being sent to the systemic circulation hepatic vein drains systemic blood from the liver and flows into the inferior vena cava inferior mesenteric artery branch of the abdominal aorta; supplies blood to the distal segment of the large intestine and rectum inferior phrenic artery branch of the abdominal aorta; supplies blood to the inferior surface of the diaphragm inferior vena cava large systemic vein that drains blood from areas largely inferior to the diaphragm; empties into the right atrium intercostal artery branch of the thoracic aorta; supplies blood to the muscles of the thoracic cavity and vertebral column intercostal vein drains the muscles of the thoracic wall and leads to the azygos vein internal carotid artery arises from the common carotid artery and begins with the carotid sinus; goes through the carotid canal of the temporal bone to the base of the brain; combines with branches of the vertebral artery forming the arterial circle; supplies blood to the brain internal iliac artery branch from the common iliac arteries; supplies blood to the urinary bladder, walls of the pelvis, external genitalia, and the medial portion of the femoral region; in females, also provide blood to the uterus and vagina internal iliac vein drains the pelvic organs and integument; formed from several smaller veins in the region; leads to the common iliac vein internal jugular vein one of a pair of major veins located in the neck region that passes through the jugular foramen and canal, flows parallel to the common carotid artery that is more or less its counterpart; primarily drains blood from the brain, receives the superficial facial vein, and empties into the subclavian vein internal thoracic artery (also, mammary artery) arises from the subclavian artery; supplies blood to the thymus, pericardium of the heart, and the anterior chest wall internal thoracic vein (also, internal mammary vein) drains the anterior surface of the chest wall and leads to the brachiocephalic vein lateral circumflex artery branch of the deep femoral artery; supplies blood to the deep muscles of the thigh and the ventral and lateral regions of the integument lateral plantar artery arises from the bifurcation of the posterior tibial arteries; supplies blood to the lateral plantar surfaces of the foot left gastric artery branch of the celiac trunk; supplies blood to the stomach lumbar arteries branches of the abdominal aorta; supply blood to the lumbar region, the abdominal wall, and spinal cord lumbar veins drain the lumbar portion of the abdominal wall and spinal cord; the superior lumbar veins drain into the azygos vein on the right or the hemiazygos vein on the left; blood from these vessels is returned to the superior vena cava rather than the inferior vena cava maxillary vein drains blood from the maxillary region and leads to the external jugular vein medial plantar artery arises from the bifurcation of the posterior tibial arteries; supplies blood to the medial plantar surfaces of the foot median antebrachial vein vein that parallels the ulnar vein but is more medial in location; intertwines with the palmar venous arches median cubital vein superficial vessel located in the antecubital region that links the cephalic vein to the basilic vein in the form of a v; a frequent site for a blood draw median sacral artery continuation of the aorta into the sacrum mediastinal artery branch of the thoracic aorta; supplies blood to the mediastinum middle cerebral artery another branch of the internal carotid artery; supplies blood to the temporal and parietal lobes of the cerebrum middle sacral vein drains the sacral region and leads to the left common iliac vein occipital sinus enlarged vein that drains the occipital region near the falx cerebelli and flows into the left and right transverse sinuses, and also into the vertebral veins ophthalmic artery branch of the internal carotid artery; supplies blood to the eyes ovarian artery branch of the abdominal aorta; supplies blood to the ovary, uterine (Fallopian) tube, and uterus ovarian vein drains the ovary; the right ovarian vein leads to the inferior vena cava and the left ovarian vein leads to the left renal vein palmar arches superficial and deep arches formed from anastomoses of the radial and ulnar arteries; supply blood to the hand and digital arteries palmar venous arches drain the hand and digits, and feed into the radial and ulnar veins parietal branches (also, somatic branches) group of arterial branches of the thoracic aorta; includes those that supply blood to the thoracic cavity, vertebral column, and the superior surface of the diaphragm pericardial artery branch of the thoracic aorta; supplies blood to the pericardium petrosal sinus enlarged vein that receives blood from the cavernous sinus and flows into the internal jugular vein phrenic vein drains the diaphragm; the right phrenic vein flows into the inferior vena cava and the left phrenic vein leads to the left renal vein plantar arch formed from the anastomosis of the dorsalis pedis artery and medial and plantar arteries; branches supply the distal portions of the foot and digits plantar veins drain the foot and lead to the plantar venous arch plantar venous arch formed from the plantar veins; leads to the anterior and posterior tibial veins through anastomoses popliteal artery continuation of the femoral artery posterior to the knee; branches into the anterior and posterior tibial arteries popliteal vein continuation of the femoral vein behind the knee; drains the region behind the knee and forms from the fusion of the fibular and anterior and posterior tibial veins posterior cerebral artery branch of the basilar artery that forms a portion of the posterior segment of the arterial circle; supplies blood to the posterior portion of the cerebrum and brain stem posterior communicating artery branch of the posterior cerebral artery that forms part of the posterior portion of the arterial circle; supplies blood to the brain posterior tibial artery branch from the popliteal artery that gives rise to the fibular or peroneal artery; supplies blood to the posterior tibial region posterior tibial vein forms from the dorsal venous arch; drains the area near the posterior surface of the tibia and leads to the popliteal vein pulmonary artery one of two branches, left and right, that divides off from the pulmonary trunk and leads to smaller arterioles and eventually to the pulmonary capillaries pulmonary circuit system of blood vessels that provide gas exchange via a network of arteries, veins, and capillaries that run from the heart, through the body, and back to the lungs pulmonary trunk single large vessel exiting the right ventricle that divides to form the right and left pulmonary arteries pulmonary veins two sets of paired vessels, one pair on each side, that are formed from the small venules leading away from the pulmonary capillaries that flow into the left atrium radial artery formed at the bifurcation of the brachial artery; parallels the radius; gives off smaller branches until it reaches the carpal region where it fuses with the ulnar artery to form the superficial and deep palmar arches; supplies blood to the lower arm and carpal region radial vein parallels the radius and radial artery; arises from the palmar venous arches and leads to the brachial vein renal artery branch of the abdominal aorta; supplies each kidney renal vein largest vein entering the inferior vena cava; drains the kidneys and leads to the inferior vena cava right gastric artery branch of the common hepatic artery; supplies blood to the stomach sigmoid sinuses enlarged veins that receive blood from the transverse sinuses; flow through the jugular foramen and into the internal jugular vein small saphenous vein located on the lateral surface of the leg; drains blood from the superficial regions of the lower leg and foot, and leads to the popliteal vein splenic artery branch of the celiac trunk; supplies blood to the spleen straight sinus enlarged vein that drains blood from the brain; receives most of the blood from the great cerebral vein and flows into the left or right transverse sinus subclavian artery right subclavian arises from the brachiocephalic artery, whereas the left subclavian artery arises from the aortic arch; gives rise to the internal thoracic, vertebral, and thyrocervical arteries; supplies blood to the arms, chest, shoulders, back, and central nervous system subclavian vein located deep in the thoracic cavity; becomes the axillary vein as it enters the axillary region; drains the axillary and smaller local veins near the scapular region; leads to the brachiocephalic vein subscapular vein drains blood from the subscapular region and leads to the axillary vein superior mesenteric artery branch of the abdominal aorta; supplies blood to the small intestine (duodenum, jejunum, and ileum), the pancreas, and a majority of the large intestine superior phrenic artery branch of the thoracic aorta; supplies blood to the superior surface of the diaphragm superior sagittal sinus enlarged vein located midsagittally between the meningeal and periosteal layers of the dura mater within the falx cerebri; receives most of the blood drained from the superior surface of the cerebrum and leads to the inferior jugular vein and the vertebral vein superior vena cava large systemic vein; drains blood from most areas superior to the diaphragm; empties into the right atrium temporal vein drains blood from the temporal region and leads to the external jugular vein testicular artery branch of the abdominal aorta; will ultimately travel outside the body cavity to the testes and form one component of the spermatic cord testicular vein drains the testes and forms part of the spermatic cord; the right testicular vein empties directly into the inferior vena cava and the left testicular vein empties into the left renal vein thoracic aorta portion of the descending aorta superior to the aortic hiatus thyrocervical artery arises from the subclavian artery; supplies blood to the thyroid, the cervical region, the upper back, and shoulder transient ischemic attack (TIA) temporary loss of neurological function caused by a brief interruption in blood flow; also known as a mini-stroke transverse sinuses pair of enlarged veins near the lambdoid suture that drain the occipital, Sagittal, and straight sinuses, and leads to the sigmoid sinuses trunk large vessel that gives rise to smaller vessels ulnar artery formed at the bifurcation of the brachial artery; parallels the ulna; gives off smaller branches until it reaches the carpal region where it fuses with the radial artery to form the superficial and deep palmar arches; supplies blood to the lower arm and carpal region ulnar vein parallels the ulna and ulnar artery; arises from the palmar venous arches and leads to the brachial vein vertebral artery arises from the subclavian artery and passes through the vertebral foramen through the foramen magnum to the brain; joins with the internal carotid artery to form the arterial circle; supplies blood to the brain and spinal cord vertebral vein arises from the base of the brain and the cervical region of the spinal cord; passes through the intervertebral foramina in the cervical vertebrae; drains smaller veins from the cranium, spinal cord, and vertebrae, and leads to the brachiocephalic vein; counterpart of the vertebral artery visceral branches branches of the descending aorta that supply blood to the viscera An Overview of Blood By the end of this section, you will be able to: e Identify the primary functions of blood in transportation, defense, and maintenance of homeostasis e Name the fluid component of blood and the three major types of formed elements, and identify their relative proportions in a blood sample e Discuss the unique physical characteristics of blood e Identify the composition of blood plasma, including its most important solutes and plasma proteins Recall that blood is a connective tissue. Like all connective tissues, it is made up of cellular elements and an extracellular matrix. The cellular elements—referred to as the formed elements—include red blood cells (RBCs), white blood cells (WBCs), and cell fragments called platelets. The extracellular matrix, called plasma, makes blood unique among connective tissues because it is fluid. This fluid, which is mostly water, perpetually suspends the formed elements and enables them to circulate throughout the body within the cardiovascular system. Functions of Blood The primary function of blood is to deliver oxygen and nutrients to and remove wastes from body cells, but that is only the beginning of the story. The specific functions of blood also include defense, distribution of heat, and maintenance of homeostasis. Transportation Nutrients from the foods you eat are absorbed in the digestive tract. Most of these travel in the bloodstream directly to the liver, where they are processed and released back into the bloodstream for delivery to body cells. Oxygen from the air you breathe diffuses into the blood, which moves from the lungs to the heart, which then pumps it out to the rest of the body. Moreover, endocrine glands scattered throughout the body release their products, called hormones, into the bloodstream, which carries them to distant target cells. Blood also picks up cellular wastes and byproducts, and transports them to various organs for removal. For instance, blood moves carbon dioxide to the lungs for exhalation from the body, and various waste products are transported to the kidneys and liver for excretion from the body in the form of urine or bile. Defense Many types of WBCs protect the body from external threats, such as disease-causing bacteria that have entered the bloodstream in a wound. Other WBCs seek out and destroy internal threats, such as cells with mutated DNA that could multiply to become cancerous, or body cells infected with viruses. When damage to the vessels results in bleeding, blood platelets and certain proteins dissolved in the plasma, the fluid portion of the blood, interact to block the ruptured areas of the blood vessels involved. This protects the body from further blood loss. Maintenance of Homeostasis Recall that body temperature is regulated via a classic negative-feedback loop. If you were exercising on a warm day, your rising core body temperature would trigger several homeostatic mechanisms, including increased transport of blood from your core to your body periphery, which is typically cooler. As blood passes through the vessels of the skin, heat would be dissipated to the environment, and the blood returning to your body core would be cooler. In contrast, on a cold day, blood is diverted away from the skin to maintain a warmer body core. In extreme cases, this may result in frostbite. Blood also helps to maintain the chemical balance of the body. Proteins and other compounds in blood act as buffers, which thereby help to regulate the pH of body tissues. Blood also helps to regulate the water content of body cells. Composition of Blood You have probably had blood drawn from a superficial vein in your arm, which was then sent to a lab for analysis. Some of the most common blood tests—for instance, those measuring lipid or glucose levels in plasma— determine which substances are present within blood and in what quantities. Other blood tests check for the composition of the blood itself, including the quantities and types of formed elements. One such test, called a hematocrit, measures the percentage of RBCs, clinically known as erythrocytes, in a blood sample. It is performed by spinning the blood sample in a specialized centrifuge, a process that causes the heavier elements suspended within the blood sample to separate from the lightweight, liquid plasma ({link]). Because the heaviest elements in blood are the erythrocytes, these settle at the very bottom of the hematocrit tube. Located above the erythrocytes is a pale, thin layer composed of the remaining formed elements of blood. These are the WBCGs, clinically known as leukocytes, and the platelets, cell fragments also called thrombocytes. This layer is referred to as the buffy coat because of its color; it normally constitutes less than 1 percent of a blood sample. Above the buffy coat is the blood plasma, normally a pale, straw-colored fluid, which constitutes the remainder of the sample. The volume of erythrocytes after centrifugation is also commonly referred to as packed cell volume (PCV). In normal blood, about 45 percent of a sample is erythrocytes. The hematocrit of any one sample can vary significantly, however, about 36—50 percent, according to gender and other factors. Normal hematocrit values for females range from 37 to 47, with a mean value of 41; for males, hematocrit ranges from 42 to 52, with a mean of 47. The percentage of other formed elements, the WBCs and platelets, is extremely small so it is not normally considered with the hematocrit. So the mean plasma percentage is the percent of blood that is not erythrocytes: for females, it is approximately 59 (or 100 minus 41), and for males, it is approximately 53 (or 100 minus 47). Composition of Blood Plasma: - Water, proteins, nutrients, hormones, etc. Buffy coat: - White blood cells, platelets Hematocrit: - Red blood cells Normal Blood: Anemia: Polycythemia: Q. 37%-47% hematocrit Depressed Elevated O" 42%-52% hematocrit hematocrit % hematocrit % The cellular elements of blood include a vast number of erythrocytes and comparatively fewer leukocytes and platelets. Plasma is the fluid in which the formed elements are suspended. A sample of blood spun in a centrifuge reveals that plasma is the lightest component. It floats at the top of the tube separated from the heaviest elements, the erythrocytes, by a buffy coat of leukocytes and platelets. Hematocrit is the percentage of the total sample that is comprised of erythrocytes. Depressed and elevated hematocrit levels are shown for comparison. Characteristics of Blood When you think about blood, the first characteristic that probably comes to mind is its color. Blood that has just taken up oxygen in the lungs is bright red, and blood that has released oxygen in the tissues is a more dusky red. This is because hemoglobin is a pigment that changes color, depending upon the degree of oxygen saturation. Blood is viscous and somewhat sticky to the touch. It has a viscosity approximately five times greater than water. Viscosity is a measure of a fluid’s thickness or resistance to flow, and is influenced by the presence of the plasma proteins and formed elements within the blood. The viscosity of blood has a dramatic impact on blood pressure and flow. Consider the difference in flow between water and honey. The more viscous honey would demonstrate a greater resistance to flow than the less viscous water. The same principle applies to blood. The normal temperature of blood is slightly higher than normal body temperature—about 38 °C (or 100.4 °F), compared to 37 °C (or 98.6 °F) for an internal body temperature reading, although daily variations of 0.5 °C are normal. Although the surface of blood vessels is relatively smooth, as blood flows through them, it experiences some friction and resistance, especially as vessels age and lose their elasticity, thereby producing heat. This accounts for its slightly higher temperature. The pH of blood averages about 7.4; however, it can range from 7.35 to 7.45 in a healthy person. Blood is therefore somewhat more basic (alkaline) on a chemical scale than pure water, which has a pH of 7.0. Blood contains numerous buffers that actually help to regulate pH. Blood constitutes approximately 8 percent of adult body weight. Adult males typically average about 5 to 6 liters of blood. Females average 4—5 liters. Blood Plasma Like other fluids in the body, plasma is composed primarily of water: In fact, it is about 92 percent water. Dissolved or suspended within this water is a mixture of substances, most of which are proteins. There are literally hundreds of substances dissolved or suspended in the plasma, although many of them are found only in very small quantities. Note: Ase een en ara inten Visit this site for a list of normal levels established for many of the substances found in a sample of blood. Serum, one of the specimen types included, refers to a sample of plasma after clotting factors have been removed. What types of measurements are given for levels of glucose in the blood? Plasma Proteins About 7 percent of the volume of plasma—nearly all that is not water—is made of proteins. These include several plasma proteins (proteins that are unique to the plasma), plus a much smaller number of regulatory proteins, including enzymes and some hormones. The major components of plasma are summarized in [link]. The three major groups of plasma proteins are as follows: e Albumin is the most abundant of the plasma proteins. Manufactured by the liver, albumin molecules serve as binding proteins—transport vehicles for fatty acids and steroid hormones. Recall that lipids are hydrophobic; however, their binding to albumin enables their transport in the watery plasma. Albumin is also the most significant contributor to the osmotic pressure of blood; that is, its presence holds water inside the blood vessels and draws water from the tissues, across blood vessel walls, and into the bloodstream. This in turn helps to maintain both blood volume and blood pressure. Albumin normally accounts for approximately 54 percent of the total plasma protein content, in clinical levels of 3.5-5.0 g/dL blood. e The second most common plasma proteins are the globulins. A heterogeneous group, there are three main subgroups known as alpha, beta, and gamma globulins. The alpha and beta globulins transport iron, lipids, and the fat-soluble vitamins A, D, E, and K to the cells; like albumin, they also contribute to osmotic pressure. The gamma globulins are proteins involved in immunity and are better known as an antibodies or immunoglobulins. Although other plasma proteins are produced by the liver, immunoglobulins are produced by specialized leukocytes known as plasma cells. (Seek additional content for more information about immunoglobulins.) Globulins make up approximately 38 percent of the total plasma protein volume, in clinical levels of 1.0—1.5 g/dL blood. e The least abundant plasma protein is fibrinogen. Like albumin and the alpha and beta globulins, fibrinogen is produced by the liver. It is essential for blood clotting, a process described later in this chapter. Fibrinogen accounts for about 7 percent of the total plasma protein volume, in clinical levels of 0.2—-0.45 g/dL blood. Other Plasma Solutes In addition to proteins, plasma contains a wide variety of other substances. These include various electrolytes, such as sodium, potassium, and calcium ions; dissolved gases, such as oxygen, carbon dioxide, and nitrogen; various organic nutrients, such as vitamins, lipids, glucose, and amino acids; and metabolic wastes. All of these nonprotein solutes combined contribute approximately 1 percent to the total volume of plasma. Major Blood Components Component Subcomponent Type and % and % and % of (where Site of production Major function(s) of blood component appropriate) Absorbed by intestinal tract or produced by metabolism Water 92 percent Transport medium Maintain osmotic Albumin concentration, 54-60 percent transport lipid molecules Transport, maintain osmotic concentration Alpha globulins— liver Plasma proteins Plasma 7 percent Globulins Beta globulins— 46-63 35-38 percent liver percent Transport, maintain osmotic concentration Gamma globulins (immunoglobulins) —plasma cells Immune responses Fibrinogen Blood clotting in 4-7 percent hemostasis Regulatory proteins Hormones : Regulate various and enzymes Various sources body functions Absorbed by intestinal Other solutes Nutrients, gases, tract, exchanged in Numerous 1 percent and wastes respiratory system, and varied or produced by cells Transport gases, Erythrocytes primarily oxygen 99 percent Erytwocyive Red bone marrow and some carbon dioxide Granular leukocytes: neutrophils Red bone marrow eosinophils basophils Nonspecific Formed immunity elements Leukocytes 37-54 <1 percent percent Platelets Lymphocytes: Lymphocytes: <1 percent Agranular bone marrow and specific leukocytes: lymphatic tissue immunity lymphocytes monocytes Monocytes: Monocytes: red bone marrow nonspecific immunity Platelets Megakaryocytes: : Note: Career Connection Phlebotomy and Medical Lab Technology Phlebotomists are professionals trained to draw blood (phleb- = “a blood vessel”; -tomy = “to cut”). When more than a few drops of blood are required, phlebotomists perform a venipuncture, typically of a surface vein in the arm. They perform a capillary stick on a finger, an earlobe, or the heel of an infant when only a small quantity of blood is required. An arterial stick is collected from an artery and used to analyze blood gases. After collection, the blood may be analyzed by medical laboratories or perhaps used for transfusions, donations, or research. While many allied health professionals practice phlebotomy, the American Society of Phlebotomy Technicians issues certificates to individuals passing a national examination, and some large labs and hospitals hire individuals expressly for their skill in phlebotomy. Medical or clinical laboratories employ a variety of individuals in technical positions: e Medical technologists (MT), also known as clinical laboratory technologists (CLT), typically hold a bachelor’s degree and certification from an accredited training program. They perform a wide variety of tests on various body fluids, including blood. The information they provide is essential to the primary care providers in determining a diagnosis and in monitoring the course of a disease and response to treatment. ¢ Medical laboratory technicians (MLT) typically have an associate’s degree but may perform duties similar to those of an MT. e Medical laboratory assistants (MLA) spend the majority of their time processing samples and carrying out routine assignments within the lab. Clinical training is required, but a degree may not be essential to obtaining a position. Chapter Review Blood is a fluid connective tissue critical to the transportation of nutrients, gases, and wastes throughout the body; to defend the body against infection and other threats; and to the homeostatic regulation of pH, temperature, and other internal conditions. Blood is composed of formed elements— erythrocytes, leukocytes, and cell fragments called platelets—and a fluid extracellular matrix called plasma. More than 90 percent of plasma is water. The remainder is mostly plasma proteins—mainly albumin, globulins, and fibrinogen—and other dissolved solutes such as glucose, lipids, electrolytes, and dissolved gases. Because of the formed elements and the plasma proteins and other solutes, blood is sticky and more viscous than water. It is also slightly alkaline, and its temperature is slightly higher than normal body temperature. Interactive Link Questions Exercise: Problem: Visit this site for a list of normal levels established for many of the substances found in a sample of blood. Serum, one of the specimen types included, refers to a sample of plasma after clotting factors have been removed. What types of measurements are given for levels of glucose in the blood? Solution: There are values given for percent saturation, tension, and blood gas, and there are listings for different types of hemoglobin. Review Questions Exercise: Problem: Which of the following statements about blood is true? a. Blood is about 92 percent water. b. Blood is slightly more acidic than water. c. Blood is slightly more viscous than water. d. Blood is slightly more salty than seawater. Solution: C Exercise: Problem: Which of the following statements about albumin is true? a. It draws water out of the blood vessels and into the body’s tissues. b. It is the most abundant plasma protein. c. It is produced by specialized leukocytes called plasma cells. d. All of the above are true. Solution: B Exercise: Problem: Which of the following plasma proteins is not produced by the liver? a. fibrinogen b. alpha globulin c. beta globulin d. immunoglobulin Solution: D Critical Thinking Questions Exercise: Problem: A patient’s hematocrit is 42 percent. Approximately what percentage of the patient’s blood is plasma? Solution: The patient’s blood is approximately 58 percent plasma (since the buffy coat is less than 1 percent). Exercise: Problem: Why would it be incorrect to refer to the formed elements as cells? Solution: The formed elements include erythrocytes and leukocytes, which are cells (although mature erythrocytes do not have a nucleus); however, the formed elements also include platelets, which are not true cells but cell fragments. Exercise: Problem: True or false: The buffy coat is the portion of a blood sample that is made up of its proteins. Solution: False. The buffy coat is the portion of blood that is made up of its leukocytes and platelets. Glossary albumin most abundant plasma protein, accounting for most of the osmotic pressure of plasma antibodies (also, immunoglobulins or gamma globulins) antigen-specific proteins produced by specialized B lymphocytes that protect the body by binding to foreign objects such as bacteria and viruses blood liquid connective tissue composed of formed elements—erythrocytes, leukocytes, and platelets—and a fluid extracellular matrix called plasma; component of the cardiovascular system buffy coat thin, pale layer of leukocytes and platelets that separates the erythrocytes from the plasma in a sample of centrifuged blood fibrinogen plasma protein produced in the liver and involved in blood clotting formed elements cellular components of blood; that is, erythrocytes, leukocytes, and platelets globulins heterogeneous group of plasma proteins that includes transport proteins, clotting factors, immune proteins, and others hematocrit (also, packed cell volume) volume percentage of erythrocytes in a sample of centrifuged blood immunoglobulins (also, antibodies or gamma globulins) antigen-specific proteins produced by specialized B lymphocytes that protect the body by binding to foreign objects such as bacteria and viruses packed cell volume (PCV) (also, hematocrit) volume percentage of erythrocytes present in a sample of centrifuged blood plasma in blood, the liquid extracellular matrix composed mostly of water that circulates the formed elements and dissolved materials throughout the cardiovascular system platelets (also, thrombocytes) one of the formed elements of blood that consists of cell fragments broken off from megakaryocytes red blood cells (RBCs) (also, erythrocytes) one of the formed elements of blood that transports oxygen white blood cells (WBCs) (also, leukocytes) one of the formed elements of blood that provides defense against disease agents and foreign materials Production of the Formed Elements By the end of this section, you will be able to: e Trace the generation of the formed elements of blood from bone marrow stem cells e Discuss the role of hemopoietic growth factors in promoting the production of the formed elements The lifespan of the formed elements is very brief. Although one type of leukocyte called memory cells can survive for years, most erythrocytes, leukocytes, and platelets normally live only a few hours to a few weeks. Thus, the body must form new blood cells and platelets quickly and continuously. When you donate a unit of blood during a blood drive (approximately 475 mL, or about 1 pint), your body typically replaces the donated plasma within 24 hours, but it takes about 4 to 6 weeks to replace the blood cells. This restricts the frequency with which donors can contribute their blood. The process by which this replacement occurs is called hemopoiesis, or hematopoiesis (from the Greek root haima- = “blood”; -poiesis = “production”). Sites of Hemopoiesis Prior to birth, hemopoiesis occurs in a number of tissues, beginning with the yolk sac of the developing embryo, and continuing in the fetal liver, spleen, lymphatic tissue, and eventually the red bone marrow. Following birth, most hemopoiesis occurs in the red marrow, a connective tissue within the spaces of spongy (cancellous) bone tissue. In children, hemopoiesis can occur in the medullary cavity of long bones; in adults, the process is largely restricted to the cranial and pelvic bones, the vertebrae, the sternum, and the proximal epiphyses of the femur and humerus. Throughout adulthood, the liver and spleen maintain their ability to generate the formed elements. This process is referred to as extramedullary hemopoiesis (meaning hemopoiesis outside the medullary cavity of adult bones). When a disease such as bone cancer destroys the bone marrow, causing hemopoiesis to fail, extramedullary hemopoiesis may be initiated. Differentiation of Formed Elements from Stem Cells All formed elements arise from stem cells of the red bone marrow. Recall that stem cells undergo mitosis plus cytokinesis (cellular division) to give rise to new daughter cells: One of these remains a stem cell and the other differentiates into one of any number of diverse cell types. Stem cells may be viewed as occupying a hierarchal system, with some loss of the ability to diversify at each step. The totipotent stem cell is the zygote, or fertilized egg. The totipotent (toti- = “all”) stem cell gives rise to all cells of the human body. The next level is the pluripotent stem cell, which gives rise to multiple types of cells of the body and some of the supporting fetal membranes. Beneath this level, the mesenchymal cell is a stem cell that develops only into types of connective tissue, including fibrous connective tissue, bone, cartilage, and blood, but not epithelium, muscle, and nervous tissue. One step lower on the hierarchy of stem cells is the hemopoietic stem cell, or hemocytoblast. All of the formed elements of blood originate from this specific type of cell. Hemopoiesis begins when the hemopoietic stem cell is exposed to appropriate chemical stimuli collectively called hemopoietic growth factors, which prompt it to divide and differentiate. One daughter cell remains a hemopoietic stem cell, allowing hemopoiesis to continue. The other daughter cell becomes either of two types of more specialized stem cells ([link]): ¢ Lymphoid stem cells give rise to a class of leukocytes known as lymphocytes, which include the various T cells, B cells, and natural killer (NK) cells, all of which function in immunity. However, hemopoiesis of lymphocytes progresses somewhat differently from the process for the other formed elements. In brief, lymphoid stem cells quickly migrate from the bone marrow to lymphatic tissues, including the lymph nodes, spleen, and thymus, where their production and differentiation continues. B cells are so named since they mature in the bone marrow, while T cells mature in the thymus. ¢ Myeloid stem cells give rise to all the other formed elements, including the erythrocytes; megakaryocytes that produce platelets; and a myeloblast lineage that gives rise to monocytes and three forms of granular leukocytes: neutrophils, eosinophils, and basophils. Hematopoietic System of Bone Marrow @ Multipotent hematopoietic stem cell (hemocytoblast) After division, some cells remain stem cells. @ i Ng The remaining cell goes down one of two paths depending on the chemical signals received. OF Myeloid stem cell Lymphoid stem cell rs rs o ~) @ @ ‘ isn a a a a Megakaryocyte Erythrocyte Basophil Neutrophil Eosinophil Monocyte Q. T lymphocyte —_B lymphocyte Megakaryoblast Proerythroblast Myeloblast Monoblast Lymphoblast Reticulocyte = ak rad | & i_- Natural killer cell Small lymphocyte ~ (large granular xy’ Sy wp FN Platelets Hemopoiesis is the proliferation and differentiation of the formed elements of blood. Lymphoid and myeloid stem cells do not immediately divide and differentiate into mature formed elements. As you can see in [link], there are several intermediate stages of precursor cells (literally, forerunner cells), many of which can be recognized by their names, which have the suffix - blast. For instance, megakaryoblasts are the precursors of megakaryocytes, and proerythroblasts become reticulocytes, which eject their nucleus and most other organelles before maturing into erythrocytes. Hemopoietic Growth Factors Development from stem cells to precursor cells to mature cells is again initiated by hemopoietic growth factors. These include the following: e Erythropoietin (EPO) is a glycoprotein hormone secreted by the interstitial fibroblast cells of the kidneys in response to low oxygen levels. It prompts the production of erythrocytes. Some athletes use synthetic EPO as a performance-enhancing drug (called blood doping) to increase RBC counts and subsequently increase oxygen delivery to tissues throughout the body. EPO is a banned substance in most organized sports, but it is also used medically in the treatment of certain anemia, specifically those triggered by certain types of cancer, and other disorders in which increased erythrocyte counts and oxygen levels are desirable. e¢ Thrombopoietin, another glycoprotein hormone, is produced by the liver and kidneys. It triggers the development of megakaryocytes into platelets. ¢ Cytokines are glycoproteins secreted by a wide variety of cells, including red bone marrow, leukocytes, macrophages, fibroblasts, and endothelial cells. They act locally as autocrine or paracrine factors, stimulating the proliferation of progenitor cells and helping to stimulate both nonspecific and specific resistance to disease. There are two major subtypes of cytokines known as colony-stimulating factors and interleukins. o Colony-stimulating factors (CSFs) are glycoproteins that act locally, as autocrine or paracrine factors. Some trigger the differentiation of myeloblasts into granular leukocytes, namely, neutrophils, eosinophils, and basophils. These are referred to as granulocyte CSFs. A different CSF induces the production of monocytes, called monocyte CSFs. Both granulocytes and monocytes are stimulated by GM-CSF; granulocytes, monocytes, platelets, and erythrocytes are stimulated by multi-CSF. Synthetic forms of these hormones are often administered to patients with various forms of cancer who are receiving chemotherapy to revive their WBC counts. o Interleukins are another class of cytokine signaling molecules important in hemopoiesis. They were initially thought to be secreted uniquely by leukocytes and to communicate only with other leukocytes, and were named accordingly, but are now known to be produced by a variety of cells including bone marrow and endothelium. Researchers now suspect that interleukins may play other roles in body functioning, including differentiation and maturation of cells, producing immunity and inflammation. To date, more than a dozen interleukins have been identified, with others likely to follow. They are generally numbered IL-1, IL-2, IL-3, etc. Note: Everyday Connection Blood Doping In its original intent, the term blood doping was used to describe the practice of injecting by transfusion supplemental RBCs into an individual, typically to enhance performance in a sport. Additional RBCs would deliver more oxygen to the tissues, providing extra aerobic capacity, clinically referred to as VO» max. The source of the cells was either from the recipient (autologous) or from a donor with compatible blood (homologous). This practice was aided by the well-developed techniques of harvesting, concentrating, and freezing of the RBCs that could be later thawed and injected, yet still retain their functionality. These practices are considered illegal in virtually all sports and run the risk of infection, significantly increasing the viscosity of the blood and the potential for transmission of blood-bome pathogens if the blood was collected from another individual. With the development of synthetic EPO in the 1980s, it became possible to provide additional RBCs by artificially stimulating RBC production in the bone marrow. Originally developed to treat patients suffering from anemia, renal failure, or cancer treatment, large quantities of EPO can be generated by recombinant DNA technology. Synthetic EPO is injected under the skin and can increase hematocrit for many weeks. It may also induce polycythemia and raise hematocrit to 70 or greater. This increased viscosity raises the resistance of the blood and forces the heart to pump more powerfully; in extreme cases, it has resulted in death. Other drugs such as cobalt II chloride have been shown to increase natural EPO gene expression. Blood doping has become problematic in many sports, especially cycling. Lance Armstrong, winner of seven Tour de France and many other cycling titles, was stripped of his victories and admitted to blood doping in 2013. Note: we — meee OPENStAX COLLEGE Watch this video to see doctors discuss the dangers of blood doping in sports. What are the some potential side effects of blood doping? Bone Marrow Sampling and Transplants Sometimes, a healthcare provider will order a bone marrow biopsy, a diagnostic test of a sample of red bone marrow, or a bone marrow transplant, a treatment in which a donor’s healthy bone marrow—and its stem cells—replaces the faulty bone marrow of a patient. These tests and procedures are often used to assist in the diagnosis and treatment of various severe forms of anemia, such as thalassemia major and sickle cell anemia, as well as some types of cancer, specifically leukemia. In the past, when a bone marrow sample or transplant was necessary, the procedure would have required inserting a large-bore needle into the region near the iliac crest of the pelvic bones (os coxae). This location was preferred, since its location close to the body surface makes it more accessible, and it is relatively isolated from most vital organs. Unfortunately, the procedure is quite painful. Now, direct sampling of bone marrow can often be avoided. In many cases, stem cells can be isolated in just a few hours from a sample of a patient’s blood. The isolated stem cells are then grown in culture using the appropriate hemopoietic growth factors, and analyzed or sometimes frozen for later use. For an individual requiring a transplant, a matching donor is essential to prevent the immune system from destroying the donor cells—a phenomenon known as tissue rejection. To treat patients with bone marrow transplants, it is first necessary to destroy the patient’s own diseased marrow through radiation and/or chemotherapy. Donor bone marrow stem cells are then intravenously infused. From the bloodstream, they establish themselves in the recipient’s bone marrow. Chapter Review Through the process of hemopoiesis, the formed elements of blood are continually produced, replacing the relatively short-lived erythrocytes, leukocytes, and platelets. Hemopoiesis begins in the red bone marrow, with hemopoietic stem cells that differentiate into myeloid and lymphoid lineages. Myeloid stem cells give rise to most of the formed elements. Lymphoid stem cells give rise only to the various lymphocytes designated as B and T cells, and NK cells. Hemopoietic growth factors, including erythropoietin, thrombopoietin, colony-stimulating factors, and interleukins, promote the proliferation and differentiation of formed elements. Interactive Link Questions Exercise: Problem: Watch this video to see doctors discuss the dangers of blood doping in sports. What are the some potential side effects of blood doping? Solution: Side effects can include heart disease, stroke, pulmonary embolism, and virus transmission. Review Questions Exercise: Problem: Which of the formed elements arise from myeloid stem cells? a. B cells b. natural killer cells c. platelets d. all of the above Solution: C Exercise: Problem: Which of the following statements about erythropoietin is true? a. It facilitates the proliferation and differentiation of the erythrocyte lineage. b. It is a hormone produced by the thyroid gland. c. It is a hemopoietic growth factor that prompts lymphoid stem cells to leave the bone marrow. d. Both a and b are true. Solution: A Exercise: Problem: Interleukins are associated primarily with which of the following? a. production of various lymphocytes b. immune responses c. inflammation d. all of the above Solution: D Critical Thinking Questions Exercise: Problem: Myelofibrosis is a disorder in which inflammation and scar tissue formation in the bone marrow impair hemopoiesis. One sign is an enlarged spleen. Why? Solution: When disease impairs the ability of the bone marrow to participate in hemopoiesis, extramedullary hemopoiesis begins in the patient’s liver and spleen. This causes the spleen to enlarge. Exercise: Problem: Would you expect a patient with a form of cancer called acute myelogenous leukemia to experience impaired production of erythrocytes, or impaired production of lymphocytes? Explain your choice. Solution: The adjective myelogenous suggests a condition originating from (generated by) myeloid cells. Acute myelogenous leukemia impairs the production of erythrocytes and other mature formed elements of the myeloid stem cell lineage. Lymphocytes arise from the lymphoid stem cell line. Glossary bone marrow biopsy diagnostic test of a sample of red bone marrow bone marrow transplant treatment in which a donor’s healthy bone marrow with its stem cells replaces diseased or damaged bone marrow of a patient colony-stimulating factors (CSFs) glycoproteins that trigger the proliferation and differentiation of myeloblasts into granular leukocytes (basophils, neutrophils, and eosinophils) cytokines class of proteins that act as autocrine or paracrine signaling molecules; in the cardiovascular system, they stimulate the proliferation of progenitor cells and help to stimulate both nonspecific and specific resistance to disease erythropoietin (EPO) glycoprotein that triggers the bone marrow to produce RBCs; secreted by the kidney in response to low oxygen levels hemocytoblast hemopoietic stem cell that gives rise to the formed elements of blood hemopoiesis production of the formed elements of blood hemopoietic growth factors chemical signals including erythropoietin, thrombopoietin, colony- stimulating factors, and interleukins that regulate the differentiation and proliferation of particular blood progenitor cells hemopoietic stem cell type of pluripotent stem cell that gives rise to the formed elements of blood (hemocytoblast) interleukins signaling molecules that may function in hemopoiesis, inflammation, and specific immune responses lymphoid stem cells type of hemopoietic stem cells that gives rise to lymphocytes, including various T cells, B cells, and NK cells, all of which function in immunity myeloid stem cells type of hemopoietic stem cell that gives rise to some formed elements, including erythrocytes, megakaryocytes that produce platelets, and a myeloblast lineage that gives rise to monocytes and three forms of granular leukocytes (neutrophils, eosinophils, and basophils) pluripotent stem cell stem cell that derives from totipotent stem cells and is capable of differentiating into many, but not all, cell types totipotent stem cell embryonic stem cell that is capable of differentiating into any and all cells of the body; enabling the full development of an organism thrombopoietin hormone secreted by the liver and kidneys that prompts the development of megakaryocytes into thrombocytes (platelets) Erythrocytes By the end of this section, you will be able to: e Describe the anatomy of erythrocytes e Discuss the various steps in the lifecycle of an erythrocyte e Explain the composition and function of hemoglobin The erythrocyte, commonly known as a red blood cell (or RBC), is by far the most common formed element: A single drop of blood contains millions of erythrocytes and just thousands of leukocytes. Specifically, males have about 5.4 million erythrocytes per microliter (uL) of blood, and females have approximately 4.8 million per pL. In fact, erythrocytes are estimated to make up about 25 percent of the total cells in the body. As you can imagine, they are quite small cells, with a mean diameter of only about 7-8 micrometers (um) ({link]). The primary functions of erythrocytes are to pick up inhaled oxygen from the lungs and transport it to the body’s tissues, and to pick up some (about 24 percent) carbon dioxide waste at the tissues and transport it to the lungs for exhalation. Erythrocytes remain within the vascular network. Although leukocytes typically leave the blood vessels to perform their defensive functions, movement of erythrocytes from the blood vessels is abnormal. Summary of Formed Elements in Blood Formed element Numbers Comments present per microliter (uL) and mean (range) Major subtypes Appearance ina standard blood smear Summary of functions Erythrocytes (red blood cells) . Flattened biconcave disk; no nucleus; pale red color 5.2 million (4.4-6.0 million) Transport oxygen and some carbon dioxide between tissues and lungs Leukocytes (white blood cells) 7000 (5000—10,000) Obvious dark-staining nucleus All function in body defenses Exit capillaries and move into tissues; lifespan of usually a few hours or days 4360 (1800-9950) Granulocytes including neutrophils, eosinophils, and basophils Abundant granules in cytoplasm; nucleus normally lobed Nonspecific (innate) resistance to disease Classified according to membrane-bound granules in cytoplasm Neutrophils 4150 (1800-7300) Nuclear lobes increase with age; pale lilac granules Phagocytic; particularly effective against bacteria. Release cytotoxic chemicals from Most common leukocyte; lifespan of minutes to days granules Eosinophils 165 Nucleus generally Phagocytic cells; Lifespan of (0-700) two-lobed; bright particularly effective minutes to days La red-orange granules with antigen- antibody 2640 (1700-4950) 2185 (1500-4000) 455 (200-950) Nucleus generally two-lobed but difficult to see due to presence of heavy, dense, dark purple granules Lack abundant granules in cytoplasm; have a simple- shaped nucleus that may be indented Spherical cells with a single often large nucleus occupying much of the cell’s volume; stains purple; seen in large (natural killer cells) and small (B and T cells) variants Largest leukocyte with an indented or horseshoe-shaped nucleus complexes. Release antihistamines. Increase in allergies and parasitic infections Promotes inflammation Body defenses Primarily specific (adaptive) immunity: T cells directly attack other cells (cellular immunity); B cells release antibodies (humoral immunity); natural killer cells are similar to T cells but nonspecific Very effective phagocytic cells engulfing pathogens or worn out cells; also Least common leukocyte; lifespan unknown Group consists of two major cell types from different lineages Initial cells originate in bone marrow, but secondary production occurs in lymphatic tissue; several distinct subtypes; memory cells form after exposure to a pathogen and rapidly increase responses to subsequent exposure; lifespan of many years Produced in red bone marrow; referred to as macrophages after leaving circulation serve as antigen- presenting cells (APCs) for other components of the immune system 350,000 Cellular fragments (150,000—500,000) | surrounded by a plasma membrane and containing granules; purple stain Hemostasis plus Formed from release growth factors }|megakaryocytes for repair and healing |that remain in the red of tissue bone marrow and shed platelets into circulation Platelets Shape and Structure of Erythrocytes As an erythrocyte matures in the red bone marrow, it extrudes its nucleus and most of its other organelles. During the first day or two that it is in the circulation, an immature erythrocyte, known as a reticulocyte, will still typically contain remnants of organelles. Reticulocytes should comprise approximately 1—2 percent of the erythrocyte count and provide a rough estimate of the rate of RBC production, with abnormally low or high rates indicating deviations in the production of these cells. These remnants, primarily of networks (reticulum) of ribosomes, are quickly shed, however, and mature, circulating erythrocytes have few internal cellular structural components. Lacking mitochondria, for example, they rely on anaerobic respiration. This means that they do not utilize any of the oxygen they are transporting, so they can deliver it all to the tissues. They also lack endoplasmic reticula and do not synthesize proteins. Erythrocytes do, however, contain some structural proteins that help the blood cells maintain their unique structure and enable them to change their shape to squeeze through capillaries. This includes the protein spectrin, a cytoskeletal protein element. Erythrocytes are biconcave disks; that is, they are plump at their periphery and very thin in the center ({link]). Since they lack most organelles, there is more interior space for the presence of the hemoglobin molecules that, as you will see shortly, transport gases. The biconcave shape also provides a greater surface area across which gas exchange can occur, relative to its volume; a sphere of a similar diameter would have a lower surface area-to- volume ratio. In the capillaries, the oxygen carried by the erythrocytes can diffuse into the plasma and then through the capillary walls to reach the cells, whereas some of the carbon dioxide produced by the cells as a waste product diffuses into the capillaries to be picked up by the erythrocytes. Capillary beds are extremely narrow, slowing the passage of the erythrocytes and providing an extended opportunity for gas exchange to occur. However, the space within capillaries can be so minute that, despite their own small size, erythrocytes may have to fold in on themselves if they are to make their way through. Fortunately, their structural proteins like spectrin are flexible, allowing them to bend over themselves to a surprising degree, then spring back again when they enter a wider vessel. In wider vessels, erythrocytes may stack up much like a roll of coins, forming a rouleaux, from the French word for “roll.” Shape of Red Blood Cells Erythrocytes are biconcave discs with very shallow centers. This shape optimizes the ratio of surface area to volume, facilitating gas exchange. It also enables them to fold up as they move through narrow blood vessels. Hemoglobin Hemoglobin is a large molecule made up of proteins and iron. It consists of four folded chains of a protein called globin, designated alpha 1 and 2, and beta 1 and 2 ({link]a). Each of these globin molecules is bound to a red pigment molecule called heme, which contains an ion of iron (Fe**) ([link]b). Hemoglobin B chain 1 B chain 2 a chain 1 (a) A molecule of hemoglobin contains four globin proteins, each of which is bound to one molecule of the iron-containing pigment heme. (b) A single erythrocyte can contain 300 million hemoglobin molecules, and thus more than 1 billion oxygen molecules. Each iron ion in the heme can bind to one oxygen molecule; therefore, each hemoglobin molecule can transport four oxygen molecules. An individual erythrocyte may contain about 300 million hemoglobin molecules, and therefore can bind to and transport up to 1.2 billion oxygen molecules (see [link]b). In the lungs, hemoglobin picks up oxygen, which binds to the iron ions, forming oxyhemoglobin. The bright red, oxygenated hemoglobin travels to the body tissues, where it releases some of the oxygen molecules, becoming darker red deoxyhemoglobin, sometimes referred to as reduced hemoglobin. Oxygen release depends on the need for oxygen in the surrounding tissues, so hemoglobin rarely if ever leaves all of its oxygen behind. In the capillaries, carbon dioxide enters the bloodstream. About 76 percent dissolves in the plasma, some of it remaining as dissolved CO>, and the remainder forming bicarbonate ion. About 23—24 percent of it binds to the amino acids in hemoglobin, forming a molecule known as carbaminohemoglobin. From the capillaries, the hemoglobin carries carbon dioxide back to the lungs, where it releases it for exchange of oxygen. Changes in the levels of RBCs can have significant effects on the body’s ability to effectively deliver oxygen to the tissues. Ineffective hematopoiesis results in insufficient numbers of RBCs and results in one of several forms of anemia. An overproduction of RBCs produces a condition called polycythemia. The primary drawback with polycythemia is not a failure to directly deliver enough oxygen to the tissues, but rather the increased viscosity of the blood, which makes it more difficult for the heart to circulate the blood. In patients with insufficient hemoglobin, the tissues may not receive sufficient oxygen, resulting in another form of anemia. In determining oxygenation of tissues, the value of greatest interest in healthcare is the percent saturation; that is, the percentage of hemoglobin sites occupied by oxygen in a patient’s blood. Clinically this value is commonly referred to simply as “percent sat.” Percent saturation is normally monitored using a device known as a pulse oximeter, which is applied to a thin part of the body, typically the tip of the patient’s finger. The device works by sending two different wavelengths of light (one red, the other infrared) through the finger and measuring the light with a photodetector as it exits. Hemoglobin absorbs light differentially depending upon its saturation with oxygen. The machine calibrates the amount of light received by the photodetector against the amount absorbed by the partially oxygenated hemoglobin and presents the data as percent saturation. Normal pulse oximeter readings range from 95-100 percent. Lower percentages reflect hypoxemia, or low blood oxygen. The term hypoxia is more generic and simply refers to low oxygen levels. Oxygen levels are also directly monitored from free oxygen in the plasma typically following an arterial stick. When this method is applied, the amount of oxygen present is expressed in terms of partial pressure of oxygen or simply pO, and is typically recorded in units of millimeters of mercury, mm Hg. The kidneys filter about 180 liters (~380 pints) of blood in an average adult each day, or about 20 percent of the total resting volume, and thus serve as ideal sites for receptors that determine oxygen saturation. In response to hypoxemia, less oxygen will exit the vessels supplying the kidney, resulting in hypoxia (low oxygen concentration) in the tissue fluid of the kidney where oxygen concentration is actually monitored. Interstitial fibroblasts within the kidney secrete EPO, thereby increasing erythrocyte production and restoring oxygen levels. In a classic negative-feedback loop, as oxygen saturation rises, EPO secretion falls, and vice versa, thereby maintaining homeostasis. Populations dwelling at high elevations, with inherently lower levels of oxygen in the atmosphere, naturally maintain a hematocrit higher than people living at sea level. Consequently, people traveling to high elevations may experience symptoms of hypoxemia, such as fatigue, headache, and shortness of breath, for a few days after their arrival. In response to the hypoxemia, the kidneys secrete EPO to step up the production of erythrocytes until homeostasis is achieved once again. To avoid the symptoms of hypoxemia, or altitude sickness, mountain climbers typically rest for several days to a week or more at a series of camps situated at increasing elevations to allow EPO levels and, consequently, erythrocyte counts to rise. When climbing the tallest peaks, such as Mt. Everest and K2 in the Himalayas, many mountain climbers rely upon bottled oxygen as they near the summit. Lifecycle of Erythrocytes Production of erythrocytes in the marrow occurs at the staggering rate of more than 2 million cells per second. For this production to occur, a number of raw materials must be present in adequate amounts. These include the same nutrients that are essential to the production and maintenance of any cell, such as glucose, lipids, and amino acids. However, erythrocyte production also requires several trace elements: e Iron. We have said that each heme group in a hemoglobin molecule contains an ion of the trace mineral iron. On average, less than 20 percent of the iron we consume is absorbed. Heme iron, from animal foods such as meat, poultry, and fish, is absorbed more efficiently than non-heme iron from plant foods. Upon absorption, iron becomes part of the body’s total iron pool. The bone marrow, liver, and spleen can store iron in the protein compounds ferritin and hemosiderin. Ferroportin transports the iron across the intestinal cell plasma membranes and from its storage sites into tissue fluid where it enters the blood. When EPO stimulates the production of erythrocytes, iron is released from storage, bound to transferrin, and carried to the red marrow where it attaches to erythrocyte precursors. e Copper. A trace mineral, copper is a component of two plasma proteins, hephaestin and ceruloplasmin. Without these, hemoglobin could not be adequately produced. Located in intestinal villi, hephaestin enables iron to be absorbed by intestinal cells. Ceruloplasmin transports copper. Both enable the oxidation of iron from Fe** to Fe**, a form in which it can be bound to its transport protein, transferrin, for transport to body cells. In a state of copper deficiency, the transport of iron for heme synthesis decreases, and iron can accumulate in tissues, where it can eventually lead to organ damage. e Zinc. The trace mineral zinc functions as a co-enzyme that facilitates the synthesis of the heme portion of hemoglobin. e B vitamins. The B vitamins folate and vitamin B,5 function as co- enzymes that facilitate DNA synthesis. Thus, both are critical for the synthesis of new cells, including erythrocytes. Erythrocytes live up to 120 days in the circulation, after which the worn-out cells are removed by a type of myeloid phagocytic cell called a macrophage, located primarily within the bone marrow, liver, and spleen. The components of the degraded erythrocytes’ hemoglobin are further processed as follows: ¢ Globin, the protein portion of hemoglobin, is broken down into amino acids, which can be sent back to the bone marrow to be used in the production of new erythrocytes. Hemoglobin that is not phagocytized is broken down in the circulation, releasing alpha and beta chains that are removed from circulation by the kidneys. e The iron contained in the heme portion of hemoglobin may be stored in the liver or spleen, primarily in the form of ferritin or hemosiderin, or carried through the bloodstream by transferrin to the red bone marrow for recycling into new erythrocytes. e The non-iron portion of heme is degraded into the waste product biliverdin, a green pigment, and then into another waste product, bilirubin, a yellow pigment. Bilirubin binds to albumin and travels in the blood to the liver, which uses it in the manufacture of bile, a compound released into the intestines to help emulsify dietary fats. In the large intestine, bacteria breaks the bilirubin apart from the bile and converts it to urobilinogen and then into stercobilin. It is then eliminated from the body in the feces. Broad-spectrum antibiotics typically eliminate these bacteria as well and may alter the color of feces. The kidneys also remove any circulating bilirubin and other related metabolic byproducts such as urobilins and secrete them into the urine. The breakdown pigments formed from the destruction of hemoglobin can be seen in a variety of situations. At the site of an injury, biliverdin from damaged RBCs produces some of the dramatic colors associated with bruising. With a failing liver, bilirubin cannot be removed effectively from circulation and causes the body to assume a yellowish tinge associated with jaundice. Stercobilins within the feces produce the typical brown color associated with this waste. And the yellow of urine is associated with the urobilins. The erythrocyte lifecycle is summarized in [link]. Erythrocyte Lifecycle Hemopoiesis of erythrocytes begins ©) Unused heme groups can be recycled and used in in the hemopoietic bone marrow. hemopoiesis, or can be converted into bilirubin and used to make bile in the liver. Iron ions can also be transferred to the protein ferritin for storage in the liver. Locations of hemopoietic bone marrow @ Stem cell @ Erythroblast Bilirubin Ferritin Iron ions Biliverdin bound to transferrin ©) The heme portion is broken down into biliverdin for transport in the blood. The iron ions bind to the blood protein transferrin for transport. Globin Heme oR ihS amino acids groups and cell @) Reticulocytes are released into the bloodstream, where they mature into erythrocytes, which circulate for an average of 120 days. \/ components Hemoglobin protein structure @) Old and damaged erythrocytes are phagocytized by is broken macrophages in down into the bone marrow, amino acids liver, and spleen. Lysosome @ The globin (protein) portion of hemoglobin is metabolized into amino acids, which are reused for protein synthesis. Erythrocytes are produced in the bone marrow and sent into the circulation. At the end of their lifecycle, they are destroyed by macrophages, and their components are recycled. Disorders of Erythrocytes The size, shape, and number of erythrocytes, and the number of hemoglobin molecules can have a major impact on a person’s health. When the number of RBCs or hemoglobin is deficient, the general condition is called anemia. There are more than 400 types of anemia and more than 3.5 million Americans suffer from this condition. Anemia can be broken down into three major groups: those caused by blood loss, those caused by faulty or decreased RBC production, and those caused by excessive destruction of RBCs. Clinicians often use two groupings in diagnosis: The kinetic approach focuses on evaluating the production, destruction, and removal of RBCs, whereas the morphological approach examines the RBCs themselves, paying particular emphasis to their size. A common test is the mean corpuscle volume (MCV), which measures size. Normal-sized cells are referred to as normocytic, smaller-than-normal cells are referred to as microcytic, and larger-than-normal cells are referred to as macrocytic. Reticulocyte counts are also important and may reveal inadequate production of RBCs. The effects of the various anemias are widespread, because reduced numbers of RBCs or hemoglobin will result in lower levels of oxygen being delivered to body tissues. Since oxygen is required for tissue functioning, anemia produces fatigue, lethargy, and an increased risk for infection. An oxygen deficit in the brain impairs the ability to think clearly, and may prompt headaches and irritability. Lack of oxygen leaves the patient short of breath, even as the heart and lungs work harder in response to the deficit. Blood loss anemias are fairly straightforward. In addition to bleeding from wounds or other lesions, these forms of anemia may be due to ulcers, hemorrhoids, inflammation of the stomach (gastritis), and some cancers of the gastrointestinal tract. The excessive use of aspirin or other nonsteroidal anti-inflammatory drugs such as ibuprofen can trigger ulceration and gastritis. Excessive menstruation and loss of blood during childbirth are also potential causes. Anemias caused by faulty or decreased RBC production include sickle cell anemia, iron deficiency anemia, vitamin deficiency anemia, and diseases of the bone marrow and stem cells. e A characteristic change in the shape of erythrocytes is seen in sickle cell disease (also referred to as sickle cell anemia). A genetic disorder, it is caused by production of an abnormal type of hemoglobin, called hemoglobin S, which delivers less oxygen to tissues and causes erythrocytes to assume a sickle (or crescent) shape, especially at low oxygen concentrations ({link]). These abnormally shaped cells can then become lodged in narrow capillaries because they are unable to fold in on themselves to squeeze through, blocking blood flow to tissues and causing a variety of serious problems from painful joints to delayed growth and even blindness and cerebrovascular accidents (strokes). Sickle cell anemia is a genetic condition particularly found in individuals of African descent. Sickle Cells Sickle cell anemia is caused by a mutation in one of the hemoglobin genes. Erythrocytes produce an abnormal type of hemoglobin, which causes the cell to take on a sickle or crescent shape. (credit: Janice Haney Carr) e Iron deficiency anemia is the most common type and results when the amount of available iron is insufficient to allow production of sufficient heme. This condition can occur in individuals with a deficiency of iron in the diet and is especially common in teens and children as well as in vegans and vegetarians. Additionally, iron deficiency anemia may be caused by either an inability to absorb and transport iron or slow, chronic bleeding. ¢ Vitamin-deficient anemias generally involve insufficient vitamin B12 and folate. o Megaloblastic anemia involves a deficiency of vitamin B12 and/or folate, and often involves diets deficient in these essential nutrients. Lack of meat or a viable alternate source, and overcooking or eating insufficient amounts of vegetables may lead to a lack of folate. o Pernicious anemia is caused by poor absorption of vitamin B12 and is often seen in patients with Crohn’s disease (a severe intestinal disorder often treated by surgery), surgical removal of the intestines or stomach (common in some weight loss surgeries), intestinal parasites, and AIDS. o Pregnancies, some medications, excessive alcohol consumption, and some diseases such as celiac disease are also associated with vitamin deficiencies. It is essential to provide sufficient folic acid during the early stages of pregnancy to reduce the risk of neurological defects, including spina bifida, a failure of the neural tube to close. e Assorted disease processes can also interfere with the production and formation of RBCs and hemoglobin. If myeloid stem cells are defective or replaced by cancer cells, there will be insufficient quantities of RBCs produced. o Aplastic anemia is the condition in which there are deficient numbers of RBC stem cells. Aplastic anemia is often inherited, or it may be triggered by radiation, medication, chemotherapy, or infection. o Thalassemia is an inherited condition typically occurring in individuals from the Middle East, the Mediterranean, African, and Southeast Asia, in which maturation of the RBCs does not proceed normally. The most severe form is called Cooley’s anemia. o Lead exposure from industrial sources or even dust from paint chips of iron-containing paints or pottery that has not been properly glazed may also lead to destruction of the red marrow. e Various disease processes also can lead to anemias. These include chronic kidney diseases often associated with a decreased production of EPO, hypothyroidism, some forms of cancer, lupus, and rheumatoid arthritis. In contrast to anemia, an elevated RBC count is called polycythemia and is detected in a patient’s elevated hematocrit. It can occur transiently in a person who is dehydrated; when water intake is inadequate or water losses are excessive, the plasma volume falls. As a result, the hematocrit rises. For reasons mentioned earlier, a mild form of polycythemia is chronic but normal in people living at high altitudes. Some elite athletes train at high elevations specifically to induce this phenomenon. Finally, a type of bone marrow disease called polycythemia vera (from the Greek vera = “true”) causes an excessive production of immature erythrocytes. Polycythemia vera can dangerously elevate the viscosity of blood, raising blood pressure and making it more difficult for the heart to pump blood throughout the body. It is a relatively rare disease that occurs more often in men than women, and is more likely to be present in elderly patients those over 60 years of age. Chapter Review The most abundant formed elements in blood, erythrocytes are red, biconcave disks packed with an oxygen-carrying compound called hemoglobin. The hemoglobin molecule contains four globin proteins bound to a pigment molecule called heme, which contains an ion of iron. In the bloodstream, iron picks up oxygen in the lungs and drops it off in the tissues; the amino acids in hemoglobin then transport carbon dioxide from the tissues back to the lungs. Erythrocytes live only 120 days on average, and thus must be continually replaced. Worn-out erythrocytes are phagocytized by macrophages and their hemoglobin is broken down. The breakdown products are recycled or removed as wastes: Globin is broken down into amino acids for synthesis of new proteins; iron is stored in the liver or spleen or used by the bone marrow for production of new erythrocytes; and the remnants of heme are converted into bilirubin, or other waste products that are taken up by the liver and excreted in the bile or removed by the kidneys. Anemia is a deficiency of RBCs or hemoglobin, whereas polycythemia is an excess of RBCs. Review Questions Exercise: Problem: Which of the following statements about mature, circulating erythrocytes is true? a. They have no nucleus. b. They are packed with mitochondria. c. They survive for an average of 4 days. d. All of the above Solution: A Exercise: Problem:A molecule of hemoglobin a. is shaped like a biconcave disk packed almost entirely with iron b. contains four glycoprotein units studded with oxygen c. consists of four globin proteins, each bound to a molecule of heme d. can carry up to 120 molecules of oxygen Solution: C Exercise: Problem: The production of healthy erythrocytes depends upon the availability of a. Copper b. zinc c. vitamin By d. copper, zinc, and vitamin By Solution: D Exercise: Problem: Aging and damaged erythrocytes are removed from the circulation by a. myeoblasts b. monocytes c. macrophages d. mast cells Solution: C Exercise: Problem: A patient has been suffering for 2 months with a chronic, watery diarrhea. A blood test is likely to reveal a. a hematocrit below 30 percent b. hypoxemia c. anemia d. polycythemia Solution: D Critical Thinking Questions Exercise: Problem: A young woman has been experiencing unusually heavy menstrual bleeding for several years. She follows a strict vegan diet (no animal foods). She is at risk for what disorder, and why? Solution: She is at risk for anemia, because her unusually heavy menstrual bleeding results in excessive loss of erythrocytes each month. At the same time, her vegan diet means that she does not have dietary sources of heme iron. The non-heme iron she consumes in plant foods is not as well absorbed as heme iron. Exercise: Problem: A patient has thalassemia, a genetic disorder characterized by abnormal synthesis of globin proteins and excessive destruction of erythrocytes. This patient is jaundiced and is found to have an excessive level of bilirubin in his blood. Explain the connection. Solution: Bilirubin is a breakdown product of the non-iron component of heme, which is cleaved from globin when erythrocytes are degraded. Excessive erythrocyte destruction would deposit excessive bilirubin in the blood. Bilirubin is a yellowish pigment, and high blood levels can manifest as yellowed skin. Glossary anemia deficiency of red blood cells or hemoglobin bilirubin yellowish bile pigment produced when iron is removed from heme and is further broken down into waste products biliverdin green bile pigment produced when the non-iron portion of heme is degraded into a waste product; converted to bilirubin in the liver carbaminohemoglobin compound of carbon dioxide and hemoglobin, and one of the ways in which carbon dioxide is carried in the blood deoxyhemoglobin molecule of hemoglobin without an oxygen molecule bound to it erythrocyte (also, red blood cell) mature myeloid blood cell that is composed mostly of hemoglobin and functions primarily in the transportation of oxygen and carbon dioxide ferritin protein-containing storage form of iron found in the bone marrow, liver, and spleen globin heme-containing globular protein that is a constituent of hemoglobin heme red, iron-containing pigment to which oxygen binds in hemoglobin hemoglobin oxygen-carrying compound in erythrocytes hemosiderin protein-containing storage form of iron found in the bone marrow, liver, and spleen hypoxemia below-normal level of oxygen saturation of blood (typically <95 percent) macrophage phagocytic cell of the myeloid lineage; a matured monocyte oxyhemoglobin molecule of hemoglobin to which oxygen is bound polycythemia elevated level of hemoglobin, whether adaptive or pathological reticulocyte immature erythrocyte that may still contain fragments of organelles sickle cell disease (also, sickle cell anemia) inherited blood disorder in which hemoglobin molecules are malformed, leading to the breakdown of RBCs that take on a characteristic sickle shape thalassemia inherited blood disorder in which maturation of RBCs does not proceed normally, leading to abnormal formation of hemoglobin and the destruction of RBCs transferrin plasma protein that binds reversibly to iron and distributes it throughout the body Leukocytes and Platelets By the end of this section, you will be able to: e Describe the general characteristics of leukocytes ¢ Classify leukocytes according to their lineage, their main structural features, and their primary functions e Discuss the most common malignancies involving leukocytes e Identify the lineage, basic structure, and function of platelets The leukocyte, commonly known as a white blood cell (or WBC), is a major component of the body’s defenses against disease. Leukocytes protect the body against invading microorganisms and body cells with mutated DNA, and they clean up debris. Platelets are essential for the repair of blood vessels when damage to them has occurred; they also provide growth factors for healing and repair. See [link] for a summary of leukocytes and platelets. Characteristics of Leukocytes Although leukocytes and erythrocytes both originate from hematopoietic stem cells in the bone marrow, they are very different from each other in many significant ways. For instance, leukocytes are far less numerous than erythrocytes: Typically there are only 5000 to 10,000 per pL. They are also larger than erythrocytes and are the only formed elements that are complete cells, possessing a nucleus and organelles. And although there is just one type of erythrocyte, there are many types of leukocytes. Most of these types have a much shorter lifespan than that of erythrocytes, some as short as a few hours or even a few minutes in the case of acute infection. One of the most distinctive characteristics of leukocytes is their movement. Whereas erythrocytes spend their days circulating within the blood vessels, leukocytes routinely leave the bloodstream to perform their defensive functions in the body’s tissues. For leukocytes, the vascular network is simply a highway they travel and soon exit to reach their true destination. When they arrive, they are often given distinct names, such as macrophage or microglia, depending on their function. As shown in [link], they leave the capillaries—the smallest blood vessels—or other small vessels through a process known as emigration (from the Latin for “removal”) or diapedesis (dia- = “through”; -pedan = “to leap”) in which they squeeze through adjacent cells in a blood vessel wall. Once they have exited the capillaries, some leukocytes will take up fixed positions in lymphatic tissue, bone marrow, the spleen, the thymus, or other organs. Others will move about through the tissue spaces very much like amoebas, continuously extending their plasma membranes, sometimes wandering freely, and sometimes moving toward the direction in which they are drawn by chemical signals. This attracting of leukocytes occurs because of positive chemotaxis (literally “movement in response to chemicals”), a phenomenon in which injured or infected cells and nearby leukocytes emit the equivalent of a chemical “911” call, attracting more leukocytes to the site. In clinical medicine, the differential counts of the types and percentages of leukocytes present are often key indicators in making a diagnosis and selecting a treatment. Emigration GQ) Leukocytes in the blood respond to chemical Eosinophil attractants released by pathogens and Injured/infected cells secrete chemical signals chemical signals from into the blood. nearby injured cells. Monocyte Neutrophil : Pathogens Leukocytes emigrate Q@) The leukocytes squeeze to site of injury and between the cells of infection. the capillary wall as they follow the chemical signals to where they are most concentrated (positive chemotaxis). Eosinophil releases cytotoxic chemicals from granules into tissue. @) Within the damaged tissue, monocytes differentiate into macrophages that phagocytize the pathogens. The eosinophils and neutrophils release chemicals that break apart pathogens. They are also capable of phagocytosis. Macrophage engulfs pathogen. Leukocytes exit the blood vessel and then move through the connective tissue of the dermis toward the site of a wound. Some leukocytes, such as the eosinophil and neutrophil, are characterized as granular leukocytes. They release chemicals from their granules that destroy pathogens; they are also capable of phagocytosis. The monocyte, an agranular leukocyte, differentiates into a macrophage that then phagocytizes the pathogens. Classification of Leukocytes When scientists first began to observe stained blood slides, it quickly became evident that leukocytes could be divided into two groups, according to whether their cytoplasm contained highly visible granules: ¢ Granular leukocytes contain abundant granules within the cytoplasm. They include neutrophils, eosinophils, and basophils (you can view their lineage from myeloid stem cells in [link]). e While granules are not totally lacking in agranular leukocytes, they are far fewer and less obvious. Agranular leukocytes include monocytes, which mature into macrophages that are phagocytic, and lymphocytes, which arise from the lymphoid stem cell line. Granular Leukocytes We will consider the granular leukocytes in order from most common to least common. All of these are produced in the red bone marrow and have a short lifespan of hours to days. They typically have a lobed nucleus and are classified according to which type of stain best highlights their granules ({link]). Granular Leukocytes és Neutrophil Eosinophil Basophil A neutrophil has small granules that stain light lilac and a nucleus with two to five lobes. An eosinophil’s granules are slightly larger and stain reddish-orange, and its nucleus has two to three lobes. A basophil has large granules that stain dark blue to purple and a two-lobed nucleus. The most common of all the leukocytes, neutrophils will normally comprise 50—70 percent of total leukocyte count. They are 10-12 pm in diameter, significantly larger than erythrocytes. They are called neutrophils because their granules show up most clearly with stains that are chemically neutral (neither acidic nor basic). The granules are numerous but quite fine and normally appear light lilac. The nucleus has a distinct lobed appearance and may have two to five lobes, the number increasing with the age of the cell. Older neutrophils have increasing numbers of lobes and are often referred to as polymorphonuclear (a nucleus with many forms), or simply “polys.” Younger and immature neutrophils begin to develop lobes and are known as “bands.” Neutrophils are rapid responders to the site of infection and are efficient phagocytes with a preference for bacteria. Their granules include lysozyme, an enzyme capable of lysing, or breaking down, bacterial cell walls; oxidants such as hydrogen peroxide; and defensins, proteins that bind to and puncture bacterial and fungal plasma membranes, so that the cell contents leak out. Abnormally high counts of neutrophils indicate infection and/or inflammation, particularly triggered by bacteria, but are also found in burn patients and others experiencing unusual stress. A burn injury increases the proliferation of neutrophils in order to fight off infection that can result from the destruction of the barrier of the skin. Low counts may be caused by drug toxicity and other disorders, and may increase an individual’s susceptibility to infection. Eosinophils typically represent 2—4 percent of total leukocyte count. They are also 10—12 pm in diameter. The granules of eosinophils stain best with an acidic stain known as eosin. The nucleus of the eosinophil will typically have two to three lobes and, if stained properly, the granules will have a distinct red to orange color. The granules of eosinophils include antihistamine molecules, which counteract the activities of histamines, inflammatory chemicals produced by basophils and mast cells. Some eosinophil granules contain molecules toxic to parasitic worms, which can enter the body through the integument, or when an individual consumes raw or undercooked fish or meat. Eosinophils are also capable of phagocytosis and are particularly effective when antibodies bind to the target and form an antigen-antibody complex. High counts of eosinophils are typical of patients experiencing allergies, parasitic wor! infestations, and some autoimmune diseases. Low counts may be due to drug toxicity and stress. Basophils are the least common leukocytes, typically comprising less than one percent of the total leukocyte count. They are slightly smaller than neutrophils and eosinophils at 8-10 yum in diameter. The granules of basophils stain best with basic (alkaline) stains. Basophils contain large granules that pick up a dark blue stain and are so common they may make it difficult to see the two-lobed nucleus. In general, basophils intensify the inflammatory response. They share this trait with mast cells. In the past, mast cells were considered to be basophils that left the circulation. However, this appears not to be the case, as the two cell types develop from different lineages. The granules of basophils release histamines, which contribute to inflammation, and heparin, which opposes blood clotting. High counts of basophils are associated with allergies, parasitic infections, and hypothyroidism. Low counts are associated with pregnancy, stress, and hyperthyroidism. Agranular Leukocytes Agranular leukocytes contain smaller, less-visible granules in their cytoplasm than do granular leukocytes. The nucleus is simple in shape, sometimes with an indentation but without distinct lobes. There are two major types of agranulocytes: lymphocytes and monocytes (see [link]). Lymphocytes are the only formed element of blood that arises from lymphoid stem cells. Although they form initially in the bone marrow, much of their subsequent development and reproduction occurs in the lymphatic tissues. Lymphocytes are the second most common type of leukocyte, accounting for about 20—30 percent of all leukocytes, and are essential for the immune response. The size range of lymphocytes is quite extensive, with some authorities recognizing two size classes and others three. Typically, the large cells are 10-14 ym and have a smaller nucleus-to- cytoplasm ratio and more granules. The smaller cells are typically 6-9 pm with a larger volume of nucleus to cytoplasm, creating a “halo” effect. A few cells may fall outside these ranges, at 14-17 pm. This finding has led to the three size range classification. The three major groups of lymphocytes include natural killer cells, B cells, and T cells. Natural killer (NK) cells are capable of recognizing cells that do not express “self” proteins on their plasma membrane or that contain foreign or abnormal markers. These “nonself” cells include cancer cells, cells infected with a virus, and other cells with atypical surface proteins. Thus, they provide generalized, nonspecific immunity. The larger lymphocytes are typically NK cells. B cells and T cells, also called B lymphocytes and T lymphocytes, play prominent roles in defending the body against specific pathogens (disease- causing microorganisms) and are involved in specific immunity. One form of B cells (plasma cells) produces the antibodies or immunoglobulins that bind to specific foreign or abnormal components of plasma membranes. This is also referred to as humoral (body fluid) immunity. T cells provide cellular-level immunity by physically attacking foreign or diseased cells. A memory cell is a variety of both B and T cells that forms after exposure to a pathogen and mounts rapid responses upon subsequent exposures. Unlike other leukocytes, memory cells live for many years. B cells undergo a maturation process in the bone marrow, whereas T cells undergo maturation in the thymus. This site of the maturation process gives rise to the name B and T cells. The functions of lymphocytes are complex and will be covered in detail in the chapter covering the lymphatic system and immunity. Smaller lymphocytes are either B or T cells, although they cannot be differentiated in a normal blood smear. Abnormally high lymphocyte counts are characteristic of viral infections as well as some types of cancer. Abnormally low lymphocyte counts are characteristic of prolonged (chronic) illness or immunosuppression, including that caused by HIV infection and drug therapies that often involve steroids. Monocytes originate from myeloid stem cells. They normally represent 2—8 percent of the total leukocyte count. They are typically easily recognized by their large size of 12—20 ym and indented or horseshoe-shaped nuclei. Macrophages are monocytes that have left the circulation and phagocytize debris, foreign pathogens, worn-out erythrocytes, and many other dead, worn out, or damaged cells. Macrophages also release antimicrobial defensins and chemotactic chemicals that attract other leukocytes to the site of an infection. Some macrophages occupy fixed locations, whereas others wander through the tissue fluid. Abnormally high counts of monocytes are associated with viral or fungal infections, tuberculosis, and some forms of leukemia and other chronic diseases. Abnormally low counts are typically caused by suppression of the bone marrow. Lifecycle of Leukocytes Most leukocytes have a relatively short lifespan, typically measured in hours or days. Production of all leukocytes begins in the bone marrow under the influence of CSFs and interleukins. Secondary production and maturation of lymphocytes occurs in specific regions of lymphatic tissue known as germinal centers. Lymphocytes are fully capable of mitosis and may produce clones of cells with identical properties. This capacity enables an individual to maintain immunity throughout life to many threats that have been encountered in the past. Disorders of Leukocytes Leukopenia is a condition in which too few leukocytes are produced. If this condition is pronounced, the individual may be unable to ward off disease. Excessive leukocyte proliferation is known as leukocytosis. Although leukocyte counts are high, the cells themselves are often nonfunctional, leaving the individual at increased risk for disease. Leukemia is a cancer involving an abundance of leukocytes. It may involve only one specific type of leukocyte from either the myeloid line (myelocytic leukemia) or the lymphoid line (lymphocytic leukemia). In chronic leukemia, mature leukocytes accumulate and fail to die. In acute leukemia, there is an overproduction of young, immature leukocytes. In both conditions the cells do not function properly. Lymphoma is a form of cancer in which masses of malignant T and/or B lymphocytes collect in lymph nodes, the spleen, the liver, and other tissues. As in leukemia, the malignant leukocytes do not function properly, and the patient is vulnerable to infection. Some forms of lymphoma tend to progress slowly and respond well to treatment. Others tend to progress quickly and require aggressive treatment, without which they are rapidly fatal. Platelets You may occasionally see platelets referred to as thrombocytes, but because this name suggests they are a type of cell, it is not accurate. A platelet is not a cell but rather a fragment of the cytoplasm of a cell called a megakaryocyte that is surrounded by a plasma membrane. Megakaryocytes are descended from myeloid stem cells (see [link]) and are large, typically 50-100 pm in diameter, and contain an enlarged, lobed nucleus. As noted earlier, thrombopoietin, a glycoprotein secreted by the kidneys and liver, stimulates the proliferation of megakaryoblasts, which mature into megakaryocytes. These remain within bone marrow tissue ([link]) and ultimately form platelet-precursor extensions that extend through the walls of bone matrow capillaries to release into the circulation thousands of cytoplasmic fragments, each enclosed by a bit of plasma membrane. These enclosed fragments are platelets. Each megakarocyte releases 2000—3000 platelets during its lifespan. Following platelet release, megakaryocyte remnants, which are little more than a cell nucleus, are consumed by macrophages. Platelets are relatively small, 2-4 ym in diameter, but numerous, with typically 150,000—160,000 per uL of blood. After entering the circulation, approximately one-third migrate to the spleen for storage for later release in response to any rupture in a blood vessel. They then become activated to perform their primary function, which is to limit blood loss. Platelets remain only about 10 days, then are phagocytized by macrophages. Platelets are critical to hemostasis, the stoppage of blood flow following damage to a vessel. They also secrete a variety of growth factors essential for growth and repair of tissue, particularly connective tissue. Infusions of concentrated platelets are now being used in some therapies to stimulate healing. Disorders of Platelets Thrombocytosis is a condition in which there are too many platelets. This may trigger formation of unwanted blood clots (thrombosis), a potentially fatal disorder. If there is an insufficient number of platelets, called thrombocytopenia, blood may not clot properly, and excessive bleeding may result. Platelets Y= Thrombopoietin oe From kidneys and liver Myeloid stem cell Megakaryoblast Megakaryocyte : 2 AS reid Platelet precursor extensions xy Lv) rn Platelets Platelets are derived from cells called megakaryocytes. Note: [=] — io rid 4 7 openstax COLLEGE Leukocytes fee ive Basophil Eosinophil Neutrophil Monocyte Lymphocyte (Micrographs provided by the Regents of University of Michigan Medical School © 2012) View University of Michigan Webscopes at http://virtualslides.med.umich.edu/Histology/Cardiovascular%20System/0 81-2 HISTO 40X.svs/view.apml? cwidth=860&cheight=733&chost=virtualslides.med.umich.edu&dlistview= 1&title=&csis=1 and explore the blood slides in greater detail. The Webscope feature allows you to move the slides as you would with a mechanical stage. You can increase and decrease the magnification. There is a chance to review each of the leukocytes individually after you have attempted to identify them from the first two blood smears. In addition, there are a few multiple choice questions. Are you able to recognize and identify the various formed elements? You will need to do this is a systematic manner, scanning along the image. The standard method is to use a grid, but this is not possible with this resource. Try constructing a simple table with each leukocyte type and then making a mark for each cell type you identify. Attempt to classify at least 50 and perhaps as many as 100 different cells. Based on the percentage of cells that you count, do the numbers represent a normal blood smear or does something appear to be abnormal? Chapter Review Leukocytes function in body defenses. They squeeze out of the walls of blood vessels through emigration or diapedesis, then may move through tissue fluid or become attached to various organs where they fight against pathogenic organisms, diseased cells, or other threats to health. Granular leukocytes, which include neutrophils, eosinophils, and basophils, originate with myeloid stem cells, as do the agranular monocytes. The other agranular leukocytes, NK cells, B cells, and T cells, arise from the lymphoid stem cell line. The most abundant leukocytes are the neutrophils, which are first responders to infections, especially with bacteria. About 20— 30 percent of all leukocytes are lymphocytes, which are critical to the body’s defense against specific threats. Leukemia and lymphoma are malignancies involving leukocytes. Platelets are fragments of cells known as megakaryocytes that dwell within the bone marrow. While many platelets are stored in the spleen, others enter the circulation and are essential for hemostasis; they also produce several growth factors important for repair and healing. Interactive Link Questions Exercise: Problem: [link] Are you able to recognize and identify the various formed elements? You will need to do this is a systematic manner, scanning along the image. The standard method is to use a grid, but this is not possible with this resource. Try constructing a simple table with each leukocyte type and then making a mark for each cell type you identify. Attempt to classify at least 50 and perhaps as many as 100 different cells. Based on the percentage of cells that you count, do the numbers represent a normal blood smear or does something appear to be abnormal? Solution: [link] This should appear to be a normal blood smear. Review Questions Exercise: Problem: The process by which leukocytes squeeze through adjacent cells in a blood vessel wall is called a. leukocytosis b. positive chemotaxis c. emigration d. cytoplasmic extending Solution: CG Exercise: Problem: Which of the following describes a neutrophil? a. abundant, agranular, especially effective against cancer cells b. abundant, granular, especially effective against bacteria c. rare, agranular, releases antimicrobial defensins d. rare, granular, contains multiple granules packed with histamine Solution: B Exercise: Problem:T and B lymphocytes a. are polymorphonuclear b. are involved with specific immune function c. proliferate excessively in leukopenia d. are most active against parasitic worms Solution: B Exercise: Problem: A patient has been experiencing severe, persistent allergy symptoms that are reduced when she takes an antihistamine. Before the treatment, this patient was likely to have had increased activity of which leukocyte? a. basophils b. neutrophils c. monocytes d. natural killer cells Solution: A Exercise: Problem:Thrombocytes are more accurately called a. clotting factors b. megakaryoblasts c. megakaryocytes d. platelets Solution: D Critical Thinking Questions Exercise: Problem: One of the more common adverse effects of cancer chemotherapy is the destruction of leukocytes. Before his next scheduled chemotherapy treatment, a patient undergoes a blood test called an absolute neutrophil count (ANC), which reveals that his neutrophil count is 1900 cells per microliter. Would his healthcare team be likely to proceed with his chemotherapy treatment? Why? Solution: A neutrophil count below 1800 cells per microliter is considered abnormal. Thus, this patient’s ANC is at the low end of the normal range and there would be no reason to delay chemotherapy. In clinical practice, most patients are given chemotherapy if their ANC is above 1000. Exercise: Problem: A patient was admitted to the burn unit the previous evening suffering from a severe burn involving his left upper extremity and shoulder. A blood test reveals that he is experiencing leukocytosis. Why is this an expected finding? Solution: Any severe stress can increase the leukocyte count, resulting in leukocytosis. A burn is especially likely to increase the proliferation of leukocytes in order to ward off infection, a significant risk when the barrier function of the skin is destroyed. Glossary agranular leukocytes leukocytes with few granules in their cytoplasm; specifically, monocytes, lymphocytes, and NK cells B lymphocytes (also, B cells) lymphocytes that defend the body against specific pathogens and thereby provide specific immunity basophils granulocytes that stain with a basic (alkaline) stain and store histamine and heparin defensins antimicrobial proteins released from neutrophils and macrophages that create openings in the plasma membranes to kill cells diapedesis (also, emigration) process by which leukocytes squeeze through adjacent cells in a blood vessel wall to enter tissues emigration (also, diapedesis) process by which leukocytes squeeze through adjacent cells in a blood vessel wall to enter tissues eosinophils granulocytes that stain with eosin; they release antihistamines and are especially active against parasitic worms granular leukocytes leukocytes with abundant granules in their cytoplasm; specifically, neutrophils, eosinophils, and basophils leukemia cancer involving leukocytes leukocyte (also, white blood cell) colorless, nucleated blood cell, the chief function of which is to protect the body from disease leukocytosis excessive leukocyte proliferation leukopenia below-normal production of leukocytes lymphocytes agranular leukocytes of the lymphoid stem cell line, many of which function in specific immunity lymphoma form of cancer in which masses of malignant T and/or B lymphocytes collect in lymph nodes, the spleen, the liver, and other tissues lysozyme digestive enzyme with bactericidal properties megakaryocyte bone marrow cell that produces platelets memory cell type of B or T lymphocyte that forms after exposure to a pathogen monocytes agranular leukocytes of the myeloid stem cell line that circulate in the bloodstream; tissue monocytes are macrophages natural killer (NK) cells cytotoxic lymphocytes capable of recognizing cells that do not express “self” proteins on their plasma membrane or that contain foreign or abnormal markers; provide generalized, nonspecific immunity neutrophils granulocytes that stain with a neutral dye and are the most numerous of the leukocytes; especially active against bacteria polymorphonuclear having a lobed nucleus, as seen in some leukocytes positive chemotaxis process in which a cell is attracted to move in the direction of chemical stimuli T lymphocytes (also, T cells) lymphocytes that provide cellular-level immunity by physically attacking foreign or diseased cells thrombocytes platelets, one of the formed elements of blood that consists of cell fragments broken off from megakaryocytes thrombocytopenia condition in which there are too few platelets, resulting in abnormal bleeding (hemophilia) thrombocytosis condition in which there are too many platelets, resulting in abnormal clotting (thrombosis) Basic Structure and Function of the Nervous System By the end of this section, you will be able to: e Identify the anatomical and functional divisions of the nervous system ¢ Relate the functional and structural differences between gray matter and white matter structures of the nervous system to the structure of neurons e List the basic functions of the nervous system The picture you have in your mind of the nervous system probably includes the brain, the nervous tissue contained within the cranium, and the spinal cord, the extension of nervous tissue within the vertebral column. That suggests it is made of two organs—and you may not even think of the spinal cord as an organ—but the nervous system is a very complex structure. Within the brain, many different and separate regions are responsible for many different and separate functions. It is as if the nervous system is composed of many organs that all look similar and can only be differentiated using tools such as the microscope or electrophysiology. In comparison, it is easy to see that the stomach is different than the esophagus or the liver, so you can imagine the digestive system as a collection of specific organs. The Central and Peripheral Nervous Systems The nervous system can be divided into two major regions: the central and peripheral nervous systems. The central nervous system (CNS) is the brain and spinal cord, and the peripheral nervous system (PNS) is everything else ([link]). The brain is contained within the cranial cavity of the skull, and the spinal cord is contained within the vertebral cavity of the vertebral column. It is a bit of an oversimplification to say that the CNS is what is inside these two cavities and the peripheral nervous system is outside of them, but that is one way to start to think about it. In actuality, there are some elements of the peripheral nervous system that are within the cranial or vertebral cavities. The peripheral nervous system is so named because it is on the periphery—meaning beyond the brain and spinal cord. Depending on different aspects of the nervous system, the dividing line between central and peripheral is not necessarily universal. Central and Peripheral Nervous System Central Nervous System Brain Spinal cord 4 vu ® =. a} > a g 2 2 ® 2 3 4 G ) < B. ea ® 3 Mest Ulf 4 fi A) Zz oO @ < oO = b ~ |Gau FP SSH d Pe Oo» s Myelin sheath External lamina Endonerium (collagen) (b) Myelinating glia wrap several layers of cell membrane around the cell membrane of an axon segment. A single Schwann cell insulates a segment of a peripheral nerve, whereas in the CNS, an oligodendrocyte may provide insulation for a few separate axon segments. EM x 1,460,000. (Micrograph provided by the Regents of University of Michigan Medical School © 2012) Note: Disorders of the... Nervous Tissue Several diseases can result from the demyelination of axons. The causes of these diseases are not the same; some have genetic causes, some are caused by pathogens, and others are the result of autoimmune disorders. Though the causes are varied, the results are largely similar. The myelin insulation of axons is compromised, making electrical signaling slower. Multiple sclerosis (MS) is one such disease. It is an example of an autoimmune disease. The antibodies produced by lymphocytes (a type of white blood cell) mark myelin as something that should not be in the body. This causes inflammation and the destruction of the myelin in the central nervous system. As the insulation around the axons is destroyed by the disease, scarring becomes obvious. This is where the name of the disease comes from; sclerosis means hardening of tissue, which is what a scar is. Multiple scars are found in the white matter of the brain and spinal cord. The symptoms of MS include both somatic and autonomic deficits. Control of the musculature is compromised, as is control of organs such as the bladder. Guillain-Barré (pronounced gee- YAN bah-RAY) syndrome is an example of a demyelinating disease of the peripheral nervous system. It is also the result of an autoimmune reaction, but the inflammation is in peripheral nerves. Sensory symptoms or motor deficits are common, and autonomic failures can lead to changes in the heart rhythm or a drop in blood pressure, especially when standing, which causes dizziness. Chapter Review Nervous tissue contains two major cell types, neurons and glial cells. Neurons are the cells responsible for communication through electrical signals. Glial cells are supporting cells, maintaining the environment around the neurons. Neurons are polarized cells, based on the flow of electrical signals along their membrane. Signals are received at the dendrites, are passed along the cell body, and propagate along the axon towards the target, which may be another neuron, muscle tissue, or a gland. Many axons are insulated by a lipid-rich substance called myelin. Specific types of glial cells provide this insulation. Several types of glial cells are found in the nervous system, and they can be categorized by the anatomical division in which they are found. In the CNS, astrocytes, oligodendrocytes, microglia, and ependymal cells are found. Astrocytes are important for maintaining the chemical environment around the neuron and are crucial for regulating the blood-brain barrier. Oligodendrocytes are the myelinating glia in the CNS. Microglia act as phagocytes and play a role in immune surveillance. Ependymal cells are responsible for filtering the blood to produce cerebrospinal fluid, which is a circulatory fluid that performs some of the functions of blood in the brain and spinal cord because of the BBB. In the PNS, satellite cells are supporting cells for the neurons, and Schwann cells insulate peripheral axons. Interactive Link Questions Exercise: Problem: Visit this site to learn about how nervous tissue is composed of neurons and glial cells. The neurons are dynamic cells with the ability to make a vast number of connections and to respond incredibly quickly to stimuli and to initiate movements based on those stimuli. They are the focus of intense research as failures in physiology can lead to devastating illnesses. Why are neurons only found in animals? Based on what this article says about neuron function, why wouldn’t they be helpful for plants or microorganisms? Solution: Neurons enable thought, perception, and movement. Plants do not move, so they do not need this type of tissue. Microorganisms are too small to have a nervous system. Many are single-celled, and therefore have organelles for perception and movement. Exercise: Problem: View the University of Michigan Webscope to see an electron micrograph of a cross-section of a myelinated nerve fiber. The axon contains microtubules and neurofilaments, bounded by a plasma membrane known as the axolemma. Outside the plasma membrane of the axon is the myelin sheath, which is composed of the tightly wrapped plasma membrane of a Schwann cell. What aspects of the cells in this image react with the stain that makes them the deep, dark, black color, such as the multiple layers that are the myelin sheath? Solution: Lipid membranes, such as the cell membrane and organelle membranes. Review Questions Exercise: Problem: What type of glial cell provides myelin for the axons in a tract? a. oligodendrocyte b. astrocyte c. Schwann cell d. satellite cell Solution: A Exercise: Problem: Which part of a neuron contains the nucleus? a. dendrite b. soma c. axon d. synaptic end bulb Solution: B Exercise: Problem: Which of the following substances is least able to cross the blood-brain barrier? a. water b. sodium ions c. glucose d. white blood cells Solution: D Exercise: Problem: What type of glial cell is the resident macrophage behind the blood- brain barrier? a. microglia b. astrocyte c. Schwann cell d. satellite cell Solution: A Exercise: Problem: What two types of macromolecules are the main components of myelin? a. carbohydrates and lipids b. proteins and nucleic acids c. lipids and proteins d. carbohydrates and nucleic acids Solution: C Critical Thinking Questions Exercise: Problem: Multiple sclerosis is a demyelinating disease affecting the central nervous system. What type of cell would be the most likely target of this disease? Why? Solution: The disease would target oligodendrocytes. In the CNS, oligodendrocytes provide the myelin for axons. Exercise: Problem: Which type of neuron, based on its shape, is best suited for relaying information directly from one neuron to another? Explain why. Solution: Bipolar cells, because they have one dendrite that receives input and one axon that provides output, would be a direct relay between two other cells. Glossary astrocyte glial cell type of the CNS that provides support for neurons and maintains the blood-brain barrier axon hillock tapering of the neuron cell body that gives rise to the axon axon segment single stretch of the axon insulated by myelin and bounded by nodes of Ranvier at either end (except for the first, which is after the initial segment, and the last, which is followed by the axon terminal) axon terminal end of the axon, where there are usually several branches extending toward the target cell axoplasm cytoplasm of an axon, which is different in composition than the cytoplasm of the neuronal cell body bipolar shape of a neuron with two processes extending from the neuron cell body—the axon and one dendrite blood-brain barrier (BBB) physiological barrier between the circulatory system and the central nervous system that establishes a privileged blood supply, restricting the flow of substances into the CNS cerebrospinal fluid (CSF) circulatory medium within the CNS that is produced by ependymal cells in the choroid plexus filtering the blood choroid plexus specialized structure containing ependymal cells that line blood capillaries and filter blood to produce CSF in the four ventricles of the brain ependymal cell glial cell type in the CNS responsible for producing cerebrospinal fluid initial segment first part of the axon as it emerges from the axon hillock, where the electrical signals known as action potentials are generated microglia glial cell type in the CNS that serves as the resident component of the immune system multipolar shape of a neuron that has multiple processes—the axon and two or more dendrites myelin sheath lipid-rich layer of insulation that surrounds an axon, formed by oligodendrocytes in the CNS and Schwann cells in the PNS; facilitates the transmission of electrical signals node of Ranvier gap between two myelinated regions of an axon, allowing for strengthening of the electrical signal as it propagates down the axon oligodendrocyte glial cell type in the CNS that provides the myelin insulation for axons in tracts satellite cell glial cell type in the PNS that provides support for neurons in the ganglia Schwann cell glial cell type in the PNS that provides the myelin insulation for axons in nerves synapse narrow junction across which a chemical signal passes from neuron to the next, initiating a new electrical signal in the target cell synaptic end bulb swelling at the end of an axon where neurotransmitter molecules are released onto a target cell across a synapse unipolar shape of a neuron which has only one process that includes both the axon and dendrite ventricle central cavity within the brain where CSF is produced and circulates Anatomy of the CNS By the end of this section, you will be able to: e Name the major regions of the adult brain e Describe the connections between the cerebrum and brain stem through the diencephalon, and from those regions into the spinal cord e Recognize the complex connections within the subcortical structures of the basal nuclei e Explain the arrangement of gray and white matter in the spinal cord The brain and the spinal cord are the central nervous system, and they represent the main organs of the nervous system. The spinal cord is a single structure, whereas the adult brain is described in terms of four major regions: the cerebrum, the diencephalon, the brain stem, and the cerebellum. A person’s conscious experiences are based on neural activity in the brain. The regulation of homeostasis is governed by a specialized region in the brain. The coordination of reflexes depends on the integration of sensory and motor pathways in the spinal cord. The Cerebrum The iconic gray mantle of the human brain, which appears to make up most of the mass of the brain, is the cerebrum ((link]). The wrinkled portion is the cerebral cortex, and the rest of the structure is beneath that outer covering. There is a large separation between the two sides of the cerebrum called the longitudinal fissure. It separates the cerebrum into two distinct halves, a right and left cerebral hemisphere. Deep within the cerebrum, the white matter of the corpus callosum provides the major pathway for communication between the two hemispheres of the cerebral cortex. The Cerebrum Cerebrum Corpus callosum Longitudinal fissure Right hemisphere Cerebral cortex hemisphere Lateral view Anterior view The cerebrum is a large component of the CNS in humans, and the most obvious aspect of it is the folded surface called the cerebral cortex. Many of the higher neurological functions, such as memory, emotion, and consciousness, are the result of cerebral function. The complexity of the cerebrum is different across vertebrate species. The cerebrum of the most primitive vertebrates is not much more than the connection for the sense of smell. In mammals, the cerebrum comprises the outer gray matter that is the cortex (from the Latin word meaning “bark of a tree”) and several deep nuclei that belong to three important functional groups. The basal nuclei are responsible for cognitive processing, the most important function being that associated with planning movements. The basal forebrain contains nuclei that are important in learning and memory. The limbic cortex is the region of the cerebral cortex that is part of the limbic system, a collection of structures involved in emotion, memory, and behavior. Cerebral Cortex The cerebrum is covered by a continuous layer of gray matter that wraps around either side of the forebrain—the cerebral cortex. This thin, extensive region of wrinkled gray matter is responsible for the higher functions of the nervous system. A gyrus (plural = gyri) is the ridge of one of those wrinkles, and a sulcus (plural = sulci) is the groove between two gyri. The pattern of these folds of tissue indicates specific regions of the cerebral cortex. The head is limited by the size of the birth canal, and the brain must fit inside the cranial cavity of the skull. Extensive folding in the cerebral cortex enables more gray matter to fit into this limited space. If the gray matter of the cortex were peeled off of the cerebrum and laid out flat, its surface area would be roughly equal to one square meter. The folding of the cortex maximizes the amount of gray matter in the cranial cavity. During embryonic development, as the telencephalon expands within the skull, the brain goes through a regular course of growth that results in everyone’s brain having a similar pattern of folds. The surface of the brain can be mapped on the basis of the locations of large gyri and sulci. Using these landmarks, the cortex can be separated into four major regions, or lobes ((link]). The lateral sulcus that separates the temporal lobe from the other regions is one such landmark. Superior to the lateral sulcus are the parietal lobe and frontal lobe, which are separated from each other by the central sulcus. The posterior region of the cortex is the occipital lobe, which has no obvious anatomical border between it and the parietal or temporal lobes on the lateral surface of the brain. From the medial surface, an obvious landmark separating the parietal and occipital lobes is called the parieto-occipital sulcus. The fact that there is no obvious anatomical border between these lobes is consistent with the functions of these regions being interrelated. Lobes of the Cerebral Cortex Central sulcus Precentral gyrus Postcentral gyrus Frontal lobe Parietal lobe Parieto-occipital sulcus Lateral sulcus Occipital lobe Temporal lobe The cerebral cortex is divided into four lobes. Extensive folding increases the surface area available for cerebral functions. Different regions of the cerebral cortex can be associated with particular functions, a concept known as localization of function. In the early 1900s, a German neuroscientist named Korbinian Brodmann performed an extensive study of the microscopic anatomy—the cytoarchitecture—of the cerebral cortex and divided the cortex into 52 separate regions on the basis of the histology of the cortex. His work resulted in a system of classification known as Brodmann’s areas, which is still used today to describe the anatomical distinctions within the cortex ({link]). The results from Brodmann’s work on the anatomy align very well with the functional differences within the cortex. Areas 17 and 18 in the occipital lobe are responsible for primary visual perception. That visual information is complex, so it is processed in the temporal and parietal lobes as well. The temporal lobe is associated with primary auditory sensation, known as Brodmann’s areas 41 and 42 in the superior temporal lobe. Because regions of the temporal lobe are part of the limbic system, memory is an important function associated with that lobe. Memory is essentially a sensory function; memories are recalled sensations such as the smell of Mom’s baking or the sound of a barking dog. Even memories of movement are really the memory of sensory feedback from those movements, such as stretching muscles or the movement of the skin around a joint. Structures in the temporal lobe are responsible for establishing long-term memory, but the ultimate location of those memories is usually in the region in which the sensory perception was processed. The main sensation associated with the parietal lobe is somatosensation, meaning the general sensations associated with the body. Posterior to the central sulcus is the postcentral gyrus, the primary somatosensory cortex, which is identified as Brodmann’s areas 1, 2, and 3. All of the tactile senses are processed in this area, including touch, pressure, tickle, pain, itch, and vibration, as well as more general senses of the body such as proprioception and kinesthesia, which are the senses of body position and movement, respectively. Anterior to the central sulcus is the frontal lobe, which is primarily associated with motor functions. The precentral gyrus is the primary motor cortex. Cells from this region of the cerebral cortex are the upper motor neurons that instruct cells in the spinal cord to move skeletal muscles. Anterior to this region are a few areas that are associated with planned movements. The premotor area is responsible for thinking of a movement to be made. The frontal eye fields are important in eliciting eye movements and in attending to visual stimuli. Broca’s area is responsible for the production of language, or controlling movements responsible for speech; in the vast majority of people, it is located only on the left side. Anterior to these regions is the prefrontal lobe, which serves cognitive functions that can be the basis of personality, short-term memory, and consciousness. The prefrontal lobotomy is an outdated mode of treatment for personality disorders (psychiatric conditions) that profoundly affected the personality of the patient. Brodmann's Areas of the Cerebral Cortex Areas 1, 2,3 Primary somatosensory cortex Area 4 Primary motor cortex Areas 44, 45 Broca’s area Area 4 Primary motor cortex Areas 39, 40 Wernicke’s area d Area 22 % Area 17 : ; SB oe ee ok A : Primary auditory “Sel fe 2» Primary visual cortex cortex Brodmann’s cytotechtonic map (1909): Brodmann’s cytotechtonic map (1909): Lateral surface Medial surface Brodmann mapping of functionally distinct regions of the cortex was based on its cytoarchitecture at a microscopic level. Subcortical structures Beneath the cerebral cortex are sets of nuclei known as subcortical nuclei that augment cortical processes. The nuclei of the basal forebrain serve as the primary location for acetylcholine production, which modulates the overall activity of the cortex, possibly leading to greater attention to sensory stimuli. Alzheimer’s disease is associated with a loss of neurons in the basal forebrain. The hippocampus and amygdala are medial-lobe structures that, along with the adjacent cortex, are involved in long-term memory formation and emotional responses. The basal nuclei are a set of nuclei in the cerebrum responsible for comparing cortical processing with the general state of activity in the nervous system to influence the likelihood of movement taking place. For example, while a student is sitting in a classroom listening to a lecture, the basal nuclei will keep the urge to jump up and scream from actually happening. (The basal nuclei are also referred to as the basal ganglia, although that is potentially confusing because the term ganglia is typically used for peripheral structures.) The major structures of the basal nuclei that control movement are the caudate, putamen, and globus pallidus, which are located deep in the cerebrum. The caudate is a long nucleus that follows the basic C-shape of the cerebrum from the frontal lobe, through the parietal and occipital lobes, into the temporal lobe. The putamen is mostly deep in the anterior regions of the frontal and parietal lobes. Together, the caudate and putamen are called the striatum. The globus pallidus is a layered nucleus that lies just medial to the putamen; they are called the lenticular nuclei because they look like curved pieces fitting together like lenses. The globus pallidus has two subdivisions, the external and internal segments, which are lateral and medial, respectively. These nuclei are depicted in a frontal section of the brain in [link]. Frontal Section of Cerebral Cortex and Basal Nuclei Lateral ventricle Striatum: Caudate Putamen Corpus callosum Globus pallidus Frontal section The major components of the basal nuclei, shown in a frontal section of the brain, are the caudate (just lateral to the lateral ventricle), the putamen (inferior to the caudate and separated by the large white-matter structure called the internal capsule), and the globus pallidus (medial to the putamen). The basal nuclei in the cerebrum are connected with a few more nuclei in the brain stem that together act as a functional group that forms a motor pathway. Two streams of information processing take place in the basal nuclei. All input to the basal nuclei is from the cortex into the striatum ({link]). The direct pathway is the projection of axons from the striatum to the globus pallidus internal segment (GPi) and the substantia nigra pars reticulata (SNr). The GPi/SNr then projects to the thalamus, which projects back to the cortex. The indirect pathway is the projection of axons from the striatum to the globus pallidus external segment (GPe), then to the subthalamic nucleus (STN), and finally to GPi/SNr. The two streams both target the GPi/SNr, but one has a direct projection and the other goes through a few intervening nuclei. The direct pathway causes the disinhibition of the thalamus (inhibition of one cell on a target cell that then inhibits the first cell), whereas the indirect pathway causes, or reinforces, the normal inhibition of the thalamus. The thalamus then can either excite the cortex (as a result of the direct pathway) or fail to excite the cortex (as a result of the indirect pathway). Connections of Basal Nuclei Basal nuclei } t = Glutamate = Dopamine A i [Thalarws |<—] Input to the basal nuclei is from the cerebral cortex, which is an excitatory connection releasing glutamate as a neurotransmitter. This input is to the striatum, or the caudate and putamen. In the direct pathway, the striatum projects to the internal segment of the globus pallidus and the substantia nigra pars reticulata (GPi/SNr). This is an inhibitory pathway, in which GABA is released at the synapse, and the target cells are hyperpolarized and less likely to fire. The output from the basal nuclei is to the thalamus, which is an inhibitory projection using GABA. The switch between the two pathways is the substantia nigra pars compacta, which projects to the striatum and releases the neurotransmitter dopamine. Dopamine receptors are either excitatory (D1-type receptors) or inhibitory (D2-type receptors). The direct pathway is activated by dopamine, and the indirect pathway is inhibited by dopamine. When the substantia nigra pars compacta is firing, it signals to the basal nuclei that the body is in an active state, and movement will be more likely. When the substantia nigra pars compacta is silent, the body is in a passive state, and movement is inhibited. To illustrate this situation, while a student is sitting listening to a lecture, the substantia nigra pars compacta would be silent and the student less likely to get up and walk around. Likewise, while the professor is lecturing, and walking around at the front of the classroom, the professor’s substantia nigra pars compacta would be active, in keeping with his or her activity level. Watch this video to learn about the basal nuclei (also known as the basal ganglia), which have two pathways that process information within the cerebrum. As shown in this video, the direct pathway is the shorter pathway through the system that results in increased activity in the cerebral cortex and increased motor activity. The direct pathway is described as resulting in “disinhibition” of the thalamus. What does disinhibition mean? What are the two neurons doing individually to cause this? Note: [elie — mss Openstax COLLEGE ro-e." Watch this video to learn about the basal nuclei (also known as the basal ganglia), which have two pathways that process information within the cerebrum. As shown in this video, the indirect pathway is the longer pathway through the system that results in decreased activity in the cerebral cortex, and therefore less motor activity. The indirect pathway has an extra couple of connections in it, including disinhibition of the subthalamic nucleus. What is the end result on the thalamus, and therefore on movement initiated by the cerebral cortex? Note: Everyday Connections The Myth of Left Brain/Right Brain There is a persistent myth that people are “right-brained” or “left-brained,” which is an oversimplification of an important concept about the cerebral hemispheres. There is some lateralization of function, in which the left side of the brain is devoted to language function and the right side is devoted to spatial and nonverbal reasoning. Whereas these functions are predominantly associated with those sides of the brain, there is no monopoly by either side on these functions. Many pervasive functions, such as language, are distributed globally around the cerebrum. Some of the support for this misconception has come from studies of split brains. A drastic way to deal with a rare and devastating neurological condition (intractable epilepsy) is to separate the two hemispheres of the brain. After sectioning the corpus callosum, a split-brained patient will have trouble producing verbal responses on the basis of sensory information processed on the right side of the cerebrum, leading to the idea that the left side is responsible for language function. However, there are well-documented cases of language functions lost from damage to the right side of the brain. The deficits seen in damage to the left side of the brain are classified as aphasia, a loss of speech function; damage on the right side can affect the use of language. Right-side damage can result in a loss of ability to understand figurative aspects of speech, such as jokes, irony, or metaphors. Nonverbal aspects of speech can be affected by damage to the right side, such as facial expression or body language, and right-side damage can lead to a “flat affect” in speech, or a loss of emotional expression in speech—sounding like a robot when talking. The Diencephalon The diencephalon is the one region of the adult brain that retains its name from embryologic development. The etymology of the word diencephalon translates to “through brain.” It is the connection between the cerebrum and the rest of the nervous system, with one exception. The rest of the brain, the spinal cord, and the PNS all send information to the cerebrum through the diencephalon. Output from the cerebrum passes through the diencephalon. The single exception is the system associated with olfaction, or the sense of smell, which connects directly with the cerebrum. In the earliest vertebrate species, the cerebrum was not much more than olfactory bulbs that received peripheral information about the chemical environment (to call it smell in these organisms is imprecise because they lived in the ocean). The diencephalon is deep beneath the cerebrum and constitutes the walls of the third ventricle. The diencephalon can be described as any region of the brain with “thalamus” in its name. The two major regions of the diencephalon are the thalamus itself and the hypothalamus ([link]). There are other structures, such as the epithalamus, which contains the pineal gland, or the subthalamus, which includes the subthalamic nucleus that is part of the basal nuclei. Thalamus The thalamus is a collection of nuclei that relay information between the cerebral cortex and the periphery, spinal cord, or brain stem. All sensory information, except for the sense of smell, passes through the thalamus before processing by the cortex. Axons from the peripheral sensory organs, or intermediate nuclei, synapse in the thalamus, and thalamic neurons project directly to the cerebrum. It is a requisite synapse in any sensory pathway, except for olfaction. The thalamus does not just pass the information on, it also processes that information. For example, the portion of the thalamus that receives visual information will influence what visual stimuli are important, or what receives attention. The cerebrum also sends information down to the thalamus, which usually communicates motor commands. This involves interactions with the cerebellum and other nuclei in the brain stem. The cerebrum interacts with the basal nuclei, which involves connections with the thalamus. The primary output of the basal nuclei is to the thalamus, which relays that output to the cerebral cortex. The cortex also sends information to the thalamus that will then influence the effects of the basal nuclei. Hypothalamus Inferior and slightly anterior to the thalamus is the hypothalamus, the other major region of the diencephalon. The hypothalamus is a collection of nuclei that are largely involved in regulating homeostasis. The hypothalamus is the executive region in charge of the autonomic nervous system and the endocrine system through its regulation of the anterior pituitary gland. Other parts of the hypothalamus are involved in memory and emotion as part of the limbic system. The Diencephalon Thalamus Hypothalamus Pituitary gland The diencephalon is composed primarily of the thalamus and hypothalamus, which together define the walls of the third ventricle. The thalami are two elongated, ovoid structures on either side of the midline that make contact in the middle. The hypothalamus is inferior and anterior to the thalamus, culminating in a sharp angle to which the pituitary gland is attached. Brain Stem The midbrain and hindbrain (composed of the pons and the medulla) are collectively referred to as the brain stem ({link]). The structure emerges from the ventral surface of the forebrain as a tapering cone that connects the brain to the spinal cord. Attached to the brain stem, but considered a separate region of the adult brain, is the cerebellum. The midbrain coordinates sensory representations of the visual, auditory, and somatosensory perceptual spaces. The pons is the main connection with the cerebellum. The pons and the medulla regulate several crucial functions, including the cardiovascular and respiratory systems and rates. The cranial nerves connect through the brain stem and provide the brain with the sensory input and motor output associated with the head and neck, including most of the special senses. The major ascending and descending pathways between the spinal cord and brain, specifically the cerebrum, pass through the brain stem. The Brain Stem Midbrain Pons Medulla The brain stem comprises three regions: the midbrain, the pons, and the medulla. Midbrain One of the original regions of the embryonic brain, the midbrain is a small region between the thalamus and pons. It is separated into the tectum and tegmentum, from the Latin words for roof and floor, respectively. The cerebral aqueduct passes through the center of the midbrain, such that these regions are the roof and floor of that canal. The tectum is composed of four bumps known as the colliculi (singular = colliculus), which means “little hill” in Latin. The inferior colliculus is the inferior pair of these enlargements and is part of the auditory brain stem pathway. Neurons of the inferior colliculus project to the thalamus, which then sends auditory information to the cerebrum for the conscious perception of sound. The superior colliculus is the superior pair and combines sensory information about visual space, auditory space, and somatosensory space. Activity in the superior colliculus is related to orienting the eyes to a sound or touch stimulus. If you are walking along the sidewalk on campus and you hear chirping, the superior colliculus coordinates that information with your awareness of the visual location of the tree right above you. That is the correlation of auditory and visual maps. If you suddenly feel something wet fall on your head, your superior colliculus integrates that with the auditory and visual maps and you know that the chirping bird just relieved itself on you. You want to look up to see the culprit, but do not. The tegmentum is continuous with the gray matter of the rest of the brain stem. Throughout the midbrain, pons, and medulla, the tegmentum contains the nuclei that receive and send information through the cranial nerves, as well as regions that regulate important functions such as those of the cardiovascular and respiratory systems. Pons The word pons comes from the Latin word for bridge. It is visible on the anterior surface of the brain stem as the thick bundle of white matter attached to the cerebellum. The pons is the main connection between the cerebellum and the brain stem. The bridge-like white matter is only the anterior surface of the pons; the gray matter beneath that is a continuation of the tegmentum from the midbrain. Gray matter in the tegmentum region of the pons contains neurons receiving descending input from the forebrain that is sent to the cerebellum. Medulla The medulla is the region known as the myelencephalon in the embryonic brain. The initial portion of the name, “myel,” refers to the significant white matter found in this region—especially on its exterior, which is continuous with the white matter of the spinal cord. The tegmentum of the midbrain and pons continues into the medulla because this gray matter is responsible for processing cranial nerve information. A diffuse region of gray matter throughout the brain stem, known as the reticular formation, is related to sleep and wakefulness, such as general brain activity and attention. The Cerebellum The cerebellum, as the name suggests, is the “little brain.” It is covered in gyri and sulci like the cerebrum, and looks like a miniature version of that part of the brain ([link]). The cerebellum is largely responsible for comparing information from the cerebrum with sensory feedback from the periphery through the spinal cord. It accounts for approximately 10 percent of the mass of the brain. The Cerebellum Cerebellum Deep cerebellar white matter (arbor vitae) Inferior olive The cerebellum is situated on the posterior surface of the brain stem. Descending input from the cerebellum enters through the large white matter structure of the pons. Ascending input from the periphery and spinal cord enters through the fibers of the inferior olive. Output goes to the midbrain, which sends a descending signal to the spinal cord. Descending fibers from the cerebrum have branches that connect to neurons in the pons. Those neurons project into the cerebellum, providing a copy of motor commands sent to the spinal cord. Sensory information from the periphery, which enters through spinal or cranial nerves, is copied to a nucleus in the medulla known as the inferior olive. Fibers from this nucleus enter the cerebellum and are compared with the descending commands from the cerebrum. If the primary motor cortex of the frontal lobe sends a command down to the spinal cord to initiate walking, a copy of that instruction is sent to the cerebellum. Sensory feedback from the muscles and joints, proprioceptive information about the movements of walking, and sensations of balance are sent to the cerebellum through the inferior olive and the cerebellum compares them. If walking is not coordinated, perhaps because the ground is uneven or a strong wind is blowing, then the cerebellum sends out a corrective command to compensate for the difference between the original cortical command and the sensory feedback. The output of the cerebellum is into the midbrain, which then sends a descending input to the spinal cord to correct the messages going to skeletal muscles. The Spinal Cord The description of the CNS is concentrated on the structures of the brain, but the spinal cord is another major organ of the system. Whereas the brain develops out of expansions of the neural tube into primary and then secondary vesicles, the spinal cord maintains the tube structure and is only specialized into certain regions. As the spinal cord continues to develop in the newborn, anatomical features mark its surface. The anterior midline is marked by the anterior median fissure, and the posterior midline is marked by the posterior median sulcus. Axons enter the posterior side through the dorsal (posterior) nerve root, which marks the posterolateral sulcus on either side. The axons emerging from the anterior side do so through the ventral (anterior) nerve root. Note that it is common to see the terms dorsal (dorsal = “back”) and ventral (ventral = “belly”) used interchangeably with posterior and anterior, particularly in reference to nerves and the structures of the spinal cord. You should learn to be comfortable with both. On the whole, the posterior regions are responsible for sensory functions and the anterior regions are associated with motor functions. This comes from the initial development of the spinal cord, which is divided into the basal plate and the alar plate. The basal plate is closest to the ventral midline of the neural tube, which will become the anterior face of the spinal cord and gives rise to motor neurons. The alar plate is on the dorsal side of the neural tube and gives rise to neurons that will receive sensory input from the periphery. The length of the spinal cord is divided into regions that correspond to the regions of the vertebral column. The name of a spinal cord region corresponds to the level at which spinal nerves pass through the intervertebral foramina. Immediately adjacent to the brain stem is the cervical region, followed by the thoracic, then the lumbar, and finally the sacral region. The spinal cord is not the full length of the vertebral column because the spinal cord does not grow significantly longer after the first or second year, but the skeleton continues to grow. The nerves that emerge from the spinal cord pass through the intervertebral formina at the respective levels. As the vertebral column grows, these nerves grow with it and result in a long bundle of nerves that resembles a horse’s tail and is named the cauda equina. The sacral spinal cord is at the level of the upper lumbar vertebral bones. The spinal nerves extend from their various levels to the proper level of the vertebral column. Gray Horns In cross-section, the gray matter of the spinal cord has the appearance of an ink-blot test, with the spread of the gray matter on one side replicated on the other—a shape reminiscent of a bulbous capital “H.” As shown in [link], the gray matter is subdivided into regions that are referred to as horns. The posterior horn is responsible for sensory processing. The anterior horn sends out motor signals to the skeletal muscles. The lateral horn, which is only found in the thoracic, upper lumbar, and sacral regions, is the central component of the sympathetic division of the autonomic nervous system. Some of the largest neurons of the spinal cord are the multipolar motor neurons in the anterior horn. The fibers that cause contraction of skeletal muscles are the axons of these neurons. The motor neuron that causes contraction of the big toe, for example, is located in the sacral spinal cord. The axon that has to reach all the way to the belly of that muscle may be a meter in length. The neuronal cell body that maintains that long fiber must be quite large, possibly several hundred micrometers in diameter, making it one of the largest cells in the body. Cross-section of Spinal Cord Posterior (dorsal) columns Gray matter: Posterior (dorsal) ~¢ horn Lateral columns Lateral horn Central canal Anterior (ventral) Anterior (ventral) columns horn The cross-section of a thoracic spinal cord segment shows the posterior, anterior, and lateral horns of gray matter, as well as the posterior, anterior, and lateral columns of white matter. LM x 40. (Micrograph provided by the Regents of University of Michigan Medical School © 2012) White Columns Just as the gray matter is separated into horns, the white matter of the spinal cord is separated into columns. Ascending tracts of nervous system fibers in these columns carry sensory information up to the brain, whereas descending tracts carry motor commands from the brain. Looking at the spinal cord longitudinally, the columns extend along its length as continuous bands of white matter. Between the two posterior horns of gray matter are the posterior columns. Between the two anterior horns, and bounded by the axons of motor neurons emerging from that gray matter area, are the anterior columns. The white matter on either side of the spinal cord, between the posterior horn and the axons of the anterior horn neurons, are the lateral columns. The posterior columns are composed of axons of ascending tracts. The anterior and lateral columns are composed of many different groups of axons of both ascending and descending tracts— the latter carrying motor commands down from the brain to the spinal cord to control output to the periphery. Note: ie Hr openstax COLLEGE O) a Watch this video to learn about the gray matter of the spinal cord that receives input from fibers of the dorsal (posterior) root and sends information out through the fibers of the ventral (anterior) root. As discussed in this video, these connections represent the interactions of the CNS with peripheral structures for both sensory and motor functions. The cervical and lumbar spinal cords have enlargements as a result of larger populations of neurons. What are these enlargements responsible for? Note: Disorders of the... Basal Nuclei Parkinson’s disease is a disorder of the basal nuclei, specifically of the substantia nigra, that demonstrates the effects of the direct and indirect pathways. Parkinson’s disease is the result of neurons in the substantia nigra pars compacta dying. These neurons release dopamine into the striatum. Without that modulatory influence, the basal nuclei are stuck in the indirect pathway, without the direct pathway being activated. The direct pathway is responsible for increasing cortical movement commands. The increased activity of the indirect pathway results in the hypokinetic disorder of Parkinson’s disease. Parkinson’s disease is neurodegenerative, meaning that neurons die that cannot be replaced, so there is no cure for the disorder. Treatments for Parkinson’s disease are aimed at increasing dopamine levels in the striatum. Currently, the most common way of doing that is by providing the amino acid L-DOPA, which is a precursor to the neurotransmitter dopamine and can cross the blood-brain barrier. With levels of the precursor elevated, the remaining cells of the substantia nigra pars compacta can make more neurotransmitter and have a greater effect. Unfortunately, the patient will become less responsive to L-DOPA treatment as time progresses, and it can cause increased dopamine levels elsewhere in the brain, which are associated with psychosis or schizophrenia. Note: [=] [a] ih => Openstax COLLEGE h -_—- Visit this site for a thorough explanation of Parkinson’s disease. Note: — meee OPENStAX COLLEGE Compared with the nearest evolutionary relative, the chimpanzee, the human has a brain that is huge. At a point in the past, a common ancestor gave rise to the two species of humans and chimpanzees. That evolutionary history is long and is still an area of intense study. But something happened to increase the size of the human brain relative to the chimpanzee. Read this article in which the author explores the current understanding of why this happened. According to one hypothesis about the expansion of brain size, what tissue might have been sacrificed so energy was available to grow our larger brain? Based on what you know about that tissue and nervous tissue, why would there be a trade-off between them in terms of energy use? Chapter Review The adult brain is separated into four major regions: the cerebrum, the diencephalon, the brain stem, and the cerebellum. The cerebrum is the largest portion and contains the cerebral cortex and subcortical nuclei. It is divided into two halves by the longitudinal fissure. The cortex is separated into the frontal, parietal, temporal, and occipital lobes. The frontal lobe is responsible for motor functions, from planning movements through executing commands to be sent to the spinal cord and periphery. The most anterior portion of the frontal lobe is the prefrontal cortex, which is associated with aspects of personality through its influence on motor responses in decision-making. The other lobes are responsible for sensory functions. The parietal lobe is where somatosensation is processed. The occipital lobe is where visual processing begins, although the other parts of the brain can contribute to visual function. The temporal lobe contains the cortical area for auditory processing, but also has regions crucial for memory formation. Nuclei beneath the cerebral cortex, known as the subcortical nuclei, are responsible for augmenting cortical functions. The basal nuclei receive input from cortical areas and compare it with the general state of the individual through the activity of a dopamine-releasing nucleus. The output influences the activity of part of the thalamus that can then increase or decrease cortical activity that often results in changes to motor commands. The basal forebrain is responsible for modulating cortical activity in attention and memory. The limbic system includes deep cerebral nuclei that are responsible for emotion and memory. The diencephalon includes the thalamus and the hypothalamus, along with some other structures. The thalamus is a relay between the cerebrum and the rest of the nervous system. The hypothalamus coordinates homeostatic functions through the autonomic and endocrine systems. The brain stem is composed of the midbrain, pons, and medulla. It controls the head and neck region of the body through the cranial nerves. There are control centers in the brain stem that regulate the cardiovascular and respiratory systems. The cerebellum is connected to the brain stem, primarily at the pons, where it receives a copy of the descending input from the cerebrum to the spinal cord. It can compare this with sensory feedback input through the medulla and send output through the midbrain that can correct motor commands for coordination. Interactive Link Questions Exercise: Problem: Watch this video to learn about the basal nuclei (also known as the basal ganglia), which have two pathways that process information within the cerebrum. As shown in this video, the direct pathway is the shorter pathway through the system that results in increased activity in the cerebral cortex and increased motor activity. The direct pathway is described as resulting in “disinhibition” of the thalamus. What does disinhibition mean? What are the two neurons doing individually to cause this? Solution: Both cells are inhibitory. The first cell inhibits the second one. Therefore, the second cell can no longer inhibit its target. This is disinhibition of that target across two synapses. Exercise: Problem: Watch this video to learn about the basal nuclei (also known as the basal ganglia), which have two pathways that process information within the cerebrum. As shown in this video, the indirect pathway is the longer pathway through the system that results in decreased activity in the cerebral cortex, and therefore less motor activity. The indirect pathway has an extra couple of connections in it, including disinhibition of the subthalamic nucleus. What is the end result on the thalamus, and therefore on movement initiated by the cerebral cortex? Solution: By disinhibiting the subthalamic nucleus, the indirect pathway increases excitation of the globus pallidus internal segment. That, in turn, inhibits the thalamus, which is the opposite effect of the direct pathway that disinhibits the thalamus. Exercise: Problem: Watch this video to learn about the gray matter of the spinal cord that receives input from fibers of the dorsal (posterior) root and sends information out through the fibers of the ventral (anterior) root. As discussed in this video, these connections represent the interactions of the CNS with peripheral structures for both sensory and motor functions. The cervical and lumbar spinal cords have enlargements as a result of larger populations of neurons. What are these enlargements responsible for? Solution: There are more motor neurons in the anterior horns that are responsible for movement in the limbs. The cervical enlargement is for the arms, and the lumbar enlargement is for the legs. Exercise: Problem: Compared with the nearest evolutionary relative, the chimpanzee, the human has a brain that is huge. At a point in the past, a common ancestor gave rise to the two species of humans and chimpanzees. That evolutionary history is long and is still an area of intense study. But something happened to increase the size of the human brain relative to the chimpanzee. Read this article in which the author explores the current understanding of why this happened. According to one hypothesis about the expansion of brain size, what tissue might have been sacrificed so energy was available to grow our larger brain? Based on what you know about that tissue and nervous tissue, why would there be a trade-off between them in terms of energy use? Solution: Energy is needed for the brain to develop and perform higher cognitive functions. That energy is not available for the muscle tissues to develop and function. The hypothesis suggests that humans have larger brains and less muscle mass, and chimpanzees have the smaller brains but more muscle mass. Review Questions Exercise: Problem: Which lobe of the cerebral cortex is responsible for generating motor commands? a. temporal b. parietal c. occipital d. frontal Solution: D Exercise: Problem: What region of the diencephalon coordinates homeostasis? a. thalamus b. epithalamus c. hypothalamus d. subthalamus Solution: C Exercise: Problem: What level of the brain stem is the major input to the cerebellum? a. midbrain b. pons c. medulla d. spinal cord Solution: B Exercise: Problem: What region of the spinal cord contains motor neurons that direct the movement of skeletal muscles? a. anterior horn b. posterior horn c. lateral horn d. alar plate Solution: A Exercise: Problem: Brodmann’s areas map different regions of the to particular functions. a. cerebellum b. cerebral cortex c. basal forebrain d. corpus callosum Solution: B Critical Thinking Questions Exercise: Problem: Damage to specific regions of the cerebral cortex, such as through a stroke, can result in specific losses of function. What functions would likely be lost by a stroke in the temporal lobe? Solution: The temporal lobe has sensory functions associated with hearing and vision, as well as being important for memory. A stroke in the temporal lobe can result in specific sensory deficits in these systems (known as agnosias) or losses in memory. Exercise: Problem: Why do the anatomical inputs to the cerebellum suggest that it can compare motor commands and sensory feedback? Solution: A copy of descending input from the cerebrum to the spinal cord, through the pons, and sensory feedback from the spinal cord and special senses like balance, through the medulla, both go to the cerebellum. It can therefore send output through the midbrain that will correct spinal cord control of skeletal muscle movements. Glossary alar plate developmental region of the spinal cord that gives rise to the posterior horn of the gray matter amygdala nucleus deep in the temporal lobe of the cerebrum that is related to memory and emotional behavior anterior column white matter between the anterior horns of the spinal cord composed of many different groups of axons of both ascending and descending tracts anterior horn gray matter of the spinal cord containing multipolar motor neurons, sometimes referred to as the ventral horn anterior median fissure deep midline feature of the anterior spinal cord, marking the separation between the right and left sides of the cord ascending tract central nervous system fibers carrying sensory information from the spinal cord or periphery to the brain basal forebrain nuclei of the cerebrum related to modulation of sensory stimuli and attention through broad projections to the cerebral cortex, loss of which is related to Alzheimer’s disease basal nuclei nuclei of the cerebrum (with a few components in the upper brain stem and diencephalon) that are responsible for assessing cortical movement commands and comparing them with the general state of the individual through broad modulatory activity of dopamine neurons; largely related to motor functions, as evidenced through the symptoms of Parkinson’s and Huntington’s diseases basal plate developmental region of the spinal cord that gives rise to the lateral and anterior horns of gray matter Broca’s area region of the frontal lobe associated with the motor commands necessary for speech production and located only in the cerebral hemisphere responsible for language production, which is the left side in approximately 95 percent of the population Brodmann’s areas mapping of regions of the cerebral cortex based on microscopic anatomy that relates specific areas to functional differences, as described by Brodmann in the early 1900s cauda equina bundle of spinal nerve roots that descend from the lower spinal cord below the first lumbar vertebra and lie within the vertebral cavity; has the appearance of a horse's tail caudate nucleus deep in the cerebrum that is part of the basal nuclei; along with the putamen, it is part of the striatum central sulcus surface landmark of the cerebral cortex that marks the boundary between the frontal and parietal lobes cerebral cortex outer gray matter covering the forebrain, marked by wrinkles and folds known as gyri and sulci cerebrum region of the adult brain that develops from the telencephalon and is responsible for higher neurological functions such as memory, emotion, and consciousness cerebellum region of the adult brain connected primarily to the pons that developed from the metencephalon (along with the pons) and is largely responsible for comparing information from the cerebrum with sensory feedback from the periphery through the spinal cord cerebral hemisphere one half of the bilaterally symmetrical cerebrum corpus callosum large white matter structure that connects the right and left cerebral hemispheres descending tract central nervous system fibers carrying motor commands from the brain to the spinal cord or periphery direct pathway connections within the basal nuclei from the striatum to the globus pallidus internal segment and substantia nigra pars reticulata that disinhibit the thalamus to increase cortical control of movement disinhibition disynaptic connection in which the first synapse inhibits the second cell, which then stops inhibiting the final target dorsal (posterior) nerve root axons entering the posterior horn of the spinal cord epithalamus region of the diecephalon containing the pineal gland frontal eye field region of the frontal lobe associated with motor commands to orient the eyes toward an object of visual attention frontal lobe region of the cerebral cortex directly beneath the frontal bone of the cranium globus pallidus nuclei deep in the cerebrum that are part of the basal nuclei and can be divided into the internal and external segments gyrus ridge formed by convolutions on the surface of the cerebrum or cerebellum hippocampus gray matter deep in the temporal lobe that is very important for long- term memory formation hypothalamus major region of the diencephalon that is responsible for coordinating autonomic and endocrine control of homeostasis indirect pathway connections within the basal nuclei from the striatum through the globus pallidus external segment and subthalamic nucleus to the globus pallidus internal segment/substantia nigra pars compacta that result in inhibition of the thalamus to decrease cortical control of movement inferior colliculus half of the midbrain tectum that is part of the brain stem auditory pathway inferior olive nucleus in the medulla that is involved in processing information related to motor control kinesthesia general sensory perception of movement of the body lateral column white matter of the spinal cord between the posterior horn on one side and the axons from the anterior horn on the same side; composed of many different groups of axons, of both ascending and descending tracts, carrying motor commands to and from the brain lateral hor region of the spinal cord gray matter in the thoracic, upper lumbar, and sacral regions that is the central component of the sympathetic division of the autonomic nervous system lateral sulcus surface landmark of the cerebral cortex that marks the boundary between the temporal lobe and the frontal and parietal lobes limbic cortex collection of structures of the cerebral cortex that are involved in emotion, memory, and behavior and are part of the larger limbic system limbic system structures at the edge (limit) of the boundary between the forebrain and hindbrain that are most associated with emotional behavior and memory formation longitudinal fissure large separation along the midline between the two cerebral hemispheres occipital lobe region of the cerebral cortex directly beneath the occipital bone of the cranium olfaction special sense responsible for smell, which has a unique, direct connection to the cerebrum parietal lobe region of the cerebral cortex directly beneath the parietal bone of the cranium parieto-occipital sulcus groove in the cerebral cortex representing the border between the parietal and occipital cortices postcentral gyrus primary motor cortex located in the frontal lobe of the cerebral cortex posterior columns white matter of the spinal cord that lies between the posterior horns of the gray matter, sometimes referred to as the dorsal column; composed of axons of ascending tracts that carry sensory information up to the brain posterior hor gray matter region of the spinal cord in which sensory input arrives, sometimes referred to as the dorsal horn posterior median sulcus midline feature of the posterior spinal cord, marking the separation between right and left sides of the cord posterolateral sulcus feature of the posterior spinal cord marking the entry of posterior nerve roots and the separation between the posterior and lateral columns of the white matter precentral gyrus ridge just posterior to the central sulcus, in the parietal lobe, where somatosensory processing initially takes place in the cerebrum prefrontal lobe specific region of the frontal lobe anterior to the more specific motor function areas, which can be related to the early planning of movements and intentions to the point of being personality-type functions premotor area region of the frontal lobe responsible for planning movements that will be executed through the primary motor cortex proprioception general sensory perceptions providing information about location and movement of body parts; the “sense of the self” putamen nucleus deep in the cerebrum that is part of the basal nuclei; along with the caudate, it is part of the striatum reticular formation diffuse region of gray matter throughout the brain stem that regulates Sleep, wakefulness, and states of consciousness somatosensation general senses related to the body, usually thought of as the senses of touch, which would include pain, temperature, and proprioception striatum the caudate and putamen collectively, as part of the basal nuclei, which receive input from the cerebral cortex subcortical nucleus all the nuclei beneath the cerebral cortex, including the basal nuclei and the basal forebrain substantia nigra pars compacta nuclei within the basal nuclei that release dopamine to modulate the function of the striatum; part of the motor pathway substantia nigra pars reticulata nuclei within the basal nuclei that serve as an output center of the nuclei; part of the motor pathway subthalamus nucleus within the basal nuclei that is part of the indirect pathway sulcus groove formed by convolutions in the surface of the cerebral cortex superior colliculus half of the midbrain tectum that is responsible for aligning visual, auditory, and somatosensory spatial perceptions tectum region of the midbrain, thought of as the roof of the cerebral aqueduct, which is subdivided into the inferior and superior colliculi tegmentum region of the midbrain, thought of as the floor of the cerebral aqueduct, which continues into the pons and medulla as the floor of the fourth ventricle temporal lobe region of the cerebral cortex directly beneath the temporal bone of the cranium thalamus major region of the diencephalon that is responsible for relaying information between the cerebrum and the hindbrain, spinal cord, and periphery ventral (anterior) nerve root axons emerging from the anterior or lateral horns of the spinal cord Circulation and the Central Nervous System By the end of this section, you will be able to: e Describe the vessels that supply the CNS with blood e Name the components of the ventricular system and the regions of the brain in which each is located e Explain the production of cerebrospinal fluid and its flow through the ventricles e Explain how a disruption in circulation would result in a stroke The CNS is crucial to the operation of the body, and any compromise in the brain and spinal cord can lead to severe difficulties. The CNS has a privileged blood supply, as suggested by the blood-brain barrier. The function of the tissue in the CNS is crucial to the survival of the organism, so the contents of the blood cannot simply pass into the central nervous tissue. To protect this region from the toxins and pathogens that may be traveling through the blood stream, there is strict control over what can move out of the general systems and into the brain and spinal cord. Because of this privilege, the CNS needs specialized structures for the maintenance of circulation. This begins with a unique arrangement of blood vessels carrying fresh blood into the CNS. Beyond the supply of blood, the CNS filters that blood into cerebrospinal fluid (CSF), which is then circulated through the cavities of the brain and spinal cord called ventricles. Blood Supply to the Brain A lack of oxygen to the CNS can be devastating, and the cardiovascular system has specific regulatory reflexes to ensure that the blood supply is not interrupted. There are multiple routes for blood to get into the CNS, with specializations to protect that blood supply and to maximize the ability of the brain to get an uninterrupted perfusion. Arterial Supply The major artery carrying recently oxygenated blood away from the heart is the aorta. The very first branches off the aorta supply the heart with nutrients and oxygen. The next branches give rise to the common carotid arteries, which further branch into the internal carotid arteries. The external carotid arteries supply blood to the tissues on the surface of the cranium. The bases of the common carotids contain stretch receptors that immediately respond to the drop in blood pressure upon standing. The orthostatic reflex is a reaction to this change in body position, so that blood pressure is maintained against the increasing effect of gravity (orthostatic means “standing up”). Heart rate increases—a reflex of the sympathetic division of the autonomic nervous system—and this raises blood pressure. The internal carotid artery enters the cranium through the carotid canal in the temporal bone. A second set of vessels that supply the CNS are the vertebral arteries, which are protected as they pass through the neck region by the transverse foramina of the cervical vertebrae. The vertebral arteries enter the cranium through the foramen magnum of the occipital bone. Branches off the left and right vertebral arteries merge into the anterior spinal artery supplying the anterior aspect of the spinal cord, found along the anterior median fissure. The two vertebral arteries then merge into the basilar artery, which gives rise to branches to the brain stem and cerebellum. The left and right internal carotid arteries and branches of the basilar artery all become the circle of Willis, a confluence of arteries that can maintain perfusion of the brain even if narrowing or a blockage limits flow through one part ({link]). Circle of Willis Anterior cerebral Anterior artery communicating artery Ophthalmic artery Middle cerebral Anterior artery choroidal Internal artery carotid Posterior artery cerebral arte! Posterior me communicating artery Superior cerebellar artery Pontine ; arteries Basilar artery Anterior Vertebral inferior artery cerebellar artery . Posterior inferior cerebellar Anterior artely spinal artery The blood supply to the brain enters through the internal carotid arteries and the vertebral arteries, eventually giving rise to the circle of Willis. Watch this animation to see how blood flows to the brain and passes through the circle of Willis before being distributed through the cerebrum. The circle of Willis is a specialized arrangement of arteries that ensure constant perfusion of the cerebrum even in the event of a blockage of one of the arteries in the circle. The animation shows the normal direction of flow through the circle of Willis to the middle cerebral artery. Where would the blood come from if there were a blockage just posterior to the middle cerebral artery on the left? Venous Return After passing through the CNS, blood returns to the circulation through a series of dural sinuses and veins ({link]). The superior sagittal sinus runs in the groove of the longitudinal fissure, where it absorbs CSF from the meninges. The superior sagittal sinus drains to the confluence of sinuses, along with the occipital sinuses and straight sinus, to then drain into the transverse sinuses. The transverse sinuses connect to the sigmoid sinuses, which then connect to the jugular veins. From there, the blood continues toward the heart to be pumped to the lungs for reoxygenation. Dural Sinuses and Veins Superior sagittal sinus Cranium Inferior sagittal Dura mater sinus Straight sinus Cerebral veins Tranverse sinus Great cerebral vein Confluence of sinuses Occipital sinus Blood returns to jugular vein via the sigmoid sinus Blood drains from the brain through a series of sinuses that connect to the jugular veins. Protective Coverings of the Brain and Spinal Cord The outer surface of the CNS is covered by a series of membranes composed of connective tissue called the meninges, which protect the brain. The dura mater is a thick fibrous layer and a strong protective sheath over the entire brain and spinal cord. It is anchored to the inner surface of the cranium and vertebral cavity. The arachnoid mater is a membrane of thin fibrous tissue that forms a loose sac around the CNS. Beneath the arachnoid is a thin, filamentous mesh called the arachnoid trabeculae, which looks like a spider web, giving this layer its name. Directly adjacent to the surface of the CNS is the pia mater, a thin fibrous membrane that follows the convolutions of gyri and sulci in the cerebral cortex and fits into other grooves and indentations ([link]). Meningeal Layers of Superior Sagittal Sinus Superior sagittal sinus Arachnoid mater Dura mater Subdural space Shee Arachnoid Pia mater granulation villi Arachnoid trabeculae Longitudinal fissure Cerebral cortex The layers of the meninges in the longitudinal fissure of the superior sagittal sinus are shown, with the dura mater adjacent to the inner surface of the cranium, the pia mater adjacent to the surface of the brain, and the arachnoid and subarachnoid space between them. An arachnoid villus is shown emerging into the dural sinus to allow CSF to filter back into the blood for drainage. Dura Mater Like a thick cap covering the brain, the dura mater is a tough outer covering. The name comes from the Latin for “tough mother” to represent its physically protective role. It encloses the entire CNS and the major blood vessels that enter the cranium and vertebral cavity. It is directly attached to the inner surface of the bones of the cranium and to the very end of the vertebral cavity. There are infoldings of the dura that fit into large crevasses of the brain. Two infoldings go through the midline separations of the cerebrum and cerebellum; one forms a shelf-like tent between the occipital lobes of the cerebrum and the cerebellum, and the other surrounds the pituitary gland. The dura also surrounds and supports the venous sinuses. Arachnoid Mater The middle layer of the meninges is the arachnoid, named for the spider-web-like trabeculae between it and the pia mater. The arachnoid defines a sac-like enclosure around the CNS. The trabeculae are found in the subarachnoid space, which is filled with circulating CSF. The arachnoid emerges into the dural sinuses as the arachnoid granulations, where the CSF is filtered back into the blood for drainage from the nervous system. The subarachnoid space is filled with circulating CSF, which also provides a liquid cushion to the brain and spinal cord. Similar to clinical blood work, a sample of CSF can be withdrawn to find chemical evidence of neuropathology or metabolic traces of the biochemical functions of nervous tissue. Pia Mater The outer surface of the CNS is covered in the thin fibrous membrane of the pia mater. It is thought to have a continuous layer of cells providing a fluid-impermeable membrane. The name pia mater comes from the Latin for “tender mother,” suggesting the thin membrane is a gentle covering for the brain. The pia extends into every convolution of the CNS, lining the inside of the sulci in the cerebral and cerebellar cortices. At the end of the spinal cord, a thin filament extends from the inferior end of CNS at the upper lumbar region of the vertebral column to the sacral end of the vertebral column. Because the spinal cord does not extend through the lower lumbar region of the vertebral column, a needle can be inserted through the dura and arachnoid layers to withdraw CSF. This procedure is called a lumbar puncture and avoids the risk of damaging the central tissue of the spinal cord. Blood vessels that are nourishing the central nervous tissue are between the pia mater and the nervous tissue. Note: Disorders of the... Meninges Meningitis is an inflammation of the meninges, the three layers of fibrous membrane that surround the CNS. Meningitis can be caused by infection by bacteria or viruses. The particular pathogens are not special to meningitis; it is just an inflammation of that specific set of tissues from what might be a broader infection. Bacterial meningitis can be caused by Streptococcus, Staphylococcus, or the tuberculosis pathogen, among many others. Viral meningitis is usually the result of common enteroviruses (such as those that cause intestinal disorders), but may be the result of the herpes virus or West Nile virus. Bacterial meningitis tends to be more severe. The symptoms associated with meningitis can be fever, chills, nausea, vomiting, light sensitivity, soreness of the neck, or severe headache. More important are the neurological symptoms, such as changes in mental state (confusion, memory deficits, and other dementia-type symptoms). A serious risk of meningitis can be damage to peripheral structures because of the nerves that pass through the meninges. Hearing loss is a common result of meningitis. The primary test for meningitis is a lumbar puncture. A needle inserted into the lumbar region of the spinal column through the dura mater and arachnoid membrane into the subarachnoid space can be used to withdraw the fluid for chemical testing. Fatality occurs in 5 to 40 percent of children and 20 to 50 percent of adults with bacterial meningitis. Treatment of bacterial meningitis is through antibiotics, but viral meningitis cannot be treated with antibiotics because viruses do not respond to that type of drug. Fortunately, the viral forms are milder. Note: ees = openstax couse mite Watch this video that describes the procedure known as the lumbar puncture, a medical procedure used to sample the CSF. Because of the anatomy of the CNS, it is a relative safe location to insert a needle. Why is the lumbar puncture performed in the lower lumbar area of the vertebral column? The Ventricular System Cerebrospinal fluid (CSF) circulates throughout and around the CNS. In other tissues, water and small molecules are filtered through capillaries as the major contributor to the interstitial fluid. In the brain, CSF is produced in special structures to perfuse through the nervous tissue of the CNS and is continuous with the interstitial fluid. Specifically, CSF circulates to remove metabolic wastes from the interstitial fluids of nervous tissues and return them to the blood stream. The ventricles are the open spaces within the brain where CSF circulates. In some of these spaces, CSF is produced by filtering of the blood that is performed by a specialized membrane known as a choroid plexus. The CSF circulates through all of the ventricles to eventually emerge into the subarachnoid space where it will be reabsorbed into the blood. The Ventricles There are four ventricles within the brain, all of which developed from the original hollow space within the neural tube, the central canal. The first two are named the lateral ventricles and are deep within the cerebrum. These ventricles are connected to the third ventricle by two openings called the interventricular foramina. The third ventricle is the space between the left and right sides of the diencephalon, which opens into the cerebral aqueduct that passes through the midbrain. The aqueduct opens into the fourth ventricle, which is the space between the cerebellum and the pons and upper medulla ([link]). Cerebrospinal Fluid Circulation Superior sagittal sinus Arachnoid granulation Subarachnoid space Choroid plexus Meningeal dura mater Right lateral ventricle Interventricular foramen Third ventricle Cerebral aqueduct AN : Lateral aperture SS Median aperture Fourth ventricle Central canal The choroid plexus in the four ventricles produce CSF, which is circulated through the ventricular system and then enters the subarachnoid space through the median and lateral apertures. The CSF is then reabsorbed into the blood at the arachnoid granulations, where the arachnoid membrane emerges into the dural sinuses. As the telencephalon enlarges and grows into the cranial cavity, it is limited by the space within the skull. The telencephalon is the most anterior region of what was the neural tube, but cannot grow past the limit of the frontal bone of the skull. Because the cerebrum fits into this space, it takes on a C-shaped formation, through the frontal, parietal, occipital, and finally temporal regions. The space within the telencephalon is stretched into this same C- shape. The two ventricles are in the left and right sides, and were at one time referred to as the first and second ventricles. The interventricular foramina connect the frontal region of the lateral ventricles with the third ventricle. The third ventricle is the space bounded by the medial walls of the hypothalamus and thalamus. The two thalami touch in the center in most brains as the massa intermedia, which is surrounded by the third ventricle. The cerebral aqueduct opens just inferior to the epithalamus and passes through the midbrain. The tectum and tegmentum of the midbrain are the roof and floor of the cerebral aqueduct, respectively. The aqueduct opens up into the fourth ventricle. The floor of the fourth ventricle is the dorsal surface of the pons and upper medulla (that gray matter making a continuation of the tegmentum of the midbrain). The fourth ventricle then narrows into the central canal of the spinal cord. The ventricular system opens up to the subarachnoid space from the fourth ventricle. The single median aperture and the pair of lateral apertures connect to the subarachnoid space so that CSF can flow through the ventricles and around the outside of the CNS. Cerebrospinal fluid is produced within the ventricles by a type of specialized membrane called a choroid plexus. Ependymal cells (one of the types of glial cells described in the introduction to the nervous system) surround blood capillaries and filter the blood to make CSF. The fluid is a clear solution with a limited amount of the constituents of blood. It is essentially water, small molecules, and electrolytes. Oxygen and carbon dioxide are dissolved into the CSF, as they are in blood, and can diffuse between the fluid and the nervous tissue. Cerebrospinal Fluid Circulation The choroid plexuses are found in all four ventricles. Observed in dissection, they appear as soft, fuzzy structures that may still be pink, depending on how well the circulatory system is cleared in preparation of the tissue. The CSF is produced from components extracted from the blood, so its flow out of the ventricles is tied to the pulse of cardiovascular circulation. From the lateral ventricles, the CSF flows into the third ventricle, where more CSF is produced, and then through the cerebral aqueduct into the fourth ventricle where even more CSF is produced. A very small amount of CSF is filtered at any one of the plexuses, for a total of about 500 milliliters daily, but it is continuously made and pulses through the ventricular system, keeping the fluid moving. From the fourth ventricle, CSF can continue down the central canal of the spinal cord, but this is essentially a cul-de-sac, so more of the fluid leaves the ventricular system and moves into the subarachnoid space through the median and lateral apertures. Within the subarachnoid space, the CSF flows around all of the CNS, providing two important functions. As with elsewhere in its circulation, the CSF picks up metabolic wastes from the nervous tissue and moves it out of the CNS. It also acts as a liquid cushion for the brain and spinal cord. By surrounding the entire system in the subarachnoid space, it provides a thin buffer around the organs within the strong, protective dura mater. The arachnoid granulations are outpocketings of the arachnoid membrane into the dural sinuses so that CSF can be reabsorbed into the blood, along with the metabolic wastes. From the dural sinuses, blood drains out of the head and neck through the jugular veins, along with the rest of the circulation for blood, to be reoxygenated by the lungs and wastes to be filtered out by the kidneys ((link]). Note: Watch this animation that shows the flow of CSF through the brain and spinal cord, and how it originates from the ventricles and then spreads into the space within the meninges, where the fluids then move into the venous sinuses to return to the cardiovascular circulation. What are the structures that produce CSF and where are they found? How are the structures indicated in this animation? Components of CSF Circulation Lateral Third Cerebral Fourth Central Subarachnoid ventricles ventricle aqueduct ventricle canal space Between : pons/upper : Location : . 7 Spinal External to in CNS Cerebrum Diencephalon Midbrain aes cond entire CNS cerebellum Blood Choroid Choroid Choroid Arachnoid vessel None None : plexus plexus plexus granulations structure Note: Disorders of the... Central Nervous System The supply of blood to the brain is crucial to its ability to perform many functions. Without a steady supply of oxygen, and to a lesser extent glucose, the nervous tissue in the brain cannot keep up its extensive electrical activity. These nutrients get into the brain through the blood, and if blood flow is interrupted, neurological function is compromised. The common name for a disruption of blood supply to the brain is a stroke. It is caused by a blockage to an artery in the brain. The blockage is from some type of embolus: a blood clot, a fat embolus, or an air bubble. When the blood cannot travel through the artery, the surrounding tissue that is deprived starves and dies. Strokes will often result in the loss of very specific functions. A stroke in the lateral medulla, for example, can cause a loss in the ability to swallow. Sometimes, seemingly unrelated functions will be lost because they are dependent on structures in the same region. Along with the swallowing in the previous example, a stroke in that region could affect sensory functions from the face or extremities because important white matter pathways also pass through the lateral medulla. Loss of blood flow to specific regions of the cortex can lead to the loss of specific higher functions, from the ability to recognize faces to the ability to move a particular region of the body. Severe or limited memory loss can be the result of a temporal lobe stroke. Related to strokes are transient ischemic attacks (TIAs), which can also be called “mini-strokes.” These are events in which a physical blockage may be temporary, cutting off the blood supply and oxygen to a region, but not to the extent that it causes cell death in that region. While the neurons in that area are recovering from the event, neurological function may be lost. Function can return if the area is able to recover from the event. Recovery from a stroke (or TIA) is strongly dependent on the speed of treatment. Often, the person who is present and notices something is wrong must then make a decision. The mnemonic FAST helps people remember what to look for when someone is dealing with sudden losses of neurological function. If someone complains of feeling “funny,” check these things quickly: Look at the person’s face. Does he or she have problems moving Face muscles and making regular facial expressions? Ask the person to raise his or her Arms above the head. Can the person lift one arm but not the other? Has the person’s Speech changed? Is he or she slurring words or having trouble saying things? If any of these things have happened, then it is ‘Time to call for help. Sometimes, treatment with blood-thinning drugs can alleviate the problem, and recovery is possible. If the tissue is damaged, the amazing thing about the nervous system is that it is adaptable. With physical, occupational, and speech therapy, victims of strokes can recover, or more accurately relearn, functions. Chapter Review The CNS has a privileged blood supply established by the blood-brain barrier. Establishing this barrier are anatomical structures that help to protect and isolate the CNS. The arterial blood to the brain comes from the internal carotid and vertebral arteries, which both contribute to the unique circle of Willis that provides constant perfusion of the brain even if one of the blood vessels is blocked or narrowed. That blood is eventually filtered to make a separate medium, the CSF, that circulates within the spaces of the brain and then into the surrounding space defined by the meninges, the protective covering of the brain and spinal cord. The blood that nourishes the brain and spinal cord is behind the glial-cell-enforced blood-brain barrier, which limits the exchange of material from blood vessels with the interstitial fluid of the nervous tissue. Thus, metabolic wastes are collected in cerebrospinal fluid that circulates through the CNS. This fluid is produced by filtering blood at the choroid plexuses in the four ventricles of the brain. It then circulates through the ventricles and into the subarachnoid space, between the pia mater and the arachnoid mater. From the arachnoid granulations, CSF is reabsorbed into the blood, removing the waste from the privileged central nervous tissue. The blood, now with the reabsorbed CSF, drains out of the cranium through the dural sinuses. The dura mater is the tough outer covering of the CNS, which is anchored to the inner surface of the cranial and vertebral cavities. It surrounds the venous space known as the dural sinuses, which connect to the jugular veins, where blood drains from the head and neck. Interactive Link Questions Exercise: Problem: Watch this animation to see how blood flows to the brain and passes through the circle of Willis before being distributed through the cerebrum. The circle of Willis is a specialized arrangement of arteries that ensure constant perfusion of the cerebrum even in the event of a blockage of one of the arteries in the circle. The animation shows the normal direction of flow through the circle of Willis to the middle cerebral artery. Where would the blood come from if there were a blockage just posterior to the middle cerebral artery on the left? Solution: If blood could not get to the middle cerebral artery through the posterior circulation, the blood would flow around the circle of Willis to reach that artery from an anterior vessel. Blood flow would just reverse within the circle. Exercise: Problem: Watch this video that describes the procedure known as the lumbar puncture, a medical procedure used to sample the CSF. Because of the anatomy of the CNS, it is a relative safe location to insert a needle. Why is the lumbar puncture performed in the lower lumbar area of the vertebral column? Solution: The spinal cord ends in the upper lumbar area of the vertebral column, so a needle inserted lower than that will not damage the nervous tissue of the CNS. Exercise: Problem: Watch this animation that shows the flow of CSF through the brain and spinal cord, and how it originates from the ventricles and then spreads into the space within the meninges, where the fluids then move into the venous sinuses to return to the cardiovascular circulation. What are the structures that produce CSF and where are they found? How are the structures indicated in this animation? Solution: The choroid plexuses of the ventricles make CSF. As shown, there is a little of the blue color appearing in each ventricle that is joined by the color flowing from the other ventricles. Review Questions Exercise: Problem: What blood vessel enters the cranium to supply the brain with fresh, oxygenated blood? a. common carotid artery b. jugular vein c. internal carotid artery d. aorta Solution: C Exercise: Problem: Which layer of the meninges surrounds and supports the sinuses that form the route through which blood drains from the CNS? a. dura mater b. arachnoid mater c. subarachnoid d. pia mater Solution: A Exercise: Problem: What type of glial cell is responsible for filtering blood to produce CSF at the choroid plexus? a. ependymal cell b. astrocyte c. oligodendrocyte d. Schwann cell Solution: A Exercise: Problem: Which portion of the ventricular system is found within the diencephalon? a. lateral ventricles b. third ventricle c. cerebral aqueduct d. fourth ventricle Solution: B Exercise: Problem:What condition causes a stroke? a. inflammation of meninges b. lumbar puncture c. infection of cerebral spinal fluid d. disruption of blood to the brain Solution: D Critical Thinking Questions Exercise: Problem: Why can the circle of Willis maintain perfusion of the brain even if there is a blockage in one part of the structure? Solution: The structure is a circular connection of blood vessels, so that blood coming up from one of the arteries can flow in either direction around the circle and avoid any blockage or narrowing of the blood vessels. Exercise: Problem: Meningitis is an inflammation of the meninges that can have severe effects on neurological function. Why is infection of this structure potentially so dangerous? Solution: The nerves that connect the periphery to the CNS pass through these layers of tissue and can be damaged by that inflammation, causing a loss of important neurological functions. Glossary anterior spinal artery blood vessel from the merged branches of the vertebral arteries that runs along the anterior surface of the spinal cord arachnoid granulation outpocket of the arachnoid membrane into the dural sinuses that allows for reabsorption of CSF into the blood arachnoid mater middle layer of the meninges named for the spider-web-like trabeculae that extend between it and the pia mater arachnoid trabeculae filaments between the arachnoid and pia mater within the subarachnoid space basilar artery blood vessel from the merged vertebral arteries that runs along the dorsal surface of the brain stem carotid canal opening in the temporal bone through which the internal carotid artery enters the cranium central canal hollow space within the spinal cord that is the remnant of the center of the neural tube cerebral aqueduct connection of the ventricular system between the third and fourth ventricles located in the midbrain choroid plexus specialized structures containing ependymal cells lining blood capillaries that filter blood to produce CSF in the four ventricles of the brain circle of Willis unique anatomical arrangement of blood vessels around the base of the brain that maintains perfusion of blood into the brain even if one component of the structure is blocked or narrowed common carotid artery blood vessel that branches off the aorta (or the brachiocephalic artery on the right) and supplies blood to the head and neck dura mater tough, fibrous, outer layer of the meninges that is attached to the inner surface of the cranium and vertebral column and surrounds the entire CNS dural sinus any of the venous structures surrounding the brain, enclosed within the dura mater, which drain blood from the CNS to the common venous return of the jugular veins foramen magnum large opening in the occipital bone of the skull through which the spinal cord emerges and the vertebral arteries enter the cranium fourth ventricle the portion of the ventricular system that is in the region of the brain stem and opens into the subarachnoid space through the median and lateral apertures internal carotid artery branch from the common carotid artery that enters the cranium and supplies blood to the brain interventricular foramina openings between the lateral ventricles and third ventricle allowing for the passage of CSF jugular veins blood vessels that return “used” blood from the head and neck lateral apertures pair of openings from the fourth ventricle to the subarachnoid space on either side and between the medulla and cerebellum lateral ventricles portions of the ventricular system that are in the region of the cerebrum lumbar puncture procedure used to withdraw CSF from the lower lumbar region of the vertebral column that avoids the risk of damaging CNS tissue because the spinal cord ends at the upper lumbar vertebrae median aperture singular opening from the fourth ventricle into the subarachnoid space at the midline between the medulla and cerebellum meninges protective outer coverings of the CNS composed of connective tissue occipital sinuses dural sinuses along the edge of the occipital lobes of the cerebrum orthostatic reflex sympathetic function that maintains blood pressure when standing to offset the increased effect of gravity pia mater thin, innermost membrane of the meninges that directly covers the surface of the CNS sigmoid sinuses dural sinuses that drain directly into the jugular veins straight sinus dural sinus that drains blood from the deep center of the brain to collect with the other sinuses subarachnoid space space between the arachnoid mater and pia mater that contains CSF and the fibrous connections of the arachnoid trabeculae superior sagittal sinus dural sinus that runs along the top of the longitudinal fissure and drains blood from the majority of the outer cerebrum third ventricle portion of the ventricular system that is in the region of the diencephalon transverse sinuses dural sinuses that drain along either side of the occipital—-cerebellar space ventricles remnants of the hollow center of the neural tube that are spaces for cerebrospinal fluid to circulate through the brain vertebral arteries arteries that ascend along either side of the vertebral column through the transverse foramina of the cervical vertebrae and enter the cranium through the foramen magnum Nerves and ganglia By the end of this section, you will be able to: e Describe the structures found in the PNS e Distinguish between somatic and autonomic structures, including the special peripheral structures of the enteric nervous system e Name the twelve cranial nerves and explain the functions associated with each e Describe the sensory and motor components of spinal nerves and the plexuses that they pass through The PNS is not as contained as the CNS because it is defined as everything that is not the CNS. Some peripheral structures are incorporated into the other organs of the body. In describing the anatomy of the PNS, it is necessary to describe the common structures, the nerves and the ganglia, as they are found in various parts of the body. Many of the neural structures that are incorporated into other organs are features of the digestive system; these structures are known as the enteric nervous system and are a special subset of the PNS. Ganglia A ganglion is a group of neuron cell bodies in the periphery. Ganglia can be categorized, for the most part, as either sensory ganglia or autonomic ganglia, referring to their primary functions. The most common type of sensory ganglion is a dorsal (posterior) root ganglion. These ganglia are the cell bodies of neurons with axons that are sensory endings in the periphery, such as in the skin, and that extend into the CNS through the dorsal nerve root. The ganglion is an enlargement of the nerve root. Under microscopic inspection, it can be seen to include the cell bodies of the neurons, as well as bundles of fibers that are the posterior nerve root ([link]). The cells of the dorsal root ganglion are unipolar cells, classifying them by shape. Also, the small round nuclei of satellite cells can be seen surrounding—as if they were orbiting—the neuron cell bodies. Dorsal Root Ganglion Pas. na The cell bodies of sensory neurons, which are unipolar neurons by shape, are seen in this photomicrograph. Also, the fibrous region is composed of the axons of these neurons that are passing through the ganglion to be part of the dorsal nerve root (tissue source: canine). LM x 40. (Micrograph provided by the Regents of University of Michigan Medical School © 2012) Spinal Cord and Root Ganglion The slide includes both a cross-section of the lumbar spinal cord and a section of the dorsal root ganglion (see also [link]) (tissue source: canine). LM x 1600. (Micrograph provided by the Regents of University of Michigan Medical School © 2012) Note: a => openstax coLLece L = q ogy ae View the University of Michigan WebScope to explore the tissue sample in greater detail. If you zoom in on the dorsal root ganglion, you can see smaller satellite glial cells surrounding the large cell bodies of the sensory neurons. From what structure do satellite cells derive during embryologic development? Another type of sensory ganglion is a cranial nerve ganglion. This is analogous to the dorsal root ganglion, except that it is associated with a cranial nerve instead of a spinal nerve. The roots of cranial nerves are within the cranium, whereas the ganglia are outside the skull. For example, the trigeminal ganglion is superficial to the temporal bone whereas its associated nerve is attached to the mid-pons region of the brain stem. The neurons of cranial nerve ganglia are also unipolar in shape with associated satellite cells. The other major category of ganglia are those of the autonomic nervous system, which is divided into the sympathetic and parasympathetic nervous systems. The sympathetic chain ganglia constitute a row of ganglia along the vertebral column that receive central input from the lateral horn of the thoracic and upper lumbar spinal cord. Superior to the chain ganglia are three paravertebral ganglia in the cervical region. Three other autonomic ganglia that are related to the sympathetic chain are the prevertebral ganglia, which are located outside of the chain but have similar functions. They are referred to as prevertebral because they are anterior to the vertebral column. The neurons of these autonomic ganglia are multipolar in shape, with dendrites radiating out around the cell body where synapses from the spinal cord neurons are made. The neurons of the chain, paravertebral, and prevertebral ganglia then project to organs in the head and neck, thoracic, abdominal, and pelvic cavities to regulate the sympathetic aspect of homeostatic mechanisms. Another group of autonomic ganglia are the terminal ganglia that receive input from cranial nerves or sacral spinal nerves and are responsible for regulating the parasympathetic aspect of homeostatic mechanisms. These two sets of ganglia, sympathetic and parasympathetic, often project to the same organs—one input from the chain ganglia and one input from a terminal ganglion—to regulate the overall function of an organ. For example, the heart receives two inputs such as these; one increases heart rate, and the other decreases it. The terminal ganglia that receive input from cranial nerves are found in the head and neck, as well as the thoracic and upper abdominal cavities, whereas the terminal ganglia that receive sacral input are in the lower abdominal and pelvic cavities. Terminal ganglia below the head and neck are often incorporated into the wall of the target organ as a plexus. A plexus, in a general sense, is a network of fibers or vessels. This can apply to nervous tissue (as in this instance) or structures containing blood vessels (such as a choroid plexus). For example, the enteric plexus is the extensive network of axons and neurons in the wall of the small and large intestines. The enteric plexus is actually part of the enteric nervous system, along with the gastric plexuses and the esophageal plexus. Though the enteric nervous system receives input originating from central neurons of the autonomic nervous system, it does not require CNS input to function. In fact, it operates independently to regulate the digestive system. Nerves Bundles of axons in the PNS are referred to as nerves. These structures in the periphery are different than the central counterpart, called a tract. Nerves are composed of more than just nervous tissue. They have connective tissues invested in their structure, as well as blood vessels supplying the tissues with nourishment. The outer surface of a nerve is a surrounding layer of fibrous connective tissue called the epineurium. Within the nerve, axons are further bundled into fascicles, which are each surrounded by their own layer of fibrous connective tissue called perineurium. Finally, individual axons are surrounded by loose connective tissue called the endoneurium ({link]). These three layers are similar to the connective tissue sheaths for muscles. Nerves are associated with the region of the CNS to which they are connected, either as cranial nerves connected to the brain or spinal nerves connected to the spinal cord. Nerve Structure Spinal nerve Epineurium Perineurium Blood vessels Axion Perineurium Endoneurium Perineurium Fascicles (b) The structure of a nerve is organized by the layers of connective tissue on the outside, around each fascicle, and surrounding the individual nerve fibers (tissue source: simian). LM x 40. (Micrograph provided by the Regents of University of Michigan Medical School mana © 2U12) Close-Up of Nerve Trunk ; Wp asf SST Zoom in on this slide of a nerve trunk to examine the endoneurium, perineurium, and epineurium in greater detail (tissue source: simian). LM x 1600. (Micrograph provided by the Regents of University of Michigan Medical School © 2012) Note: — openstax coLLece* View the University of Michigan WebScope to explore the tissue sample in greater detail. With what structures in a skeletal muscle are the endoneurium, perineurium, and epineurium comparable? Cranial Nerves The nerves attached to the brain are the cranial nerves, which are primarily responsible for the sensory and motor functions of the head and neck (one of these nerves targets organs in the thoracic and abdominal cavities as part of the parasympathetic nervous system). There are twelve cranial nerves, which are designated CNI through CNXII for “Cranial Nerve,” using Roman numerals for 1 through 12. They can be classified as sensory nerves, motor nerves, or a combination of both, meaning that the axons in these nerves originate out of sensory ganglia external to the cranium or motor nuclei within the brain stem. Sensory axons enter the brain to synapse in a nucleus. Motor axons connect to skeletal muscles of the head or neck. Three of the nerves are solely composed of sensory fibers; five are strictly motor; and the remaining four are mixed nerves. Learning the cranial nerves is a tradition in anatomy courses, and students have always used mnemonic devices to remember the nerve names. A traditional mnemonic is the rhyming couplet, “On Old Olympus’ Towering Tops/A Finn And German Viewed Some Hops,” in which the initial letter of each word corresponds to the initial letter in the name of each nerve. The names of the nerves have changed over the years to reflect current usage and more accurate naming. An exercise to help learn this sort of information is to generate a mnemonic using words that have personal significance. The names of the cranial nerves are listed in [link] along with a brief description of their function, their source (sensory ganglion or motor nucleus), and their target (sensory nucleus or skeletal muscle). They are listed here with a brief explanation of each nerve ((Link]). The olfactory nerve and optic nerve are responsible for the sense of smell and vision, respectively. The oculomotor nerve is responsible for eye movements by controlling four of the extraocular muscles. It is also responsible for lifting the upper eyelid when the eyes point up, and for pupillary constriction. The trochlear nerve and the abducens nerve are both responsible for eye movement, but do so by controlling different extraocular muscles. The trigeminal nerve is responsible for cutaneous sensations of the face and controlling the muscles of mastication. The facial nerve is responsible for the muscles involved in facial expressions, as well as part of the sense of taste and the production of saliva. The vestibulocochlear nerve is responsible for the senses of hearing and balance. The glossopharyngeal nerve is responsible for controlling muscles in the oral cavity and upper throat, as well as part of the sense of taste and the production of saliva. The vagus nerve is responsible for contributing to homeostatic control of the organs of the thoracic and upper abdominal cavities. The spinal accessory nerve is responsible for controlling the muscles of the neck, along with cervical spinal nerves. The hypoglossal nerve is responsible for controlling the muscles of the lower throat and tongue. The Cranial Nerves Oculomotor nerve III Optic nerve II Trochlear IV nerve Trigeminal nerve V Abducens nerve VI . Facial nerve VII Vestibulocochlear nerve VIII Glossopharyngeal nerve IX Hypoglossal nerve XII Vagus nerve X The anatomical arrangement of the roots of the cranial nerves observed from an inferior view of the brain. Three of the cranial nerves also contain autonomic fibers, and a fourth is almost purely a component of the autonomic system. The oculomotor, facial, and glossopharyngeal nerves contain fibers that contact autonomic ganglia. The oculomotor fibers initiate pupillary constriction, whereas the facial and glossopharyngeal fibers both initiate salivation. The vagus nerve primarily targets autonomic ganglia in the thoracic and upper abdominal cavities. Note: CRS i] * => Snensae COLLEGE" Os Ba Visit this site to read about a man who wakes with a headache and a loss of vision. His regular doctor sent him to an ophthalmologist to address the vision loss. The ophthalmologist recognizes a greater problem and immediately sends him to the emergency room. Once there, the patient undergoes a large battery of tests, but a definite cause cannot be found. A specialist recognizes the problem as meningitis, but the question is what caused it originally. How can that be cured? The loss of vision comes from swelling around the optic nerve, which probably presented as a bulge on the inside of the eye. Why is swelling related to meningitis going to push on the optic nerve? Another important aspect of the cranial nerves that lends itself to a mnemonic is the functional role each nerve plays. The nerves fall into one of three basic groups. They are sensory, motor, or both (see [link]). The sentence, “Some Say Marry Money But My Brother Says Brains Beauty Matter More,” corresponds to the basic function of each nerve. The first, second, and eighth nerves are purely sensory: the olfactory (CNI), optic (CNII), and vestibulocochlear (CNVIID nerves. The three eye-movement nerves are all motor: the oculomotor (CNHI), trochlear (CNIV), and abducens (CNVI). The spinal accessory (CNXI) and hypoglossal (CNXII) nerves are also strictly motor. The remainder of the nerves contain both sensory and motor fibers. They are the trigeminal (CNV), facial (CNVID, glossopharyngeal (CNIX), and vagus (CNX) nerves. The nerves that convey both are often related to each other. The trigeminal and facial nerves both concern the face; one concerns the sensations and the other concems the muscle movements. The facial and glossopharyngeal nerves are both responsible for conveying gustatory, or taste, sensations as well as controlling salivary glands. The vagus nerve is involved in visceral responses to taste, namely the gag reflex. This is not an exhaustive list of what these combination nerves do, but there is a thread of relation between them. Cranial Nerves Function Mnemonic # Name (S/M/B) Central connection (nuclei) On I Olfactory Smell (S) Olfactory bulb Old II Optic Vision (S) Hypothalamus/thalamus/midbrain Eye movements Olympus’ Il Oculomotor (M) Oculomotor nucleus Towering IV Trochlear Re movements Trochlear nucleus . F Sensory/motor Trigeminal nuclei in the Ops ” oo — face (B) midbrain, pons, and medulla A VI Abducens oe pvcnese Abducens nucleus (M) Cranial Nerves Function Mnemonic # Name (S/M/B) Central connection (nuclei) Finn VII Facial Motor — face, Facial nucleus, solitary nucleus, Taste (B) superior salivatory nucleus aa VII Auditory Hearing/balance Cochlear nucleus, Vestibular (Vestibulocochlear) (S) nucleus/cerebellum Motor teat Solitary nucleus, inferior German Ix Glossopharyngeal salivatory nucleus, nucleus Taste (B) : ambiguus Motor/sensory — Viewed Xx Vagus viscera Medulla (autonomic) (B) . Motor — head : Some XI Spinal Accessory and neck (M) Spinal accessory nucleus Hops XI Hypoglossal Motor lower Hypoglossal nucleus throat (M) Spinal Nerves The nerves connected to the spinal cord are the spinal nerves. The arrangement of these nerves is much more regular than that of the cranial nerves. All of the spinal nerves are combined sensory and motor axons that separate into two nerve roots. The sensory axons enter the spinal cord as the dorsal nerve root. The motor fibers, both somatic and autonomic, emerge as the ventral nerve root. The dorsal root ganglion for each nerve is an enlargement of the spinal nerve. There are 31 spinal nerves, named for the level of the spinal cord at which each one emerges. There are eight pairs of cervical nerves designated C1 to C8, twelve thoracic nerves designated T1 to T12, five pairs of lumbar nerves designated L1 to LS, five pairs of sacral nerves designated S1 to S5, and one pair of coccygeal nerves. The nerves are numbered from the superior to inferior positions, and each emerges from the vertebral column through the intervertebral foramen at its level. The first nerve, C1, emerges between the first cervical vertebra and the occipital bone. The second nerve, C2, emerges between the first and second cervical vertebrae. The same occurs for C3 to C7, but C8 emerges between the seventh cervical vertebra and the first thoracic vertebra. For the thoracic and lumbar nerves, each one emerges between the vertebra that has the same designation and the next vertebra in the column. The sacral nerves emerge from the sacral foramina along the length of that unique vertebra. Spinal nerves extend outward from the vertebral column to enervate the periphery. The nerves in the periphery are not straight continuations of the spinal nerves, but rather the reorganization of the axons in those nerves to follow different courses. Axons from different spinal nerves will come together into a systemic nerve. This occurs at four places along the length of the vertebral column, each identified as a nerve plexus, whereas the other spinal nerves directly correspond to nerves at their respective levels. In this instance, the word plexus is used to describe networks of nerve fibers with no associated cell bodies. Of the four nerve plexuses, two are found at the cervical level, one at the lumbar level, and one at the sacral level ({link]). The cervical plexus is composed of axons from spinal nerves C1 through C5 and branches into nerves in the posterior neck and head, as well as the phrenic nerve, which connects to the diaphragm at the base of the thoracic cavity. The other plexus from the cervical level is the brachial plexus. Spinal nerves C4 through T1 reorganize through this plexus to give rise to the nerves of the arms, as the name brachial suggests. A large nerve from this plexus is the radial nerve from which the axillary nerve branches to go to the armpit region. The radial nerve continues through the arm and is paralleled by the ulnar nerve and the median nerve. The lumbar plexus arises from all the lumbar spinal nerves and gives rise to nerves enervating the pelvic region and the anterior leg. The femoral nerve is one of the major nerves from this plexus, which gives rise to the saphenous nerve as a branch that extends through the anterior lower leg. The sacral plexus comes from the lower lumbar nerves L4 and L5 and the sacral nerves S1 to S4. The most significant systemic nerve to come from this plexus is the sciatic nerve, which is a combination of the tibial nerve and the fibular nerve. The sciatic nerve extends across the hip joint and is most commonly associated with the condition sciatica, which is the result of compression or irritation of the nerve or any of the spinal nerves giving rise to it. These plexuses are described as arising from spinal nerves and giving rise to certain systemic nerves, but they contain fibers that serve sensory functions or fibers that serve motor functions. This means that some fibers extend from cutaneous or other peripheral sensory surfaces and send action potentials into the CNS. Those are axons of sensory neurons in the dorsal root ganglia that enter the spinal cord through the dorsal nerve root. Other fibers are the axons of motor neurons of the anterior horn of the spinal cord, which emerge in the ventral nerve root and send action potentials to cause skeletal muscles to contract in their target regions. For example, the radial nerve contains fibers of cutaneous sensation in the arm, as well as motor fibers that move muscles in the arm. Spinal nerves of the thoracic region, T2 through T11, are not part of the plexuses but rather emerge and give rise to the intercostal nerves found between the ribs, which articulate with the vertebrae surrounding the spinal nerve. Nerve Plexuses of the Body Cervical plexus Phrenic nerve Brachial plexus Axillary nerve Median nerve ———_— Radial nerve — b Ulnar nerve ; @) © a} Ce CTa/ ealgeeyrraee ey 1 f OQ Lumbar plexus Femoral nerve Obturator nerve Sacral plexus Common fibular nerve There are four main nerve plexuses in the human body. The cervical plexus supplies nerves to the posterior head and neck, as well as to the diaphragm. The brachial plexus supplies nerves to the arm. The lumbar plexus supplies nerves to the anterior leg. The sacral plexus supplies nerves to the posterior leg. Note: Aging and the... Nervous System Anosmia is the loss of the sense of smell. It is often the result of the olfactory nerve being severed, usually because of blunt force trauma to the head. The sensory neurons of the olfactory epithelium have a limited lifespan of approximately one to four months, and new ones are made on a regular basis. The new neurons extend their axons into the CNS by growing along the existing fibers of the olfactory nerve. The ability of these neurons to be replaced is lost with age. Age-related anosmia is not the result of impact trauma to the head, but rather a slow loss of the sensory neurons with no new neurons born to replace them. Smell is an important sense, especially for the enjoyment of food. There are only five tastes sensed by the tongue, and two of them are generally thought of as unpleasant tastes (sour and bitter). The rich sensory experience of food is the result of odor molecules associated with the food, both as food is moved into the mouth, and therefore passes under the nose, and when it is chewed and molecules are released to move up the pharynx into the posterior nasal cavity. Anosmia results in a loss of the enjoyment of food. As the replacement of olfactory neurons declines with age, anosmia can set in. Without the sense of smell, many sufferers complain of food tasting bland. Often, the only way to enjoy food is to add seasoning that can be sensed on the tongue, which usually means adding table salt. The problem with this solution, however, is that this increases sodium intake, which can lead to cardiovascular problems through water retention and the associated increase in blood pressure. Chapter Review The PNS is composed of the groups of neurons (ganglia) and bundles of axons (nerves) that are outside of the brain and spinal cord. Ganglia are of two types, sensory or autonomic. Sensory ganglia contain unipolar sensory neurons and are found on the dorsal root of all spinal nerves as well as associated with many of the cranial nerves. Autonomic ganglia are in the sympathetic chain, the associated paravertebral or prevertebral ganglia, or in terminal ganglia near or within the organs controlled by the autonomic nervous system. Nerves are classified as cranial nerves or spinal nerves on the basis of their connection to the brain or spinal cord, respectively. The twelve cranial nerves can be strictly sensory in function, strictly motor in function, or a combination of the two functions. Sensory fibers are axons of sensory ganglia that carry sensory information into the brain and target sensory nuclei. Motor fibers are axons of motor neurons in motor nuclei of the brain stem and target skeletal muscles of the head and neck. Spinal nerves are all mixed nerves with both sensory and motor fibers. Spinal nerves emerge from the spinal cord and reorganize through plexuses, which then give rise to systemic nerves. Thoracic spinal nerves are not part of any plexus, but give rise to the intercostal nerves directly. Interactive Link Questions Exercise: Problem: [link] If you zoom in on the DRG, you can see smaller satellite glial cells surrounding the large cell bodies of the sensory neurons. From what structure do satellite cells derive during embryologic development? Solution: [link] They derive from the neural crest. Exercise: Problem: [link] To what structures in a skeletal muscle are the endoneurium, perineurium, and epineurium comparable? Solution: [link] The endoneurium surrounding individual nerve fibers is comparable to the endomysium surrounding myofibrils, the perineurium bundling axons into fascicles is comparable to the perimysium bundling muscle fibers into fascicles, and the epineurium surrounding the whole nerve is comparable to the epimysium surrounding the muscle. Exercise: Problem: Visit this site to read about a man who wakes with a headache and a loss of vision. His regular doctor sent him to an ophthalmologist to address the vision loss. The ophthalmologist recognizes a greater problem and immediately sends him to the emergency room. Once there, the patient undergoes a large battery of tests, but a definite cause cannot be found. A specialist recognizes the problem as meningitis, but the question is what caused it originally. How can that be cured? The loss of vision comes from swelling around the optic nerve, which probably presented as a bulge on the inside of the eye. Why is swelling related to meningitis going to push on the optic nerve? Solution: The optic nerve enters the CNS in its projection from the eyes in the periphery, which means that it crosses through the meninges. Meningitis will include swelling of those protective layers of the CNS, resulting in pressure on the optic nerve, which can compromise vision. Review Questions Exercise: Problem: What type of ganglion contains neurons that control homeostatic mechanisms of the body? a. sensory ganglion b. dorsal root ganglion c. autonomic ganglion d. cranial nerve ganglion Solution: GC Exercise: Problem:Which ganglion is responsible for cutaneous sensations of the face? a. otic ganglion b. vestibular ganglion c. geniculate ganglion d. trigeminal ganglion Solution: D Exercise: Problem:What is the name for a bundle of axons within a nerve? a. fascicle b. tract c. nerve root d. epineurium Solution: A Exercise: Problem:Which cranial nerve does not control functions in the head and neck? a. olfactory b. trochlear c. glossopharyngeal d. vagus Solution: D Exercise: Problem: Which of these structures is not under direct control of the peripheral nervous system? a. trigeminal ganglion b. gastric plexus c. sympathetic chain ganglia d. cervical plexus Solution: B Critical Thinking Questions Exercise: Problem:Why are ganglia and nerves not surrounded by protective structures like the meninges of the CNS? Solution: The peripheral nervous tissues are out in the body, sometimes part of other organ systems. There is not a privileged blood supply like there is to the brain and spinal cord, so peripheral nervous tissues do not need the same sort of protections. Exercise: Problem: Testing for neurological function involves a series of tests of functions associated with the cranial nerves. What functions, and therefore which nerves, are being tested by asking a patient to follow the tip of a pen with their eyes? Solution: The contraction of extraocular muscles is being tested, which is the function of the oculomotor, trochlear, and abducens nerves. Glossary abducens nerve sixth cranial nerve; responsible for contraction of one of the extraocular muscles axillary nerve systemic nerve of the arm that arises from the brachial plexus brachial plexus nerve plexus associated with the lower cervical spinal nerves and first thoracic spinal nerve cervical plexus nerve plexus associated with the upper cervical spinal nerves cranial nerve one of twelve nerves connected to the brain that are responsible for sensory or motor functions of the head and neck cranial nerve ganglion sensory ganglion of cranial nerves dorsal (posterior) root ganglion sensory ganglion attached to the posterior nerve root of a spinal nerve endoneurium innermost layer of connective tissue that surrounds individual axons within a nerve enteric nervous system peripheral structures, namely ganglia and nerves, that are incorporated into the digestive system organs enteric plexus neuronal plexus in the wall of the intestines, which is part of the enteric nervous system epineurium outermost layer of connective tissue that surrounds an entire nerve esophageal plexus neuronal plexus in the wall of the esophagus that is part of the enteric nervous system extraocular muscles six skeletal muscles that control eye movement within the orbit facial nerve seventh cranial nerve; responsible for contraction of the facial muscles and for part of the sense of taste, as well as causing saliva production fascicle small bundles of nerve or muscle fibers enclosed by connective tissue femoral nerve systemic nerve of the anterior leg that arises from the lumbar plexus fibular nerve systemic nerve of the posterior leg that begins as part of the sciatic nerve gastric plexuses neuronal networks in the wall of the stomach that are part of the enteric nervous system glossopharyngeal nerve ninth cranial nerve; responsible for contraction of muscles in the tongue and throat and for part of the sense of taste, as well as causing saliva production hypoglossal nerve twelfth cranial nerve; responsible for contraction of muscles of the tongue intercostal nerve systemic nerve in the thoracic cavity that is found between two ribs lumbar plexus nerve plexus associated with the lumbar spinal nerves median nerve systemic nerve of the arm, located between the ulnar and radial nerves nerve plexus network of nerves without neuronal cell bodies included oculomotor nerve third cranial nerve; responsible for contraction of four of the extraocular muscles, the muscle in the upper eyelid, and pupillary constriction olfactory nerve first cranial nerve; responsible for the sense of smell optic nerve second cranial nerve; responsible for visual sensation paravertebral ganglia autonomic ganglia superior to the sympathetic chain ganglia perineurium layer of connective tissue surrounding fascicles within a nerve phrenic nerve systemic nerve from the cervical plexus that enervates the diaphragm plexus network of nerves or nervous tissue prevertebral ganglia autonomic ganglia that are anterior to the vertebral column and functionally related to the sympathetic chain ganglia radial nerve systemic nerve of the arm, the distal component of which is located near the radial bone sacral plexus nerve plexus associated with the lower lumbar and sacral spinal nerves saphenous nerve systemic nerve of the lower anterior leg that is a branch from the femoral nerve sciatic nerve systemic nerve from the sacral plexus that is a combination of the tibial and fibular nerves and extends across the hip joint and gluteal region into the upper posterior leg sciatica painful condition resulting from inflammation or compression of the sciatic nerve or any of the spinal nerves that contribute to it spinal accessory nerve eleventh cranial nerve; responsible for contraction of neck muscles spinal nerve one of 31 nerves connected to the spinal cord sympathetic chain ganglia autonomic ganglia in a chain along the anterolateral aspect of the vertebral column that are responsible for contributing to homeostatic mechanisms of the autonomic nervous system systemic nerve nerve in the periphery distal to a nerve plexus or spinal nerve terminal ganglion autonomic ganglia that are near or within the walls of organs that are responsible for contributing to homeostatic mechanisms of the autonomic nervous system tibial nerve systemic nerve of the posterior leg that begins as part of the sciatic nerve trigeminal ganglion sensory ganglion that contributes sensory fibers to the trigeminal nerve trigeminal nerve fifth cranial nerve; responsible for cutaneous sensation of the face and contraction of the muscles of mastication trochlear nerve fourth cranial nerve; responsible for contraction of one of the extraocular muscles ulnar nerve systemic nerve of the arm located close to the ulna, a bone of the forearm vagus nerve tenth cranial nerve; responsible for the autonomic control of organs in the thoracic and upper abdominal cavities vestibulocochlear nerve eighth cranial nerve; responsible for the sensations of hearing and balance Sensory Perception By the end of this section, you will be able to: e Describe different types of sensory receptors e Describe the structures responsible for the special senses of taste, smell, hearing, balance, and vision e Distinguish how different tastes are transduced e Describe the means of mechanoreception for hearing and balance e List the supporting structures around the eye and describe the structure of the eyeball e Describe the processes of phototransduction A major role of sensory receptors is to help us learn about the environment around us, or about the state of our internal environment. Stimuli from varying sources, and of different types, are received and changed into the electrochemical signals of the nervous system. This occurs when a stimulus changes the cell membrane potential of a sensory neuron. The stimulus causes the sensory cell to produce an action potential that is relayed into the central nervous system (CNS), where it is integrated with other sensory information— or sometimes higher cognitive functions—to become a conscious perception of that stimulus. The central integration may then lead to a motor response. Describing sensory function with the term sensation or perception is a deliberate distinction. Sensation is the activation of sensory receptor cells at the level of the stimulus. Perception is the central processing of sensory stimuli into a meaningful pattern. Perception is dependent on sensation, but not all sensations are perceived. Receptors are the cells or structures that detect sensations. A receptor cell is changed directly by a stimulus. A transmembrane protein receptor is a protein in the cell membrane that mediates a physiological change in a neuron, most often through the opening of ion channels or changes in the cell signaling processes. Transmembrane receptors are activated by chemicals called ligands. For example, a molecule in food can serve as a ligand for taste receptors. Other transmembrane proteins, which are not accurately called receptors, are sensitive to mechanical or thermal changes. Physical changes in these proteins increase ion flow across the membrane, and can generate an action potential or a graded potential in the sensory neurons. Sensory Receptors Stimuli in the environment activate specialized receptor cells in the peripheral nervous system. Different types of stimuli are sensed by different types of receptor cells. Receptor cells can be classified into types on the basis of three different criteria: cell type, position, and function. Receptors can be classified structurally on the basis of cell type and their position in relation to stimuli they sense. They can also be classified functionally on the basis of the transduction of stimuli, or how the mechanical stimulus, light, or chemical changed the cell membrane potential. Structural Receptor Types The cells that interpret information about the environment can be either (1) a neuron that has a free nerve ending, with dendrites embedded in tissue that would receive a sensation; (2) a neuron that has an encapsulated ending in which the sensory nerve endings are encapsulated in connective tissue that enhances their sensitivity; or (3) a specialized receptor cell, which has distinct structural components that interpret a specific type of stimulus ((link]). The pain and temperature receptors in the dermis of the skin are examples of neurons that have free nerve endings. Also located in the dermis of the skin are lamellated corpuscles, neurons with encapsulated nerve endings that respond to pressure and touch. The cells in the retina that respond to light stimuli are an example of a specialized receptor, a photoreceptor. Receptor Classification by Cell Type Free nerve endings (dendrites) Axon Cell body L esaseaacats a a a a (a) Neuron (receptor) with free nerve endings Dendrite Axon Dee keeeedreernrsts (b) Neuron (receptor) with encapsulated nerve endings Encapsulated nerve ending Bipolar cell (c) Specialized receptor cell Receptor cell types can be classified on the basis of their structure. Sensory neurons can have either (a) free nerve endings or (b) encapsulated endings. Photoreceptors in the eyes, such as rod cells, are examples of (c) specialized receptor cells. These cells release neurotransmitters onto a bipolar cell, which then synapses with the optic nerve neurons. Another way that receptors can be classified is based on their location relative to the stimuli. An exteroceptor is a receptor that is located near a stimulus in the external environment, such as the somatosensory receptors that are located in the skin. An interoceptor is one that interprets stimuli from internal organs and tissues, such as the receptors that sense the increase in blood pressure in the aorta or carotid sinus. Finally, a proprioceptor is a receptor located near a moving part of the body, such as a muscle, that interprets the positions of the tissues as they move. Functional Receptor Types A third classification of receptors is by how the receptor transduces stimuli into membrane potential changes. Stimuli are of three general types. Some stimuli are ions and macromolecules that affect transmembrane receptor proteins when these chemicals diffuse across the cell membrane. Some stimuli are physical variations in the environment that affect receptor cell membrane potentials. Other stimuli include the electromagnetic radiation from visible light. For humans, the only electromagnetic energy that is perceived by our eyes is visible light. Some other organisms have receptors that humans lack, such as the heat sensors of snakes, the ultraviolet light sensors of bees, or magnetic receptors in migratory birds. Receptor cells can be further categorized on the basis of the type of stimuli they transduce. Chemical stimuli can be interpreted by a chemoreceptor that interprets chemical stimuli, such as an object’s taste or smell. Osmoreceptors respond to solute concentrations of body fluids. Additionally, pain is primarily a chemical sense that interprets the presence of chemicals from tissue damage, or similar intense stimuli, through a nociceptor. Physical stimuli, such as pressure and vibration, as well as the sensation of sound and body position (balance), are interpreted through a mechanoreceptor. Another physical stimulus that has its own type of receptor is temperature, which is sensed through a thermoreceptor that is either sensitive to temperatures above (heat) or below (cold) normal body temperature. Sensory Modalities Ask anyone what the senses are, and they are likely to list the five major senses —taste, smell, touch, hearing, and sight. However, these are not all of the senses. The most obvious omission from this list is balance. Also, what is referred to simply as touch can be further subdivided into pressure, vibration, stretch, and hair-follicle position, on the basis of the type of mechanoreceptors that perceive these touch sensations. Other overlooked senses include temperature perception by thermoreceptors and pain perception by nociceptors. Within the realm of physiology, senses can be classified as either general or specific. A general sense is one that is distributed throughout the body and has receptor cells within the structures of other organs. Mechanoreceptors in the skin, muscles, or the walls of blood vessels are examples of this type. General senses often contribute to the sense of touch, as described above, or to proprioception (body movement) and kinesthesia (body movement), or to a visceral sense, which is most important to autonomic functions. A special sense is one that has a specific organ devoted to it, namely the eye, inner ear, tongue, or nose. Each of the senses is referred to as a sensory modality. Modality refers to the way that information is encoded, which is similar to the idea of transduction. The main sensory modalities can be described on the basis of how each is transduced. The chemical senses are taste and smell. The general sense that is usually referred to as touch includes chemical sensation in the form of nociception, or pain. Pressure, vibration, muscle stretch, and the movement of hair by an external stimulus, are all sensed by mechanoreceptors. Hearing and balance are also sensed by mechanoreceptors. Finally, vision involves the activation of photoreceptors. Listing all the different sensory modalities, which can number as many as 17, involves separating the five major senses into more specific categories, or submodalities, of the larger sense. An individual sensory modality represents the sensation of a specific type of stimulus. For example, the general sense of touch, which is known as somatosensation, can be separated into light pressure, deep pressure, vibration, itch, pain, temperature, or hair movement. Gustation (Taste) Only a few recognized submodalities exist within the sense of taste, or gustation. Until recently, only four tastes were recognized: sweet, salty, sour, and bitter. Research at the turn of the 20th century led to recognition of the fifth taste, umami, during the mid-1980s. Umamii is a Japanese word that means “delicious taste,” and is often translated to mean savory. Very recent research has suggested that there may also be a sixth taste for fats, or lipids. Gustation is the special sense associated with the tongue. The surface of the tongue, along with the rest of the oral cavity, is lined by a stratified squamous epithelium. Raised bumps called papillae (singular = papilla) contain the structures for gustatory transduction. There are four types of papillae, based on their appearance ([link]): circumvallate, foliate, filiform, and fungiform. Within the structure of the papillae are taste buds that contain specialized gustatory receptor cells for the transduction of taste stimuli. These receptor cells are sensitive to the chemicals contained within foods that are ingested, and they release neurotransmitters based on the amount of the chemical in the food. Neurotransmitters from the gustatory cells can activate sensory neurons in the facial, glossopharyngeal, and vagus cranial nerves. The Tongue Taste buds Circumvallate papilla Taste hairs Taste pore Fungiform papilla Filiform papilla Foliate papilla Basal cell Transitional cell Gustatory cell The tongue is covered with small bumps, called papillae, which contain taste buds that are sensitive to chemicals in ingested food or drink. Different types of papillae are found in different regions of the tongue. The taste buds contain specialized gustatory receptor cells that respond to chemical stimuli dissolved in the saliva. These receptor cells activate sensory neurons that are part of the facial and glossopharyngeal nerves. LM x 1600. (Micrograph provided by the Regents of University of Michigan Medical School © 2012) Salty taste is simply the perception of sodium ions (Na’) in the saliva. When you eat something salty, the salt crystals dissociate into the component ions Na’ and CI, which dissolve into the saliva in your mouth. The Na* concentration becomes high outside the gustatory cells, creating a strong concentration gradient that drives the diffusion of the ion into the cells. The entry of Na* into these cells results in the depolarization of the cell membrane and the generation of a receptor potential. Sour taste is the perception of H* concentration. Just as with sodium ions in salty flavors, these hydrogen ions enter the cell and trigger depolarization. Sour flavors are, essentially, the perception of acids in our food. Increasing hydrogen ion concentrations in the saliva (lowering saliva pH) triggers progressively stronger graded potentials in the gustatory cells. For example, orange juice—which contains citric acid—will taste sour because it has a pH value of approximately 3. Of course, it is often sweetened so that the sour taste is masked. The first two tastes (salty and sour) are triggered by the cations Na” and H’. The other tastes result from food molecules binding to a G protein-coupled receptor. A G protein signal transduction system ultimately leads to depolarization of the gustatory cell. The sweet taste is the sensitivity of gustatory cells to the presence of glucose dissolved in the saliva. Other monosaccharides such as fructose, or artificial sweeteners such as aspartame (NutraSweet™), saccharine, or sucralose (Splenda™) also activate the sweet receptors. The affinity for each of these molecules varies, and some will taste sweeter than glucose because they bind to the G protein-coupled receptor differently. Bitter taste is similar to sweet in that food molecules bind to G protein—coupled receptors. However, there are a number of different ways in which this can happen because there are a large diversity of bitter-tasting molecules. Some bitter molecules depolarize gustatory cells, whereas others hyperpolarize gustatory cells. Likewise, some bitter molecules increase G protein activation within the gustatory cells, whereas other bitter molecules decrease G protein activation. The specific response depends on which molecule is binding to the receptor. One major group of bitter-tasting molecules are alkaloids. Alkaloids are nitrogen containing molecules that are commonly found in bitter-tasting plant products, such as coffee, hops (in beer), tannins (in wine), tea, and aspirin. By containing toxic alkaloids, the plant is less susceptible to microbe infection and less attractive to herbivores. Therefore, the function of bitter taste may primarily be related to stimulating the gag reflex to avoid ingesting poisons. Because of this, many bitter foods that are normally ingested are often combined with a sweet component to make them more palatable (cream and sugar in coffee, for example). The highest concentration of bitter receptors appear to be in the posterior tongue, where a gag reflex could still spit out poisonous food. The taste known as umami is often referred to as the savory taste. Like sweet and bitter, it is based on the activation of G protein-coupled receptors by a specific molecule. The molecule that activates this receptor is the amino acid L-glutamate. Therefore, the umami flavor is often perceived while eating protein-rich foods. Not surprisingly, dishes that contain meat are often described as savory. Once the gustatory cells are activated by the taste molecules, they release neurotransmitters onto the dendrites of sensory neurons. These neurons are part of the facial and glossopharyngeal cranial nerves, as well as a component within the vagus nerve dedicated to the gag reflex. The facial nerve connects to taste buds in the anterior third of the tongue. The glossopharyngeal nerve connects to taste buds in the posterior two thirds of the tongue. The vagus nerve connects to taste buds in the extreme posterior of the tongue, verging on the pharynx, which are more sensitive to noxious stimuli such as bitterness. Note: openstax COLLEGE 3 Watch this video to learn about Dr. Danielle Reed of the Monell Chemical Senses Center in Philadelphia, Pennsylvania, who became interested in science at an early age because of her sensory experiences. She recognized that her sense of taste was unique compared with other people she knew. Now, she studies the genetic differences between people and their sensitivities to taste stimuli. In the video, there is a brief image of a person sticking out their tongue, which has been covered with a colored dye. This is how Dr. Reed is able to visualize and count papillae on the surface of the tongue. People fall into two groups known as “tasters” and “non-tasters” based on the density of papillae on their tongue, which also indicates the number of taste buds. Non- tasters can taste food, but they are not as sensitive to certain tastes, such as bitterness. Dr. Reed discovered that she is a non-taster, which explains why she perceived bitterness differently than other people she knew. Are you very sensitive to tastes? Can you see any similarities among the members of your family? Olfaction (Smell) Like taste, the sense of smell, or olfaction, is also responsive to chemical stimuli. The olfactory receptor neurons are located in a small region within the superior nasal cavity ({link]). This region is referred to as the olfactory epithelium and contains bipolar sensory neurons. Each olfactory sensory neuron has dendrites that extend from the apical surface of the epithelium into the mucus lining the cavity. As airborne molecules are inhaled through the nose, they pass over the olfactory epithelial region and dissolve into the mucus. These odorant molecules bind to proteins that keep them dissolved in the mucus and help transport them to the olfactory dendrites. The odorant—protein complex binds to a receptor protein within the cell membrane of an olfactory dendrite. These receptors are G protein—coupled, and will produce a graded membrane potential in the olfactory neurons. The axon of an olfactory neuron extends from the basal surface of the epithelium, through an olfactory foramen in the cribriform plate of the ethmoid bone, and into the brain. The group of axons called the olfactory tract connect to the olfactory bulb on the ventral surface of the frontal lobe. From there, the axons split to travel to several brain regions. Some travel to the cerebrum, specifically to the primary olfactory cortex that is located in the inferior and medial areas of the temporal lobe. Others project to structures within the limbic system and hypothalamus, where smells become associated with long-term memory and emotional responses. This is how certain smells trigger emotional memories, such as the smell of food associated with one’s birthplace. Smell is the one sensory modality that does not synapse in the thalamus before connecting to the cerebral cortex. This intimate connection between the olfactory system and the cerebral cortex is one reason why smell can be a potent trigger of memories and emotion. The nasal epithelium, including the olfactory cells, can be harmed by airborne toxic chemicals. Therefore, the olfactory neurons are regularly replaced within the nasal epithelium, after which the axons of the new neurons must find their appropriate connections in the olfactory bulb. These new axons grow along the axons that are already in place in the cranial nerve. The Olfactory System Olfactory tract Olfactory bulb Olfactory tract Mitral cells Olfactory Olfactory neurons epithelium Nasalconchae = : Ethmoid bone Path of inhaled air Filaments of olfactory nerve Connective tissue Olfactory gland Olfactory receptor Dendrite Olfactory cilia Mucus Path of inhaled air containing odorant molecules (a) Nasal cavity (b) Olfactory system (c) Olfactory epithelium (a) The olfactory system begins in the peripheral structures of the nasal cavity. (b) The olfactory receptor neurons are within the olfactory epithelium. (c) Axons of the olfactory receptor neurons project through the cribriform plate of the ethmoid bone and synapse with the neurons of the olfactory bulb (tissue source: simian). LM x 812. (Micrograph provided by the Regents of University of Michigan Medical School © 2012) Note: Disorders of the... Olfactory System: Anosmia Blunt force trauma to the face, such as that common in many car accidents, can lead to the loss of the olfactory nerve, and subsequently, loss of the sense of smell. This condition is known as anosmia. When the frontal lobe of the brain moves relative to the ethmoid bone, the olfactory tract axons may be sheared apart. Professional fighters often experience anosmia because of repeated trauma to face and head. In addition, certain pharmaceuticals, such as antibiotics, can cause anosmia by killing all the olfactory neurons at once. If no axons are in place within the olfactory nerve, then the axons from newly formed olfactory neurons have no guide to lead them to their connections within the olfactory bulb. There are temporary causes of anosmia, as well, such as those caused by inflammatory responses related to respiratory infections or allergies. Loss of the sense of smell can result in food tasting bland. A person with an impaired sense of smell may require additional spice and seasoning levels for food to be tasted. Anosmia may also be related to some presentations of mild depression, because the loss of enjoyment of food may lead to a general sense of despair. The ability of olfactory neurons to replace themselves decreases with age, leading to age-related anosmia. This explains why some elderly people salt their food more than younger people do. However, this increased sodium intake can increase blood volume and blood pressure, increasing the risk of cardiovascular diseases in the elderly. Audition (Hearing) Hearing, or audition, is the transduction of sound waves into a neural signal that is made possible by the structures of the ear ([link]). The large, fleshy structure on the lateral aspect of the head is known as the auricle. Some sources will also refer to this structure as the pinna, though that term is more appropriate for a structure that can be moved, such as the external ear of a cat. The C-shaped curves of the auricle direct sound waves toward the auditory canal. The canal enters the skull through the external auditory meatus of the temporal bone. At the end of the auditory canal is the tympanic membrane, or ear drum, which vibrates after it is struck by sound waves. The auricle, ear canal, and tympanic membrane are often referred to as the external ear. The middle ear consists of a space spanned by three small bones called the ossicles. The three ossicles are the malleus, incus, and stapes, which are Latin names that roughly translate to hammer, anvil, and stirrup. The malleus is attached to the tympanic membrane and articulates with the incus. The incus, in turn, articulates with the stapes. The stapes is then attached to the inner ear, where the sound waves will be transduced into a neural signal. The middle ear is connected to the pharynx through the Eustachian tube, which helps equilibrate air pressure across the tympanic membrane. The tube is normally closed but will pop open when the muscles of the pharynx contract during swallowing or yawning. Structures of the Ear Malleus —_Incus Stapes (attached to oval window) Vestibule Auricle Vestibular nerve Cochlear nerve Round window Ear canal Cochlea : Eustachian tube Tympanic membrane cavity External ear Middle ear Inner ear Tympanic The external ear contains the auricle, ear canal, and tympanic membrane. The middle ear contains the ossicles and is connected to the pharynx by the Eustachian tube. The inner ear contains the cochlea and vestibule, which are responsible for audition and equilibrium, respectively. The inner ear is often described as a bony labyrinth, as it is composed of a series of canals embedded within the temporal bone. It has two separate regions, the cochlea and the vestibule, which are responsible for hearing and balance, respectively. The neural signals from these two regions are relayed to the brain stem through separate fiber bundles. However, these two distinct bundles travel together from the inner ear to the brain stem as the vestibulocochlear nerve. Sound is transduced into neural signals within the cochlear region of the inner ear, which contains the sensory neurons of the spiral ganglia. These ganglia are located within the spiral-shaped cochlea of the inner ear. The cochlea is attached to the stapes through the oval window. The oval window is located at the beginning of a fluid-filled tube within the cochlea called the scala vestibuli. The scala vestibuli extends from the oval window, travelling above the cochlear duct, which is the central cavity of the cochlea that contains the sound-transducing neurons. At the uppermost tip of the cochlea, the scala vestibuli curves over the top of the cochlear duct. The fluid-filled tube, now called the scala tympani, returns to the base of the cochlea, this time travelling under the cochlear duct. The scala tympani ends at the round window, which is covered by a membrane that contains the fluid within the scala. As vibrations of the ossicles travel through the oval window, the fluid of the scala vestibuli and scala tympani moves in a wave-like motion. The frequency of the fluid waves match the frequencies of the sound waves ({link]). The membrane covering the round window will bulge out or pucker in with the movement of the fluid within the scala tympani. Transmission of Sound Waves to Cochlea @ Tympanic membrane _—_@) Vibrations are vibrates in response amplified across to sound wave. ossicles. (0) Sound wave represents alternating areas of high and low pressure. 139d) } \ \\ | | Wavelength wy , Ne Z oe Frequency of sound wave : , , - ; measured in Hz (cycles ® Vibrations against oval window set up standing per second) wave in fluid of vestibuli. Scala ’ vestibuli ima ; Cochlear nf duct Organ of Corti Basilar membrane Scala tympani © Pressure bends the membrane of the cochlear duct at a point of maximum WENEVAVAVAAVE vibration for a given frequency, causing hair cells in the basilar membrane to Frequency of standing vibrate. wave is the same as sound wave A sound wave causes the tympanic membrane to vibrate. This vibration is amplified as it moves across the malleus, incus, and stapes. The amplified vibration is picked up by the oval window causing pressure waves in the fluid of the scala vestibuli and scala tympani. The complexity of the pressure waves is determined by the changes in amplitude and frequency of the sound waves entering the ear. A cross-sectional view of the cochlea shows that the scala vestibuli and scala tympani run along both sides of the cochlear duct ([link]). The cochlear duct contains several organs of Corti, which tranduce the wave motion of the two scala into neural signals. The organs of Corti lie on top of the basilar membrane, which is the side of the cochlear duct located between the organs of Corti and the scala tympani. As the fluid waves move through the scala vestibuli and scala tympani, the basilar membrane moves at a specific spot, depending on the frequency of the waves. Higher frequency waves move the region of the basilar membrane that is close to the base of the cochlea. Lower frequency waves move the region of the basilar membrane that is near the tip of the cochlea. Cross Section of the Cochlea Bony cochlear wall Scala vestibuli Cochlear duct Tectorial membrane Basilar membrane Scala tympani — ———— Cochlear branch of N VIII The three major spaces within the cochlea are highlighted. The scala tympani and scala vestibuli lie on either side of the cochlear duct. The organ of Corti, containing the mechanoreceptor hair cells, is adjacent to the scala tympani, where it sits atop the basilar membrane. The organs of Corti contain hair cells, which are named for the hair-like stereocilia extending from the cell’s apical surfaces ([{link]). The stereocilia are an array of microvilli-like structures arranged from tallest to shortest. Protein fibers tether adjacent hairs together within each array, such that the array will bend in response to movements of the basilar membrane. The stereocilia extend up from the hair cells to the overlying tectorial membrane, which is attached medially to the organ of Corti. When the pressure waves from the scala move the basilar membrane, the tectorial membrane slides across the stereocilia. This bends the stereocilia either toward or away from the tallest member of each array. When the stereocilia bend toward the tallest member of their array, tension in the protein tethers opens ion channels in the hair cell membrane. This will depolarize the hair cell membrane, triggering nerve impulses that travel down the afferent nerve fibers attached to the hair cells. When the stereocilia bend toward the shortest member of their array, the tension on the tethers slackens and the ion channels close. When no sound is present, and the stereocilia are standing straight, a small amount of tension still exists on the tethers, keeping the membrane potential of the hair cell slightly depolarized. Hair Cell ge tia Tether Stereocilia Hair cell The hair cell is a mechanoreceptor with an array of stereocilia emerging from its apical surface. The stereocilia are tethered together by proteins that open ion channels when the array is bent toward the tallest member of their array, and closed when the array is bent toward the shortest member of their array. Cochlea and Organ of Corti LM x 412. (Micrograph provided by the Regents of University of Michigan Medical School © 2012) Note: ae — openstax COLLEGE View the University of Michigan WebScope to explore the tissue sample in greater detail. The basilar membrane is the thin membrane that extends from the central core of the cochlea to the edge. What is anchored to this membrane so that they can be activated by movement of the fluids within the cochlea? As stated above, a given region of the basilar membrane will only move if the incoming sound is at a specific frequency. Because the tectorial membrane only moves where the basilar membrane moves, the hair cells in this region will also only respond to sounds of this specific frequency. Therefore, as the frequency of a sound changes, different hair cells are activated all along the basilar membrane. The cochlea encodes auditory stimuli for frequencies between 20 and 20,000 Hz, which is the range of sound that human ears can detect. The unit of Hertz measures the frequency of sound waves in terms of cycles produced per second. Frequencies as low as 20 Hz are detected by hair cells at the apex, or tip, of the cochlea. Frequencies in the higher ranges of 20 KHz are encoded by hair cells at the base of the cochlea, close to the round and oval windows ((link]). Most auditory stimuli contain a mixture of sounds at a variety of frequencies and intensities (represented by the amplitude of the sound wave). The hair cells along the length of the cochlear duct, which are each sensitive to a particular frequency, allow the cochlea to separate auditory stimuli by frequency, just as a prism separates visible light into its component colors. Frequency Coding in the Cochlea Oval window base Tectorial membrane 20,000 Hz 1500 Hz 20 Hz (high frequency) = (medium frequency) (low frequency) _— Relative length of fibers in basilar membrane Basilar membrane Round window The standing sound wave generated in the cochlea by the movement of the oval window deflects the basilar membrane on the basis of the frequency of sound. Therefore, hair cells at the base of the cochlea are activated only by high frequencies, whereas those at the apex of the cochlea are activated only by low frequencies. Note: cerae pene Ea) _ openstax COLLEGE” . 7, 1 oe rat Watch this video to learn more about how the structures of the ear convert sound waves into a neural signal by moving the “hairs,” or stereocilia, of the cochlear duct. Specific locations along the length of the duct encode specific frequencies, or pitches. The brain interprets the meaning of the sounds we hear as music, speech, noise, etc. Which ear structures are responsible for the amplification and transfer of sound from the external ear to the inner ear? Note: mms Openstax COLLEGE | Te on Fa tae Watch this animation to learn more about the inner ear and to see the cochlea unroll, with the base at the back of the image and the apex at the front. Specific wavelengths of sound cause specific regions of the basilar membrane to vibrate, much like the keys of a piano produce sound at different frequencies. Based on the animation, where do frequencies—from high to low pitches—cause activity in the hair cells within the cochlear duct? Equilibrium (Balance) Along with audition, the inner ear is responsible for encoding information about equilibrium, the sense of balance. A similar mechanoreceptor—a hair cell with stereocilia—senses head position, head movement, and whether our bodies are in motion. These cells are located within the vestibule of the inner ear. Head position is sensed by the utricle and saccule, whereas head movement is sensed by the semicircular canals. The neural signals generated in the vestibular ganglion are transmitted through the vestibulocochlear nerve to the brain stem and cerebellum. The utricle and saccule are both largely composed of macula tissue (plural = maculae). The macula is composed of hair cells surrounded by support cells. The stereocilia of the hair cells extend into a viscous gel called the otolithic membrane ((link]). On top of the otolithic membrane is a layer of calcium carbonate crystals, called otoliths. The otoliths essentially make the otolithic membrane top-heavy. The otolithic membrane moves separately from the macula in response to head movements. Tilting the head causes the otolithic membrane to slide over the macula in the direction of gravity. The moving otolithic membrane, in turn, bends the sterocilia, causing some hair cells to depolarize as others hyperpolarize. The exact position of the head is interpreted by the brain based on the pattern of hair-cell depolarization. Linear Acceleration Coding by Maculae Otolithic Otoliths membrane i 2 a Teh = 2-5 Head upright Endolymph Macula Otoliths Otolithic membrane Hair cells Vestibular division of ‘ - vestibulocochlear nerve Head tilted forward aC \ The maculae are specialized for sensing linear acceleration, such as when gravity acts on the tilting head, or if the head starts moving in a straight line. The difference in inertia between the hair cell stereocilia and the otolithic membrane in which they are embedded leads to a shearing force that causes the stereocilia to bend in the direction of that linear acceleration. The semicircular canals are three ring-like extensions of the vestibule. One is oriented in the horizontal plane, whereas the other two are oriented in the vertical plane. The anterior and posterior vertical canals are oriented at approximately 45 degrees relative to the sagittal plane ([link]). The base of each semicircular canal, where it meets with the vestibule, connects to an enlarged region known as the ampulla. The ampulla contains the hair cells that respond to rotational movement, such as turning the head while saying “no.” The stereocilia of these hair cells extend into the cupula, a membrane that attaches to the top of the ampulla. As the head rotates in a plane parallel to the semicircular canal, the fluid lags, deflecting the cupula in the direction opposite to the head movement. The semicircular canals contain several ampullae, with some oriented horizontally and others oriented vertically. By comparing the relative movements of both the horizontal and vertical ampullae, the vestibular system can detect the direction of most head movements within three-dimensional (3-D) space. Rotational Coding by Semicircular Canals h As the head rotates, Ampullary nerve \\\\\) cupula bends in opposite direction of the rotation Rotational movement of the head is encoded by the hair cells in the base of the semicircular canals. As one of the canals moves in an arc with the head, the internal fluid moves in the opposite direction, causing the cupula and stereocilia to bend. The movement of two canals within a plane results in information about the direction in which the head is moving, and activation of all six canals can give a very precise indication of head movement in three dimensions. Somatosensation (Touch) Somatosensation is considered a general sense, as opposed to the special senses discussed in this section. Somatosensation is the group of sensory modalities that are associated with touch, proprioception, and interoception. These modalities include pressure, vibration, light touch, tickle, itch, temperature, pain, proprioception, and kinesthesia. This means that its receptors are not associated with a specialized organ, but are instead spread throughout the body in a variety of organs. Many of the somatosensory receptors are located in the skin, but receptors are also found in muscles, tendons, joint capsules, ligaments, and in the walls of visceral organs. Two types of somatosensory signals that are transduced by free nerve endings are pain and temperature. These two modalities use thermoreceptors and nociceptors to transduce temperature and pain stimuli, respectively. Temperature receptors are stimulated when local temperatures differ from body temperature. Some thermoreceptors are sensitive to just cold and others to just heat. Nociception is the sensation of potentially damaging stimuli. Mechanical, chemical, or thermal stimuli beyond a set threshold will elicit painful sensations. Stressed or damaged tissues release chemicals that activate receptor proteins in the nociceptors. For example, the sensation of heat associated with spicy foods involves capsaicin, the active molecule in hot peppers. Capsaicin molecules bind to a transmembrane ion channel in nociceptors that is sensitive to temperatures above 37°C. The dynamics of capsaicin binding with this transmembrane ion channel is unusual in that the molecule remains bound for a long time. Because of this, it will decrease the ability of other stimuli to elicit pain sensations through the activated nociceptor. For this reason, capsaicin can be used as a topical analgesic, such as in products such as Icy Hot™. If you drag your finger across a textured surface, the skin of your finger will vibrate. Such low frequency vibrations are sensed by mechanoreceptors called Merkel cells, also known as type I cutaneous mechanoreceptors. Merkel cells are located in the stratum basale of the epidermis. Deep pressure and vibration is transduced by lamellated (Pacinian) corpuscles, which are receptors with encapsulated endings found deep in the dermis, or subcutaneous tissue. Light touch is transduced by the encapsulated endings known as tactile (Meissner) corpuscles. Follicles are also wrapped in a plexus of nerve endings known as the hair follicle plexus. These nerve endings detect the movement of hair at the surface of the skin, such as when an insect may be walking along the skin. Stretching of the skin is transduced by stretch receptors known as bulbous corpuscles. Bulbous corpuscles are also known as Ruffini corpuscles, or type II cutaneous mechanoreceptors. Other somatosensory receptors are found in the joints and muscles. Stretch receptors monitor the stretching of tendons, muscles, and the components of joints. For example, have you ever stretched your muscles before or after exercise and noticed that you can only stretch so far before your muscles spasm back to a less stretched state? This spasm is a reflex that is initiated by stretch receptors to avoid muscle tearing. Such stretch receptors can also prevent over-contraction of a muscle. In skeletal muscle tissue, these stretch receptors are called muscle spindles. Golgi tendon organs similarly transduce the stretch levels of tendons. Bulbous corpuscles are also present in joint capsules, where they measure stretch in the components of the skeletal system within the joint. The types of nerve endings, their locations, and the stimuli they transduce are presented in [link]. Mechanoreceptors of Somatosensation Name Free nerve endings Mechanoreceptors Bulbous corpuscle Historical (eponymous) name Merkel’s discs Ruffini’s corpuscle Location(s) Dermis, cornea, tongue, joint capsules, visceral organs Epidermal— dermal junction, mucosal membranes Dermis, joint capsules Stimuli Pain, temperature, mechanical deformation Low frequency vibration (5-15 Hz) Stretch Mechanoreceptors of Somatosensation Name Tactile corpuscle Lamellated corpuscle Hair follicle plexus Muscle spindle Tendon stretch organ Historical (eponymous) name Meissner’s corpuscle Pacinian corpuscle Golgi tendon organ *No corresponding eponymous name. Vision Location(s) Papillary dermis, especially in the fingertips and lips Deep dermis, subcutaneous tissue Wrapped around hair follicles in the dermis In line with skeletal muscle fibers In line with tendons Stimuli Light touch, vibrations below 50 Hz Deep pressure, high- frequency vibration (around 250 Hz) Movement of hair Muscle contraction and stretch Stretch of tendons Vision is the special sense of sight that is based on the transduction of light stimuli received through the eyes. The eyes are located within either orbit in the skull. The bony orbits surround the eyeballs, protecting them and anchoring the soft tissues of the eye ({link]). The eyelids, with lashes at their leading edges, help to protect the eye from abrasions by blocking particles that may land on the surface of the eye. The inner surface of each lid is a thin membrane known as the palpebral conjunctiva. The conjunctiva extends over the white areas of the eye (the sclera), connecting the eyelids to the eyeball. Tears are produced by the lacrimal gland, located beneath the lateral edges of the nose. Tears produced by this gland flow through the lacrimal duct to the medial corner of the eye, where the tears flow over the conjunctiva, washing away foreign particles. The Eye in the Orbit ign _— ; Eyebrow i we Aas Orbicularis oculi muscle Levator palpebrae superioris muscle Palpebral conjunctiva Eyelashes Cornea Conjunctiva The eye is located within the orbit and surrounded by soft tissues that protect and support its function. The orbit is surrounded by cranial bones of the skull. Movement of the eye within the orbit is accomplished by the contraction of six extraocular muscles that originate from the bones of the orbit and insert into the surface of the eyeball ([link]). Four of the muscles are arranged at the cardinal points around the eye and are named for those locations. They are the superior rectus, medial rectus, inferior rectus, and lateral rectus. When each of these muscles contract, the eye to moves toward the contracting muscle. For example, when the superior rectus contracts, the eye rotates to look up. The superior oblique originates at the posterior orbit, near the origin of the four rectus muscles. However, the tendon of the oblique muscles threads through a pulley-like piece of cartilage known as the trochlea. The tendon inserts obliquely into the superior surface of the eye. The angle of the tendon through the trochlea means that contraction of the superior oblique rotates the eye medially. The inferior oblique muscle originates from the floor of the orbit and inserts into the inferolateral surface of the eye. When it contracts, it laterally rotates the eye, in opposition to the superior oblique. Rotation of the eye by the two oblique muscles is necessary because the eye is not perfectly aligned on the sagittal plane. When the eye looks up or down, the eye must also rotate slightly to compensate for the superior rectus pulling at approximately a 20-degree angle, rather than straight up. The same is true for the inferior rectus, which is compensated by contraction of the inferior oblique. A seventh muscle in the orbit is the levator palpebrae superioris, which is responsible for elevating and retracting the upper eyelid, a movement that usually occurs in concert with elevation of the eye by the superior rectus (see [link]). The extraocular muscles are innervated by three cranial nerves. The lateral rectus, which causes abduction of the eye, is innervated by the abducens nerve. The superior oblique is innervated by the trochlear nerve. All of the other muscles are innervated by the oculomotor nerve, as is the levator palpebrae superioris. The motor nuclei of these cranial nerves connect to the brain stem, which coordinates eye movements. Extraocular Muscles Superior oblique muscle Trochlea Superior x Trochlea ; ‘ rectus ae Superior oblique tendon Superior Superior rectus oblique muscle Lateral rectus muscle r =; Lateral rectus a Medial rectus Inferior Inferior oblique rectus Common Inferior rectus Inferior oblique SS tendinous ring muscle muscle Lateral view of the right eye Anterior view of the right eye The extraocular muscles move the eye within the orbit. The eye itself is a hollow sphere composed of three layers of tissue. The outermost layer is the fibrous tunic, which includes the white sclera and clear cornea. The sclera accounts for five sixths of the surface of the eye, most of which is not visible, though humans are unique compared with many other species in having so much of the “white of the eye” visible ({link]). The transparent cornea covers the anterior tip of the eye and allows light to enter the eye. The middle layer of the eye is the vascular tunic, which is mostly composed of the choroid, ciliary body, and iris. The choroid is a layer of highly vascularized connective tissue that provides a blood supply to the eyeball. The choroid is posterior to the ciliary body, a muscular structure that is attached to the lens by suspensory ligaments, or zonule fibers. These two structures bend the lens, allowing it to focus light on the back of the eye. Overlaying the ciliary body, and visible in the anterior eye, is the iris—the colored part of the eye. The iris is a smooth muscle that opens or closes the pupil, which is the hole at the center of the eye that allows light to enter. The iris constricts the pupil in response to bright light and dilates the pupil in response to dim light. The innermost layer of the eye is the neural tunic, or retina, which contains the nervous tissue responsible for photoreception. The eye is also divided into two cavities: the anterior cavity and the posterior cavity. The anterior cavity is the space between the cornea and lens, including the iris and ciliary body. It is filled with a watery fluid called the aqueous humor. The posterior cavity is the space behind the lens that extends to the posterior side of the interior eyeball, where the retina is located. The posterior cavity is filled with a more viscous fluid called the vitreous humor. The retina is composed of several layers and contains specialized cells for the initial processing of visual stimuli. The photoreceptors (rods and cones) change their membrane potential when stimulated by light energy. The change in membrane potential alters the amount of neurotransmitter that the photoreceptor cells release onto bipolar cells in the outer synaptic layer. It is the bipolar cell in the retina that connects a photoreceptor to a retinal ganglion cell (RGC) in the inner synaptic layer. There, amacrine cells additionally contribute to retinal processing before an action potential is produced by the RGC. The axons of RGCs, which lie at the innermost layer of the retina, collect at the optic disc and leave the eye as the optic nerve (see [link]). Because these axons pass through the retina, there are no photoreceptors at the very back of the eye, where the optic nerve begins. This creates a “blind spot” in the retina, and a corresponding blind spot in our visual field. Structure of the Eye Lateral Posterior cavity Vitreous chamber ; N Scleral venous sinus = . (canal of Schlemm) | / A \ Suspensory ligaments Lens Lateral rectus muscle Sclera Choroid ; ———— Cornea Retina Iris ] Pupil Anterior cavity (contains aqueous humor): . Posterior chamber | J Anterior chamber @ | r > Suspensory ligaments ff [ la Ciliary body: ' Ciliary process Ciliary muscle Fovea centralis Optic (II) nerve — Central retinal artery and vein Optic disc (blind spot) Medial rectus muscle Medial The sphere of the eye can be divided into anterior and posterior chambers. The wall of the eye is composed of three layers: the fibrous tunic, vascular tunic, and neural tunic. Within the neural tunic is the retina, with three layers of cells and two synaptic layers in between. The center of the retina has a small indentation known as the fovea. Note that the photoreceptors in the retina (rods and cones) are located behind the axons, RGCs, bipolar cells, and retinal blood vessels. A significant amount of light is absorbed by these structures before the light reaches the photoreceptor cells. However, at the exact center of the retina is a small area known as the fovea. At the fovea, the retina lacks the supporting cells and blood vessels, and only contains photoreceptors. Therefore, visual acuity, or the sharpness of vision, is greatest at the fovea. This is because the fovea is where the least amount of incoming light is absorbed by other retinal structures (see [link]). As one moves in either direction from this central point of the retina, visual acuity drops significantly. In addition, each photoreceptor cell of the fovea is connected to a single RGC. Therefore, this RGC does not have to integrate inputs from multiple photoreceptors, which reduces the accuracy of visual transduction. Toward the edges of the retina, several photoreceptors converge on RGCs (through the bipolar cells) up to a ratio of 50 to 1. The difference in visual acuity between the fovea and peripheral retina is easily evidenced by looking directly at a word in the middle of this paragraph. The visual stimulus in the middle of the field of view falls on the fovea and is in the sharpest focus. Without moving your eyes off that word, notice that words at the beginning or end of the paragraph are not in focus. The images in your peripheral vision are focused by the peripheral retina, and have vague, blurry edges and words that are not as clearly identified. As a result, a large part of the neural function of the eyes is concerned with moving the eyes and head so that important visual stimuli are centered on the fovea. Light falling on the retina causes chemical changes to pigment molecules in the photoreceptors, ultimately leading to a change in the activity of the RGCs. Photoreceptor cells have two parts, the inner segment and the outer segment ([link]). The inner segment contains the nucleus and other common organelles of a cell, whereas the outer segment is a specialized region in which photoreception takes place. There are two types of photoreceptors—rods and cones—which differ in the shape of their outer segment. The rod-shaped outer segments of the rod photoreceptor contain a stack of membrane-bound discs that contain the photosensitive pigment rhodopsin. The cone-shaped outer segments of the cone photoreceptor contain their photosensitive pigments in infoldings of the cell membrane. There are three cone photopigments, called opsins, which are each sensitive to a particular wavelength of light. The wavelength of visible light determines its color. The pigments in human eyes are specialized in perceiving three different primary colors: red, green, and blue. Photoreceptor y Pigment epithelium Melanin granules Discs Connecting stalks Mitochondria Rods Golgi apparatus Cone Nuclei Bipolar cell Ganglion cell (a) Choroid Pigment epithelium Rods and cones Bipolar cells Ganglion cells Optic nerve axons (b) (a) All photoreceptors have inner segments containing the nucleus and other important organelles and outer segments with membrane arrays containing the photosensitive opsin molecules. Rod outer segments are long columnar shapes with stacks of membrane-bound discs that contain the rhodopsin pigment. Cone outer segments are short, tapered shapes with folds of membrane in place of the discs in the rods. (b) Tissue of the retina shows a dense layer of nuclei of the rods and cones. LM x 800. (Micrograph provided by the Regents of University of Michigan Medical School © 2012) At the molecular level, visual stimuli cause changes in the photopigment molecule that lead to changes in membrane potential of the photoreceptor cell. A single unit of light is called a photon, which is described in physics as a packet of energy with properties of both a particle and a wave. The energy of a photon is represented by its wavelength, with each wavelength of visible light corresponding to a particular color. Visible light is electromagnetic radiation with a wavelength between 380 and 720 nm. Wavelengths of electromagnetic radiation longer than 720 nm fall into the infrared range, whereas wavelengths shorter than 380 nm fall into the ultraviolet range. Light with a wavelength of 380 nm is blue whereas light with a wavelength of 720 nm is dark red. All other colors fall between red and blue at various points along the wavelength scale. Opsin pigments are actually transmembrane proteins that contain a cofactor known as retinal. Retinal is a hydrocarbon molecule related to vitamin A. When a photon hits retinal, the long hydrocarbon chain of the molecule is biochemically altered. Specifically, photons cause some of the double-bonded carbons within the chain to switch from a cis to a trans conformation. This process is called photoisomerization. Before interacting with a photon, retinal’s flexible double-bonded carbons are in the cis conformation. This molecule is referred to as 11-cis-retinal. A photon interacting with the molecule causes the flexible double-bonded carbons to change to the trans- conformation, forming all-trans-retinal, which has a straight hydrocarbon chain ((link]). The shape change of retinal in the photoreceptors initiates visual transduction in the retina. Activation of retinal and the opsin proteins result in activation of a G protein. The G protein changes the membrane potential of the photoreceptor cell, which then releases less neurotransmitter into the outer synaptic layer of the retina. Until the retinal molecule is changed back to the 11-cis-retinal shape, the opsin cannot respond to light energy, which is called bleaching. When a large group of photopigments is bleached, the retina will send information as if opposing visual information is being perceived. After a bright flash of light, afterimages are usually seen in negative. The photoisomerization is reversed by a series of enzymatic changes so that the retinal responds to more light energy. Retinal Isomers Photon 11-trans-retinal 11-cis-retinal Fai “eA and opsin are reassembled to form rhodopsin Regeneration Rhodopsin molecules Rod bar of 11-cis- tra, (a) 11-cis-retinal (b) all-trans-retinal The retinal molecule has two isomers, (a) one before a photon interacts with it and (b) one that is altered through photoisomerization. The opsins are sensitive to limited wavelengths of light. Rhodopsin, the photopigment in rods, is most sensitive to light at a wavelength of 498 nm. The three color opsins have peak sensitivities of 564 nm, 534 nm, and 420 nm corresponding roughly to the primary colors of red, green, and blue ({link]). The absorbance of rhodopsin in the rods is much more sensitive than in the cone opsins; specifically, rods are sensitive to vision in low light conditions, and cones are sensitive to brighter conditions. In normal sunlight, rhodopsin will be constantly bleached while the cones are active. In a darkened room, there is not enough light to activate cone opsins, and vision is entirely dependent on rods. Rods are so sensitive to light that a single photon can result in an action potential from a rod’s corresponding RGC. The three types of cone opsins, being sensitive to different wavelengths of light, provide us with color vision. By comparing the activity of the three different cones, the brain can extract color information from visual stimuli. For example, a bright blue light that has a wavelength of approximately 450 nm would activate the “red” cones minimally, the “green” cones marginally, and the “blue” cones predominantly. The relative activation of the three different cones is calculated by the brain, which perceives the color as blue. However, cones cannot react to low-intensity light, and rods do not sense the color of light. Therefore, our low-light vision is—in essence—in grayscale. In other words, in a dark room, everything appears as a shade of gray. If you think that you can see colors in the dark, it is most likely because your brain knows what color something is and is relying on that memory. Comparison of Color Sensitivity of Photopigments 420 nm 498nm 534nm 564nm Green Red Blue cones Rods cones cones Normalized absorbance 400 500 600 700 Violet Blue Cyan Green Yellow Red Wavelength (nm) Comparing the peak sensitivity and absorbance spectra of the four photopigments suggests that they are most sensitive to particular wavelengths. OR page | Watch this video to learn more about a transverse section through the brain that depicts the visual pathway from the eye to the occipital cortex. The first half of the pathway is the projection from the RGCs through the optic nerve to the lateral geniculate nucleus in the thalamus on either side. This first fiber in the pathway synapses on a thalamic cell that then projects to the visual cortex in the occipital lobe where “seeing,” or visual perception, takes place. This video gives an abbreviated overview of the visual system by concentrating on the pathway from the eyes to the occipital lobe. The video makes the statement (at 0:45) that “specialized cells in the retina called ganglion cells convert the light rays into electrical signals.” What aspect of retinal processing is simplified by that statement? Explain your answer. Sensory Nerves Once any sensory cell transduces a stimulus into a nerve impulse, that impulse has to travel along axons to reach the CNS. In many of the special senses, the axons leaving the sensory receptors have a topographical arrangement, meaning that the location of the sensory receptor relates to the location of the axon in the nerve. For example, in the retina, axons from RGCs in the fovea are located at the center of the optic nerve, where they are surrounded by axons from the more peripheral RGCs. Spinal Nerves Generally, spinal nerves contain afferent axons from sensory receptors in the periphery, such as from the skin, mixed with efferent axons travelling to the muscles or other effector organs. As the spinal nerve nears the spinal cord, it splits into dorsal and ventral roots. The dorsal root contains only the axons of sensory neurons, whereas the ventral roots contain only the axons of the motor neurons. Some of the branches will synapse with local neurons in the dorsal root ganglion, posterior (dorsal) horn, or even the anterior (ventral) horn, at the level of the spinal cord where they enter. Other branches will travel a short distance up or down the spine to interact with neurons at other levels of the spinal cord. A branch may also turn into the posterior (dorsal) column of the white matter to connect with the brain. For the sake of convenience, we will use the terms ventral and dorsal in reference to structures within the spinal cord that are part of these pathways. This will help to underscore the relationships between the different components. Typically, spinal nerve systems that connect to the brain are contralateral, in that the right side of the body is connected to the left side of the brain and the left side of the body to the right side of the brain. Cranial Nerves Cranial nerves convey specific sensory information from the head and neck directly to the brain. For sensations below the neck, the right side of the body is connected to the left side of the brain and the left side of the body to the right side of the brain. Whereas spinal information is contralateral, cranial nerve systems are mostly ipsilateral, meaning that a cranial nerve on the right side of the head is connected to the right side of the brain. Some cranial nerves contain only sensory axons, such as the olfactory, optic, and vestibulocochlear nerves. Other cranial nerves contain both sensory and motor axons, including the trigeminal, facial, glossopharyngeal, and vagus nerves (however, the vagus nerve is not associated with the somatic nervous system). The general senses of somatosensation for the face travel through the trigeminal system. Chapter Review The senses are olfaction (smell), gustation (taste), somatosensation (sensations associated with the skin and body), audition (hearing), equilibrium (balance), and vision. With the exception of somatosensation, this list represents the special senses, or those systems of the body that are associated with specific organs such as the tongue or eye. Somatosensation belongs to the general senses, which are those sensory structures that are distributed throughout the body and in the walls of various organs. The special senses are all primarily part of the somatic nervous system in that they are consciously perceived through cerebral processes, though some special senses contribute to autonomic function. The general senses can be divided into somatosensation, which is commonly considered touch, but includes tactile, pressure, vibration, temperature, and pain perception. The general senses also include the visceral senses, which are separate from the somatic nervous system function in that they do not normally rise to the level of conscious perception. The cells that transduce sensory stimuli into the electrochemical signals of the nervous system are classified on the basis of structural or functional aspects of the cells. The structural classifications are either based on the anatomy of the cell that is interacting with the stimulus (free nerve endings, encapsulated endings, or specialized receptor cell), or where the cell is located relative to the stimulus (interoceptor, exteroceptor, proprioceptor). Thirdly, the functional classification is based on how the cell transduces the stimulus into a neural signal. Chemoreceptors respond to chemical stimuli and are the basis for olfaction and gustation. Related to chemoreceptors are osmoreceptors and nociceptors for fluid balance and pain reception, respectively. Mechanoreceptors respond to mechanical stimuli and are the basis for most aspects of somatosensation, as well as being the basis of audition and equilibrium in the inner ear. Thermoreceptors are sensitive to temperature changes, and photoreceptors are sensitive to light energy. The nerves that convey sensory information from the periphery to the CNS are either spinal nerves, connected to the spinal cord, or cranial nerves, connected to the brain. Spinal nerves have mixed populations of fibers; some are motor fibers and some are sensory. The sensory fibers connect to the spinal cord through the dorsal root, which is attached to the dorsal root ganglion. Sensory information from the body that is conveyed through spinal nerves will project to the opposite side of the brain to be processed by the cerebral cortex. The cranial nerves can be strictly sensory fibers, such as the olfactory, optic, and vestibulocochlear nerves, or mixed sensory and motor nerves, such as the trigeminal, facial, glossopharyngeal, and vagus nerves. The cranial nerves are connected to the same side of the brain from which the sensory information originates. Interactive Link Questions Exercise: Problem: Watch this video to learn about Dr. Danielle Reed of the Monell Chemical Senses Center in Philadelphia, PA, who became interested in science at an early age because of her sensory experiences. She recognized that her sense of taste was unique compared with other people she knew. Now, she studies the genetic differences between people and their sensitivities to taste stimuli. In the video, there is a brief image of a person sticking out their tongue, which has been covered with a colored dye. This is how Dr. Reed is able to visualize and count papillae on the surface of the tongue. People fall into two large groups known as “tasters” and “non-tasters” on the basis of the density of papillae on their tongue, which also indicates the number of taste buds. Non-tasters can taste food, but they are not as sensitive to certain tastes, such as bitterness. Dr. Reed discovered that she is anon-taster, which explains why she perceived bitterness differently than other people she knew. Are you very sensitive to tastes? Can you see any similarities among the members of your family? Solution: Answers will vary, but a typical answer might be: I can eat most anything (except mushrooms! ), so I don’t think that I’m that sensitive to tastes. My whole family likes eating a variety of foods, so it seems that we all have the same level of sensitivity. Exercise: Problem: [link] The basilar membrane is the thin membrane that extends from the central core of the cochlea to the edge. What is anchored to this membrane so that they can be activated by movement of the fluids within the cochlea? Solution: [link] The hair cells are located in the organ of Corti, which is located on the basilar membrane. The stereocilia of those cells would normally be attached to the tectorial membrane (though they are detached in the micrograph because of processing of the tissue). Exercise: Problem: Watch this video to learn more about how the structures of the ear convert sound waves into a neural signal by moving the “hairs,” or stereocilia, of the cochlear duct. Specific locations along the length of the duct encode specific frequencies, or pitches. The brain interprets the meaning of the sounds we hear as music, speech, noise, etc. Which ear structures are responsible for the amplification and transfer of sound from the external ear to the inner ear? Solution: The small bones in the middle ear, the ossicles, amplify and transfer sound between the tympanic membrane of the external ear and the oval window of the inner ear. Exercise: Problem: Watch this animation to learn more about the inner ear and to see the cochlea unroll, with the base at the back of the image and the apex at the front. Specific wavelengths of sound cause specific regions of the basilar membrane to vibrate, much like the keys of a piano produce sound at different frequencies. Based on the animation, where do frequencies— from high to low pitches—cause activity in the hair cells within the cochlear duct? Solution: High frequencies activate hair cells toward the base of the cochlea, and low frequencies activate hair cells toward the apex of the cochlea. Exercise: Problem: Watch this video to learn more about a transverse section through the brain that depicts the visual pathway from the eye to the occipital cortex. The first half of the pathway is the projection from the RGCs through the optic nerve to the lateral geniculate nucleus in the thalamus on either side. This first fiber in the pathway synapses on a thalamic cell that then projects to the visual cortex in the occipital lobe where “seeing,” or visual perception, takes place. This video gives an abbreviated overview of the visual system by concentrating on the pathway from the eyes to the occipital lobe. The video makes the statement (at 0:45) that “specialized cells in the retina called ganglion cells convert the light rays into electrical signals.” What aspect of retinal processing is simplified by that statement? Explain your answer. Solution: Photoreceptors convert light energy, or photons, into an electrochemical signal. The retina contains bipolar cells and the RGCs that finally convert it into action potentials that are sent from the retina to the CNS. It is important to recognize when popular media and online sources oversimplify complex physiological processes so that misunderstandings are not generated. This video was created by a medical device manufacturer who might be trying to highlight other aspects of the visual system than retinal processing. The statement they make is not incorrect, it just bundles together several steps, which makes it sound like RGCs are the transducers, rather than photoreceptors. Review Questions Exercise: Problem: What type of receptor cell is responsible for transducing pain stimuli? a. mechanoreceptor b. nociceptor c. osmoreceptor d. photoreceptor Solution: B Exercise: Problem: Which of these cranial nerves is part of the gustatory system? a. olfactory b. trochlear c. trigeminal d. facial Solution: D Exercise: Problem:Which submodality of taste is sensitive to the pH of saliva? a. umMami b. sour c. bitter d. sweet Solution: B Exercise: Problem: Axons from which neuron in the retina make up the optic nerve? a. amacrine cells b. photoreceptors c. bipolar cells d. retinal ganglion cells Solution: D Exercise: Problem: What type of receptor cell is involved in the sensations of sound and balance? a. photoreceptor b. chemoreceptor c. mechanoreceptor d. nociceptor Solution: C Critical Thinking Questions Exercise: Problem: The sweetener known as stevia can replace glucose in food. What does the molecular similarity of stevia to glucose mean for the gustatory sense? Solution: The stevia molecule is similar to glucose such that it will bind to the glucose receptor in sweet-sensitive taste buds. However, it is not a substrate for the ATP-generating metabolism within cells. Exercise: Problem: Why does the blind spot from the optic disc in either eye not result in a blind spot in the visual field? Solution: The visual field for each eye is projected onto the retina as light is focused by the lens. The visual information from the right visual field falls on the left side of the retina and vice versa. The optic disc in the right eye is on the medial side of the fovea, which would be the left side of the retina. However, the optic disc in the left eye would be on the right side of that fovea, so the right visual field falls on the side of the retina in the left field where there is no blind spot. Glossary alkaloid substance, usually from a plant source, that is chemically basic with respect to pH and will stimulate bitter receptors amacrine cell type of cell in the retina that connects to the bipolar cells near the outer synaptic layer and provides the basis for early image processing within the retina ampulla in the ear, the structure at the base of a semicircular canal that contains the hair cells and cupula for transduction of rotational movement of the head anosmia loss of the sense of smell; usually the result of physical disruption of the first cranial nerve aqueous humor watery fluid that fills the anterior chamber containing the cornea, iris, ciliary body, and lens of the eye audition sense of hearing auricle fleshy external structure of the ear basilar membrane in the ear, the floor of the cochlear duct on which the organ of Corti sits bipolar cell cell type in the retina that connects the photoreceptors to the RGCs capsaicin molecule that activates nociceptors by interacting with a temperature- sensitive ion channel and is the basis for “hot” sensations in spicy food chemoreceptor sensory receptor cell that is sensitive to chemical stimuli, such as in taste, smell, or pain choroid highly vascular tissue in the wall of the eye that supplies the outer retina with blood ciliary body smooth muscle structure on the interior surface of the iris that controls the shape of the lens through the zonule fibers cochlea auditory portion of the inner ear containing structures to transduce sound stimuli cochlear duct space within the auditory portion of the inner ear that contains the organ of Corti and is adjacent to the scala tympani and scala vestibuli on either side cone photoreceptor one of the two types of retinal receptor cell that is specialized for color vision through the use of three photopigments distributed through three separate populations of cells contralateral word meaning “on the opposite side,” as in axons that cross the midline in a fiber tract cornea fibrous covering of the anterior region of the eye that is transparent so that light can pass through it cupula specialized structure within the base of a semicircular canal that bends the stereocilia of hair cells when the head rotates by way of the relative movement of the enclosed fluid encapsulated ending configuration of a sensory receptor neuron with dendrites surrounded by specialized structures to aid in transduction of a particular type of sensation, such as the lamellated corpuscles in the deep dermis and subcutaneous tissue equilibrium sense of balance that includes sensations of position and movement of the head external ear structures on the lateral surface of the head, including the auricle and the ear canal back to the tympanic membrane exteroceptor sensory receptor that is positioned to interpret stimuli from the external environment, such as photoreceptors in the eye or somatosensory receptors in the skin extraocular muscle one of six muscles originating out of the bones of the orbit and inserting into the surface of the eye which are responsible for moving the eye fibrous tunic outer layer of the eye primarily composed of connective tissue known as the sclera and cornea fovea exact center of the retina at which visual stimuli are focused for maximal acuity, where the retina is thinnest, at which there is nothing but photoreceptors free nerve ending configuration of a sensory receptor neuron with dendrites in the connective tissue of the organ, such as in the dermis of the skin, that are most often sensitive to chemical, thermal, and mechanical stimuli general sense any sensory system that is distributed throughout the body and incorporated into organs of multiple other systems, such as the walls of the digestive organs or the skin gustation sense of taste gustatory receptor cells sensory cells in the taste bud that transduce the chemical stimuli of gustation hair cells mechanoreceptor cells found in the inner ear that transduce stimuli for the senses of hearing and balance incus (also, anvil) ossicle of the middle ear that connects the malleus to the stapes inferior oblique extraocular muscle responsible for lateral rotation of the eye inferior rectus extraocular muscle responsible for looking down inner ear structure within the temporal bone that contains the sensory apparati of hearing and balance inner segment in the eye, the section of a photoreceptor that contains the nucleus and other major organelles for normal cellular functions inner synaptic layer layer in the retina where bipolar cells connect to RGCs interoceptor sensory receptor that is positioned to interpret stimuli from internal organs, such as stretch receptors in the wall of blood vessels ipsilateral word meaning on the same side, as in axons that do not cross the midline in a fiber tract iris colored portion of the anterior eye that surrounds the pupil kinesthesia sense of body movement based on sensations in skeletal muscles, tendons, joints, and the skin lacrimal duct duct in the medial corner of the orbit that drains tears into the nasal cavity lacrimal gland gland lateral to the orbit that produces tears to wash across the surface of the eye lateral rectus extraocular muscle responsible for abduction of the eye lens component of the eye that focuses light on the retina levator palpebrae superioris muscle that causes elevation of the upper eyelid, controlled by fibers in the oculomotor nerve macula enlargement at the base of a semicircular canal at which transduction of equilibrium stimuli takes place within the ampulla malleus (also, hammer) ossicle that is directly attached to the tympanic membrane mechanoreceptor receptor cell that transduces mechanical stimuli into an electrochemical signal medial rectus extraocular muscle responsible for adduction of the eye middle ear space within the temporal bone between the ear canal and bony labyrinth where the ossicles amplify sound waves from the tympanic membrane to the oval window neural tunic layer of the eye that contains nervous tissue, namely the retina nociceptor receptor cell that senses pain stimuli odorant molecules volatile chemicals that bind to receptor proteins in olfactory neurons to stimulate the sense of smell olfaction sense of smell olfactory bulb central target of the first cranial nerve; located on the ventral surface of the frontal lobe in the cerebrum olfactory epithelium region of the nasal epithelium where olfactory neurons are located olfactory sensory neuron receptor cell of the olfactory system, sensitive to the chemical stimuli of smell, the axons of which compose the first cranial nerve opsin protein that contains the photosensitive cofactor retinal for phototransduction optic disc spot on the retina at which RGC axons leave the eye and blood vessels of the inner retina pass optic nerve second cranial nerve, which is responsible visual sensation organ of Corti structure in the cochlea in which hair cells transduce movements from sound waves into electrochemical signals osmoreceptor receptor cell that senses differences in the concentrations of bodily fluids on the basis of osmotic pressure ossicles three small bones in the middle ear otolith layer of calcium carbonate crystals located on top of the otolithic membrane otolithic membrane gelatinous substance in the utricle and saccule of the inner ear that contains calcium carbonate crystals and into which the stereocilia of hair cells are embedded outer segment in the eye, the section of a photoreceptor that contains opsin molecules that transduce light stimuli outer synaptic layer layer in the retina at which photoreceptors connect to bipolar cells oval window membrane at the base of the cochlea where the stapes attaches, marking the beginning of the scala vestibuli palpebral conjunctiva membrane attached to the inner surface of the eyelids that covers the anterior surface of the cornea papilla for gustation, a bump-like projection on the surface of the tongue that contains taste buds photoisomerization chemical change in the retinal molecule that alters the bonding so that it switches from the 11-cis-retinal isomer to the all-trans-retinal isomer photon individual “packet” of light photoreceptor receptor cell specialized to respond to light stimuli proprioception sense of position and movement of the body proprioceptor receptor cell that senses changes in the position and kinesthetic aspects of the body pupil open hole at the center of the iris that light passes through into the eye receptor cell cell that transduces environmental stimuli into neural signals retina nervous tissue of the eye at which phototransduction takes place retinal cofactor in an opsin molecule that undergoes a biochemical change when struck by a photon (pronounced with a stress on the last syllable) retinal ganglion cell (RGC) neuron of the retina that projects along the second cranial nerve rhodopsin photopigment molecule found in the rod photoreceptors rod photoreceptor one of the two types of retinal receptor cell that is specialized for low- light vision round window membrane that marks the end of the scala tympani saccule structure of the inner ear responsible for transducing linear acceleration in the vertical plane scala tympani portion of the cochlea that extends from the apex to the round window scala vestibuli portion of the cochlea that extends from the oval window to the apex sclera white of the eye semicircular canals structures within the inner ear responsible for transducing rotational movement information sensory modality a particular system for interpreting and perceiving environmental stimuli by the nervous system somatosensation general sense associated with modalities lumped together as touch special sense any sensory system associated with a specific organ structure, namely smell, taste, sight, hearing, and balance spiral ganglion location of neuronal cell bodies that transmit auditory information along the eighth cranial nerve stapes (also, stirrup) ossicle of the middle ear that is attached to the inner ear stereocilia array of apical membrane extensions in a hair cell that transduce movements when they are bent submodality specific sense within a broader major sense such as sweet as a part of the sense of taste, or color as a part of vision superior oblique extraocular muscle responsible for medial rotation of the eye superior rectus extraocular muscle responsible for looking up taste buds structures within a papilla on the tongue that contain gustatory receptor cells tectorial membrane component of the organ of Corti that lays over the hair cells, into which the stereocilia are embedded thermoreceptor sensory receptor specialized for temperature stimuli topographical relating to positional information transduction process of changing an environmental stimulus into the electrochemical signals of the nervous system trochlea cartilaginous structure that acts like a pulley for the superior oblique muscle tympanic membrane ear drum umami taste submodality for sensitivity to the concentration of amino acids; also called the savory sense utricle structure of the inner ear responsible for transducing linear acceleration in the horizontal plane vascular tunic middle layer of the eye primarily composed of connective tissue with a rich blood supply vestibular ganglion location of neuronal cell bodies that transmit equilibrium information along the eighth cranial nerve vestibule in the ear, the portion of the inner ear responsible for the sense of equilibrium visceral sense sense associated with the internal organs vision special sense of sight based on transduction of light stimuli visual acuity property of vision related to the sharpness of focus, which varies in relation to retinal position vitreous humor viscous fluid that fills the posterior chamber of the eye zonule fibers fibrous connections between the ciliary body and the lens Divisions of the ANS By the end of this section, you will be able to: e Name the components that generate the sympathetic and parasympathetic responses of the autonomic nervous system e Explain the differences in output connections within the two divisions of the autonomic nervous system e Describe the signaling molecules and receptor proteins involved in communication within the two divisions of the autonomic nervous system The nervous system can be divided into two functional parts: the somatic nervous system and the autonomic nervous system. The major differences between the two systems are evident in the responses that each produces. The somatic nervous system causes contraction of skeletal muscles. The autonomic nervous system controls cardiac and smooth muscle, as well as glandular tissue. The somatic nervous system is associated with voluntary responses (though many can happen without conscious awareness, like breathing), and the autonomic nervous system is associated with involuntary responses, such as those related to homeostasis. The autonomic nervous system regulates many of the internal organs through a balance of two aspects, or divisions. In addition to the endocrine system, the autonomic nervous system is instrumental in homeostatic mechanisms in the body. The two divisions of the autonomic nervous system are the sympathetic division and the parasympathetic division. The sympathetic system is associated with the fight-or-flight response, and parasympathetic activity is referred to by the epithet of rest and digest. Homeostasis is the balance between the two systems. At each target effector, dual innervation determines activity. For example, the heart receives connections from both the sympathetic and parasympathetic divisions. One causes heart rate to increase, whereas the other causes heart rate to decrease. Note: — wees Openstax COLLEGE ie Batt Watch this video to learn more about adrenaline and the fight-or-flight response. When someone is said to have a rush of adrenaline, the image of bungee jumpers or skydivers usually comes to mind. But adrenaline, also known as epinephrine, is an important chemical in coordinating the body’s fight-or-flight response. In this video, you look inside the physiology of the fight-or-flight response, as envisioned for a firefighter. His body’s reaction is the result of the sympathetic division of the autonomic nervous system causing system-wide changes as it prepares for extreme responses. What two changes does adrenaline bring about to help the skeletal muscle response? Sympathetic Division of the Autonomic Nervous System To respond to a threat—to fight or to run away—the sympathetic system causes divergent effects as many different effector organs are activated together for a common purpose. More oxygen needs to be inhaled and delivered to skeletal muscle. The respiratory, cardiovascular, and musculoskeletal systems are all activated together. Additionally, sweating keeps the excess heat that comes from muscle contraction from causing the body to overheat. The digestive system shuts down so that blood is not absorbing nutrients when it should be delivering oxygen to skeletal muscles. To coordinate all these responses, the connections in the sympathetic system diverge from a limited region of the central nervous system (CNS) to a wide array of ganglia that project to the many effector organs simultaneously. The complex set of structures that compose the output of the sympathetic system make it possible for these disparate effectors to come together in a coordinated, systemic change. The sympathetic division of the autonomic nervous system influences the various organ systems of the body through connections emerging from the thoracic and upper lumbar spinal cord. It is referred to as the thoracolumbar system to reflect this anatomical basis. A central neuron in the lateral horn of any of these spinal regions projects to ganglia adjacent to the vertebral column through the ventral spinal roots. The majority of ganglia of the sympathetic system belong to a network of sympathetic chain ganglia that runs alongside the vertebral column. The ganglia appear as a series of clusters of neurons linked by axonal bridges. There are typically 23 ganglia in the chain on either side of the spinal column. Three correspond to the cervical region, 12 are in the thoracic region, four are in the lumbar region, and four correspond to the sacral region. The cervical and sacral levels are not connected to the spinal cord directly through the spinal roots, but through ascending or descending connections through the bridges within the chain. A diagram that shows the connections of the sympathetic system is somewhat like a circuit diagram that shows the electrical connections between different receptacles and devices. In [link], the “circuits” of the sympathetic system are intentionally simplified. Connections of Sympathetic Division of the Autonomic Nervous System Region of spinal cord Left chain ganglia Medulla ee ‘ # Ze Right chain ‘ « .o ganglia gee gi WX 7° 1-7 cet 2 ex\0 ¢} ey? mM Spinal cord a Coccygeal ganglia fused together (ganglion impar) } Associated nerves and prevertebral ganglia eee eee cress oc Superior mesenteric ganglion i Inferior ganglion 3 Celiac ganglion Cs) . % . “ x mesenteric N14 “S Target organs (effectors) Lacrinal gland -“‘G@eepEp Mucous membrane- Siete nose and palate Submaxillary gland ~~~ Gaex Sublingual gland = Mucous membrane- mouth Parotid gland Heart Larynx Trachea Bronchi Esophagus Stomach Abdominal blood vessels Large intestine Rectum Kidney Bladder Gonads External genitalia Neurons from the lateral horn of the spinal cord (preganglionic nerve fibers - solid lines)) project to the chain ganglia on either side of the vertebral column or to collateral (prevertebral) ganglia that are anterior to the vertebral column in the abdominal cavity. Axons from these ganglionic neurons (postganglionic nerve fibers - dotted lines) then project to target effectors throughout the body. To continue with the analogy of the circuit diagram, there are three different types of “junctions” that operate within the sympathetic system ([link]). The first type is most direct: the sympathetic nerve projects to the chain ganglion at the same level as the target effector (the organ, tissue, or gland to be innervated). An example of this type is spinal nerve T1 that synapses with the T1 chain ganglion to innervate the trachea. The fibers of this branch are called white rami communicantes (singular = ramus communicans); they are myelinated and therefore referred to as white (see [link]a). The axon from the central neuron (the preganglionic fiber shown as a Solid line) synapses with the ganglionic neuron (with the postganglionic fiber shown as a dashed line). This neuron then projects to a target effector—in this case, the trachea—via gray rami communicantes, which are unmyelinated axons. In some cases, the target effectors are located superior or inferior to the spinal segment at which the preganglionic fiber emerges. With respect to the “wiring” involved, the synapse with the ganglionic neuron occurs at chain ganglia superior or inferior to the location of the central neuron. An example of this is spinal nerve T1 that innervates the eye. The spinal nerve tracks up through the chain until it reaches the superior cervical ganglion, where it synapses with the postganglionic neuron (see [link |b). The cervical ganglia are referred to as paravertebral ganglia, given their location adjacent to prevertebral ganglia in the sympathetic chain. Not all axons from the central neurons terminate in the chain ganglia. Additional branches from the ventral nerve root continue through the chain and on to one of the collateral ganglia as the greater splanchnic nerve or lesser splanchnic nerve. For example, the greater splanchnic nerve at the level of TS synapses with a collateral ganglion outside the chain before making the connection to the postganglionic nerves that innervate the stomach (see [link]c). Collateral ganglia, also called prevertebral ganglia, are situated anterior to the vertebral column and receive inputs from splanchnic nerves as well as central sympathetic neurons. They are associated with controlling organs in the abdominal cavity, and are also considered part of the enteric nervous system. The three collateral ganglia are the celiac ganglion, the superior mesenteric ganglion, and the inferior mesenteric ganglion (see [link]). The word celiac is derived from the Latin word “coelom,” which refers to a body cavity (in this case, the abdominal cavity), and the word mesenteric refers to the digestive system. Sympathetic Connections and Chain Ganglia (a) A central neuron synapses with a ganglion at the same z level within the chain ganglia. a ) Sympathetic chain ganglion ; White ramus Spinal | communicans Spinal cord —_ nerve ql ---= To target effector Dorsal root ganglion \ To target effector | BZ Gray ramus (b) A central neuron N 4 ; : communicans synapses within a : } AN more superior or inferior Ww Spinal ganglion in the chain. Seb Spinal cord nerve (c) Acentral neuron projects through the white ramus but does not synapse in a chain ganglion. Instead, it continues through one of the splanchnic nerves to synapse within a prevertebral ganglion. Sympathetic chain ganglion 2 No synapse in Spinal . spinal ganglion inal cor ner ; apineheere ne Prevertebral : ~Sx. ganglion /~ To target effector Splanchnic nerve Axon of central neuron ---- Axon of ganglionic neuron e Central neuron body © Ganglionic neuron body —© Synapse The axon from a central sympathetic neuron in the spinal cord can project to the periphery in a number of different ways. (a) The fiber can project out to the ganglion at the same level and synapse on a ganglionic neuron. (b) A branch can project to more superior or inferior ganglion in the chain. (c) A branch can project through the white ramus communicans, but not terminate on a ganglionic neuron in the chain. Instead, it projects through one of the splanchnic nerves to a collateral ganglion or the adrenal medulla (not pictured). An axon from the central neuron that projects to a sympathetic ganglion is referred to as a preganglionic fiber or neuron, and represents the output from the CNS to the ganglion. Because the sympathetic ganglia are adjacent to the vertebral column, preganglionic sympathetic fibers are relatively short, and they are myelinated. A postganglionic fiber—the axon from a ganglionic neuron that projects to the target effector—represents the output of a ganglion that directly influences the organ. Compared with the preganglionic fibers, postganglionic sympathetic fibers are long because of the relatively greater distance from the ganglion to the target effector. These fibers are unmyelinated. (Note that the term “postganglionic neuron” may be used to describe the projection from a ganglion to the target. The problem with that usage is that the cell body is in the ganglion, and only the fiber is postganglionic. Typically, the term neuron applies to the entire cell.) One type of preganglionic sympathetic fiber does not terminate in a ganglion. These are the axons from central sympathetic neurons that project to the adrenal medulla, the interior portion of the adrenal gland. These axons are still referred to as preganglionic fibers, but the target is not a ganglion. The adrenal medulla releases signaling molecules into the bloodstream, rather than using axons to communicate with target structures. The cells in the adrenal medulla that are contacted by the preganglionic fibers are called chromaffin cells. These cells are neurosecretory cells that develop from the neural crest along with the sympathetic ganglia, reinforcing the idea that the gland is, functionally, a sympathetic ganglion. The projections of the sympathetic division of the autonomic nervous system diverge widely, resulting in a broad influence of the system throughout the body. As a response to a threat, the sympathetic system would increase heart rate and breathing rate and cause blood flow to the skeletal muscle to increase and blood flow to the digestive system to decrease. Sweat gland secretion should also increase as part of an integrated response. All of those physiological changes are going to be required to occur together to run away from the hunting lioness, or the modern equivalent. This divergence is seen in the branching patterns of preganglionic sympathetic neurons—a single preganglionic sympathetic neuron may have 10—20 targets. An axon that leaves a central neuron of the lateral horn in the thoracolumbar spinal cord will pass through the white ramus communicans and enter the sympathetic chain, where it will branch toward a variety of targets. At the level of the spinal cord at which the preganglionic sympathetic fiber exits the spinal cord, a branch will synapse on a neuron in the adjacent chain ganglion. Some branches will extend up or down to a different level of the chain ganglia. Other branches will pass through the chain ganglia and project through one of the splanchnic nerves to a collateral ganglion. Finally, some branches may project through the splanchnic nerves to the adrenal medulla. All of these branches mean that one preganglionic neuron can influence different regions of the sympathetic system very broadly, by acting on widely distributed organs. Parasympathetic Division of the Autonomic Nervous System The parasympathetic division of the autonomic nervous system is named because its central neurons are located on either side of the thoracolumbar region of the spinal cord (para- = “beside” or “near”). The parasympathetic system can also be referred to as the craniosacral system (or outflow) because the preganglionic neurons are located in nuclei of the brain stem and the lateral horn of the sacral spinal cord. The connections, or “circuits,” of the parasympathetic division are similar to the general layout of the sympathetic division with a few specific differences ([{link]). The preganglionic fibers from the cranial region travel in cranial nerves, whereas preganglionic fibers from the sacral region travel in spinal nerves. The targets of these fibers are terminal ganglia, which are located near—or even within—the target effector. These ganglia are often referred to as intramural ganglia when they are found within the walls of the target organ. The postganglionic fiber projects from the terminal ganglia a short distance to the target effector, or to the specific target tissue within the organ. Comparing the relative lengths of axons in the parasympathetic system, the preganglionic fibers are long and the postganglionic fibers are short because the ganglia are close to—and sometimes within—the target effectors. The cranial component of the parasympathetic system is based in particular nuclei of the brain stem. In the midbrain, the Edinger—Westphal nucleus is part of the oculomotor complex, and axons from those neurons travel with the fibers in the oculomotor nerve (cranial nerve III) that innervate the extraocular muscles. The preganglionic parasympathetic fibers within cranial nerve III terminate in the ciliary ganglion, which is located in the posterior orbit. The postganglionic parasympathetic fibers then project to the smooth muscle of the iris to control pupillary size. In the upper medulla, the salivatory nuclei contain neurons with axons that project through the facial and glossopharyngeal nerves to ganglia that control salivary glands. Tear production is influenced by parasympathetic fibers in the facial nerve, which activate a ganglion, and ultimately the lacrimal (tear) gland. Neurons in the dorsal nucleus of the vagus nerve and the nucleus ambiguus project through the vagus nerve (cranial nerve X) to the terminal ganglia of the thoracic and abdominal cavities. Parasympathetic preganglionic fibers primarily influence the heart, bronchi, and esophagus in the thoracic cavity and the stomach, liver, pancreas, gall bladder, and small intestine of the abdominal cavity. The postganglionic fibers from the ganglia activated by the vagus nerve are often incorporated into the structure of the organ, such as the mesenteric plexus of the digestive tract organs and the intramural ganglia. Connections of Parasympathetic Division of the Autonomic Nervous System Eddinger—Westpha nucleus Super salivatory nucleus Inferior salivatory nucleus Dorsal nucleus of the vagus and nucleus ambiguus Oe ee ee es Spinal cord Region of Associated nerves Target organs (effectors) spinal cord and terminal ganglia Ciliary ganglion a é , A __\ Cranial nerve Ill eS #) SS Pterygopalatine | Cranial nerve VII ganglion G2) =) \ —} Submandibular ganglion S5 0 \ AJ Coccygeal v ganglia fused together DreoyG@ (ganglion impar) a Parasympathetic fibers —— Sympathetic fibers Eye Lacrinal gland Mucous membrane (nose and palate) Submaxillary gland Sublingual gland Mucous membrane (mouth) Parotid gland Heart Larynx Trachea Bronchi Esophagus Stomach Abdominal blood vessels Liver and bile duct Pancreas Adrenal gland Small intestine Large intestine Rectum Kidney Bladder Gonads External genitalia Neurons from brain-stem nuclei, or from the lateral horn of the sacral spinal cord, project to terminal ganglia near or within the various organs of the body. Axons from these ganglionic neurons then project the short distance to those target effectors. Chemical Signaling in the Autonomic Nervous System Where an autonomic neuron connects with a target, there is a synapse. The electrical signal of the action potential causes the release of a signaling molecule, which will bind to receptor proteins on the target cell. Synapses of the autonomic system are classified as either cholinergic, meaning that acetylcholine (ACh) is released, or adrenergic, meaning that norepinephrine is released. The terms cholinergic and adrenergic refer not only to the signaling molecule that is released but also to the class of receptors that each binds. The cholinergic system includes two classes of receptor: the nicotinic receptor and the muscarinic receptor. Both receptor types bind to ACh and cause changes in the target cell. The nicotinic receptor is a ligand- gated cation channel and the muscarinic receptor is a G protein—coupled receptor. The receptors are named for, and differentiated by, other molecules that bind to them. Whereas nicotine will bind to the nicotinic receptor, and muscarine will bind to the muscarinic receptor, there is no cross-reactivity between the receptors. The situation is similar to locks and keys. Imagine two locks—one for a classroom and the other for an office— that are opened by two separate keys. The classroom key will not open the office door and the office key will not open the classroom door. This is similar to the specificity of nicotine and muscarine for their receptors. However, a master key can open multiple locks, such as a master key for the Biology Department that opens both the classroom and the office doors. This is similar to ACh that binds to both types of receptors. The molecules that define these receptors are not crucial—they are simply tools for researchers to use in the laboratory. These molecules are exogenous, meaning that they are made outside of the human body, so a researcher can use them without any confounding endogenous results (results caused by the molecules produced in the body). The adrenergic system also has two types of receptors, named the alpha (a)-adrenergic receptor and beta (f)-adrenergic receptor. Unlike cholinergic receptors, these receptor types are not classified by which drugs can bind to them. All of them are G protein-coupled receptors. There are three types of a-adrenergic receptors, termed a1, Q, and a3, and there are two types of f-adrenergic receptors, termed 8, and B5. An additional aspect of the adrenergic system is that there is a second signaling molecule called epinephrine. The chemical difference between norepinephrine and epinephrine is the addition of a methyl group (CH3) in epinephrine. The prefix “nor-” actually refers to this chemical difference, in which a methyl group is missing. The term adrenergic should remind you of the word adrenaline, which is associated with the fight-or-flight response described at the beginning of the chapter. Adrenaline and epinephrine are two names for the same molecule. The adrenal gland (in Latin, ad- = “on top of”; renal = “kidney”) secretes adrenaline. The ending “-ine” refers to the chemical being derived, or extracted, from the adrenal gland. A similar construction from Greek instead of Latin results in the word epinephrine (epi- = “above”; nephr- = “kidney”). In scientific usage, epinephrine is preferred in the United States, whereas adrenaline is preferred in Great Britain, because “adrenalin” was once a registered, proprietary drug name in the United States. Though the drug is no longer sold, the convention of referring to this molecule by the two different names persists. Similarly, norepinephrine and noradrenaline are two names for the same molecule. Having understood the cholinergic and adrenergic systems, their role in the autonomic system is relatively simple to understand. All preganglionic fibers, both sympathetic and parasympathetic, release ACh. All ganglionic neurons—the targets of these preganglionic fibers—have nicotinic receptors in their cell membranes. The nicotinic receptor is a ligand-gated cation channel that results in depolarization of the postsynaptic membrane. The postganglionic parasympathetic fibers also release ACh, but the receptors on their targets are muscarinic receptors, which are G protein—coupled receptors and do not exclusively cause depolarization of the postsynaptic membrane. Postganglionic sympathetic fibers release norepinephrine, except for fibers that project to sweat glands and to blood vessels associated with skeletal muscles, which release ACh ((link]). Autonomic System Signaling Molecules Sympathetic Parasympathetic hs Acetylcholine > Acetylcholine > Preganglionic ete tay een nicotinic receptor nicotinic receptor Norepinephrine — a- or B-adrenergic receptors Acetylcholine > secon Acetylcholine > a muscarinic receptor a Postganglionic muscarinic (associated with sweat receptor glands and the blood vessels associated with skeletal muscles only Signaling molecules can belong to two broad groups. Neurotransmitters are released at synapses, whereas hormones are released into the bloodstream. These are simplistic definitions, but they can help to clarify this point. Acetylcholine can be considered a neurotransmitter because it is released by axons at synapses. The adrenergic system, however, presents a challenge. Postganglionic sympathetic fibers release norepinephrine, which can be considered a neurotransmitter. But the adrenal medulla releases epinephrine and norepinephrine into circulation, so they should be considered hormones. What are referred to here as synapses may not fit the strictest definition of synapse. Some sources will refer to the connection between a postganglionic fiber and a target effector as neuroeffector junctions; neurotransmitters, as defined above, would be called neuromodulators. The structure of postganglionic connections are not the typical synaptic end bulb that is found at the neuromuscular junction, but rather are chains of swellings along the length of a postganglionic fiber called a varicosity ((link]). Autonomic Varicosities Synaptic vesicles > Postganglionic varicosities Sarcolemma The connection between autonomic fibers and target effectors is not the same as the typical synapse, such as the neuromuscular junction. Instead of a synaptic end bulb, a neurotransmitter is released from swellings along the length of a fiber that makes an extended network of connections in the target effector. Note: Everyday Connections Fight or Flight? What About Fright and Freeze? The original usage of the epithet “fight or flight” comes from a scientist named Walter Cannon who worked at Harvard in 1915. The concept of homeostasis and the functioning of the sympathetic system had been introduced in France in the previous century. Cannon expanded the idea, and introduced the idea that an animal responds to a threat by preparing to stand and fight or run away. The nature of this response was thoroughly explained in a book on the physiology of pain, hunger, fear, and rage. When students learn about the sympathetic system and the fight-or-flight response, they often stop and wonder about other responses. If you were faced with a lioness running toward you as pictured at the beginning of this chapter, would you run or would you stand your ground? Some people would say that they would freeze and not know what to do. So isn’t there really more to what the autonomic system does than fight, flight, rest, or digest. What about fear and paralysis in the face of a threat? The common epithet of “fight or flight” is being enlarged to be “fight, flight, or fright” or even “fight, flight, fright, or freeze.” Cannon’s original contribution was a catchy phrase to express some of what the nervous system does in response to a threat, but it is incomplete. The sympathetic system is responsible for the physiological responses to emotional states. The name “sympathetic” can be said to mean that (sym- = “together”; - pathos = “pain,” “suffering,” or “emotion”). Note: Oia) apr = cpenstax couece” ony ‘ io re Watch this video to learn more about the nervous system. As described in this video, the nervous system has a way to deal with threats and stress that is separate from the conscious control of the somatic nervous system. The system comes from a time when threats were about survival, but in the modern age, these responses become part of stress and anxiety. This video describes how the autonomic system is only part of the response to threats, or stressors. What other organ system gets involved, and what part of the brain coordinates the two systems for the entire response, including epinephrine (adrenaline) and cortisol? Chapter Review The primary responsibilities of the autonomic nervous system are to regulate homeostatic mechanisms in the body, which is also part of what the endocrine system does. The key to understanding the autonomic system is to explore the response pathways—the output of the nervous system. The way we respond to the world around us, to manage the internal environment on the basis of the external environment, is divided between two parts of the autonomic nervous system. The sympathetic division responds to threats and produces a readiness to confront the threat or to run away: the fight-or- flight response. The parasympathetic division plays the opposite role. When the external environment does not present any immediate danger, a restful mode descends on the body, and the digestive system is more active. The sympathetic output of the nervous system originates out of the lateral horn of the thoracolumbar spinal cord. An axon from one of these central neurons projects by way of the ventral spinal nerve root and spinal nerve to a sympathetic ganglion, either in the sympathetic chain ganglia or one of the collateral locations, where it synapses on a ganglionic neuron. These preganglionic fibers release ACh, which excites the ganglionic neuron through the nicotinic receptor. The axon from the ganglionic neuron—the postganglionic fiber—then projects to a target effector where it will release norepinephrine to bind to an adrenergic receptor, causing a change in the physiology of that organ in keeping with the broad, divergent sympathetic response. The postganglionic connections to sweat glands in the skin and blood vessels supplying skeletal muscle are, however, exceptions; those fibers release ACh onto muscarinic receptors. The sympathetic system has a specialized preganglionic connection to the adrenal medulla that causes epinephrine and norepinephrine to be released into the bloodstream rather than exciting a neuron that contacts an organ directly. This hormonal component means that the sympathetic chemical signal can spread throughout the body very quickly and affect many organ systems at once. The parasympathetic output is based in the brain stem and sacral spinal cord. Neurons from particular nuclei in the brain stem or from the lateral horn of the sacral spinal cord (preganglionic neurons) project to terminal (intramural) ganglia located close to or within the wall of target effectors. These preganglionic fibers also release ACh onto nicotinic receptors to excite the ganglionic neurons. The postganglionic fibers then contact the target tissues within the organ to release ACh, which binds to muscarinic receptors to induce rest-and-digest responses. Signaling molecules utilized by the autonomic nervous system are released from axons and can be considered as either neurotransmitters (when they directly interact with the effector) or as hormones (when they are released into the bloodstream). The same molecule, such as norepinephrine, could be considered either a neurotransmitter or a hormone on the basis of whether it is released from a postganglionic sympathetic axon or from the adrenal gland. The synapses in the autonomic system are not always the typical type of connection first described in the neuromuscular junction. Instead of having synaptic end bulbs at the very end of an axonal fiber, they may have swellings—called varicosities—along the length of a fiber so that it makes a network of connections within the target tissue. Interactive Link Questions Exercise: Problem: Watch this video to learn more about adrenaline and the fight-or-flight response. When someone is said to have a rush of adrenaline, the image of bungee jumpers or skydivers usually comes to mind. But adrenaline, also known as epinephrine, is an important chemical in coordinating the body’s fight-or-flight response. In this video, you look inside the physiology of the fight-or-flight response, as envisioned for a firefighter. His body’s reaction is the result of the sympathetic division of the autonomic nervous system causing system-wide changes as it prepares for extreme responses. What two changes does adrenaline bring about to help the skeletal muscle response? Solution: The heart rate increases to send more blood to the muscles, and the liver releases stored glucose to fuel the muscles. Exercise: Problem: Watch this video to learn more about the nervous system. As described in this video, the nervous system has a way to deal with threats and stress that is separate from the conscious control of the somatic nervous system. The system comes from a time when threats were about survival, but in the modern age, these responses become part of stress and anxiety. This video describes how the autonomic system is only part of the response to threats, or stressors. What other organ system gets involved, and what part of the brain coordinates the two systems for the entire response, including epinephrine (adrenaline) and cortisol? Solution: The endocrine system is also responsible for responses to stress in our lives. The hypothalamus coordinates the autonomic response through projections into the spinal cord and through influence over the pituitary gland, the effective center of the endocrine system. Review Questions Exercise: Problem: Which of these physiological changes would not be considered part of the sympathetic fight-or-flight response? a. increased heart rate b. increased sweating c. dilated pupils d. increased stomach motility Solution: D Exercise: Problem: Which type of fiber could be considered the longest? a. preganglionic parasympathetic b. preganglionic sympathetic c. postganglionic parasympathetic d. postganglionic sympathetic Solution: A Exercise: Problem: Which signaling molecule is most likely responsible for an increase in digestive activity? a. epinephrine b. norepinephrine c. acetylcholine d. adrenaline Solution: C Exercise: Problem: Which of these cranial nerves contains preganglionic parasympathetic fibers? a. optic, CN II b. facial, CN VII c. trigeminal, CN V d. hypoglossal, CN XII Solution: B Exercise: Problem: Which of the following is not a target of a sympathetic preganglionic fiber? a. intermural ganglion b. collateral ganglion c. adrenal gland d. chain ganglion Solution: A Critical Thinking Questions Exercise: Problem: In the context of a lioness hunting on the savannah, why would the sympathetic system not activate the digestive system? Solution: Whereas energy is needed for running away from the threat, blood needs to be sent to the skeletal muscles for oxygen supply. The additional fuel, in the form of carbohydrates, probably wouldn’t improve the ability to escape the threat as much as the diversion of oxygen-rich blood would hinder it. Exercise: Problem: A target effector, such as the heart, receives input from the sympathetic and parasympathetic systems. What is the actual difference between the sympathetic and parasympathetic divisions at the level of those connections (i.e., at the synapse)? Solution: The postganglionic sympathetic fiber releases norepinephrine, whereas the postganglionic parasympathetic fiber releases acetylcholine. Specific locations in the heart have adrenergic receptors and muscarinic receptors. Which receptors are bound is the signal that determines how the heart responds. Glossary alpha (a)-adrenergic receptor one of the receptors to which epinephrine and norepinephrine bind, which comes in three subtypes: a, Q>, and a3 acetylcholine (ACh) neurotransmitter that binds at a motor end-plate to trigger depolarization adrenal medulla interior portion of the adrenal (or suprarenal) gland that releases epinephrine and norepinephrine into the bloodstream as hormones adrenergic synapse where norepinephrine is released, which binds to a- or B- adrenergic receptors beta (B)-adrenergic receptor one of the receptors to which epinephrine and norepinephrine bind, which comes in two subtypes: 8, and B»5 celiac ganglion one of the collateral ganglia of the sympathetic system that projects to the digestive system central neuron specifically referring to the cell body of a neuron in the autonomic system that is located in the central nervous system, specifically the lateral horn of the spinal cord or a brain stem nucleus cholinergic synapse at which acetylcholine is released and binds to the nicotinic or muscarinic receptor chromaffin cells neuroendocrine cells of the adrenal medulla that release epinephrine and norepinephrine into the bloodstream as part of sympathetic system activity ciliary ganglion one of the terminal ganglia of the parasympathetic system, located in the posterior orbit, axons from which project to the iris collateral ganglia ganglia outside of the sympathetic chain that are targets of sympathetic preganglionic fibers, which are the celiac, inferior mesenteric, and superior mesenteric ganglia craniosacral system alternate name for the parasympathetic division of the autonomic nervous system that is based on the anatomical location of central neurons in brain-stem nuclei and the lateral horn of the sacral spinal cord; also referred to as craniosacral outflow dorsal nucleus of the vagus nerve location of parasympathetic neurons that project through the vagus nerve to terminal ganglia in the thoracic and abdominal cavities Eddinger—Westphal nucleus location of parasympathetic neurons that project to the ciliary ganglion endogenous describes substance made in the human body epinephrine signaling molecule released from the adrenal medulla into the bloodstream as part of the sympathetic response exogenous describes substance made outside of the human body fight-or-flight response set of responses induced by sympathetic activity that lead to either fleeing a threat or standing up to it, which in the modem world is often associated with anxious feelings G protein—coupled receptor membrane protein complex that consists of a receptor protein that binds to a signaling molecule—a G protein—that is activated by that binding and in tur activates an effector protein (enzyme) that creates a second-messenger molecule in the cytoplasm of the target cell ganglionic neuron specifically refers to the cell body of a neuron in the autonomic system that is located in a ganglion gray rami Communicantes (singular = ramus communicans) unmyelinated structures that provide a short connection from a sympathetic chain ganglion to the spinal nerve that contains the postganglionic sympathetic fiber greater splanchnic nerve nerve that contains fibers of the central sympathetic neurons that do not synapse in the chain ganglia but project onto the celiac ganglion inferior mesenteric ganglion one of the collateral ganglia of the sympathetic system that projects to the digestive system intramural ganglia terminal ganglia of the parasympathetic system that are found within the walls of the target effector lesser splanchnic nerve nerve that contains fibers of the central sympathetic neurons that do not synapse in the chain ganglia but project onto the inferior mesenteric ganglion ligand-gated cation channel ion channel, such as the nicotinic receptor, that is specific to positively charged ions and opens when a molecule such as a neurotransmitter binds to it mesenteric plexus nervous tissue within the wall of the digestive tract that contains neurons that are the targets of autonomic preganglionic fibers and that project to the smooth muscle and glandular tissues in the digestive organ muscarinic receptor type of acetylcholine receptor protein that is characterized by also binding to muscarine and is a metabotropic receptor nicotinic receptor type of acetylcholine receptor protein that is characterized by also binding to nicotine and is an ionotropic receptor norepinephrine signaling molecule released as a neurotransmitter by most postganglionic sympathetic fibers as part of the sympathetic response, or as a hormone into the bloodstream from the adrenal medulla nucleus ambiguus brain-stem nucleus that contains neurons that project through the vagus nerve to terminal ganglia in the thoracic cavity; specifically associated with the heart parasympathetic division division of the autonomic nervous system responsible for restful and digestive functions paravertebral ganglia autonomic ganglia superior to the sympathetic chain ganglia postganglionic fiber axon from a ganglionic neuron in the autonomic nervous system that projects to and synapses with the target effector; sometimes referred to as a postganglionic neuron preganglionic fiber axon from a central neuron in the autonomic nervous system that projects to and synapses with a ganglionic neuron; sometimes referred to as a preganglionic neuron prevertebral ganglia autonomic ganglia that are anterior to the vertebral column and functionally related to the sympathetic chain ganglia rest and digest set of functions associated with the parasympathetic system that lead to restful actions and digestion superior cervical ganglion one of the paravertebral ganglia of the sympathetic system that projects to the head superior mesenteric ganglion one of the collateral ganglia of the sympathetic system that projects to the digestive system sympathetic chain ganglia series of ganglia adjacent to the vertebral column that receive input from central sympathetic neurons sympathetic division division of the autonomic nervous system associated with the fight-or- flight response target effector organ, tissue, or gland that will respond to the control of an autonomic or somatic or endocrine signal terminal ganglia ganglia of the parasympathetic division of the autonomic system, which are located near or within the target effector, the latter also known as intramural ganglia thoracolumbar system alternate name for the sympathetic division of the autonomic nervous system that is based on the anatomical location of central neurons in the lateral horn of the thoracic and upper lumbar spinal cord varicosity structure of some autonomic connections that is not a typical synaptic end bulb, but a string of swellings along the length of a fiber that makes a network of connections with the target effector white rami communicantes (singular = ramus communicans) myelinated structures that provide a short connection from a sympathetic chain ganglion to the spinal nerve that contains the preganglionic sympathetic fiber Central Control By the end of this section, you will be able to: ¢ Describe the role of higher centers of the brain in autonomic regulation e Explain the connection of the hypothalamus to homeostasis ¢ Describe the regions of the CNS that link the autonomic system with emotion e Describe the pathways important to descending control of the autonomic system The pupillary light reflex ((link]) begins when light hits the retina and causes a signal to travel along the optic nerve. This is visual sensation, because the afferent branch of this reflex is simply sharing the special sense pathway. Bright light hitting the retina leads to the parasympathetic response, through the oculomotor nerve, followed by the postganglionic fiber from the ciliary ganglion, which stimulates the circular fibers of the iris to contract and constrict the pupil. When light hits the retina in one eye, both pupils contract. When that light is removed, both pupils dilate again back to the resting position. When the stimulus is unilateral (presented to only one eye), the response is bilateral (both eyes). The same is not true for somatic reflexes. If you touch a hot radiator, you only pull that arm back, not both. Central control of autonomic reflexes is different than for somatic reflexes. The hypothalamus, along with other CNS locations, controls the autonomic system. Pupillary Reflex Pathways Pretectal Action potentials from right eye reach both right and left pretectal nuclei. Oculomotor SY, VA nerves (III) Ciliary ganglia The pretectal nuclei stimulate both sides of the Eddinger—Westphal nucleus even though the light was perceived only in () The right and left sides the right eye. of the Eddinger—Westphal nuclei generate action @) Light is shined on potentials through the right and left oculomotor right eye only. _<) bb Ree Thalamus Hippocampus . . Structures arranged around the edge of the cerebrum constitute the limbic lobe, which includes the amygdala, hippocampus, and cingulate gyrus, and connects to the hypothalamus. The Medulla The medulla contains nuclei referred to as the cardiovascular center, which controls the smooth and cardiac muscle of the cardiovascular system through autonomic connections. When the homeostasis of the cardiovascular system shifts, such as when blood pressure changes, the coordination of the autonomic system can be accomplished within this region. Furthermore, when descending inputs from the hypothalamus stimulate this area, the sympathetic system can increase activity in the cardiovascular system, such as in response to anxiety or stress. The preganglionic sympathetic fibers that are responsible for increasing heart rate are referred to as the cardiac accelerator nerves, whereas the preganglionic sympathetic fibers responsible for constricting blood vessels compose the vasomotor nerves. Several brain stem nuclei are important for the visceral control of major organ systems. One brain stem nucleus involved in cardiovascular function is the solitary nucleus. It receives sensory input about blood pressure and cardiac function from the glossopharyngeal and vagus nerves, and its output will activate sympathetic stimulation of the heart or blood vessels through the upper thoracic lateral horn. Another brain stem nucleus important for visceral control is the dorsal motor nucleus of the vagus nerve, which is the motor nucleus for the parasympathetic functions ascribed to the vagus nerve, including decreasing the heart rate, relaxing bronchial tubes in the lungs, and activating digestive function through the enteric nervous system. The nucleus ambiguus, which is named for its ambiguous histology, also contributes to the parasympathetic output of the vagus nerve and targets muscles in the pharynx and larynx for swallowing and speech, as well as contributing to the parasympathetic tone of the heart along with the dorsal motor nucleus of the vagus. Note: Everyday Connections Exercise and the Autonomic System In addition to its association with the fight-or-flight response and rest-and- digest functions, the autonomic system is responsible for certain everyday functions. For example, it comes into play when homeostatic mechanisms dynamically change, such as the physiological changes that accompany exercise. Getting on the treadmill and putting in a good workout will cause the heart rate to increase, breathing to be stronger and deeper, sweat glands to activate, and the digestive system to suspend activity. These are the same physiological changes associated with the fight-or-flight response, but there is nothing chasing you on that treadmill. This is not a simple homeostatic mechanism at work because “maintaining the internal environment” would mean getting all those changes back to their set points. Instead, the sympathetic system has become active during exercise so that your body can cope with what is happening. A homeostatic mechanism is dealing with the conscious decision to push the body away from a resting state. The heart, actually, is moving away from its homeostatic set point. Without any input from the autonomic system, the heart would beat at approximately 100 bpm, and the parasympathetic system slows that down to the resting rate of approximately 70 bpm. But in the middle of a good workout, you should see your heart rate at 120-140 bpm. You could say that the body is stressed because of what you are doing to it. Homeostatic mechanisms are trying to keep blood pH in the normal range, or to keep body temperature under control, but those are in response to the choice to exercise. Note: Ct [= os mss" OPENStax COLLEGE A “a r r. io ane Watch this video to learn about physical responses to emotion. The autonomic system, which is important for regulating the homeostasis of the organ systems, is also responsible for our physiological responses to emotions such as fear. The video summarizes the extent of the body’s reactions and describes several effects of the autonomic system in response to fear. On the basis of what you have already studied about autonomic function, which effect would you expect to be associated with parasympathetic, rather than sympathetic, activity? Chapter Review The autonomic system integrates sensory information and higher cognitive processes to generate output, which balances homeostatic mechanisms. The central autonomic structure is the hypothalamus, which coordinates sympathetic and parasympathetic efferent pathways to regulate activities of the organ systems of the body. The majority of hypothalamic output travels through the medial forebrain bundle and the dorsal longitudinal fasciculus to influence brain stem and spinal components of the autonomic nervous system. The medial forebrain bundle also connects the hypothalamus with higher centers of the limbic system where emotion can influence visceral responses. The amygdala is a structure within the limbic system that influences the hypothalamus in the regulation of the autonomic system, as well as the endocrine system. These higher centers have descending control of the autonomic system through brain stem centers, primarily in the medulla, such as the cardiovascular center. This collection of medullary nuclei regulates cardiac function, as well as blood pressure. Sensory input from the heart, aorta, and carotid sinuses project to these regions of the medulla. The solitary nucleus increases sympathetic tone of the cardiovascular system through the cardiac accelerator and vasomotor nerves. The nucleus ambiguus and the dorsal motor nucleus both contribute fibers to the vagus nerve, which exerts parasympathetic control of the heart by decreasing heart rate. Interactive Link Questions Exercise: Problem: Watch this video to learn about physical responses to emotion. The autonomic system, which is important for regulating the homeostasis of the organ systems, is also responsible for our physiological responses to emotions such as fear. The video summarizes the extent of the body’s reactions and describes several effects of the autonomic system in response to fear. On the basis of what you have already studied about autonomic function, which effect would you expect to be associated with parasympathetic, rather than sympathetic, activity? Solution: The release of urine in extreme fear. The sympathetic system normally constricts sphincters such as that of the urethra. Review Questions Exercise: Problem: Which of these locations in the forebrain is the master control center for homeostasis through the autonomic and endocrine systems? a. hypothalamus b. thalamus c. amygdala d. cerebral cortex Solution: A Exercise: Problem: Which nerve projects to the hypothalamus to indicate the level of light stimuli in the retina? a. glossopharyngeal b. oculomotor c. optic d. vagus Solution: C Exercise: Problem: What region of the limbic lobe is responsible for generating stress responses via the hypothalamus? a. hippocampus b. amygdala c. mammillary bodies d. prefrontal cortex Solution: B Exercise: Problem: What is another name for the preganglionic sympathetic fibers that project to the heart? a. solitary tract b. vasomotor nerve c. vagus nerve d. cardiac accelerator nerve Solution: D Exercise: Problem: What central fiber tract connects forebrain and brain stem structures with the hypothalamus? a. cardiac accelerator nerve b. medial forebrain bundle c. dorsal longitudinal fasciculus d. corticospinal tract Solution: B Critical Thinking Questions Exercise: Problem: Horner’s syndrome is a condition that presents with changes in one eye, such as pupillary constriction and dropping of eyelids, as well as decreased sweating in the face. Why could a tumor in the thoracic cavity have an effect on these autonomic functions? Solution: Pupillary dilation and sweating, two functions lost in Horner’s syndrome, are caused by the sympathetic system. A tumor in the thoracic cavity may interrupt the output of the thoracic ganglia that project to the head and face. Exercise: Problem: The cardiovascular center is responsible for regulating the heart and blood vessels through homeostatic mechanisms. What tone does each component of the cardiovascular system have? What connections does the cardiovascular center invoke to keep these two systems in their resting tone? Solution: The heart—based on the resting heart rate—is under parasympathetic tone, and the blood vessels—based on the lack of parasympathetic input—are under sympathetic tone. The vagus nerve contributes to the lowered resting heart rate, whereas the vasomotor nerves maintain the slight constriction of systemic blood vessels. Glossary cardiac accelerator nerves preganglionic sympathetic fibers that cause the heart rate to increase when the cardiovascular center in the medulla initiates a signal cardiovascular center region in the medulla that controls the cardiovascular system through cardiac accelerator nerves and vasomotor nerves, which are components of the sympathetic division of the autonomic nervous system dorsal longitudinal fasciculus major output pathway of the hypothalamus that descends through the gray matter of the brain stem and into the spinal cord limbic lobe structures arranged around the edges of the cerebrum that are involved in memory and emotion medial forebrain bundle fiber pathway that extends anteriorly into the basal forebrain, passes through the hypothalamus, and extends into the brain stem and spinal cord vasomotor nerves preganglionic sympathetic fibers that cause the constriction of blood vessels in response to signals from the cardiovascular center Organs and Structures of the Respiratory System By the end of this section, you will be able to: e List the structures that make up the respiratory system e Describe how the respiratory system processes oxygen and CO» e Compare and contrast the functions of upper respiratory tract with the lower respiratory tract The major organs of the respiratory system function primarily to provide oxygen to body tissues for cellular respiration, remove the waste product carbon dioxide, and help to maintain acid-base balance. Portions of the respiratory system are also used for non-vital functions, such as sensing odors, speech production, and for straining, such as during childbirth or coughing ([link]). Major Respiratory Structures Nasal cavity Nostril Oral cavit J Pharynx Larynx Trachea Left main bronchus Right main bronchus Right lun : Left lung Diaphragm The major respiratory structures span the nasal cavity to the diaphragm. Functionally, the respiratory system can be divided into a conducting zone and a respiratory zone. The conducting zone of the respiratory system includes the organs and structures not directly involved in gas exchange. The gas exchange occurs in the respiratory zone. Conducting Zone The major functions of the conducting zone are to provide a route for incoming and outgoing air, remove debris and pathogens from the incoming air, and warm and humidify the incoming air. Several structures within the conducting zone perform other functions as well. The epithelium of the nasal passages, for example, is essential to sensing odors, and the bronchial epithelium that lines the lungs can metabolize some airborne carcinogens. The Nose and its Adjacent Structures The major entrance and exit for the respiratory system is through the nose. When discussing the nose, it is helpful to divide it into two major sections: the external nose, and the nasal cavity or internal nose. The external nose consists of the surface and skeletal structures that result in the outward appearance of the nose and contribute to its numerous functions ([link]). The root is the region of the nose located between the eyebrows. The bridge is the part of the nose that connects the root to the rest of the nose. The dorsum nasi is the length of the nose. The apex is the tip of the nose. On either side of the apex, the nostrils are formed by the alae (singular = ala). An ala is a cartilaginous structure that forms the lateral side of each naris (plural = nares), or nostril opening. The philtrum is the concave surface that connects the apex of the nose to the upper lip. Nose i ee Dorsum nasi a a as ———— es _—_ 7" i ea Philtrum ew 4 Frontal bone Nasal bone Maxillary bone Septal cartilage Major alar cartilage Septal cartilage This illustration shows features of the external nose (top) and skeletal features of the nose (bottom). Underneath the thin skin of the nose are its skeletal features (see [link], lower illustration). While the root and bridge of the nose consist of bone, the protruding portion of the nose is composed of cartilage. As a result, when looking at a skull, the nose is missing. The nasal bone is one of a pair of bones that lies under the root and bridge of the nose. The nasal bone articulates superiorly with the frontal bone and laterally with the maxillary bones. Septal cartilage is flexible hyaline cartilage connected to the nasal bone, forming the dorsum nasi. The alar cartilage consists of the apex of the nose; it surrounds the naris. The nares open into the nasal cavity, which is separated into left and right sections by the nasal septum ([link]). The nasal septum is formed anteriorly by a portion of the septal cartilage (the flexible portion you can touch with your fingers) and posteriorly by the perpendicular plate of the ethmoid bone (a cranial bone located just posterior to the nasal bones) and the thin vomer bones (whose name refers to its plough shape). Each lateral wall of the nasal cavity has three bony projections, called the superior, middle, and inferior nasal conchae. The inferior conchae are separate bones, whereas the superior and middle conchae are portions of the ethmoid bone. Conchae serve to increase the surface area of the nasal cavity and to disrupt the flow of air as it enters the nose, causing air to bounce along the epithelium, where it is cleaned and warmed. The conchae and meatuses also conserve water and prevent dehydration of the nasal epithelium by trapping water during exhalation. The floor of the nasal cavity is composed of the palate. The hard palate at the anterior region of the nasal cavity is composed of bone. The soft palate at the posterior portion of the nasal cavity consists of muscle tissue. Air exits the nasal cavities via the internal nares and moves into the pharynx. Upper Airway Sphenoidal sinus ee Ethmoid bone Nasal meatuses Olfactory epithelium (superior, middle, and inferior) Nasal conchae Pharyngeal tonsil Nasal vestibule Opening of ase auditory tube Nasopharynx Hard palate Uvula Soft palate Tounge Palatine tonsil ; Lingual tonsil se Epiglottis Oropharynx Hyoid bone Laryngopharynx Vestibular fold Vocal fold Thyroid cartilage Cricoid cartilage Esophagus Thyroid gland Trachea Several bones that help form the walls of the nasal cavity have air- containing spaces called the paranasal sinuses, which serve to warm and humidify incoming air. Sinuses are lined with a mucosa. Each paranasal sinus is named for its associated bone: frontal sinus, maxillary sinus, sphenoidal sinus, and ethmoidal sinus. The sinuses produce mucus and lighten the weight of the skull. The nares and anterior portion of the nasal cavities are lined with mucous membranes, containing sebaceous glands and hair follicles that serve to prevent the passage of large debris, such as dirt, through the nasal cavity. An olfactory epithelium used to detect odors is found deeper in the nasal cavity. The conchae, meatuses, and paranasal sinuses are lined by respiratory epithelium composed of pseudostratified ciliated columnar epithelium ({link]). The epithelium contains goblet cells, one of the specialized, columnar epithelial cells that produce mucus to trap debris. The cilia of the respiratory epithelium help remove the mucus and debris from the nasal cavity with a constant beating motion, sweeping materials towards the throat to be swallowed. Interestingly, cold air slows the movement of the cilia, resulting in accumulation of mucus that may in turn lead to a runny nose during cold weather. This moist epithelium functions to warm and humidify incoming air. Capillaries located just beneath the nasal epithelium warm the air by convection. Serous and mucus-producing cells also secrete the lysozyme enzyme and proteins called defensins, which have antibacterial properties. Immune cells that patrol the connective tissue deep to the respiratory epithelium provide additional protection. Pseudostratified Ciliated Columnar Epithelium Lumen of Goblet cell trachea Cilia Pseudostratified columnar epithelia Seromucous gland in submucosa Respiratory epithelium is pseudostratified ciliated columnar epithelium. Seromucous glands provide lubricating mucus. LM x 680. (Micrograph provided by the Regents of University of Michigan Medical School © 2012) View the University of Michigan WebScope to explore the tissue sample in greater detail. Pharynx The pharynx is a tube formed by skeletal muscle and lined by mucous membrane that is continuous with that of the nasal cavities (see [link]). The pharynx is divided into three major regions: the nasopharynx, the oropharynx, and the laryngopharynx ([link]). Divisions of the Pharynx Nasal cavity Hard palate Soft palate Tongue Epiglottis Larynx (voice box) Esophagus Trachea The pharynx is divided into three regions: the nasopharynx, the oropharynx, and the laryngopharynx. The nasopharynx is flanked by the conchae of the nasal cavity, and it serves only as an airway. At the top of the nasopharynx are the pharyngeal tonsils. A pharyngeal tonsil, also called an adenoid, is an aggregate of lymphoid reticular tissue similar to a lymph node that lies at the superior portion of the nasopharynx. The function of the pharyngeal tonsil is not well understood, but it contains a rich supply of lymphocytes and is covered with ciliated epithelium that traps and destroys invading pathogens that enter during inhalation. The pharyngeal tonsils are large in children, but interestingly, tend to regress with age and may even disappear. The uvula is a small bulbous, teardrop-shaped structure located at the apex of the soft palate. Both the uvula and soft palate move like a pendulum during swallowing, swinging upward to close off the nasopharynx to prevent ingested materials from entering the nasal cavity. In addition, auditory (Eustachian) tubes that connect to each middle ear cavity open into the nasopharynx. This connection is why colds often lead to ear infections. The oropharynx is a passageway for both air and food. The oropharynx is bordered superiorly by the nasopharynx and anteriorly by the oral cavity. The fauces is the opening at the connection between the oral cavity and the oropharynx. As the nasopharynx becomes the oropharynx, the epithelium changes from pseudostratified ciliated columnar epithelium to stratified squamous epithelium. The oropharynx contains two distinct sets of tonsils, the palatine and lingual tonsils. A palatine tonsil is one of a pair of structures located laterally in the oropharynx in the area of the fauces. The lingual tonsil is located at the base of the tongue. Similar to the pharyngeal tonsil, the palatine and lingual tonsils are composed of lymphoid tissue, and trap and destroy pathogens entering the body through the oral or nasal Cavities. The laryngopharynx is inferior to the oropharynx and posterior to the larynx. It continues the route for ingested material and air until its inferior end, where the digestive and respiratory systems diverge. The stratified squamous epithelium of the oropharynx is continuous with the laryngopharynx. Anteriorly, the laryngopharynx opens into the larynx, whereas posteriorly, it enters the esophagus. Larynx The larynx is a cartilaginous structure inferior to the laryngopharynx that connects the pharynx to the trachea and helps regulate the volume of air that enters and leaves the lungs ({link]). The structure of the larynx is formed by several pieces of cartilage. Three large cartilage pieces—the thyroid cartilage (anterior), epiglottis (superior), and cricoid cartilage (inferior)— form the major structure of the larynx. The thyroid cartilage is the largest piece of cartilage that makes up the larynx. The thyroid cartilage consists of the laryngeal prominence, or “Adam’s apple,” which tends to be more prominent in males. The thick cricoid cartilage forms a ring, with a wide posterior region and a thinner anterior region. Three smaller, paired cartilages—the arytenoids, corniculates, and cuneiforms—attach to the epiglottis and the vocal cords and muscle that help move the vocal cords to produce speech. Larynx Epiglottis Body of hyoid bone Thyrohyoid membrane Thyroid cartilage Laryngeal prominence Cricothyroid ligament - 9 Cricoid cartilage Cricotracheal ligament Tracheal cartilages Epiglottis Thyrohyoid membrane Body of hyoid bone Fatty pad Thyrohyoid membrane Vestibular fold Vocal fold Thyroid cartilage Cricothyroid ligament Cuneiform cartilage Corniculate cartilage Arytenoid cartilage Cricoid cartilage Cricotracheal ligament Tracheal cartilages Right lateral view The larynx extends from the laryngopharynx and the hyoid bone to the trachea. The epiglottis, attached to the thyroid cartilage, is a very flexible piece of elastic cartilage that covers the opening of the trachea (see [link]). When in the “closed” position, the unattached end of the epiglottis rests on the glottis. The glottis is composed of the vestibular folds, the true vocal cords, and the space between these folds ([link]). A vestibular fold, or false vocal cord, is one of a pair of folded sections of mucous membrane. A true vocal cord is one of the white, membranous folds attached by muscle to the thyroid and arytenoid cartilages of the larynx on their outer edges. The inner edges of the true vocal cords are free, allowing oscillation to produce sound. The size of the membranous folds of the true vocal cords differs between individuals, producing voices with different pitch ranges. Folds in males tend to be larger than those in females, which create a deeper voice. The act of swallowing causes the pharynx and larynx to lift upward, allowing the pharynx to expand and the epiglottis of the larynx to swing downward, closing the opening to the trachea. These movements produce a larger area for food to pass through, while preventing food and beverages from entering the trachea. Vocal Cords Esophagus Pyriform fossa Trachea True vocal cord Vestibular fold Glottis Epiglottis Tongue The true vocal cords and vestibular folds of the larynx are viewed inferiorly from the laryngopharynx. Continuous with the laryngopharynx, the superior portion of the larynx is lined with stratified squamous epithelium, transitioning into pseudostratified ciliated columnar epithelium that contains goblet cells. Similar to the nasal cavity and nasopharynx, this specialized epithelium produces mucus to trap debris and pathogens as they enter the trachea. The cilia beat the mucus upward towards the laryngopharynx, where it can be swallowed down the esophagus. Trachea The trachea (windpipe) extends from the larynx toward the lungs ((link]a). The trachea is formed by 16 to 20 stacked, C-shaped pieces of hyaline cartilage that are connected by dense connective tissue. The trachealis muscle and elastic connective tissue together form the fibroelastic membrane, a flexible membrane that closes the posterior surface of the trachea, connecting the C-shaped cartilages. The fibroelastic membrane allows the trachea to stretch and expand slightly during inhalation and exhalation, whereas the rings of cartilage provide structural support and prevent the trachea from collapsing. In addition, the trachealis muscle can be contracted to force air through the trachea during exhalation. The trachea is lined with pseudostratified ciliated columnar epithelium, which is continuous with the larynx. The esophagus borders the trachea posteriorly. Trachea (a) The tracheal tube is formed by stacked, C-shaped pieces of hyaline cartilage. (b) The layer visible in this cross-section of tracheal wall tissue between the hyaline cartilage and the lumen of the trachea is the mucosa, which is composed of pseudostratified ciliated columnar epithelium that contains goblet cells. LM x 1220. (Micrograph provided by the Regents of University of Michigan Medical School © 2012) Larynx Trachea =) Tracheal cartilages Submucosal Pseudostratified seromucous glands columnar epithelia Cilia =©Lumen Primary bronchi Right lung Left lung Hyaline cartilage Secondary bronchi (a) (b) Bronchial Tree The trachea branches into the right and left primary bronchi at the carina. These bronchi are also lined by pseudostratified ciliated columnar epithelium containing mucus-producing goblet cells ((link]b). The carina is a raised structure that contains specialized nervous tissue that induces violent coughing if a foreign body, such as food, is present. Rings of cartilage, similar to those of the trachea, support the structure of the bronchi and prevent their collapse. The primary bronchi enter the lungs at the hilum, a concave region where blood vessels, lymphatic vessels, and nerves also enter the lungs. The bronchi continue to branch into bronchial a tree. A bronchial tree (or respiratory tree) is the collective term used for these multiple-branched bronchi. The main function of the bronchi, like other conducting zone structures, is to provide a passageway for air to move into and out of each lung. In addition, the mucous membrane traps debris and pathogens. A bronchiole branches from the tertiary bronchi. Bronchioles, which are about 1 mm in diameter, further branch until they become the tiny terminal bronchioles, which lead to the structures of gas exchange. There are more than 1000 terminal bronchioles in each lung. The muscular walls of the bronchioles do not contain cartilage like those of the bronchi. This muscular wall can change the size of the tubing to increase or decrease airflow through the tube. Respiratory Zone In contrast to the conducting zone, the respiratory zone includes structures that are directly involved in gas exchange. The respiratory zone begins where the terminal bronchioles join a respiratory bronchiole, the smallest type of bronchiole ({link]), which then leads to an alveolar duct, opening into a cluster of alveoli. Respiratory Zone Terminal bronchiole Smooth muscle Deoxygenated blood from pulmonary artery Oxygenated blood to pulmonary vein Respiratory bronchiole Alveolar Alveolus sac Capillaries Alveolar pores Bronchioles lead to alveolar sacs in the respiratory zone, where gas exchange occurs. Alveoli An alveolar duct is a tube composed of smooth muscle and connective tissue, which opens into a cluster of alveoli. An alveolus is one of the many small, grape-like sacs that are attached to the alveolar ducts. An alveolar sac is a cluster of many individual alveoli that are responsible for gas exchange. An alveolus is approximately 200 pm in diameter with elastic walls that allow the alveolus to stretch during air intake, which greatly increases the surface area available for gas exchange. Alveoli are connected to their neighbors by alveolar pores, which help maintain equal air pressure throughout the alveoli and lung ([link]). Structures of the Respiratory Zone (a) The alveolus is responsible for gas exchange. (b) A micrograph shows the alveolar structures within lung tissue. LM x 178. (Micrograph provided by the Regents of University of Michigan Medical School © 2012) Alveoli Alveolar duct Blood vessels Lumen of bronchiole U> Alveolar pores Capillary Respiratory membrane * Type | alveolar cell Macrophage Alveolus (gas-filled space) Type II alveolar cell Alveolar sac (a) (b) The alveolar wall consists of three major cell types: type I alveolar cells, type II alveolar cells, and alveolar macrophages. A type I alveolar cell is a squamous epithelial cell of the alveoli, which constitute up to 97 percent of the alveolar surface area. These cells are about 25 nm thick and are highly permeable to gases. A type II alveolar cell is interspersed among the type I cells and secretes pulmonary surfactant, a substance composed of phospholipids and proteins that reduces the surface tension of the alveoli. Roaming around the alveolar wall is the alveolar macrophage, a phagocytic cell of the immune system that removes debris and pathogens that have reached the alveoli. The simple squamous epithelium formed by type I alveolar cells is attached to a thin, elastic basement membrane. This epithelium is extremely thin and borders the endothelial membrane of capillaries. Taken together, the alveoli and capillary membranes form a respiratory membrane that is approximately 0.5 mm thick. The respiratory membrane allows gases to cross by simple diffusion, allowing oxygen to be picked up by the blood for transport and CO; to be released into the air of the alveoli. Note: Diseases of the... Respiratory System: Asthma Asthma is common condition that affects the lungs in both adults and children. Approximately 8.2 percent of adults (18.7 million) and 9.4 percent of children (7 million) in the United States suffer from asthma. In addition, asthma is the most frequent cause of hospitalization in children. Asthma is a chronic disease characterized by inflammation and edema of the airway, and bronchospasms (that is, constriction of the bronchioles), which can inhibit air from entering the lungs. In addition, excessive mucus secretion can occur, which further contributes to airway occlusion ((link]). Cells of the immune system, such as eosinophils and mononuclear cells, may also be involved in infiltrating the walls of the bronchi and bronchioles. Bronchospasms occur periodically and lead to an “asthma attack.” An attack may be triggered by environmental factors such as dust, pollen, pet hair, or dander, changes in the weather, mold, tobacco smoke, and respiratory infections, or by exercise and stress. Normal and Bronchial Asthma Tissues Mucus Goblet cell Epithelium Basement membrane Lamina propria O Smooth muscle (e) 2 Es - y © 6 Gland Cartilage Mucus Goblet cell Basement membrane Mast cell Lamina propria Macrophage Eosinophil Smooth muscle Neutrophil (a) Normal lung tissue does not have the characteristics of lung tissue during (b) an asthma attack, which include thickened mucosa, increased mucus-producing goblet cells, and eosinophil infiltrates. Symptoms of an asthma attack involve coughing, shortness of breath, wheezing, and tightness of the chest. Symptoms of a severe asthma attack that requires immediate medical attention would include difficulty breathing that results in blue (cyanotic) lips or face, confusion, drowsiness, a rapid pulse, sweating, and severe anxiety. The severity of the condition, frequency of attacks, and identified triggers influence the type of medication that an individual may require. Longer-term treatments are used for those with more severe asthma. Short-term, fast-acting drugs that are used to treat an asthma attack are typically administered via an inhaler. For young children or individuals who have difficulty using an inhaler, asthma medications can be administered via a nebulizer. In many cases, the underlying cause of the condition is unknown. However, recent research has demonstrated that certain viruses, such as human rhinovirus C (HRVC), and the bacteria Mycoplasma pneumoniae and Chlamydia pneumoniae that are contracted in infancy or early childhood, may contribute to the development of many cases of asthma. Note: | [a = epehstan coulece ‘ My Oe Visit this site to learn more about what happens during an asthma attack. What are the three changes that occur inside the airways during an asthma attack? Chapter Review The respiratory system is responsible for obtaining oxygen and getting rid of carbon dioxide, and aiding in speech production and in sensing odors. From a functional perspective, the respiratory system can be divided into two major areas: the conducting zone and the respiratory zone. The conducting zone consists of all of the structures that provide passageways for air to travel into and out of the lungs: the nasal cavity, pharynx, trachea, bronchi, and most bronchioles. The nasal passages contain the conchae and meatuses that expand the surface area of the cavity, which helps to warm and humidify incoming air, while removing debris and pathogens. The pharynx is composed of three major sections: the nasopharynx, which is continuous with the nasal cavity; the oropharynx, which borders the nasopharynx and the oral cavity; and the laryngopharynx, which borders the oropharynx, trachea, and esophagus. The respiratory zone includes the structures of the lung that are directly involved in gas exchange: the terminal bronchioles and alveoli. The lining of the conducting zone is composed mostly of pseudostratified ciliated columnar epithelium with goblet cells. The mucus traps pathogens and debris, whereas beating cilia move the mucus superiorly toward the throat, where it is swallowed. As the bronchioles become smaller and smaller, and nearer the alveoli, the epithelium thins and is simple squamous epithelium in the alveoli. The endothelium of the surrounding capillaries, together with the alveolar epithelium, forms the respiratory membrane. This is a blood-air barrier through which gas exchange occurs by simple diffusion. Interactive Link Questions Exercise: Problem: Visit this site to learn more about what happens during an asthma attack. What are the three changes that occur inside the airways during an asthma attack? Solution: Inflammation and the production of a thick mucus; constriction of the airway muscles, or bronchospasm; and an increased sensitivity to allergens. Review Questions Exercise: Problem: Which of the following anatomical structures is not part of the conducting zone? a. pharynx b. nasal cavity c. alveoli d. bronchi Solution: C Exercise: Problem: What is the function of the conchae in the nasal cavity? a. increase surface area b. exchange gases c. maintain surface tension d. maintain air pressure Solution: A Exercise: Problem: The fauces connects which of the following structures to the oropharynx? a. nasopharynx b. laryngopharynx c. nasal cavity d. oral cavity Solution: D Exercise: Problem: Which of the following are structural features of the trachea? a. C-shaped cartilage b. smooth muscle fibers c. cilia d. all of the above Solution: A Exercise: Problem: Which of the following structures is not part of the bronchial tree? a. alveoli b. bronchi c. terminal bronchioles d. respiratory bronchioles Solution: C Exercise: Problem: What is the role of alveolar macrophages? a. to secrete pulmonary surfactant b. to secrete antimicrobial proteins c. to remove pathogens and debris d. to facilitate gas exchange Solution: C Critical Thinking Questions Exercise: Problem: Describe the three regions of the pharynx and their functions. Solution: The pharynx has three major regions. The first region is the nasopharynx, which is connected to the posterior nasal cavity and functions as an airway. The second region is the oropharynx, which is continuous with the nasopharynx and is connected to the oral cavity at the fauces. The laryngopharynx is connected to the oropharynx and the esophagus and trachea. Both the oropharynx and laryngopharynx are passageways for air and food and drink. Exercise: Problem: If a person sustains an injury to the epiglottis, what would be the physiological result? Solution: The epiglottis is a region of the larynx that is important during the swallowing of food or drink. As a person swallows, the pharynx moves upward and the epiglottis closes over the trachea, preventing food or drink from entering the trachea. If a person’s epiglottis were injured, this mechanism would be impaired. As a result, the person may have problems with food or drink entering the trachea, and possibly, the lungs. Over time, this may cause infections such as pneumonia to set in. Exercise: Problem:Compare and contrast the conducting and respiratory zones. Solution: The conducting zone of the respiratory system includes the organs and structures that are not directly involved in gas exchange, but perform other duties such as providing a passageway for air, trapping and removing debris and pathogens, and warming and humidifying incoming air. Such structures include the nasal cavity, pharynx, larynx, trachea, and most of the bronchial tree. The respiratory zone includes all the organs and structures that are directly involved in gas exchange, including the respiratory bronchioles, alveolar ducts, and alveoli. References Bizzintino J, Lee WM, Laing IA, Vang F, Pappas T, Zhang G, Martin AC, Khoo SK, Cox DW, Geelhoed GC, et al. Association between human rhinovirus C and severity of acute asthma in children. Eur Respir J [Internet]. 2010 [cited 2013 Mar 22]; 37(5):1037—1042. Available from: submit=Go&gca=erj%3B37%2F5%2F 1037 &allch= Kumar V, Ramzi S, Robbins SL. Robbins Basic Pathology. 7th ed. Philadelphia (PA): Elsevier Ltd; 2005. Martin RJ, Kraft M, Chu HW, Berns, EA, Cassell GH. A link between chronic asthma and chronic infection. J Allergy Clin Immunol [Internet]. 2001 [cited 2013 Mar 22]; 107(4):595-601. Available from: submit=Go&gca=erj%3B37%2F5%2F 1037 &allch= Glossary ala (plural = alae) small, flaring structure of a nostril that forms the lateral side of the nares alar cartilage cartilage that supports the apex of the nose and helps shape the nares; it is connected to the septal cartilage and connective tissue of the alae alveolar duct small tube that leads from the terminal bronchiole to the respiratory bronchiole and is the point of attachment for alveoli alveolar macrophage immune system cell of the alveolus that removes debris and pathogens alveolar pore opening that allows airflow between neighboring alveoli alveolar sac cluster of alveoli alveolus small, grape-like sac that performs gas exchange in the lungs apex tip of the external nose bronchial tree collective name for the multiple branches of the bronchi and bronchioles of the respiratory system bridge portion of the external nose that lies in the area of the nasal bones bronchiole branch of bronchi that are 1 mm or less in diameter and terminate at alveolar sacs bronchus tube connected to the trachea that branches into many subsidiaries and provides a passageway for air to enter and leave the lungs conducting zone region of the respiratory system that includes the organs and structures that provide passageways for air and are not directly involved in gas exchange cricoid cartilage portion of the larynx composed of a ring of cartilage with a wide posterior region and a thinner anterior region; attached to the esophagus dorsum nasi intermediate portion of the external nose that connects the bridge to the apex and is supported by the nasal bone epiglottis leaf-shaped piece of elastic cartilage that is a portion of the larynx that swings to close the trachea during swallowing external nose region of the nose that is easily visible to others fauces portion of the posterior oral cavity that connects the oral cavity to the oropharynx fibroelastic membrane specialized membrane that connects the ends of the C-shape cartilage in the trachea; contains smooth muscle fibers glottis opening between the vocal folds through which air passes when producing speech laryngeal prominence region where the two lamina of the thyroid cartilage join, forming a protrusion known as “Adam’s apple” laryngopharynx portion of the pharynx bordered by the oropharynx superiorly and esophagus and trachea inferiorly; serves as a route for both air and food larynx cartilaginous structure that produces the voice, prevents food and beverages from entering the trachea, and regulates the volume of air that enters and leaves the lungs lingual tonsil lymphoid tissue located at the base of the tongue meatus one of three recesses (superior, middle, and inferior) in the nasal cavity attached to the conchae that increase the surface area of the nasal cavity naris (plural = nares) opening of the nostrils nasal bone bone of the skull that lies under the root and bridge of the nose and is connected to the frontal and maxillary bones nasal septum wall composed of bone and cartilage that separates the left and right nasal cavities nasopharynx portion of the pharynx flanked by the conchae and oropharynx that serves as an airway oropharynx portion of the pharynx flanked by the nasopharynx, oral cavity, and laryngopharynx that is a passageway for both air and food palatine tonsil one of the paired structures composed of lymphoid tissue located anterior to the uvula at the roof of isthmus of the fauces paranasal sinus one of the cavities within the skull that is connected to the conchae that serve to warm and humidify incoming air, produce mucus, and lighten the weight of the skull; consists of frontal, maxillary, sphenoidal, and ethmoidal sinuses pharyngeal tonsil structure composed of lymphoid tissue located in the nasopharynx pharynx region of the conducting zone that forms a tube of skeletal muscle lined with respiratory epithelium; located between the nasal conchae and the esophagus and trachea philtrum concave surface of the face that connects the apex of the nose to the top lip pulmonary surfactant substance composed of phospholipids and proteins that reduces the surface tension of the alveoli; made by type II alveolar cells respiratory bronchiole specific type of bronchiole that leads to alveolar sacs respiratory epithelium ciliated lining of much of the conducting zone that is specialized to remove debris and pathogens, and produce mucus respiratory membrane alveolar and capillary wall together, which form an air-blood barrier that facilitates the simple diffusion of gases respiratory zone includes structures of the respiratory system that are directly involved in gas exchange root region of the external nose between the eyebrows thyroid cartilage largest piece of cartilage that makes up the larynx and consists of two lamina trachea tube composed of cartilaginous rings and supporting tissue that connects the lung bronchi and the larynx; provides a route for air to enter and exit the lung trachealis muscle smooth muscle located in the fibroelastic membrane of the trachea true vocal cord one of the pair of folded, white membranes that have a free inner edge that oscillates as air passes through to produce sound type I alveolar cell squamous epithelial cells that are the major cell type in the alveolar wall; highly permeable to gases type II alveolar cell cuboidal epithelial cells that are the minor cell type in the alveolar wall; secrete pulmonary surfactant vestibular fold part of the folded region of the glottis composed of mucous membrane; supports the epiglottis during swallowing The Lungs By the end of this section, you will be able to: ¢ Describe the overall function of the lung e Summarize the blood flow pattern associated with the lungs ¢ Outline the anatomy of the blood supply to the lungs e Describe the pleura of the lungs and their function A major organ of the respiratory system, each lung houses structures of both the conducting and respiratory zones. The main function of the lungs is to perform the exchange of oxygen and carbon dioxide with air from the atmosphere. To this end, the lungs exchange respiratory gases across a very large epithelial surface area—about 70 square meters—that is highly permeable to gases. Gross Anatomy of the Lungs The lungs are pyramid-shaped, paired organs that are connected to the trachea by the right and left bronchi; on the inferior surface, the lungs are bordered by the diaphragm. The diaphragm is the flat, dome-shaped muscle located at the base of the lungs and thoracic cavity. The lungs are enclosed by the pleurae, which are attached to the mediastinum. The right lung is shorter and wider than the left lung, and the left lung occupies a smaller volume than the right. The cardiac notch is an indentation on the surface of the left lung, and it allows space for the heart ([link]). The apex of the lung is the superior region, whereas the base is the opposite region near the diaphragm. The costal surface of the lung borders the ribs. The mediastinal surface faces the midline. Gross Anatomy of the Lungs Trachea Superior lobe Main (primary) bronchus Superior lobe Lobar (secondary) ronchus Segmental (tertiary) bronchus Middle lobe Cardiac notch Inferior lobe Inferior lobe Right lung Left lung Each lung is composed of smaller units called lobes. Fissures separate these lobes from each other. The right lung consists of three lobes: the superior, middle, and inferior lobes. The left lung consists of two lobes: the superior and inferior lobes. A bronchopulmonary segment is a division of a lobe, and each lobe houses multiple bronchopulmonary segments. Each segment receives air from its own tertiary bronchus and is supplied with blood by its own artery. Some diseases of the lungs typically affect one or more bronchopulmonary segments, and in some cases, the diseased segments can be surgically removed with little influence on neighboring segments. A pulmonary lobule is a subdivision formed as the bronchi branch into bronchioles. Each lobule receives its own large bronchiole that has multiple branches. An interlobular septum is a wall, composed of connective tissue, which separates lobules from one another. Blood Supply and Nervous Innervation of the Lungs The blood supply of the lungs plays an important role in gas exchange and serves as a transport system for gases throughout the body. In addition, innervation by the both the parasympathetic and sympathetic nervous systems provides an important level of control through dilation and constriction of the airway. Blood Supply The major function of the lungs is to perform gas exchange, which requires blood from the pulmonary circulation. This blood supply contains deoxygenated blood and travels to the lungs where erythrocytes, also known as red blood cells, pick up oxygen to be transported to tissues throughout the body. The pulmonary artery is an artery that arises from the pulmonary trunk and carries deoxygenated, arterial blood to the alveoli. The pulmonary artery branches multiple times as it follows the bronchi, and each branch becomes progressively smaller in diameter. One arteriole and an accompanying venule supply and drain one pulmonary lobule. As they near the alveoli, the pulmonary arteries become the pulmonary capillary network. The pulmonary capillary network consists of tiny vessels with very thin walls that lack smooth muscle fibers. The capillaries branch and follow the bronchioles and structure of the alveoli. It is at this point that the capillary wall meets the alveolar wall, creating the respiratory membrane. Once the blood is oxygenated, it drains from the alveoli by way of multiple pulmonary veins, which exit the lungs through the hilum. Nervous Innervation Dilation and constriction of the airway are achieved through nervous control by the parasympathetic and sympathetic nervous systems. The parasympathetic system causes bronchoconstriction, whereas the sympathetic nervous system stimulates bronchodilation. Reflexes such as coughing, and the ability of the lungs to regulate oxygen and carbon dioxide levels, also result from this autonomic nervous system control. Sensory nerve fibers arise from the vagus nerve, and from the second to fifth thoracic ganglia. The pulmonary plexus is a region on the lung root formed by the entrance of the nerves at the hilum. The nerves then follow the bronchi in the lungs and branch to innervate muscle fibers, glands, and blood vessels. Pleura of the Lungs Each lung is enclosed within a cavity that is surrounded by the pleura. The pleura (plural = pleurae) is a serous membrane that surrounds the lung. The right and left pleurae, which enclose the right and left lungs, respectively, are separated by the mediastinum. The pleurae consist of two layers. The visceral pleura is the layer that is superficial to the lungs, and extends into and lines the lung fissures ({link]). In contrast, the parietal pleura is the outer layer that connects to the thoracic wall, the mediastinum, and the diaphragm. The visceral and parietal pleurae connect to each other at the hilum. The pleural cavity is the space between the visceral and parietal layers. Parietal and Visceral Pleurae of the Lungs Intercostal muscle Pleural sac | Intercostal f , Ke ae muscles <4 I ent pleura Visceral pleura Pleural cavity Diaphragm Chest wall (rib cage, sternum, thoracic vertebrae, connective tissue, intercostal muscles) The pleurae perform two major functions: They produce pleural fluid and create cavities that separate the major organs. Pleural fluid is secreted by mesothelial cells from both pleural layers and acts to lubricate their surfaces. This lubrication reduces friction between the two layers to prevent trauma during breathing, and creates surface tension that helps maintain the position of the lungs against the thoracic wall. This adhesive characteristic of the pleural fluid causes the lungs to enlarge when the thoracic wall expands during ventilation, allowing the lungs to fill with air. The pleurae also create a division between major organs that prevents interference due to the movement of the organs, while preventing the spread of infection. Note: Everyday Connection The Effects of Second-Hand Tobacco Smoke The burning of a tobacco cigarette creates multiple chemical compounds that are released through mainstream smoke, which is inhaled by the smoker, and through sidestream smoke, which is the smoke that is given off by the burning cigarette. Second-hand smoke, which is a combination of sidestream smoke and the mainstream smoke that is exhaled by the smoker, has been demonstrated by numerous scientific studies to cause disease. At least 40 chemicals in sidestream smoke have been identified that negatively impact human health, leading to the development of cancer or other conditions, such as immune system dysfunction, liver toxicity, cardiac arrhythmias, pulmonary edema, and neurological dysfunction. Furthermore, second-hand smoke has been found to harbor at least 250 compounds that are known to be toxic, carcinogenic, or both. Some major classes of carcinogens in second-hand smoke are polyaromatic hydrocarbons (PAHs), N-nitrosamines, aromatic amines, formaldehyde, and acetaldehyde. Tobacco and second-hand smoke are considered to be carcinogenic. Exposure to second-hand smoke can cause lung cancer in individuals who are not tobacco users themselves. It is estimated that the risk of developing lung cancer is increased by up to 30 percent in nonsmokers who live with an individual who smokes in the house, as compared to nonsmokers who are not regularly exposed to second-hand smoke. Children are especially affected by second-hand smoke. Children who live with an individual who smokes inside the home have a larger number of lower respiratory infections, which are associated with hospitalizations, and higher risk of sudden infant death syndrome (SIDS). Second-hand smoke in the home has also been linked to a greater number of ear infections in children, as well as worsening symptoms of asthma. Chapter Review The lungs are the major organs of the respiratory system and are responsible for performing gas exchange. The lungs are paired and separated into lobes; The left lung consists of two lobes, whereas the right lung consists of three lobes. Blood circulation is very important, as blood is required to transport oxygen from the lungs to other tissues throughout the body. The function of the pulmonary circulation is to aid in gas exchange. The pulmonary artery provides deoxygenated blood to the capillaries that form respiratory membranes with the alveoli, and the pulmonary veins return newly oxygenated blood to the heart for further transport throughout the body. The lungs are innervated by the parasympathetic and sympathetic nervous systems, which coordinate the bronchodilation and bronchoconstriction of the airways. The lungs are enclosed by the pleura, a membrane that is composed of visceral and parietal pleural layers. The space between these two layers is called the pleural cavity. The mesothelial cells of the pleural membrane create pleural fluid, which serves as both a lubricant (to reduce friction during breathing) and as an adhesive to adhere the lungs to the thoracic wall (to facilitate movement of the lungs during ventilation). Review Questions Exercise: Problem: Which of the following structures separates the lung into lobes? a. Mediastinum b. fissure c. root d. pleura Solution: B Exercise: Problem: A section of the lung that receives its own tertiary bronchus is called the a. bronchopulmonary segment b. pulmonary lobule c. interpulmonary segment d. respiratory segment Solution: A Exercise: Problem: The circulation picks up oxygen for cellular use and drops off carbon dioxide for removal from the body. a. pulmonary b. interlobular c. respiratory d. bronchial Solution: C Exercise: Problem: The pleura that surrounds the lungs consists of two layers, the a. visceral and parietal pleurae. b. mediastinum and parietal pleurae. c. visceral and mediastinum pleurae. d. none of the above Solution: A Critical Thinking Questions Exercise: Problem:Compare and contrast the right and left lungs. Solution: The right and left lungs differ in size and shape to accommodate other organs that encroach on the thoracic region. The right lung consists of three lobes and is shorter than the left lung, due to the position of the liver underneath it. The left lung consist of two lobes and is longer and narrower than the right lung. The left lung has a concave region on the mediastinal surface called the cardiac notch that allows space for the heart. Exercise: Problem: Why are the pleurae not damaged during normal breathing? Solution: There is a cavity, called the pleural cavity, between the parietal and visceral layers of the pleura. Mesothelial cells produce and secrete pleural fluid into the pleural cavity that acts as a lubricant. Therefore, as you breathe, the pleural fluid prevents the two layers of the pleura from rubbing against each other and causing damage due to friction. Glossary bronchoconstriction decrease in the size of the bronchiole due to contraction of the muscular wall bronchodilation increase in the size of the bronchiole due to contraction of the muscular wall cardiac notch indentation on the surface of the left lung that allows space for the heart hilum concave structure on the mediastinal surface of the lungs where blood vessels, lymphatic vessels, nerves, and a bronchus enter the lung lung organ of the respiratory system that performs gas exchange parietal pleura outermost layer of the pleura that connects to the thoracic wall, mediastinum, and diaphragm pleural cavity space between the visceral and parietal pleurae pleural fluid substance that acts as a lubricant for the visceral and parietal layers of the pleura during the movement of breathing pulmonary artery artery that arises from the pulmonary trunk and carries deoxygenated, arterial blood to the alveoli pulmonary plexus network of autonomic nervous system fibers found near the hilum of the lung visceral pleura innermost layer of the pleura that is superficial to the lungs and extends into the lung fissures Overview of the Digestive System By the end of this section, you will be able to: Identify the organs of the alimentary canal from proximal to distal, and briefly state their function Identify the accessory digestive organs and briefly state their function Describe the four fundamental tissue layers of the alimentary canal Contrast the contributions of the enteric and autonomic nervous systems to digestive system functioning Explain how the peritoneum anchors the digestive organs The function of the digestive system is to break down the foods you eat, release their nutrients, and absorb those nutrients into the body. Although the small intestine is the workhorse of the system, where the majority of digestion occurs, and where most of the released nutrients are absorbed into the blood or lymph, each of the digestive system organs makes a vital contribution to this process ([link]). Components of the Digestive System Salivary glands: Mouth Ca Parotid gland Tongue r Sublingual gland Pe Submandibular gland Pharynx Esophagus Liver Gallbladder Stomach y 7 Spleen Small intestine: Duodenum Jejunum lleum Pancreas Large intestine: Transverse colon Ascending colon Descending colon Cecum Sigmoid colon Appendix Rectum Anus Anal canal All digestive organs play integral roles in the life-sustaining process of digestion. As is the case with all body systems, the digestive system does not work in isolation; it functions cooperatively with the other systems of the body. Consider for example, the interrelationship between the digestive and cardiovascular systems. Arteries supply the digestive organs with oxygen and processed nutrients, and veins drain the digestive tract. These intestinal veins, constituting the hepatic portal system, are unique; they do not return blood directly to the heart. Rather, this blood is diverted to the liver where its nutrients are off-loaded for processing before blood completes its circuit back to the heart. At the same time, the digestive system provides nutrients to the heart muscle and vascular tissue to support their functioning. The interrelationship of the digestive and endocrine systems is also critical. Hormones secreted by several endocrine glands, as well as endocrine cells of the pancreas, the stomach, and the small intestine, contribute to the control of digestion and nutrient metabolism. In turn, the digestive system provides the nutrients to fuel endocrine function. [link] gives a quick glimpse at how these other systems contribute to the functioning of the digestive system. Contribution of Other Body Systems to the Digestive System Body system Cardiovascular Endocrine Integumentary Lymphatic Muscular Benefits received by the digestive system Blood supplies digestive organs with oxygen and processed nutrients Endocrine hormones help regulate secretion in digestive glands and accessory organs Skin helps protect digestive organs and synthesizes vitamin D for calcium absorption Mucosa-associated lymphoid tissue and other lymphatic tissue defend against entry of pathogens; lacteals absorb lipids; and lymphatic vessels transport lipids to bloodstream Skeletal muscles support and protect abdominal organs Contribution of Other Body Systems to the Digestive System Body system Benefits received by the digestive system Sensory and motor neurons help regulate Nervous secretions and muscle contractions in the digestive tract Respiratory organs provide oxygen and remove Respirator aa P e carbon dioxide Skeletal Bones help protect and support digestive organs ; Kidneys convert vitamin D into its active form, Urinary allowing calcium absorption in the small intestine Digestive System Organs The easiest way to understand the digestive system is to divide its organs into two main categories. The first group is the organs that make up the alimentary canal. Accessory digestive organs comprise the second group and are critical for orchestrating the breakdown of food and the assimilation of its nutrients into the body. Accessory digestive organs, despite their name, are critical to the function of the digestive system. Alimentary Canal Organs Also called the gastrointestinal (GI) tract or gut, the alimentary canal (aliment- = “to nourish’) is a one-way tube about 7.62 meters (25 feet) in length during life and closer to 10.67 meters (35 feet) in length when measured after death, once smooth muscle tone is lost. The main function of the organs of the alimentary canal is to nourish the body. This tube begins at the mouth and terminates at the anus. Between those two points, the canal is modified as the pharynx, esophagus, stomach, and small and large intestines to fit the functional needs of the body. Both the mouth and anus are open to the external environment; thus, food and wastes within the alimentary canal are technically considered to be outside the body. Only through the process of absorption do the nutrients in food enter into and 3 cc nourish the body’s “inner space.” Accessory Structures Each accessory digestive organ aids in the breakdown of food ((Link]). Within the mouth, the teeth and tongue begin mechanical digestion, whereas the salivary glands begin chemical digestion. Once food products enter the small intestine, the gallbladder, liver, and pancreas release secretions—such as bile and enzymes—essential for digestion to continue. Together, these are called accessory organs because they sprout from the lining cells of the developing gut (mucosa) and augment its function; indeed, you could not live without their vital contributions, and many significant diseases result from their malfunction. Even after development is complete, they maintain a connection to the gut by way of ducts. Histology of the Alimentary Canal Throughout its length, the alimentary tract is composed of the same four tissue layers; the details of their structural arrangements vary to fit their specific functions. Starting from the lumen and moving outwards, these layers are the mucosa, submucosa, muscularis, and serosa, which is continuous with the mesentery (see [link]). Layers of the Alimentary Canal Vein } Submucosal plexus a’, (| | (plexus of Meissner) | i Mesentery Glands in pelaly submucosa <= — y Nerve Submucosa | y a Gland in mucosa Duct of gland outside tract Myenteric plexus Lymphatic tissue Serosa: Areolar connective tissue Epithelium Lumen Mucosa: Epithelium Lamina propria Muscularis mucosae Muscularis: Circular muscle Longitudinal muscle The wall of the alimentary canal has four basic tissue layers: the mucosa, submucosa, muscularis, and serosa. The mucosa is referred to as a mucous membrane, because mucus production is a characteristic feature of gut epithelium. The membrane consists of epithelium, which is in direct contact with ingested food, and the lamina propria, a layer of connective tissue analogous to the dermis. In addition, the mucosa has a thin, smooth muscle layer, called the muscularis mucosa (not to be confused with the muscularis layer, described below). Epithelium—lIn the mouth, pharynx, esophagus, and anal canal, the epithelium is primarily a non-keratinized, stratified squamous epithelium. In the stomach and intestines, it is a simple columnar epithelium. Notice that the epithelium is in direct contact with the lumen, the space inside the alimentary canal. Interspersed among its epithelial cells are goblet cells, which secrete mucus and fluid into the lumen, and enteroendocrine cells, which secrete hormones into the interstitial spaces between cells. Epithelial cells have a very brief lifespan, averaging from only a couple of days (in the mouth) to about a week (in the gut). This process of rapid renewal helps preserve the health of the alimentary canal, despite the wear and tear resulting from continued contact with foodstuffs. Lamina propria—In addition to loose connective tissue, the lamina propria contains numerous blood and lymphatic vessels that transport nutrients absorbed through the alimentary canal to other parts of the body. The lamina propria also serves an immune function by housing clusters of lymphocytes, making up the mucosa-associated lymphoid tissue (MALT). These lymphocyte clusters are particularly substantial in the distal ileum where they are known as Peyer’s patches. When you consider that the alimentary canal is exposed to foodborne bacteria and other foreign matter, it is not hard to appreciate why the immune system has evolved a means of defending against the pathogens encountered within it. Muscularis mucosa—This thin layer of smooth muscle is in a constant state of tension, pulling the mucosa of the stomach and small intestine into undulating folds. These folds dramatically increase the surface area available for digestion and absorption. As its name implies, the submucosa lies immediately beneath the mucosa. A broad layer of dense connective tissue, it connects the overlying mucosa to the underlying muscularis. It includes blood and lymphatic vessels (which transport absorbed nutrients), and a scattering of submucosal glands that release digestive secretions. Additionally, it serves as a conduit for a dense branching network of nerves, the submucosal plexus, which functions as described below. The third layer of the alimentary canal is the muscularis (also called the muscularis externa). The muscularis in the small intestine is made up of a double layer of smooth muscle: an inner circular layer and an outer longitudinal layer. The contractions of these layers promote mechanical digestion, expose more of the food to digestive chemicals, and move the food along the canal. In the most proximal and distal regions of the alimentary canal, including the mouth, pharynx, anterior part of the esophagus, and external anal sphincter, the muscularis is made up of skeletal muscle, which gives you voluntary control over swallowing and defecation. The basic two-layer structure found in the small intestine is modified in the organs proximal and distal to it. The stomach is equipped for its churning function by the addition of a third layer, the oblique muscle. While the colon has two layers like the small intestine, its longitudinal layer is segregated into three narrow parallel bands, the tenia coli, which make it look like a series of pouches rather than a simple tube. The serosa is the portion of the alimentary canal superficial to the muscularis. Present only in the region of the alimentary canal within the abdominal cavity, it consists of a layer of visceral peritoneum overlying a layer of loose connective tissue. Instead of serosa, the mouth, pharynx, and esophagus have a dense sheath of collagen fibers called the adventitia. These tissues serve to hold the alimentary canal in place near the ventral surface of the vertebral column. Nerve Supply As soon as food enters the mouth, it is detected by receptors that send impulses along the sensory neurons of cranial nerves. Without these nerves, not only would your food be without taste, but you would also be unable to feel either the food or the structures of your mouth, and you would be unable to avoid biting yourself as you chew, an action enabled by the motor branches of cranial nerves. Intrinsic innervation of much of the alimentary canal is provided by the enteric nervous system, which runs from the esophagus to the anus, and contains approximately 100 million motor, sensory, and interneurons (unique to this system compared to all other parts of the peripheral nervous system). These enteric neurons are grouped into two plexuses. The myenteric plexus (plexus of Auerbach) lies in the muscularis layer of the alimentary canal and is responsible for motility, especially the rhythm and force of the contractions of the muscularis. The submucosal plexus (plexus of Meissner) lies in the submucosal layer and is responsible for regulating digestive secretions and reacting to the presence of food (see [link]). Extrinsic innervations of the alimentary canal are provided by the autonomic nervous system, which includes both sympathetic and parasympathetic nerves. In general, sympathetic activation (the fight-or- flight response) restricts the activity of enteric neurons, thereby decreasing GI secretion and motility. In contrast, parasympathetic activation (the rest- and-digest response) increases GI secretion and motility by stimulating neurons of the enteric nervous system. Blood Supply The blood vessels serving the digestive system have two functions. They transport the protein and carbohydrate nutrients absorbed by mucosal cells after food is digested in the lumen. Lipids are absorbed via lacteals, tiny structures of the lymphatic system. The blood vessels’ second function is to supply the organs of the alimentary canal with the nutrients and oxygen needed to drive their cellular processes. Specifically, the more anterior parts of the alimentary canal are supplied with blood by arteries branching off the aortic arch and thoracic aorta. Below this point, the alimentary canal is supplied with blood by arteries branching from the abdominal aorta. The celiac trunk services the liver, stomach, and duodenum, whereas the superior and inferior mesenteric arteries supply blood to the remaining small and large intestines. The veins that collect nutrient-rich blood from the small intestine (where most absorption occurs) empty into the hepatic portal system. This venous network takes the blood into the liver where the nutrients are either processed or stored for later use. Only then does the blood drained from the alimentary canal viscera circulate back to the heart. To appreciate just how demanding the digestive process is on the cardiovascular system, consider that while you are “resting and digesting,” about one-fourth of the blood pumped with each heartbeat enters arteries serving the intestines. The Peritoneum The digestive organs within the abdominal cavity are held in place by the peritoneum, a broad serous membranous sac made up of squamous epithelial tissue surrounded by connective tissue. It is composed of two different regions: the parietal peritoneum, which lines the abdominal wall, and the visceral peritoneum, which envelopes the abdominal organs ([link]). The peritoneal cavity is the space bounded by the visceral and parietal peritoneal surfaces. A few milliliters of watery fluid act as a lubricant to minimize friction between the serosal surfaces of the peritoneum. The Peritoneum Spinal cord Vertebra Kidney Kidney Pancreas Spleen Liver Small intestine Gallbladder Large intestine Large intestine Stomach Small intestine Visceral peritoneum Peritoneal cavity Parietal peritoneum A cross-section of the abdomen shows the relationship between abdominal organs and the peritoneum (darker lines). Note: Disorders of the... Digestive System: Peritonitis Inflammation of the peritoneum is called peritonitis. Chemical peritonitis can develop any time the wall of the alimentary canal is breached, allowing the contents of the lumen entry into the peritoneal cavity. For example, when an ulcer perforates the stomach wall, gastric juices spill into the peritoneal cavity. Hemorrhagic peritonitis occurs after a ruptured tubal pregnancy or traumatic injury to the liver or spleen fills the peritoneal cavity with blood. Even more severe peritonitis is associated with bacterial infections seen with appendicitis, colonic diverticulitis, and pelvic inflammatory disease (infection of uterine tubes, usually by sexually transmitted bacteria). Peritonitis is life threatening and often results in emergency surgery to correct the underlying problem and intensive antibiotic therapy. When your great grandparents and even your parents were young, the mortality from peritonitis was high. Aggressive surgery, improvements in anesthesia safety, the advance of critical care expertise, and antibiotics have greatly improved the mortality rate from this condition. Even so, the mortality rate still ranges from 30 to 40 percent. The visceral peritoneum includes multiple large folds that envelope various abdominal organs, holding them to the dorsal surface of the body wall. Within these folds are blood vessels, lymphatic vessels, and nerves that innervate the organs with which they are in contact, supplying their adjacent organs. The five major peritoneal folds are described in [link]. Note that during fetal development, certain digestive structures, including the first portion of the small intestine (called the duodenum), the pancreas, and portions of the large intestine (the ascending and descending colon, and the rectum) remain completely or partially posterior to the peritoneum. Thus, the location of these organs is described as retroperitoneal. The Five Major Peritoneal Folds Fold Description ore Apron-like structure that lies superficial to the small intestine and transverse colon; a site of fat deposition omentum ; ; in people who are overweight Falciform Anchors the liver to the anterior abdominal wall and ligament inferior border of the diaphragm The Five Major Peritoneal Folds Fold Description heccar Suspends the stomach from the inferior border of the liver; provides a pathway for structures connecting to omentum the liver Vertical band of tissue anterior to the lumbar Mesentery vertebrae and anchoring all of the small intestine except the initial portion (the duodenum) Attaches two portions of the large intestine (the Mesocolon transverse and sigmoid colon) to the posterior abdominal wall Note: wees Openstax COLLEGE oe ee - By clicking on this link you can watch a short video of what happens to the food you eat, as it passes from your mouth to your intestine. Along the way, note how the food changes consistency and form. How does this change in consistency facilitate your gaining nutrients from food? Chapter Review The digestive system includes the organs of the alimentary canal and accessory structures. The alimentary canal forms a continuous tube that is open to the outside environment at both ends. The organs of the alimentary canal are the mouth, pharynx, esophagus, stomach, small intestine, and large intestine. The accessory digestive structures include the teeth, tongue, salivary glands, liver, pancreas, and gallbladder. The wall of the alimentary canal is composed of four basic tissue layers: mucosa, submucosa, muscularis, and serosa. The enteric nervous system provides intrinsic innervation, and the autonomic nervous system provides extrinsic innervation. Interactive Link Questions Exercise: Problem: By clicking on this link, you can watch a short video of what happens to the food you eat as it passes from your mouth to your intestine. Along the way, note how the food changes consistency and form. How does this change in consistency facilitate your gaining nutrients from food? Solution: Answers may vary. Review Questions Exercise: Problem: Which of these organs is not considered an accessory digestive structure? a. mouth b. salivary glands c. pancreas d. liver Solution: A Exercise: Problem: Which of the following organs is supported by a layer of adventitia rather than serosa? a. esophagus b. stomach c. small intestine d. large intestine Solution: A Exercise: Problem: Which of the following membranes covers the stomach? a. falciform ligament b. mesocolon c. parietal peritoneum d. visceral peritoneum Solution: D Critical Thinking Questions Exercise: Problem: Explain how the enteric nervous system supports the digestive system. What might occur that could result in the autonomic nervous system having a negative impact on digestion? Solution: The enteric nervous system helps regulate alimentary canal motility and the secretion of digestive juices, thus facilitating digestion. If a person becomes overly anxious, sympathetic innervation of the alimentary canal is stimulated, which can result in a slowing of digestive activity. Exercise: Problem: What layer of the alimentary canal tissue is capable of helping to protect the body against disease, and through what mechanism? Solution: The lamina propria of the mucosa contains lymphoid tissue that makes up the MALT and responds to pathogens encountered in the alimentary canal. Glossary accessory digestive organ includes teeth, tongue, salivary glands, gallbladder, liver, and pancreas alimentary canal continuous muscular digestive tube that extends from the mouth to the anus motility movement of food through the GI tract mucosa innermost lining of the alimentary canal muscularis muscle (skeletal or smooth) layer of the alimentary canal wall myenteric plexus (plexus of Auerbach) major nerve supply to alimentary canal wall; controls motility retroperitoneal located posterior to the peritoneum serosa outermost layer of the alimentary canal wall present in regions within the abdominal cavity submucosa layer of dense connective tissue in the alimentary canal wall that binds the overlying mucosa to the underlying muscularis submucosal plexus (plexus of Meissner) nerve supply that regulates activity of glands and smooth muscle The Mouth, Pharynx, and Esophagus By the end of this section, you will be able to: e Describe the structures of the mouth, including its three accessory digestive organs e Group the 32 adult teeth according to name, location, and function e Describe the process of swallowing, including the roles of the tongue, upper esophageal sphincter, and epiglottis e Trace the pathway food follows from ingestion into the mouth through release into the stomach In this section, you will examine the anatomy and functions of the three main organs of the upper alimentary canal—the mouth, pharynx, and esophagus—as well as three associated accessory organs—the tongue, salivary glands, and teeth. The Mouth The cheeks, tongue, and palate frame the mouth, which is also called the oral cavity (or buccal cavity). The structures of the mouth are illustrated in [link]. At the entrance to the mouth are the lips, or labia (singular = labium). Their outer covering is skin, which transitions to a mucous membrane in the mouth proper. Lips are very vascular with a thin layer of keratin; hence, the reason they are "red." They have a huge representation on the cerebral cortex, which probably explains the human fascination with kissing! The lips cover the orbicularis oris muscle, which regulates what comes in and goes out of the mouth. The labial frenulum is a midline fold of mucous membrane that attaches the inner surface of each lip to the gum. The cheeks make up the oral cavity’s sidewalls. While their outer covering is skin, their inner covering is mucous membrane. This membrane is made up of non- keratinized, stratified squamous epithelium. Between the skin and mucous membranes are connective tissue and buccinator muscles. The next time you eat some food, notice how the buccinator muscles in your cheeks and the orbicularis oris muscle in your lips contract, helping you keep the food from falling out of your mouth. Additionally, notice how these muscles work when you are speaking. The pocket-like part of the mouth that is framed on the inside by the gums and teeth, and on the outside by the cheeks and lips is called the oral vestibule. Moving farther into the mouth, the opening between the oral cavity and throat (oropharynx) is called the fauces (like the kitchen faucet"). The main open area of the mouth, or oral cavity proper, runs from the gums and teeth to the fauces. When you are chewing, you do not find it difficult to breathe simultaneously. The next time you have food in your mouth, notice how the arched shape of the roof of your mouth allows you to handle both digestion and respiration at the same time. This arch is called the palate. The anterior region of the palate serves as a wall (or septum) between the oral and nasal cavities as well as a rigid shelf against which the tongue can push food. It is created by the maxillary and palatine bones of the skull and, given its bony structure, is known as the hard palate. If you run your tongue along the roof of your mouth, you’! notice that the hard palate ends in the posterior oral cavity, and the tissue becomes fleshier. This part of the palate, known as the soft palate, is composed mainly of skeletal muscle. You can therefore manipulate, subconsciously, the soft palate—for instance, to yawn, swallow, or sing (see [link]). Mouth Superior lip Superior labial frenulum wr ages AK, Gingivae (gums) fg Sy Palatoglossal arch Fauces Hard palate a oe Palatopharyngeal arch Soft palate ih 42 via & [ -\ <0 4 Palatine tonsil Cheek ———————— Vv \ Sa ea | Tongue (underside) IN Molars Sa We Lingual frenulum ~ yee (ix Opening duct of Premolars ey ~~) FE py submandibular gland y AT) : Gingivae (gums) Cuspid (canine) we Incisors =a ea / NS es as, Inferior labial frenulum Oral vestibule Ne, ee Inferior lip Anterior view The mouth includes the lips, tongue, palate, gums, and teeth. A fleshy bead of tissue called the uvula drops down from the center of the posterior edge of the soft palate. Although some have suggested that the uvula is a vestigial organ, it serves an important purpose. When you swallow, the soft palate and uvula move upward, helping to keep foods and liquid from entering the nasal cavity. Unfortunately, it can also contribute to the sound produced by snoring. Two muscular folds extend downward from the soft palate, on either side of the uvula. Toward the front, the palatoglossal arch lies next to the base of the tongue; behind it, the palatopharyngeal arch forms the superior and lateral margins of the fauces. Between these two arches are the palatine tonsils, clusters of lymphoid tissue that protect the pharynx. The lingual tonsils are located at the base of the tongue. The Tongue Perhaps you have heard it said that the tongue is the strongest muscle in the body. Those who stake this claim cite its strength proportionate to its size. Although it is difficult to quantify the relative strength of different muscles, it remains indisputable that the tongue is a workhorse, facilitating ingestion, mechanical digestion, chemical digestion (lingual lipase), sensation (of taste, texture, and temperature of food), swallowing, and vocalization. The tongue is attached to the mandible, the styloid processes of the temporal bones, and the hyoid bone. The hyoid is unique in that it only distantly/indirectly articulates with other bones. The tongue is positioned over the floor of the oral cavity. A medial septum extends the entire length of the tongue, dividing it into symmetrical halves. Beneath its mucous membrane covering, each half of the tongue is composed of the same number and type of intrinsic and extrinsic skeletal muscles. The intrinsic muscles (those within the tongue) are the longitudinalis inferior, longitudinalis superior, transversus linguae, and verticalis linguae muscles. These allow you to change the size and shape of your tongue, as well as to stick it out, if you wish. Having such a flexible tongue facilitates both swallowing and speech. As you learned in your study of the muscular system, the extrinsic muscles of the tongue are the mylohyoid, hyoglossus, styloglossus, and genioglossus muscles. These muscles originate outside the tongue and insert into connective tissues within the tongue. The mylohyoid is responsible for raising the tongue, the hyoglossus pulls it down and back, the styloglossus pulls it up and back, and the genioglossus pulls it forward. Working in concert, these muscles perform three important digestive functions in the mouth: (1) position food for optimal chewing, (2) gather food into a bolus (rounded mass), and (3) position food so it can be swallowed. The top and sides of the tongue are studded with papillae, extensions of lamina propria of the mucosa, which are covered in stratified squamous epithelium ({link]). Fungiform papillae, which are mushroom shaped, cover a large area of the tongue; they tend to be larger toward the rear of the tongue and smaller on the tip and sides. In contrast, filiform papillae are long and thin. Fungiform papillae contain taste buds, and filiform papillae have touch receptors that help the tongue move food around in the mouth. The filiform papillae create an abrasive surface that performs mechanically, much like a cat’s rough tongue that is used for grooming. Lingual glands in the lamina propria of the tongue secrete mucus and a watery serous fluid that contains the enzyme lingual lipase, which plays a minor role in breaking down triglycerides but does not begin working until it is activated in the stomach. A fold of mucous membrane on the underside of the tongue, the lingual frenulum, tethers the tongue to the floor of the mouth. People with the congenital anomaly ankyloglossia, also known by the non-medical term “tongue tie,” have a lingual frenulum that is too short or otherwise malformed. Severe ankyloglossia can impair speech and must be corrected with surgery. Tongue Epiglottis Palatopharyngeal arch Palatine tonsil Lingual tonsil Palatoglossal arch Cg <9 © Terminal sulcus Foliate papillae Circumvallate papilla Dorsum of tongue Fungiform papilla Filiform papilla This superior view of the tongue shows the locations and types of lingual papillae. The Salivary Glands Many small salivary glands are housed within the mucous membranes of the mouth and tongue. These minor exocrine glands are constantly secreting saliva, either directly into the oral cavity or indirectly through ducts, even while you sleep. In fact, an average of 1 to 1.5 liters of saliva is secreted each day. Usually just enough saliva is present to moisten the mouth and teeth. Secretion increases when you eat, because saliva is essential to moisten food and initiate the chemical breakdown of carbohydrates. Small amounts of saliva are also secreted by the labial glands in the lips. In addition, the buccal glands in the cheeks, palatal glands in the palate, and lingual glands in the tongue help ensure that all areas of the mouth are supplied with adequate saliva. The Major Salivary Glands Outside the oral mucosa are three pairs of major salivary glands, which secrete the majority of saliva into ducts that open into the mouth: e The submandibular glands, which are in the floor of the mouth, secrete saliva into the mouth through the submandibular ducts. e The sublingual glands, which lie below the tongue, use the lesser sublingual ducts to secrete saliva into the oral cavity. e The parotid glands lie between the skin and the masseter muscle, near the ears. They secrete saliva into the mouth through the parotid duct, which is located near the second upper molar tooth ((Link]). Saliva Saliva is essentially (98 to 99.5 percent) water. The remaining 4.5 percent is a complex mixture of ions, glycoproteins, enzymes, growth factors, and waste products. Perhaps the most important ingredient in saliva from the perspective of digestion is the enzyme salivary amylase, which initiates the breakdown of carbohydrates. Food does not spend enough time in the mouth to allow all the carbohydrates to break down, but salivary amylase continues acting until it is inactivated by stomach acids. Bicarbonate and phosphate ions function as chemical buffers, maintaining saliva at a pH between 6.35 and 6.85. Salivary mucus helps lubricate food, facilitating movement in the mouth, bolus formation, and swallowing. Saliva contains immunoglobulin A, which prevents microbes from penetrating the epithelium, and lysozyme, which makes saliva antimicrobial. Saliva also contains epidermal growth factor, which might have given rise to the adage “a mother’s kiss can heal a wound.” Each of the major salivary glands secretes a unique formulation of saliva according to its cellular makeup. For example, the parotid glands secrete a watery solution that contains salivary amylase. The submandibular glands have cells similar to those of the parotid glands, as well as mucus-secreting cells. Therefore, saliva secreted by the submandibular glands also contains amylase but in a liquid thickened with mucus. The sublingual glands contain mostly mucous cells, and they secrete the thickest saliva with the least amount of salivary amylase. Salivary glands Parotid salivary gland Parotid duct Sublingual ducts Sublingual salivary gland Submandibular salivary gland Submandibular duct The major salivary glands are located outside the oral mucosa and deliver saliva into the mouth through ducts. Note: Homeostatic Imbalances The Parotid Glands: Mumps Infections of the nasal passages and pharynx can attack any salivary gland. The parotid glands are the usual site of infection with the virus that causes mumps (paramyxovirus). Mumps manifests by enlargement and inflammation of the parotid glands, causing a characteristic swelling between the ears and the jaw. Symptoms include fever and throat pain, which can be severe when swallowing acidic substances such as orange juice. In about one-third of men who are past puberty, mumps also causes testicular inflammation, typically affecting only one testis and rarely resulting in sterility. With the increasing use and effectiveness of mumps vaccines, the incidence of mumps has decreased dramatically. According to the U.S. Centers for Disease Control and Prevention (CDC), the number of mumps cases dropped from more than 150,000 in 1968 to fewer than 1700 in 1993 to only 11 reported cases in 2011. Regulation of Salivation The autonomic nervous system regulates salivation (the secretion of Saliva). In the absence of food, parasympathetic stimulation keeps saliva flowing at just the right level for comfort as you speak, swallow, sleep, and generally go about life. Over-salivation can occur, for example, if you are stimulated by the smell of food, but that food is not available for you to eat. Drooling is an extreme instance of the overproduction of saliva. During times of stress, such as before speaking in public, sympathetic stimulation takes over, reducing salivation and producing the symptom of dry mouth often associated with anxiety. When you are dehydrated, salivation is reduced, causing the mouth to feel dry and prompting you to take action to quench your thirst. Salivation can be stimulated by the sight, smell, and taste of food. It can even be stimulated by thinking about food. You might notice whether reading about food and salivation right now has had any effect on your production of saliva. How does the salivation process work while you are eating? Food contains chemicals that stimulate taste receptors on the tongue, which send impulses to the superior and inferior salivatory nuclei in the brain stem. These two nuclei then send back parasympathetic impulses through fibers in the glossopharyngeal and facial nerves, which stimulate salivation. Even after you swallow food, salivation is increased to cleanse the mouth and to water down and neutralize any irritating chemical remnants, such as that hot sauce in your burrito. Most saliva is swallowed along with food and is reabsorbed, so that fluid is not lost. The Teeth The teeth, or dentes (singular = dens), are organs similar to bones that you use to tear, grind, and otherwise mechanically break down food. Types of Teeth During the course of your lifetime, you have two sets of teeth (one set of teeth is a dentition). Your 20 deciduous teeth, or baby teeth, first begin to appear at about 6 months of age. Between approximately age 6 and 12, these teeth are replaced by 32 permanent teeth. Moving from the center of the mouth toward the side, these are as follows ([link]): e The eight incisors, four top and four bottom, are the sharp front teeth you use for biting into food. e The four cuspids (or canines) flank the incisors and have a pointed edge (cusp) to tear up food. These fang-like teeth are superb for piercing tough or fleshy foods. e Posterior to the cuspids are the eight premolars (or bicuspids), which have an overall flatter shape with two rounded cusps useful for mashing foods. e The most posterior and largest are the 12 molars, which have several pointed cusps used to crush food so it is ready for swallowing. The third members of each set of three molars, top and bottom, are commonly referred to as the wisdom teeth, because their eruption is commonly delayed until early adulthood. It is not uncommon for wisdom teeth to fail to erupt; that is, they remain impacted. In these cases, the teeth are typically removed by orthodontic surgery. Permanent and Deciduous Teeth Central incisor (7-8 yr) Lateral incisor (8-9 yr) Cuspid or canine (11-12 yr) First premolar or bicuspid (9-10 yr) Second premolar or bicuspid (10—12 yr) First molar (6-7 yr) Second molar (12-13 yr) Third molar or Central incisor wisdom tooth (8-12 mo) Lateral incisor (12-24 mo) Cuspid or canine (16-24 mo) First molar (12-16 mo) Second molar (24-32 mo) Second molar (24-32 mo) First molar (12-16 mo) Cuspid or canine (16-24 mo) Lateral incisor (12-15 mo) Central incisor (6-8 mo) Third molar or wisdom tooth Second molar (11-13 yr) First molar (6-7 yr) Second premolar or bicuspid (11-12 yr) First premolar or bicuspid (9-10 yr) Cuspid or canine (9-10 yr) Lateral incisor (7-8 yr) Central incisor (7-8 yr) This figure of two human dentitions shows the arrangement of teeth in the maxilla and mandible, and the relationship between the deciduous and permanent teeth. Anatomy of a Tooth The teeth are secured in the alveolar processes (sockets) of the maxilla and the mandible. Gingivae (commonly called the gums) are soft tissues that line the alveolar processes and surround the necks of the teeth. Teeth are also held in their sockets by a connective tissue called the periodontal ligament. The two main parts of a tooth are the crown, which is the portion projecting above the gum line, and the root, which is embedded within the maxilla and mandible. Both parts contain an inner pulp cavity, containing loose connective tissue through which run nerves and blood vessels. The region of the pulp cavity that runs through the root of the tooth is called the root canal. Surrounding the pulp cavity is dentin, a bone-like tissue. In the root of each tooth, the dentin is covered by an even harder bone-like layer called cementum. In the crown of each tooth, the dentin is covered by an outer layer of enamel, the hardest substance in the body ([link]). Although enamel protects the underlying dentin and pulp cavity, it is still nonetheless susceptible to mechanical and chemical erosion, or what is known as tooth decay. The most common form, dental caries (cavities) develops when colonies of bacteria feeding on sugars in the mouth release acids that cause soft tissue inflammation and degradation of the calcium crystals of the enamel. The digestive functions of the mouth are summarized in [link]. The Structure of the Tooth Enamel crown Dentin —— Gingiva Neck _ \ (gum) \ \ ‘ =m Pulp cavity ~; & \ (contains | |g blood vessels TD tt V&, Lf ') 7S and nerves) 16 i a | (0 Root (| ad l ime. Periodontal NL) i » ee ligament \\ “ Lt] f : HO {| 771 Root canal |G, / > Ae | V4 I . \ WM! SS Lp ey 26 7 — Bone This longitudinal section through a molar in its alveolar socket shows the relationships between enamel, dentin, Digestive Functions of the Mouth Structure Lips and cheeks Salivary glands Tongue’s extrinsic muscles and pulp. Action Confine food between teeth Secrete saliva Move tongue sideways, and in and out Outcome Food is chewed evenly during mastication Moisten and lubricate the lining of the mouth and pharynx Moisten, soften, and dissolve food Clean the mouth and teeth Salivary amylase breaks down starch Manipulate food for chewing Shape food into a bolus Manipulate food for swallowing Digestive Functions of the Mouth Structure Action Outcome Tongue’s Change intrinsic tongue Manipulate food for swallowing muscles shape Sense Nerve impulses from taste buds T food in are conducted to salivary nuclei in aste mouth the brain stem and then to salivary buds é and sense glands, stimulating saliva taste secretion ; Secrete Activated in the stomach Lingual ; ; eager fande lingual Break down triglycerides into s lipase fatty acids and diglycerides Sales Break down solid food into Teeth and crush - smaller particles for deglutition food The Pharynx The pharynx (throat) is involved in both digestion and respiration. It receives food and air from the mouth, and air from the nasal cavities. When food enters the pharynx, involuntary muscle contractions close off the air passageways. A short tube of skeletal muscle lined with a mucous membrane, the pharynx runs from the posterior oral and nasal cavities to the opening of the esophagus and larynx. It has three subdivisions. The most superior, the nasopharynx, is involved only in breathing and speech. The other two subdivisions, the oropharynx and the laryngopharynx, are used for both breathing and digestion. The oropharynx begins inferior to the nasopharynx and is continuous below with the laryngopharynx ([link]). The inferior border of the laryngopharynx connects to the esophagus, whereas the anterior portion connects to the larynx, allowing air to flow into the bronchial tree. Pharynx Soft palate Nasopharynx Hard palate Uvula Oropharynx Epiglottis Glottis \ Laryngopharynx Larynx Trachea —~ Esophagus Es Nasal cavity | Oral cavity S) —/ = Pharynx aa Larynx a ae — The pharynx runs from the nostrils to the esophagus and the larynx. Histologically, the wall of the oropharynx is similar to that of the oral cavity. The mucosa includes a stratified squamous epithelium that is endowed with mucus-producing glands. During swallowing, the elevator skeletal muscles of the pharynx contract, raising and expanding the pharynx to receive the bolus of food. Once received, these muscles relax and the constrictor muscles of the pharynx contract, forcing the bolus into the esophagus and initiating peristalsis. Usually during swallowing, the soft palate and uvula rise reflexively to close off the entrance to the nasopharynx. At the same time, the larynx is pulled superiorly and the cartilaginous epiglottis, its most superior structure, folds inferiorly, covering the glottis (the opening to the larynx); this process effectively blocks access to the trachea and bronchi. When the food “goes down the wrong way,” it goes into the trachea. When food enters the trachea, the reaction is to cough, which usually forces the food up and out of the trachea, and back into the pharynx. The Esophagus The esophagus is a muscular tube that connects the pharynx to the stomach. It is approximately 25.4 cm (10 in) in length, located posterior to the trachea, and remains in a collapsed form when not engaged in swallowing. As you can see in [link], the esophagus runs a mainly straight route through the mediastinum of the thorax. To enter the abdomen, the esophagus penetrates the diaphragm through an opening called the esophageal hiatus. Passage of Food through the Esophagus The upper esophageal sphincter, which is continuous with the inferior pharyngeal constrictor, controls the movement of food from the pharynx into the esophagus. The upper two-thirds of the esophagus consists of both smooth and skeletal muscle fibers, with the latter fading out in the bottom third of the esophagus. Rhythmic waves of peristalsis, which begin in the upper esophagus, propel the bolus of food toward the stomach. Meanwhile, secretions from the esophageal mucosa lubricate the esophagus and food. Food passes from the esophagus into the stomach at the lower esophageal sphincter (also called the gastroesophageal or cardiac sphincter). Recall that sphincters are muscles that surround tubes and serve as valves, closing the tube when the sphincters contract and opening it when they relax. The lower esophageal sphincter relaxes to let food pass into the stomach, and then contracts to prevent stomach acids from backing up into the esophagus. Surrounding this sphincter is the muscular diaphragm, which helps close off the sphincter when no food is being swallowed. When the lower esophageal sphincter does not completely close, the stomach’s contents can reflux (that is, back up into the esophagus), causing heartburn or gastroesophageal reflux disease (GERD). Esophagus Upper esophageal sphincter Esophagus Lower esophageal sphincter {8) Stomach SS TY The upper esophageal sphincter controls the movement of food from the pharynx to the esophagus. The lower esophageal sphincter controls the movement of food from the esophagus to the stomach. Histology of the Esophagus The mucosa of the esophagus is made up of an epithelial lining that contains non-keratinized, stratified squamous epithelium, with a layer of basal and parabasal cells. This epithelium protects against erosion from food particles. The mucosa’s lamina propria contains mucus-secreting glands. The muscularis layer changes according to location: In the upper third of the esophagus, the muscularis is skeletal muscle. In the middle third, it is both skeletal and smooth muscle. In the lower third, it is smooth muscle. As mentioned previously, the most superficial layer of the esophagus is called the adventitia, not the serosa. In contrast to the stomach and intestines, the loose connective tissue of the adventitia is not covered by a fold of visceral peritoneum. The digestive functions of the esophagus are identified in [link]. Digestive Functions of the Esophagus Action Outcome Upper esophageal Allows the bolus to move from the sphincter laryngopharynx to the esophagus relaxation Peristalsis Propels the bolus through the esophagus powel Allows the bolus to move from the esophagus esophageal into the stomach and prevents chime from hincter ' ee entering the esophagus relaxation Digestive Functions of the Esophagus Action Outcome Mucus Lubricates the esophagus, allowing easy passage secretion of the bolus Deglutition Deglutition is another word for swallowing—the movement of food from the mouth to the stomach. The entire process takes about 4 to 8 seconds for solid or semisolid food, and about 1 second for very soft food and liquids. Although this sounds quick and effortless, deglutition is, in fact, a complex process that involves both the skeletal muscle of the tongue and the muscles of the pharynx and esophagus. It is aided by the presence of mucus and saliva. There are three stages in deglutition: the voluntary phase, the pharyngeal phase, and the esophageal phase ([link]). The autonomic nervous system controls the latter two phases. Deglutition je = “aS Y Kg HSS bn. m _— < ¢ } Ino ) Superior pharyngeal constrictor muscle Medial pharyngeal constrictor muscle Medial and inferior \ i pharyngeal \@\\ constrictor ' \ muscles . Inferior pharyngeal | and esophageal \\\—— constrictor muscles <<“ “a \ A ( \ YW Yj Deglutition includes the voluntary phase and two involuntary phases: the pharyngeal phase and the esophageal phase. The Voluntary Phase The voluntary phase of deglutition (also known as the oral or buccal phase) is so called because you can control when you swallow food. In this phase, chewing has been completed and swallowing is set in motion. The tongue moves upward and backward against the palate, pushing the bolus to the back of the oral cavity and into the oropharynx. Other muscles keep the mouth closed and prevent food from falling out. At this point, the two involuntary phases of swallowing begin. The Pharyngeal Phase In the pharyngeal phase, stimulation of receptors in the oropharynx sends impulses to the deglutition center (a collection of neurons that controls swallowing) in the medulla oblongata. Impulses are then sent back to the uvula and soft palate, causing them to move upward and close off the nasopharynx. The laryngeal muscles also constrict to prevent aspiration of food into the trachea. At this point, deglutition apnea takes place, which means that breathing ceases for a very brief time. Contractions of the pharyngeal constrictor muscles move the bolus through the oropharynx and laryngopharynx. Relaxation of the upper esophageal sphincter then allows food to enter the esophagus. The Esophageal Phase The entry of food into the esophagus marks the beginning of the esophageal phase of deglutition and the initiation of peristalsis. As in the previous phase, the complex neuromuscular actions are controlled by the medulla oblongata. Peristalsis propels the bolus through the esophagus and toward the stomach. The circular muscle layer of the muscularis contracts, pinching the esophageal wall and forcing the bolus forward. At the same time, the longitudinal muscle layer of the muscularis also contracts, shortening this area and pushing out its walls to receive the bolus. In this way, a series of contractions keeps moving food toward the stomach. When the bolus nears the stomach, distention of the esophagus initiates a short reflex relaxation of the lower esophageal sphincter that allows the bolus to pass into the stomach. During the esophageal phase, esophageal glands secrete mucus that lubricates the bolus and minimizes friction. Note: I OR ero HS Erk ep wees OPenstax COLLEGE” one Watch this_animation to see how swallowing is a complex process that involves the nervous system to coordinate the actions of upper respiratory and digestive activities. During which stage of swallowing is there a risk of food entering respiratory pathways and how is this risk blocked? Chapter Review In the mouth, the tongue and the teeth begin mechanical digestion, and saliva begins chemical digestion. The pharynx, which plays roles in breathing and vocalization as well as digestion, runs from the nasal and oral cavities superiorly to the esophagus inferiorly (for digestion) and to the larynx anteriorly (for respiration). During deglutition (swallowing), the soft palate rises to close off the nasopharynx, the larynx elevates, and the epiglottis folds over the glottis. The esophagus includes an upper esophageal sphincter made of skeletal muscle, which regulates the movement of food from the pharynx to the esophagus. It also has a lower esophageal sphincter, made of smooth muscle, which controls the passage of food from the esophagus to the stomach. Cells in the esophageal wall secrete mucus that eases the passage of the food bolus. Interactive Link Questions Exercise: Problem: Watch this animation to see how swallowing is a complex process that involves the nervous system to coordinate the actions of upper respiratory and digestive activities. During which stage of swallowing is there a risk of food entering respiratory pathways and how is this risk blocked? Solution: Answers may vary. Review Questions Exercise: Problem: Which of these ingredients in saliva is responsible for activating salivary amylase? a. mucus b. phosphate ions c. chloride ions d. urea Solution: C Exercise: Problem: Which of these statements about the pharynx is true? a. It extends from the nasal and oral cavities superiorly to the esophagus anteriorly. b. The oropharynx is continuous superiorly with the nasopharynx. c. The nasopharynx is involved in digestion. d. The laryngopharynx is composed partially of cartilage. Solution: B Exercise: Problem: Which structure is located where the esophagus penetrates the diaphragm? a. esophageal hiatus b. cardiac orifice c. upper esophageal sphincter d. lower esophageal sphincter Solution: A Exercise: Problem: Which phase of deglutition involves contraction of the longitudinal muscle layer of the muscularis? a. voluntary phase b. buccal phase c. pharyngeal phase d. esophageal phase Solution: D Critical Thinking Questions Exercise: Problem: The composition of saliva varies from gland to gland. Discuss how saliva produced by the parotid gland differs in action from saliva produced by the sublingual gland. Solution: Parotid gland saliva is watery with little mucus but a lot of amylase, which allows it to mix freely with food during mastication and begin the digestion of carbohydrates. In contrast, sublingual gland saliva has a lot of mucus with the least amount of amylase of all the salivary glands. The high mucus content serves to lubricate the food for swallowing. Exercise: Problem: During a hockey game, the puck hits a player in the mouth, knocking out all eight of his most anterior teeth. Which teeth did the player lose and how does this loss affect food ingestion? Solution: The incisors. Since these teeth are used for tearing off pieces of food during ingestion, the player will need to ingest foods that have already been cut into bite-sized pieces until the broken teeth are replaced. Exercise: Problem: What prevents swallowed food from entering the airways? Solution: Usually when food is swallowed, involuntary muscle contractions cause the soft palate to rise and close off the nasopharynx. The larynx also is pulled up, and the epiglottis folds over the glottis. These actions block off the air passages. Exercise: Problem: Explain the mechanism responsible for gastroesophageal reflux. Solution: If the lower esophageal sphincter does not close completely, the stomach’s acidic contents can back up into the esophagus, a phenomenon known as GERD. Exercise: Problem: Describe the three processes involved in the esophageal phase of deglutition. Solution: Peristalsis moves the bolus down the esophagus and toward the stomach. Esophageal glands secrete mucus that lubricates the bolus and reduces friction. When the bolus nears the stomach, the lower esophageal sphincter relaxes, allowing the bolus to pass into the stomach. References van Loon FPL, Holmes SJ, Sirotkin B, Williams W, Cochi S, Hadler S, Lindegren ML. Morbidity and Mortality Weekly Report: Mumps surveillance -- United States, 1988—1993 [Internet]. Atlanta, GA: Center for Disease Control; [cited 2013 Apr 3]. Available from: http://www.cdc.gov/mmwr/preview/mmwrhtml/00038546.htm. Glossary bolus mass of chewed food cementum bone-like tissue covering the root of a tooth crown portion of tooth visible superior to the gum line cuspid (also, canine) pointed tooth used for tearing and shredding food deciduous tooth one of 20 “baby teeth” deglutition three-stage process of swallowing dens tooth dentin bone-like tissue immediately deep to the enamel of the crown or cementum of the root of a tooth dentition set of teeth enamel covering of the dentin of the crown of a tooth esophagus muscular tube that runs from the pharynx to the stomach fauces opening between the oral cavity and the oropharynx gingiva gum incisor midline, chisel-shaped tooth used for cutting into food labium lip labial frenulum midline mucous membrane fold that attaches the inner surface of the lips to the gums laryngopharynx part of the pharynx that functions in respiration and digestion lingual frenulum mucous membrane fold that attaches the bottom of the tongue to the floor of the mouth lingual lipase digestive enzyme from glands in the tongue that acts on triglycerides lower esophageal sphincter smooth muscle sphincter that regulates food movement from the esophagus to the stomach molar tooth used for crushing and grinding food oral cavity (also, buccal cavity) mouth oral vestibule part of the mouth bounded externally by the cheeks and lips, and internally by the gums and teeth oropharynx part of the pharynx continuous with the oral cavity that functions in respiration and digestion palatoglossal arch muscular fold that extends from the lateral side of the soft palate to the base of the tongue palatopharyngeal arch muscular fold that extends from the lateral side of the soft palate to the side of the pharynx parotid gland one of a pair of major salivary glands located inferior and anterior to the ears permanent tooth one of 32 adult teeth pharynx throat premolar (also, bicuspid) transitional tooth used for mastication, crushing, and grinding food pulp cavity deepest portion of a tooth, containing nerve endings and blood vessels root portion of a tooth embedded in the alveolar processes beneath the gum line saliva aqueous solution of proteins and ions secreted into the mouth by the salivary glands salivary amylase digestive enzyme in saliva that acts on starch salivary gland an exocrine gland that secretes a digestive fluid called saliva salivation secretion of saliva soft palate posterior region of the bottom portion of the nasal cavity that consists of skeletal muscle sublingual gland one of a pair of major salivary glands located beneath the tongue submandibular gland one of a pair of major salivary glands located in the floor of the mouth tongue accessory digestive organ of the mouth, the bulk of which is composed of skeletal muscle upper esophageal sphincter skeletal muscle sphincter that regulates food movement from the pharynx to the esophagus voluntary phase initial phase of deglutition, in which the bolus moves from the mouth to the oropharynx The Stomach By the end of this section, you will be able to: e Label on a diagram the four main regions of the stomach, its curvatures, and its sphincter e Identify the four main types of secreting cells in gastric glands, and their important products e Explain why the stomach does not digest itself ¢ Describe the mechanical and chemical digestion of food entering the stomach Although a minimal amount of carbohydrate digestion occurs in the mouth, chemical digestion really gets underway in the stomach. An expansion of the alimentary canal that lies immediately inferior to the esophagus, the stomach links the esophagus to the first part of the small intestine (the duodenum) and is relatively fixed in place at its esophageal and duodenal ends. In between, however, it can be a highly active structure, contracting and continually changing position and size. These contractions provide mechanical assistance to digestion. The empty stomach is only about the size of your fist, but can stretch to hold as much as 4 liters of food and fluid, or more than 75 times its empty volume, and then return to its resting size when empty. Although you might think that the size of a person’s stomach is related to how much food that individual consumes, body weight does not correlate with stomach size. Rather, when you eat greater quantities of food—such as at holiday dinner—you stretch the stomach more than when you eat less. Popular culture tends to refer to the stomach as the location where all digestion takes place. Of course, this is not true. An important function of the stomach is to serve as a temporary holding chamber. You can ingest a meal far more quickly than it can be digested and absorbed by the small intestine. Thus, the stomach holds food and parses only small amounts into the small intestine at a time. Foods are not processed in the order they are eaten; rather, they are mixed together with digestive juices in the stomach until they are converted into chyme, which is released into the small intestine. As you will see in the sections that follow, the stomach plays several important roles in chemical digestion, including the continued digestion of carbohydrates and the initial digestion of proteins and triglycerides. Little if any nutrient absorption occurs in the stomach, with the exception of the negligible amount of nutrients in alcohol. Structure There are four main regions in the stomach: the cardia, fundus, body, and pylorus ([link]). The cardia (or cardiac region) is the point where the esophagus connects to the stomach and through which food passes into the stomach. Located inferior to the diaphragm, above and to the left of the cardia, is the dome-shaped fundus. Below the fundus is the body, the main part of the stomach. The funnel-shaped pylorus connects the stomach to the duodenum. The wider end of the funnel, the pyloric antrum, connects to the body of the stomach. The narrower end is called the pyloric canal, which connects to the duodenum. The smooth muscle pyloric sphincter is located at this latter point of connection and controls stomach emptying. In the absence of food, the stomach deflates inward, and its mucosa and submucosa fall into a large fold called a ruga. Stomach Cardia Esophagus Muscularis externa: Longitudinal layer Circular layer Oblique layer Fundus Serosa Lesser curvature Body Pyloric sphincter (valve) at pylorus Lumen Rugae of mucosa Duodenum F ~~ Pyloric canal Pyloric antrum Greater curvature The stomach has four major regions: the cardia, fundus, body, and pylorus. The addition of an inner oblique smooth muscle layer gives the muscularis the ability to vigorously churn and mix food. The convex lateral surface of the stomach is called the greater curvature; the concave medial border is the lesser curvature. The stomach is held in place by the lesser omentum, which extends from the liver to the lesser curvature, and the greater omentum, which runs from the greater curvature to the posterior abdominal wall. Histology The wall of the stomach is made of the same four layers as most of the rest of the alimentary canal, but with adaptations to the mucosa and muscularis for the unique functions of this organ. In addition to the typical circular and longitudinal smooth muscle layers, the muscularis has an inner oblique smooth muscle layer ({link]). As a result, in addition to moving food through the canal, the stomach can vigorously churn food, mechanically breaking it down into smaller particles. Histology of the Stomach Parietal cell Surface epithelium Gastric pit Gastric gland Lamina propria Chief cell Muscularis mucosae Submucosa ———_—— aS > Oblique layer . Muscularis Circular layer —9 : » | Enteroendocrine externa Longitudinal SS / cell layer ~ y Serosa The stomach wall is adapted for the functions of the stomach. In the epithelium, gastric pits lead to gastric glands that secrete gastric juice. The gastric glands (one gland is shown enlarged on the right) contain different types of cells that secrete a variety of enzymes, including hydrochloride acid, which activates the protein-digesting enzyme pepsin. The stomach mucosa’s epithelial lining consists only of surface mucus cells, which secrete a protective coat of alkaline mucus. A vast number of gastric pits dot the surface of the epithelium, giving it the appearance of a well-used pincushion, and mark the entry to each gastric gland, which secretes a complex digestive fluid referred to as gastric juice. Although the walls of the gastric pits are made up primarily of mucus cells, the gastric glands are made up of different types of cells. The glands of the cardia and pylorus are composed primarily of mucus-secreting cells. Cells that make up the pyloric antrum secrete mucus and a number of hormones, including the majority of the stimulatory hormone, gastrin. The much larger glands of the fundus and body of the stomach, the site of most chemical digestion, produce most of the gastric secretions. These glands are made up of a variety of secretory cells. These include parietal cells, chief cells, mucous neck cells, and enteroendocrine cells. Parietal cells—Located primarily in the middle region of the gastric glands are parietal cells, which are among the most highly differentiated of the body’s epithelial cells. These relatively large cells produce both hydrochloric acid (HCI) and intrinsic factor. HCl is responsible for the high acidity (pH 1.5 to 3.5) of the stomach contents and is needed to activate the protein-digesting enzyme, pepsin. The acidity also kills much of the bacteria you ingest with food and helps to denature proteins, making them more available for enzymatic digestion. Intrinsic factor is a glycoprotein necessary for the absorption of vitamin Bj, in the small intestine. Chief cells—Located primarily in the basal regions of gastric glands are chief cells, which secrete pepsinogen, the inactive proenzyme form of pepsin. HCl is necessary for the conversion of pepsinogen to pepsin. Mucous neck cells—Gastric glands in the upper part of the stomach contain mucous neck cells that secrete thin, acidic mucus that is much different from the mucus secreted by the goblet cells of the surface epithelium. The role of this mucus is not currently known. Enteroendocrine cells—Finally, enteroendocrine cells found in the gastric glands secrete various hormones into the interstitial fluid of the lamina propria. These include gastrin, which is released mainly by enteroendocrine G cells. [link] describes the digestive functions of important hormones secreted by the stomach. Note: Op eC => openstax COLLEGE arene: Watch this animation that depicts the structure of the stomach and how this structure functions in the initiation of protein digestion. This view of the stomach shows the characteristic rugae. What is the function of these rugae? Hormones Secreted by the Stomach Production Production Hormone site stimulus Target organ Action Hormones Secreted by the Stomach Hormone Gastrin Gastrin Gastrin Gastrin Ghrelin Production site Stomach mucosa, mainly G cells of the pyloric antrum Stomach mucosa, mainly G cells of the pyloric antrum Stomach mucosa, mainly G cells of the pyloric antrum Stomach mucosa, mainly G cells of the pyloric antrum Stomach mucosa, mainly fundus Production stimulus Presence of peptides and amino acids in stomach Presence of peptides and amino acids in stomach Presence of peptides and amino acids in stomach Presence of peptides and amino acids in stomach Fasting state (levels increase just prior to meals) Target organ Stomach Small intestine Ileocecal valve Large intestine Hypothalamus Action Increases secretion by gastric glands; promotes gastric emptying Promotes intestinal muscle contraction Relaxes valve Triggers mass movements Regulates food intake, primarily by stimulating hunger and satiety Hormones Secreted by the Stomach Hormone Histamine Serotonin Somatostatin Somatostatin Somatostatin Gastric Secretion Production site Stomach mucosa Stomach mucosa Mucosa of stomach, especially pyloric antrum; also duodenum Mucosa of stomach, especially pyloric antrum; also duodenum Mucosa of stomach, especially pyloric antrum; also duodenum Production stimulus Presence of food in the stomach Presence of food in the stomach Presence of food in the stomach; sympathetic axon stimulation Presence of food in the stomach; sympathetic axon stimulation Presence of food in the stomach; sympathetic axon stimulation Target organ Stomach Stomach Stomach Pancreas Small intestine Action Stimulates parietal cells to release HCl Contracts stomach muscle Restricts all gastric secretions, gastric motility, and emptying Restricts pancreatic secretions Reduces intestinal absorption by reducing blood flow The secretion of gastric juice is controlled by both nerves and hormones. Stimuli in the brain, stomach, and small intestine activate or inhibit gastric juice production. This is why the three phases of gastric secretion are called the cephalic, gastric, and intestinal phases ([link]). However, once gastric secretion begins, all three phases can occur simultaneously. The Three Phases of Gastric Secretion Stimulates stomach secretory activity CEPHALIC PHASE Inhibits stomach secretory activity Stimulates stomach secretory activity GASTRIC PHASE Inhibits stomach secretory activity Stimulates stomach secretory activity INTESTINAL PHASE Inhibits stomach secretory activity Gastric secretion occurs in three phases: cephalic, gastric, and intestinal. During each phase, the secretion of gastric juice can be stimulated or inhibited. The cephalic phase (reflex phase) of gastric secretion, which is relatively brief, takes place before food enters the stomach. The smell, taste, sight, or thought of food triggers this phase. For example, when you bring a piece of sushi to your lips, impulses from receptors in your taste buds or the nose are relayed to your brain, which returns signals that increase gastric secretion to prepare your stomach for digestion. This enhanced secretion is a conditioned reflex, meaning it occurs only if you like or want a particular food. Depression and loss of appetite can suppress the cephalic reflex. The gastric phase of secretion lasts 3 to 4 hours, and is set in motion by local neural and hormonal mechanisms triggered by the entry of food into the stomach. For example, when your sushi reaches the stomach, it creates distention that activates the stretch receptors. This stimulates parasympathetic neurons to release acetylcholine, which then provokes increased secretion of gastric juice. Partially digested proteins, caffeine, and rising pH stimulate the release of gastrin from enteroendocrine G cells, which in turn induces parietal cells to increase their production of HCl, which is needed to create an acidic environment for the conversion of pepsinogen to pepsin, and protein digestion. Additionally, the release of gastrin activates vigorous smooth muscle contractions. However, it should be noted that the stomach does have a natural means of avoiding excessive acid secretion and potential heartburn. Whenever pH levels drop too low, cells in the stomach react by suspending HCI secretion and increasing mucous secretions. The intestinal phase of gastric secretion has both excitatory and inhibitory elements. The duodenum has a major role in regulating the stomach and its emptying. When partially digested food fills the duodenum, intestinal mucosal cells release a hormone called intestinal (enteric) gastrin, which further excites gastric juice secretion. This stimulatory activity is brief, however, because when the intestine distends with chyme, the enterogastric reflex inhibits secretion. One of the effects of this reflex is to close the pyloric sphincter, which blocks additional chyme from entering the duodenum. The Mucosal Barrier The mucosa of the stomach is exposed to the highly corrosive acidity of gastric juice. Gastric enzymes that can digest protein can also digest the stomach itself. The stomach is protected from self-digestion by the mucosal barrier. This barrier has several components. First, the stomach wall is covered by a thick coating of bicarbonate-rich mucus. This mucus forms a physical barrier, and its bicarbonate ions neutralize acid. Second, the epithelial cells of the stomach's mucosa meet at tight junctions, which block gastric juice from penetrating the underlying tissue layers. Finally, stem cells located where gastric glands join the gastric pits quickly replace damaged epithelial mucosal cells, when the epithelial cells are shed. In fact, the surface epithelium of the stomach is completely replaced every 3 to 6 days. Note: Homeostatic Imbalances Ulcers: When the Mucosal Barrier Breaks Down As effective as the mucosal barrier is, it is not a “fail-safe” mechanism. Sometimes, gastric juice eats away at the superficial lining of the stomach mucosa, creating erosions, which mostly heal on their own. Deeper and larger erosions are called ulcers. Why does the mucosal barrier break down? A number of factors can interfere with its ability to protect the stomach lining. The majority of all ulcers are caused by either excessive intake of non-steroidal anti-inflammatory drugs (NSAIDs), including aspirin, or Helicobacter pylori infection. Antacids help relieve symptoms of ulcers such as “burning” pain and indigestion. When ulcers are caused by NSAID use, switching to other classes of pain relievers allows healing. When caused by H. pylori infection, antibiotics are effective. A potential complication of ulcers is perforation: Perforated ulcers create a hole in the stomach wall, resulting in peritonitis (inflammation of the peritoneum). These ulcers must be repaired surgically. Digestive Functions of the Stomach The stomach participates in virtually all the digestive activities with the exception of ingestion and defecation. Although almost all absorption takes place in the small intestine, the stomach does absorb some nonpolar substances, such as alcohol and aspirin. Mechanical Digestion Within a few moments after food after enters your stomach, mixing waves begin to occur at intervals of approximately 20 seconds. A mixing wave is a unique type of peristalsis that mixes and softens the food with gastric juices to create chyme. The initial mixing waves are relatively gentle, but these are followed by more intense waves, starting at the body of the stomach and increasing in force as they reach the pylorus. It is fair to say that long before your sushi exits through the pyloric sphincter, it bears little resemblance to the sushi you ate. The pylorus, which holds around 30 mL (1 fluid ounce) of chyme, acts as a filter, permitting only liquids and small food particles to pass through the mostly, but not fully, closed pyloric sphincter. In a process called gastric emptying, rhythmic mixing waves force about 3 mL of chyme at a time through the pyloric sphincter and into the duodenum. Release of a greater amount of chyme at one time would overwhelm the capacity of the small intestine to handle it. The rest of the chyme is pushed back into the body of the stomach, where it continues mixing. This process is repeated when the next mixing waves force more chyme into the duodenum. Gastric emptying is regulated by both the stomach and the duodenum. The presence of chyme in the duodenum activates receptors that inhibit gastric secretion. This prevents additional chyme from being released by the stomach before the duodenum is ready to process it. Chemical Digestion The fundus plays an important role, because it stores both undigested food and gases that are released during the process of chemical digestion. Food may sit in the fundus of the stomach for a while before being mixed with the chyme. While the food is in the fundus, the digestive activities of salivary amylase continue until the food begins mixing with the acidic chyme. Ultimately, mixing waves incorporate this food with the chyme, the acidity of which inactivates salivary amylase and activates lingual lipase. Lingual lipase then begins breaking down triglycerides into free fatty acids, and mono- and diglycerides. The breakdown of protein begins in the stomach through the actions of HCl and the enzyme pepsin. During infancy, gastric glands also produce rennin, an enzyme that helps digest milk protein. Its numerous digestive functions notwithstanding, there is only one stomach function necessary to life: the production of intrinsic factor. The intestinal absorption of vitamin By», which is necessary for both the production of mature red blood cells and normal neurological functioning, cannot occur without intrinsic factor. People who undergo total gastrectomy (stomach removal)—for life-threatening stomach cancer, for example—can survive with minimal digestive dysfunction if they receive vitamin B,> injections. The contents of the stomach are completely emptied into the duodenum within 2 to 4 hours after you eat a meal. Different types of food take different amounts of time to process. Foods heavy in carbohydrates empty fastest, followed by high-protein foods. Meals with a high triglyceride content remain in the stomach the longest. Since enzymes in the small intestine digest fats slowly, food can stay in the stomach for 6 hours or longer when the duodenum is processing fatty chyme. However, note that this is still a fraction of the 24 to 72 hours that full digestion typically takes from start to finish. Chapter Review The stomach participates in all digestive activities except ingestion and defecation. It vigorously churns food. It secretes gastric juices that break down food and absorbs certain drugs, including aspirin and some alcohol. The stomach begins the digestion of protein and continues the digestion of carbohydrates and fats. It stores food as an acidic liquid called chyme, and releases it gradually into the small intestine through the pyloric sphincter. Interactive Link Questions Exercise: Problem: Watch this animation that depicts the structure of the stomach and how this structure functions in the initiation of protein digestion. This view of the stomach shows the characteristic rugae. What is the function of these rugae? Solution: Answers may vary. Review Questions Exercise: Problem: Which of these cells secrete hormones? a. parietal cells b. mucous neck cells c. enteroendocrine cells d. chief cells Solution: C Exercise: Problem:Where does the majority of chemical digestion in the stomach occur? a. fundus and body b. cardia and fundus c. body and pylorus d. body Solution: A Exercise: Problem: During gastric emptying, chyme is released into the duodenum through the a. esophageal hiatus b. pyloric antrum c. pyloric canal d. pyloric sphincter Solution: D Exercise: Problem: Parietal cells secrete a. gastrin b. hydrochloric acid c. pepsin d. pepsinogen Solution: B Critical Thinking Questions Exercise: Problem: Explain how the stomach is protected from self-digestion and why this is necessary. Solution: The mucosal barrier protects the stomach from self-digestion. It includes a thick coating of bicarbonate-rich mucus; the mucus is physically protective, and bicarbonate neutralizes gastric acid. Epithelial cells meet at tight junctions, which block gastric juice from penetrating the underlying tissue layers, and stem cells quickly replace sloughed off epithelial mucosal cells. Exercise: Problem: Describe unique anatomical features that enable the stomach to perform digestive functions. Solution: The stomach has an additional inner oblique smooth muscle layer that helps the muscularis churn and mix food. The epithelium includes gastric glands that secrete gastric fluid. The gastric fluid consists mainly of mucous, HCl, and the enzyme pepsin released as pepsinogen. Glossary body mid-portion of the stomach cardia (also, cardiac region) part of the stomach surrounding the cardiac orifice (esophageal hiatus) cephalic phase (also, reflex phase) initial phase of gastric secretion that occurs before food enters the stomach chief cell gastric gland cell that secretes pepsinogen enteroendocrine cell gastric gland cell that releases hormones fundus dome-shaped region of the stomach above and to the left of the cardia G cell gastrin-secreting enteroendocrine cell gastric emptying process by which mixing waves gradually cause the release of chyme into the duodenum gastric gland gland in the stomach mucosal epithelium that produces gastric juice gastric phase phase of gastric secretion that begins when food enters the stomach gastric pit narrow channel formed by the epithelial lining of the stomach mucosa gastrin peptide hormone that stimulates secretion of hydrochloric acid and gut motility hydrochloric acid (HCl) digestive acid secreted by parietal cells in the stomach intrinsic factor glycoprotein required for vitamin B,> absorption in the small intestine intestinal phase phase of gastric secretion that begins when chyme enters the intestine mixing wave unique type of peristalsis that occurs in the stomach mucosal barrier protective barrier that prevents gastric juice from destroying the stomach itself mucous neck cell gastric gland cell that secretes a uniquely acidic mucus parietal cell gastric gland cell that secretes hydrochloric acid and intrinsic factor pepsinogen inactive form of pepsin pyloric antrum wider, more superior part of the pylorus pyloric canal narrow, more inferior part of the pylorus pyloric sphincter sphincter that controls stomach emptying pylorus lower, funnel-shaped part of the stomach that is continuous with the duodenum ruga fold of alimentary canal mucosa and submucosa in the empty stomach and other organs stomach alimentary canal organ that contributes to chemical and mechanical digestion of food from the esophagus before releasing it, as chyme, to the small intestine The Small and Large Intestines By the end of this section, you will be able to: ¢ Compare and contrast the location and gross anatomy of the small and large intestines e Identify three main adaptations of the small intestine wall that increase its absorptive capacity e Describe the mechanical and chemical digestion of chyme upon its release into the small intestine e List three features unique to the wall of the large intestine and identify their contributions to its function e Identify the beneficial roles of the bacterial flora in digestive system functioning e Trace the pathway of food waste from its point of entry into the large intestine through its exit from the body as feces The word intestine is derived from a Latin root meaning “internal,” and indeed, the two organs together nearly fill the interior of the abdominal cavity. In addition, called the small and large bowel, or colloquially the “suts,” they constitute the greatest mass and length of the alimentary canal and, with the exception of ingestion, perform all digestive system functions. The Small Intestine Chyme released from the stomach enters the small intestine, which is the primary digestive organ in the body. Not only is this where most digestion occurs, it is also where practically all absorption occurs. The longest part of the alimentary canal, the small intestine is about 3.05 meters (10 feet) long in a living person (but about twice as long in a cadaver due to the loss of muscle tone). Since this makes it about five times longer than the large intestine, you might wonder why it is called “small.” In fact, its name derives from its relatively smaller diameter of only about 2.54 cm (1 in), compared with 7.62 cm (3 in) for the large intestine. As we’ll see shortly, in addition to its length, the folds and projections of the lining of the small intestine work to give it an enormous surface area, which is approximately 200 m?, more than 100 times the surface area of your skin. This large surface area is necessary for complex processes of digestion and absorption that occur within it. Structure The coiled tube of the small intestine is subdivided into three regions. From proximal (at the stomach) to distal, these are the duodenum, jejunum, and ileum ([link]). The shortest region is the 25.4-cm (10-in) duodenum, which begins at the pyloric sphincter. Just past the pyloric sphincter, it bends posteriorly behind the peritoneum, becoming retroperitoneal, and then makes a C-shaped curve around the head of the pancreas before ascending anteriorly again to return to the peritoneal cavity and join the jejunum. The duodenum can therefore be subdivided into four segments: the superior, descending, horizontal, and ascending duodenum. Of particular interest is the hepatopancreatic ampulla (ampulla of Vater). Located in the duodenal wall, the ampulla marks the transition from the anterior portion of the alimentary canal to the mid-region, and is where the bile duct (through which bile passes from the liver) and the main pancreatic duct (through which pancreatic juice passes from the pancreas) join. This ampulla opens into the duodenum at a tiny volcano-shaped structure called the major duodenal papilla. The hepatopancreatic sphincter (sphincter of Oddi) regulates the flow of both bile and pancreatic juice from the ampulla into the duodenum. Small Intestine Duodenum Jejunum lleum Large intestine Rectum ———___—>=_ The three regions of the small intestine are the duodenum, jejunum, and ileum. The jejunum is about 0.9 meters (3 feet) long (in life) and runs from the duodenum to the ileum. Jejunum means “empty” in Latin and supposedly was so named by the ancient Greeks who noticed it was always empty at death. No clear demarcation exists between the jejunum and the final segment of the small intestine, the ileum. The ileum is the longest part of the small intestine, measuring about 1.8 meters (6 feet) in length. It is thicker, more vascular, and has more developed mucosal folds than the jejunum. The ileum joins the cecum, the first portion of the large intestine, at the ileocecal sphincter (or valve). The jejunum and ileum are tethered to the posterior abdominal wall by the mesentery. The large intestine frames these three parts of the small intestine. Parasympathetic nerve fibers from the vagus nerve and sympathetic nerve fibers from the thoracic splanchnic nerve provide extrinsic innervation to the small intestine. The superior mesenteric artery is its main arterial supply. Veins run parallel to the arteries and drain into the superior mesenteric vein. Nutrient-rich blood from the small intestine is then carried to the liver via the hepatic portal vein. Histology The wall of the small intestine is composed of the same four layers typically present in the alimentary system. However, three features of the mucosa and submucosa are unique. These features, which increase the absorptive surface area of the small intestine more than 600-fold, include circular folds, villi, and microvilli ([link]). These adaptations are most abundant in the proximal two-thirds of the small intestine, where the majority of absorption occurs. ~~ of the Small ie Absorptive cells Capillary oe Artery (brush border) Goblet cell trl@t JURee Jet Be! ° ony OS ED Vac |i Lymphatic vesicle Muscularis mucosae Duodenal gland (a) (a) The absorptive surface of the small intestine is vastly enlarged by the presence of circular folds, villi, and microvilli. (b) Micrograph of the circular folds. (c) Micrograph of the villi. (d) Electron micrograph of the microvilli. From left to right, LM x 56, LM x 508, EM x 196,000. (credit b-d: Micrograph provided by the Regents of University of Michigan Medical School © 2012) Circular folds Also called a plica circulare, a circular fold is a deep ridge in the mucosa and submucosa. Beginning near the proximal part of the duodenum and ending near the middle of the ileum, these folds facilitate absorption. Their shape causes the chyme to spiral, rather than move in a straight line, through the small intestine. Spiraling slows the movement of chyme and provides the time needed for nutrients to be fully absorbed. Villi Within the circular folds are small (0.5—1 mm long) hairlike vascularized projections called villi (singular = villus) that give the mucosa a furry texture. There are about 20 to 40 villi per square millimeter, increasing the surface area of the epithelium tremendously. The mucosal epithelium, primarily composed of absorptive cells, covers the villi. In addition to muscle and connective tissue to support its structure, each villus contains a capillary bed composed of one arteriole and one venule, as well as a lymphatic capillary called a lacteal. The breakdown products of carbohydrates and proteins (sugars and amino acids) can enter the bloodstream directly, but lipid breakdown products are absorbed by the lacteals and transported to the bloodstream via the lymphatic system. Microvilli As their name suggests, microvilli (singular = microvillus) are much smaller (1 pm) than villi. They are cylindrical apical surface extensions of the plasma membrane of the mucosa’s epithelial cells, and are supported by microfilaments within those cells. Although their small size makes it difficult to see each microvillus, their combined microscopic appearance suggests a mass of bristles, which is termed the brush border. Fixed to the surface of the microvilli membranes are enzymes that finish digesting carbohydrates and proteins. There are an estimated 200 million microvilli per square millimeter of small intestine, greatly expanding the surface area of the plasma membrane and thus greatly enhancing absorption. Intestinal Glands In addition to the three specialized absorptive features just discussed, the mucosa between the villi is dotted with deep crevices that each lead into a tubular intestinal gland (crypt of Lieberkiihn), which is formed by cells that line the crevices (see [link]). These produce intestinal juice, a slightly alkaline (pH 7.4 to 7.8) mixture of water and mucus. Each day, about 0.95 to 1.9 liters (1 to 2 quarts) are secreted in response to the distention of the small intestine or the irritating effects of chyme on the intestinal mucosa. The submucosa of the duodenum is the only site of the complex mucus- secreting duodenal glands (Brunner’s glands), which produce a bicarbonate-rich alkaline mucus that buffers the acidic chyme as it enters from the stomach. The roles of the cells in the small intestinal mucosa are detailed in [link]. Cells of the Small Intestinal Mucosa Cells of the SmdlbhatéstialtNéucosa Cell type Cell type Absorptive Goblet Paneth G cells I cells K cells mucosa Location in the mucosa Epithelium/intestinal glands Epithelium/intestinal glands Intestinal glands Intestinal glands of duodenum Intestinal glands of duodenum Intestinal glands Function Function Digestion and absorption of nutrients in chyme Secretion of mucus Secretion of the bactericidal enzyme lysozyme; phagocytosis Secretion of the hormone intestinal gastrin Secretion of the hormone cholecystokinin, which stimulates release of pancreatic juices and bile Secretion of the hormone glucose-dependent insulinotropic peptide, which stimulates the release of insulin Cells of the Small Intestinal Mucosa Location in the Cell type mucosa Function Secretion of the hormone motilin, which accelerates Intestinal glands of ; gastric emptying, M cells duodenum and : ; ‘ oe stimulates intestinal jejunum ; : : peristalsis, and stimulates the production of pepsin ? retion of the h S cells Intestinal glands Sec eno Die OENOn’ secretin Intestinal MALT The lamina propria of the small intestine mucosa is studded with quite a bit of MALT. In addition to solitary lymphatic nodules, aggregations of intestinal MALT, which are typically referred to as Peyer’s patches, are concentrated in the distal ileum, and serve to keep bacteria from entering the bloodstream. Peyer’s patches are most prominent in young people and become less distinct as you age, which coincides with the general activity of our immune system. Note: — meee OPENStAX COLLEGE Watch this animation that depicts the structure of the small intestine, and, in particular, the villi. Epithelial cells continue the digestion and absorption of nutrients and transport these nutrients to the lymphatic and circulatory systems. In the small intestine, the products of food digestion are absorbed by different structures in the villi. Which structure absorbs and transports fats? Mechanical Digestion in the Small Intestine The movement of intestinal smooth muscles includes both segmentation and a form of peristalsis called migrating motility complexes. The kind of peristaltic mixing waves seen in the stomach are not observed here. If you could see into the small intestine when it was going through segmentation, it would look as if the contents were being shoved incrementally back and forth, as the rings of smooth muscle repeatedly contract and then relax. Segmentation in the small intestine does not force chyme through the tract. Instead, it combines the chyme with digestive juices and pushes food particles against the mucosa to be absorbed. The duodenum is where the most rapid segmentation occurs, at a rate of about 12 times per minute. In the ileum, segmentations are only about eight times per minute ([link]). Segmentation Segmentation separates chyme and then pushes it back together, mixing it and providing time for digestion and absorption. When most of the chyme has been absorbed, the small intestinal wall becomes less distended. At this point, the localized segmentation process is replaced by transport movements. The duodenal mucosa secretes the hormone motilin, which initiates peristalsis in the form of a migrating motility complex. These complexes, which begin in the duodenum, force chyme through a short section of the small intestine and then stop. The next contraction begins a little bit farther down than the first, forces chyme a bit farther through the small intestine, then stops. These complexes move slowly down the small intestine, forcing chyme on the way, taking around 90 to 120 minutes to finally reach the end of the ileum. At this point, the process is repeated, starting in the duodenum. The ileocecal valve, a sphincter, is usually in a constricted state, but when motility in the ileum increases, this sphincter relaxes, allowing food residue to enter the first portion of the large intestine, the cecum. Relaxation of the ileocecal sphincter is controlled by both nerves and hormones. First, digestive activity in the stomach provokes the gastroileal reflex, which increases the force of ileal segmentation. Second, the stomach releases the hormone gastrin, which enhances ileal motility, thus relaxing the ileocecal sphincter. After chyme passes through, backward pressure helps close the sphincter, preventing backflow into the ileum. Because of this reflex, your lunch is completely emptied from your stomach and small intestine by the time you eat your dinner. It takes about 3 to 5 hours for all chyme to leave the small intestine. Chemical Digestion in the Small Intestine The digestion of proteins and carbohydrates, which partially occurs in the stomach, is completed in the small intestine with the aid of intestinal and pancreatic juices. Lipids arrive in the intestine largely undigested, so much of the focus here is on lipid digestion, which is facilitated by bile and the enzyme pancreatic lipase. Moreover, intestinal juice combines with pancreatic juice to provide a liquid medium that facilitates absorption. The intestine is also where most water is absorbed, via osmosis. The small intestine’s absorptive cells also synthesize digestive enzymes and then place them in the plasma membranes of the microvilli. This distinguishes the small intestine from the stomach; that is, enzymatic digestion occurs not only in the lumen, but also on the luminal surfaces of the mucosal cells. For optimal chemical digestion, chyme must be delivered from the stomach slowly and in small amounts. This is because chyme from the stomach is typically hypertonic, and if large quantities were forced all at once into the small intestine, the resulting osmotic water loss from the blood into the intestinal lumen would result in potentially life-threatening low blood volume. In addition, continued digestion requires an upward adjustment of the low pH of stomach chyme, along with rigorous mixing of the chyme with bile and pancreatic juices. Both processes take time, so the pumping action of the pylorus must be carefully controlled to prevent the duodenum from being overwhelmed with chyme. Note: Disorders of the... Small Intestine: Lactose Intolerance Lactose intolerance is a condition characterized by indigestion caused by dairy products. It occurs when the absorptive cells of the small intestine do not produce enough lactase, the enzyme that digests the milk sugar lactose. In most mammals, lactose intolerance increases with age. In contrast, some human populations, most notably Caucasians, are able to maintain the ability to produce lactase as adults. In people with lactose intolerance, the lactose in chyme is not digested. Bacteria in the large intestine ferment the undigested lactose, a process that produces gas. In addition to gas, symptoms include abdominal cramps, bloating, and diarrhea. Symptom severity ranges from mild discomfort to severe pain; however, symptoms resolve once the lactose is eliminated in feces. The hydrogen breath test is used to help diagnose lactose intolerance. Lactose-tolerant people have very little hydrogen in their breath. Those with lactose intolerance exhale hydrogen, which is one of the gases produced by the bacterial fermentation of lactose in the colon. After the hydrogen is absorbed from the intestine, it is transported through blood vessels into the lungs. There are a number of lactose-free dairy products available in grocery stores. In addition, dietary supplements are available. Taken with food, they provide lactase to help digest lactose. The Large Intestine The large intestine is the terminal part of the alimentary canal. The primary function of this organ is to finish absorption of nutrients and water, synthesize certain vitamins, form feces, and eliminate feces from the body. Structure The large intestine runs from the appendix to the anus. It frames the small intestine on three sides. Despite its being about one-half as long as the small intestine, it is called large because it is more than twice the diameter of the small intestine, about 3 inches. Subdivisions The large intestine is subdivided into four main regions: the cecum, the colon, the rectum, and the anus. The ileocecal valve, located at the opening between the ileum and the large intestine, controls the flow of chyme from the small intestine to the large intestine. Cecum The first part of the large intestine is the cecum, a sac-like structure that is suspended inferior to the ileocecal valve. It is about 6 cm (2.4 in) long, receives the contents of the ileum, and continues the absorption of water and salts. The appendix (or vermiform appendix) is a winding tube that attaches to the cecum. Although the 7.6-cm (3-in) long appendix contains lymphoid tissue, suggesting an immunologic function, this organ is generally considered vestigial. However, at least one recent report postulates a survival advantage conferred by the appendix: In diarrheal illness, the appendix may serve as a bacterial reservoir to repopulate the enteric bacteria for those surviving the initial phases of the illness. Moreover, its twisted anatomy provides a haven for the accumulation and multiplication of enteric bacteria. The mesoappendix, the mesentery of the appendix, tethers it to the mesentery of the ileum. Colon The cecum blends seamlessly with the colon. Upon entering the colon, the food residue first travels up the ascending colon on the right side of the abdomen. At the inferior surface of the liver, the colon bends to form the right colic flexure (hepatic flexure) and becomes the transverse colon. The region defined as hindgut begins with the last third of the transverse colon and continues on. Food residue passing through the transverse colon travels across to the left side of the abdomen, where the colon angles sharply immediately inferior to the spleen, at the left colic flexure (splenic flexure). From there, food residue passes through the descending colon, which runs down the left side of the posterior abdominal wall. After entering the pelvis inferiorly, it becomes the s-shaped sigmoid colon, which extends medially to the midline ([{link]). The ascending and descending colon, and the rectum (discussed next) are located in the retroperitoneum. The transverse and sigmoid colon are tethered to the posterior abdominal wall by the mesocolon. Large Intestine Right colic (hepatic) flexure Left colic (splenic) flexure Transverse colon Ascending Descending colon colon lleum Cecum Vermiform Sigmoid appendix colon Anal canal Rectum The large intestine includes the cecum, colon, and rectum. Note: Homeostatic Imbalances Colorectal Cancer Each year, approximately 140,000 Americans are diagnosed with colorectal cancer, and another 49,000 die from it, making it one of the most deadly malignancies. People with a family history of colorectal cancer are at increased risk. Smoking, excessive alcohol consumption, and a diet high in animal fat and protein also increase the risk. Despite popular opinion to the contrary, studies support the conclusion that dietary fiber and calcium do not reduce the risk of colorectal cancer. Colorectal cancer may be signaled by constipation or diarrhea, cramping, abdominal pain, and rectal bleeding. Bleeding from the rectum may be either obvious or occult (hidden in feces). Since most colon cancers arise from benign mucosal growths called polyps, cancer prevention is focused on identifying these polyps. The colonoscopy is both diagnostic and therapeutic. Colonoscopy not only allows identification of precancerous polyps, the procedure also enables them to be removed before they become malignant. Screening for fecal occult blood tests and colonoscopy is recommended for those over 50 years of age. Rectum Food residue leaving the sigmoid colon enters the rectum in the pelvis, near the third sacral vertebra. The final 20.3 cm (8 in) of the alimentary canal, the rectum extends anterior to the sacrum and coccyx. Even though rectum is Latin for “straight,” this structure follows the curved contour of the sacrum and has three lateral bends that create a trio of internal transverse folds called the rectal valves. These valves help separate the feces from gas to prevent the simultaneous passage of feces and gas. Anal Canal Finally, food residue reaches the last part of the large intestine, the anal canal, which is located in the perineum, completely outside of the abdominopelvic cavity. This 3.8—5 cm (1.5—2 in) long structure opens to the exterior of the body at the anus. The anal canal includes two sphincters. The internal anal sphincter is made of smooth muscle, and its contractions are involuntary. The external anal sphincter is made of skeletal muscle, which is under voluntary control. Except when defecating, both usually remain closed. Histology There are several notable differences between the walls of the large and small intestines ([link]). For example, few enzyme-secreting cells are found in the wall of the large intestine, and there are no circular folds or villi. Other than in the anal canal, the mucosa of the colon is simple columnar epithelium made mostly of enterocytes (absorptive cells) and goblet cells. In addition, the wall of the large intestine has far more intestinal glands, which contain a vast population of enterocytes and goblet cells. These goblet cells secrete mucus that eases the movement of feces and protects the intestine from the effects of the acids and gases produced by enteric bacteria. The enterocytes absorb water and salts as well as vitamins produced by your intestinal bacteria. Histology of the large Intestine Openings of Microvilli intestinal glands ‘ond aN — = ~~ NI \\(] Absorptive cell 4 absorbs water Large intestine Goblet cell secretes mucus @We eas ] Smooth muscle fiber —————— ae Lymphatic nodule Muscularis mucosae Submucosa (a) The histologies of the large intestine and small intestine (not shown) are adapted for the digestive functions of each organ. (b) This micrograph shows the colon’s simple columnar epithelium and goblet cells. LM x 464. (credit b: Micrograph provided by the Regents of University of Michigan Medical School © 2012) Anatomy Three features are unique to the large intestine: teniae coli, haustra, and epiploic appendages ([link]). The teniae coli are three bands of smooth muscle that make up the longitudinal muscle layer of the muscularis of the large intestine, except at its terminal end. Tonic contractions of the teniae coli bunch up the colon into a succession of pouches called haustra (singular = haustrum), which are responsible for the wrinkled appearance of the colon. Attached to the teniae coli are small, fat-filled sacs of visceral peritoneum called epiploic appendages. The purpose of these is unknown. Although the rectum and anal canal have neither teniae coli nor haustra, they do have well-developed layers of muscularis that create the strong contractions needed for defecation. Teniae Coli, Haustra, and Epiploic Appendages Epiploic appendages The stratified squamous epithelial mucosa of the anal canal connects to the skin on the outside of the anus. This mucosa varies considerably from that of the rest of the colon to accommodate the high level of abrasion as feces pass through. The anal canal’s mucous membrane is organized into longitudinal folds, each called an anal column, which house a grid of arteries and veins. Two superficial venous plexuses are found in the anal canal: one within the anal columns and one at the anus. Depressions between the anal columns, each called an anal sinus, secrete mucus that facilitates defecation. The pectinate line (or dentate line) is a horizontal, jagged band that runs circumferentially just below the level of the anal sinuses, and represents the junction between the hindgut and external skin. The mucosa above this line is fairly insensitive, whereas the area below is very sensitive. The resulting difference in pain threshold is due to the fact that the upper region is innervated by visceral sensory fibers, and the lower region is innervated by somatic sensory fibers. Bacterial Flora Most bacteria that enter the alimentary canal are killed by lysozyme, defensins, HCl, or protein-digesting enzymes. However, trillions of bacteria live within the large intestine and are referred to as the bacterial flora. Most of the more than 700 species of these bacteria are nonpathogenic commensal organisms that cause no harm as long as they stay in the gut lumen. In fact, many facilitate chemical digestion and absorption, and some synthesize certain vitamins, mainly biotin, pantothenic acid, and vitamin K. Some are linked to increased immune response. A refined system prevents these bacteria from crossing the mucosal barrier. First, peptidoglycan, a component of bacterial cell walls, activates the release of chemicals by the mucosa’s epithelial cells, which draft immune cells, especially dendritic cells, into the mucosa. Dendritic cells open the tight junctions between epithelial cells and extend probes into the lumen to evaluate the microbial antigens. The dendritic cells with antigens then travel to neighboring lymphoid follicles in the mucosa where T cells inspect for antigens. This process triggers an IgA-mediated response, if warranted, in the lumen that blocks the commensal organisms from infiltrating the mucosa and setting off a far greater, widespread systematic reaction. Digestive Functions of the Large Intestine The residue of chyme that enters the large intestine contains few nutrients except water, which is reabsorbed as the residue lingers in the large intestine, typically for 12 to 24 hours. Thus, it may not surprise you that the large intestine can be completely removed without significantly affecting digestive functioning. For example, in severe cases of inflammatory bowel disease, the large intestine can be removed by a procedure known as a colectomy. Often, a new fecal pouch can be crafted from the small intestine and sutured to the anus, but if not, an ileostomy can be created by bringing the distal ileum through the abdominal wall, allowing the watery chyme to be collected in a bag-like adhesive appliance. Mechanical Digestion In the large intestine, mechanical digestion begins when chyme moves from the ileum into the cecum, an activity regulated by the ileocecal sphincter. Right after you eat, peristalsis in the ileum forces chyme into the cecum. When the cecum is distended with chyme, contractions of the ileocecal sphincter strengthen. Once chyme enters the cecum, colon movements begin. Mechanical digestion in the large intestine includes a combination of three types of movements. The presence of food residues in the colon stimulates a slow-moving haustral contraction. This type of movement involves sluggish segmentation, primarily in the transverse and descending colons. When a haustrum is distended with chyme, its muscle contracts, pushing the residue into the next haustrum. These contractions occur about every 30 minutes, and each last about 1 minute. These movements also mix the food residue, which helps the large intestine absorb water. The second type of movement is peristalsis, which, in the large intestine, is slower than in the more proximal portions of the alimentary canal. The third type is a mass movement. These strong waves start midway through the transverse colon and quickly force the contents toward the rectum. Mass movements usually occur three or four times per day, either while you eat or immediately afterward. Distension in the stomach and the breakdown products of digestion in the small intestine provoke the gastrocolic reflex, which increases motility, including mass movements, in the colon. Fiber in the diet both softens the stool and increases the power of colonic contractions, optimizing the activities of the colon. Chemical Digestion Although the glands of the large intestine secrete mucus, they do not secrete digestive enzymes. Therefore, chemical digestion in the large intestine occurs exclusively because of bacteria in the lumen of the colon. Through the process of saccharolytic fermentation, bacteria break down some of the remaining carbohydrates. This results in the discharge of hydrogen, carbon dioxide, and methane gases that create flatus (gas) in the colon; flatulence is excessive flatus. Each day, up to 1500 mL of flatus is produced in the colon. More is produced when you eat foods such as beans, which are rich in otherwise indigestible sugars and complex carbohydrates like soluble dietary fiber. Absorption, Feces Formation, and Defecation The small intestine absorbs about 90 percent of the water you ingest (either as liquid or within solid food). The large intestine absorbs most of the remaining water, a process that converts the liquid chyme residue into semisolid feces (“stool”). Feces is composed of undigested food residues, unabsorbed digested substances, millions of bacteria, old epithelial cells from the GI mucosa, inorganic salts, and enough water to let it pass smoothly out of the body. Of every 500 mL (17 ounces) of food residue that enters the cecum each day, about 150 mL (5 ounces) become feces. Feces are eliminated through contractions of the rectal muscles. You help this process by a voluntary procedure called Valsalva’s maneuver, in which you increase intra-abdominal pressure by contracting your diaphragm and abdominal wall muscles, and closing your glottis. The process of defecation begins when mass movements force feces from the colon into the rectum, stretching the rectal wall and provoking the defecation reflex, which eliminates feces from the rectum. This parasympathetic reflex is mediated by the spinal cord. It contracts the sigmoid colon and rectum, relaxes the internal anal sphincter, and initially contracts the external anal sphincter. The presence of feces in the anal canal sends a signal to the brain, which gives you the choice of voluntarily opening the external anal sphincter (defecating) or keeping it temporarily closed. If you decide to delay defecation, it takes a few seconds for the reflex contractions to stop and the rectal walls to relax. The next mass movement will trigger additional defecation reflexes until you defecate. If defecation is delayed for an extended time, additional water is absorbed, making the feces firmer and potentially leading to constipation. On the other hand, if the waste matter moves too quickly through the intestines, not enough water is absorbed, and diarrhea can result. This can be caused by the ingestion of foodborne pathogens. In general, diet, health, and stress determine the frequency of bowel movements. The number of bowel movements varies greatly between individuals, ranging from two or three per day to three or four per week. Note: eee —— mss" OPENStax COLLEGE By watching this animation you will see that for the various food groups— proteins, fats, and carbohydrates—digestion begins in different parts of the digestion system, though all end in the same place. Of the three major food classes (carbohydrates, fats, and proteins), which is digested in the mouth, the stomach, and the small intestine? Chapter Review The three main regions of the small intestine are the duodenum, the jejunum, and the ileum. The small intestine is where digestion is completed and virtually all absorption occurs. These two activities are facilitated by structural adaptations that increase the mucosal surface area by 600-fold, including circular folds, villi, and microvilli. There are around 200 million microvilli per square millimeter of small intestine, which contain brush border enzymes that complete the digestion of carbohydrates and proteins. Combined with pancreatic juice, intestinal juice provides the liquid medium needed to further digest and absorb substances from chyme. The small intestine is also the site of unique mechanical digestive movements. Segmentation moves the chyme back and forth, increasing mixing and opportunities for absorption. Migrating motility complexes propel the residual chyme toward the large intestine. The main regions of the large intestine are the cecum, the colon, and the rectum. The large intestine absorbs water and forms feces, and is responsible for defecation. Bacterial flora break down additional carbohydrate residue, and synthesize certain vitamins. The mucosa of the large intestinal wall is generously endowed with goblet cells, which secrete mucus that eases the passage of feces. The entry of feces into the rectum activates the defecation reflex. Interactive Link Questions Exercise: Problem: Watch this animation that depicts the structure of the small intestine, and, in particular, the villi. Epithelial cells continue the digestion and absorption of nutrients and transport these nutrients to the lymphatic and circulatory systems. In the small intestine, the products of food digestion are absorbed by different structures in the villi. Which structure absorbs and transports fats? Solution: Answers may vary. Exercise: Problem: By watching this animation, you will see that for the various food groups—proteins, fats, and carbohydrates—digestion begins in different parts of the digestion system, though all end in the same place. Of the three major food classes (carbohydrates, fats, and proteins), which is digested in the mouth, the stomach, and the small intestine? Solution: Answers may vary. Review Questions Exercise: Problem: In which part of the alimentary canal does most digestion occur? a. stomach b. proximal small intestine c. distal small intestine d. ascending colon Solution: B Exercise: Problem: Which of these is most associated with villi? a. haustra b. lacteals c. bacterial flora d. intestinal glands Solution: B Exercise: Problem: What is the role of the small intestine’s MALT? a. secreting mucus b. buffering acidic chyme c. activating pepsin d. preventing bacteria from entering the bloodstream Solution: D Exercise: Problem: Which part of the large intestine attaches to the appendix? a. cecum b. ascending colon c. transverse colon d. descending colon Solution: A Critical Thinking Questions Exercise: Problem: Explain how nutrients absorbed in the small intestine pass into the general circulation. Solution: Nutrients from the breakdown of carbohydrates and proteins are absorbed through a capillary bed in the villi of the small intestine. Lipid breakdown products are absorbed into a lacteal in the villi, and transported via the lymphatic system to the bloodstream. Exercise: Problem: Why is it important that chyme from the stomach is delivered to the small intestine slowly and in small amounts? Solution: If large quantities of chyme were forced into the small intestine, it would result in osmotic water loss from the blood into the intestinal lumen that could cause potentially life-threatening low blood volume and erosion of the duodenum. Exercise: Problem: Describe three of the differences between the walls of the large and small intestines. Solution: The mucosa of the small intestine includes circular folds, villi, and microvilli. The wall of the large intestine has a thick mucosal layer, and deeper and more abundant mucus-secreting glands that facilitate the smooth passage of feces. There are three features that are unique to the large intestine: teniae coli, haustra, and epiploic appendages. References American Cancer Society (US). Cancer facts and figures: colorectal cancer: 2011-2013 [Internet]. c2013 [cited 2013 Apr 3]. Available from: http://www.cancer.org/Research/CancerFactsFigures/ColorectalCancerFacts Figures/colorectal-cancer-facts-figures-2011-2013-page. The Nutrition Source. Fiber and colon cancer: following the scientific trail [Internet]. Boston (MA): Harvard School of Public Health; c2012 [cited 2013 Apr 3]. Available from: http://www.hsph.harvard.edu/nutritionsource/nutrition-news/fiber-and- colon-cancer/index. html. Centers for Disease Control and Prevention (US). Morbidity and mortality weekly report: notifiable diseases and mortality tables [Internet]. Atlanta (GA); [cited 2013 Apr 3]. Available from: http://www.cdc.gov/mmwr/preview/mmwrhtml/mm6101md.htm? s_cid=mm6101imd_w. Glossary anal canal final segment of the large intestine anal column long fold of mucosa in the anal canal anal sinus recess between anal columns appendix (vermiform appendix) coiled tube attached to the cecum ascending colon first region of the colon bacterial flora bacteria in the large intestine brush border fuzzy appearance of the small intestinal mucosa created by microvilli cecum pouch forming the beginning of the large intestine circular fold (also, plica circulare) deep fold in the mucosa and submucosa of the small intestine colon part of the large intestine between the cecum and the rectum descending colon part of the colon between the transverse colon and the sigmoid colon duodenal gland (also, Brunner’s gland) mucous-secreting gland in the duodenal submucosa duodenum first part of the small intestine, which starts at the pyloric sphincter and ends at the jejunum epiploic appendage small sac of fat-filled visceral peritoneum attached to teniae coli external anal sphincter voluntary skeletal muscle sphincter in the anal canal feces semisolid waste product of digestion flatus gas in the intestine gastrocolic reflex propulsive movement in the colon activated by the presence of food in the stomach gastroileal reflex long reflex that increases the strength of segmentation in the ileum haustrum small pouch in the colon created by tonic contractions of teniae coli haustral contraction slow segmentation in the large intestine hepatopancreatic ampulla (also, ampulla of Vater) bulb-like point in the wall of the duodenum where the bile duct and main pancreatic duct unite hepatopancreatic sphincter (also, sphincter of Oddi) sphincter regulating the flow of bile and pancreatic juice into the duodenum ileocecal sphincter sphincter located where the small intestine joins with the large intestine ileum end of the small intestine between the jejunum and the large intestine internal anal sphincter involuntary smooth muscle sphincter in the anal canal intestinal gland (also, crypt of Lieberktihn) gland in the small intestinal mucosa that secretes intestinal juice intestinal juice mixture of water and mucus that helps absorb nutrients from chyme jejunum middle part of the small intestine between the duodenum and the ileum lacteal lymphatic capillary in the villi large intestine terminal portion of the alimentary canal left colic flexure (also, splenic flexure) point where the transverse colon curves below the inferior end of the spleen main pancreatic duct (also, duct of Wirsung) duct through which pancreatic juice drains from the pancreas major duodenal papilla point at which the hepatopancreatic ampulla opens into the duodenum mass movement long, slow, peristaltic wave in the large intestine mesoappendix mesentery of the appendix microvillus small projection of the plasma membrane of the absorptive cells of the small intestinal mucosa migrating motility complex form of peristalsis in the small intestine motilin hormone that initiates migrating motility complexes pectinate line horizontal line that runs like a ring, perpendicular to the inferior margins of the anal sinuses rectal valve one of three transverse folds in the rectum where feces is separated from flatus rectum part of the large intestine between the sigmoid colon and anal canal right colic flexure (also, hepatic flexure) point, at the inferior surface of the liver, where the ascending colon turns abruptly to the left saccharolytic fermentation anaerobic decomposition of carbohydrates sigmoid colon end portion of the colon, which terminates at the rectum small intestine section of the alimentary canal where most digestion and absorption occurs tenia coli one of three smooth muscle bands that make up the longitudinal muscle layer of the muscularis in all of the large intestine except the terminal end transverse colon part of the colon between the ascending colon and the descending colon Valsalva’s maneuver voluntary contraction of the diaphragm and abdominal wall muscles and closing of the glottis, which increases intra-abdominal pressure and facilitates defecation villus projection of the mucosa of the small intestine Accessory Organs in Digestion: The Liver, Pancreas, and Gallbladder By the end of this section, you will be able to: e State the main digestive roles of the liver, pancreas, and gallbladder e Identify three main features of liver histology that are critical to its function Discuss the composition and function of bile Identify the major types of enzymes and buffers present in pancreatic juice Chemical digestion in the small intestine relies on the activities of three accessory digestive organs: the liver, pancreas, and gallbladder ({link]). The digestive role of the liver is to produce bile and export it to the duodenum. The gallbladder primarily stores, concentrates, and releases bile. The pancreas produces pancreatic juice, which contains digestive enzymes and bicarbonate ions, and delivers it to the duodenum. Accessory Organs Liver: Right lobe Quadrate lobe Left lobe Caudate lobe Gallbladder Spleen Right hepatic duct Pancreas Cystic duct Common hepatic duct Pancreatic duct Common bile duct Left hepatic duct The liver, pancreas, and gallbladder are considered accessory digestive organs, but their roles in the digestive system are vital. The Liver The liver is the largest gland in the body, weighing about three pounds in an adult. It is also one of the most important organs. In addition to being an accessory digestive organ, it plays a number of roles in metabolism and regulation. The liver lies inferior to the diaphragm in the right upper quadrant of the abdominal cavity and receives protection from the surrounding ribs. The liver is divided into two primary lobes: a large right lobe and a much smaller left lobe. In the right lobe, some anatomists also identify an inferior quadrate lobe and a posterior caudate lobe, which are defined by internal features. The liver is connected to the abdominal wall and diaphragm by five peritoneal folds referred to as ligaments. These are the falciform ligament, the coronary ligament, two lateral ligaments, and the ligamentum teres hepatis. The falciform ligament and ligamentum teres hepatis are actually remnants of the umbilical vein, and separate the right and left lobes anteriorly. The lesser omentum tethers the liver to the lesser curvature of the stomach. The porta hepatis (“gate to the liver”) is where the hepatic artery and hepatic portal vein enter the liver. These two vessels, along with the common hepatic duct, run behind the lateral border of the lesser omentum on the way to their destinations. As shown in [link], the hepatic artery delivers oxygenated blood from the heart to the liver. The hepatic portal vein delivers partially deoxygenated blood containing nutrients absorbed from the small intestine and actually supplies more oxygen to the liver than do the much smaller hepatic arteries. In addition to nutrients, drugs and toxins are also absorbed. After processing the bloodborne nutrients and toxins, the liver releases nutrients needed by other cells back into the blood, which drains into the central vein and then through the hepatic vein to the inferior vena cava. With this hepatic portal circulation, all blood from the alimentary canal passes through the liver. This largely explains why the liver is the most common site for the metastasis of cancers that originate in the alimentary canal. Microscopic Anatomy of the Liver Central vein Connective tissue Lobules Interlobular vein (to hepatic vein) Central vein Sinusoids Plates of Portal venule hepatocytes From portal vein The liver receives oxygenated blood from the hepatic artery and nutrient-rich deoxygenated blood from the hepatic portal vein. Histology The liver has three main components: hepatocytes, bile canaliculi, and hepatic sinusoids. A hepatocyte is the liver’s main cell type, accounting for around 80 percent of the liver's volume. These cells play a role in a wide variety of secretory, metabolic, and endocrine functions. Plates of hepatocytes called hepatic laminae radiate outward from the portal vein in each hepatic lobule. Between adjacent hepatocytes, grooves in the cell membranes provide room for each bile canaliculus (plural = canaliculi). These small ducts accumulate the bile produced by hepatocytes. From here, bile flows first into bile ductules and then into bile ducts. The bile ducts unite to form the larger right and left hepatic ducts, which themselves merge and exit the liver as the common hepatic duct. This duct then joins with the cystic duct from the gallbladder, forming the common bile duct through which bile flows into the small intestine. A hepatic sinusoid is an open, porous blood space formed by fenestrated capillaries from nutrient-rich hepatic portal veins and oxygen-rich hepatic arteries. Hepatocytes are tightly packed around the fenestrated endothelium of these spaces, giving them easy access to the blood. From their central position, hepatocytes process the nutrients, toxins, and waste materials carried by the blood. Materials such as bilirubin are processed and excreted into the bile canaliculi. Other materials including proteins, lipids, and carbohydrates are processed and secreted into the sinusoids or just stored in the cells until called upon. The hepatic sinusoids combine and send blood to a central vein. Blood then flows through a hepatic vein into the inferior vena cava. This means that blood and bile flow in opposite directions. The hepatic sinusoids also contain star-shaped reticuloendothelial cells (Kupffer cells), phagocytes that remove dead red and white blood cells, bacteria, and other foreign material that enter the sinusoids. The portal triad is a distinctive arrangement around the perimeter of hepatic lobules, consisting of three basic structures: a bile duct, a hepatic artery branch, and a hepatic portal vein branch. Bile Recall that lipids are hydrophobic, that is, they do not dissolve in water. Thus, before they can be digested in the watery environment of the small intestine, large lipid globules must be broken down into smaller lipid globules, a process called emulsification. Bile is a mixture secreted by the liver to accomplish the emulsification of lipids in the small intestine. Hepatocytes secrete about one liter of bile each day. A yellow-brown or yellow-green alkaline solution (pH 7.6 to 8.6), bile is a mixture of water, bile salts, bile pigments, phospholipids (such as lecithin), electrolytes, cholesterol, and triglycerides. The components most critical to emulsification are bile salts and phospholipids, which have a nonpolar (hydrophobic) region as well as a polar (hydrophilic) region. The hydrophobic region interacts with the large lipid molecules, whereas the hydrophilic region interacts with the watery chyme in the intestine. This results in the large lipid globules being pulled apart into many tiny lipid fragments of about 1 ym in diameter. This change dramatically increases the surface area available for lipid-digesting enzyme activity. This is the same way dish soap works on fats mixed with water. Bile salts act as emulsifying agents, so they are also important for the absorption of digested lipids. While most constituents of bile are eliminated in feces, bile salts are reclaimed by the enterohepatic circulation. Once bile salts reach the ileum, they are absorbed and returned to the liver in the hepatic portal blood. The hepatocytes then excrete the bile salts into newly formed bile. Thus, this precious resource is recycled. Bilirubin, the main bile pigment, is a waste product produced when the spleen removes old or damaged red blood cells from the circulation. These breakdown products, including proteins, iron, and toxic bilirubin, are transported to the liver via the splenic vein of the hepatic portal system. In the liver, proteins and iron are recycled, whereas bilirubin is excreted in the bile. It accounts for the green color of bile. Bilirubin is eventually transformed by intestinal bacteria into stercobilin, a brown pigment that gives your stool its characteristic color! In some disease states, bile does not enter the intestine, resulting in white (‘acholic’) stool with a high fat content, since virtually no fats are broken down or absorbed. Hepatocytes work non-stop, but bile production increases when fatty chyme enters the duodenum and stimulates the secretion of the gut hormone secretin. Between meals, bile is produced but conserved. The valve-like hepatopancreatic ampulla closes, allowing bile to divert to the gallbladder, where it is concentrated and stored until the next meal. Note: . . mss’ OPENStax COLLEGE Watch this video to see the structure of the liver and how this structure supports the functions of the liver, including the processing of nutrients, toxins, and wastes. At rest, about 1500 mL of blood per minute flow through the liver. What percentage of this blood flow comes from the hepatic portal system? The Pancreas The soft, oblong, glandular pancreas lies transversely in the retroperitoneum behind the stomach. Its head is nestled into the “c-shaped” curvature of the duodenum with the body extending to the left about 15.2 cm (6 in) and ending as a tapering tail in the hilum of the spleen. It is a curious mix of exocrine (secreting digestive enzymes) and endocrine (releasing hormones into the blood) functions ([link]). Exocrine and Endocrine Pancreas Common bile duct Pancreatic duct Tail of pancreas Lobules Acinar cells secrete Head of pancreas digestive enzymes. Pancreatic islet) _ ® Sie as cells secrete We = hormones. ‘@ Pancreatic duct Exocrine cells secrete pancreatic juice. The pancreas has a head, a body, and a tail. It delivers pancreatic juice to the duodenum through the pancreatic duct. The exocrine part of the pancreas arises as little grape-like cell clusters, each called an acinus (plural = acini), located at the terminal ends of pancreatic ducts. These acinar cells secrete enzyme-rich pancreatic juice into tiny merging ducts that form two dominant ducts. The larger duct fuses with the common bile duct (carrying bile from the liver and gallbladder) just before entering the duodenum via a common opening (the hepatopancreatic ampulla). The smooth muscle sphincter of the hepatopancreatic ampulla controls the release of pancreatic juice and bile into the small intestine. The second and smaller pancreatic duct, the accessory duct (duct of Santorini), runs from the pancreas directly into the duodenum, approximately 1 inch above the hepatopancreatic ampulla. When present, it is a persistent remnant of pancreatic development. Scattered through the sea of exocrine acini are small islands of endocrine cells, the islets of Langerhans. These vital cells produce the hormones pancreatic polypeptide, insulin, glucagon, and somatostatin. Pancreatic Juice The pancreas produces over a liter of pancreatic juice each day. Unlike bile, it is clear and composed mostly of water along with some salts, sodium bicarbonate, and several digestive enzymes. Sodium bicarbonate is responsible for the slight alkalinity of pancreatic juice (pH 7.1 to 8.2), which serves to buffer the acidic gastric juice in chyme, inactivate pepsin from the stomach, and create an optimal environment for the activity of pH- sensitive digestive enzymes in the small intestine. Pancreatic enzymes are active in the digestion of sugars, proteins, and fats. The pancreas produces protein-digesting enzymes in their inactive forms. These enzymes are activated in the duodenum. If produced in an active form, they would digest the pancreas (which is exactly what occurs in the disease, pancreatitis). The intestinal brush border enzyme enteropeptidase stimulates the activation of trypsin from trypsinogen of the pancreas, which in turn changes the pancreatic enzymes procarboxypeptidase and chymotrypsinogen into their active forms, carboxypeptidase and chymotrypsin. The enzymes that digest starch (amylase), fat (lipase), and nucleic acids (nuclease) are secreted in their active forms, since they do not attack the pancreas as do the protein-digesting enzymes. Pancreatic Secretion Regulation of pancreatic secretion is the job of hormones and the parasympathetic nervous system. The entry of acidic chyme into the duodenum stimulates the release of secretin, which in turn causes the duct cells to release bicarbonate-rich pancreatic juice. The presence of proteins and fats in the duodenum stimulates the secretion of CCK, which then stimulates the acini to secrete enzyme-rich pancreatic juice and enhances the activity of secretin. Parasympathetic regulation occurs mainly during the cephalic and gastric phases of gastric secretion, when vagal stimulation prompts the secretion of pancreatic juice. Usually, the pancreas secretes just enough bicarbonate to counterbalance the amount of HCl produced in the stomach. Hydrogen ions enter the blood when bicarbonate is secreted by the pancreas. Thus, the acidic blood draining from the pancreas neutralizes the alkaline blood draining from the stomach, maintaining the pH of the venous blood that flows to the liver. The Gallbladder The gallbladder is 8—10 cm (~3-4 in) long and is nested in a shallow area on the posterior aspect of the right lobe of the liver. This muscular sac stores, concentrates, and, when stimulated, propels the bile into the duodenum via the common bile duct. It is divided into three regions. The fundus is the widest portion and tapers medially into the body, which in turn narrows to become the neck. The neck angles slightly superiorly as it approaches the hepatic duct. The cystic duct is 1—2 cm (less than 1 in) long and turns inferiorly as it bridges the neck and hepatic duct. The simple columnar epithelium of the gallbladder mucosa is organized in rugae, similar to those of the stomach. There is no submucosa in the gallbladder wall. The wall’s middle, muscular coat is made of smooth muscle fibers. When these fibers contract, the gallbladder’s contents are ejected through the cystic duct and into the bile duct ([link]). Visceral peritoneum reflected from the liver capsule holds the gallbladder against the liver and forms the outer coat of the gallbladder. The gallbladder's mucosa absorbs water and ions from bile, concentrating it by up to 10-fold. Gallbladder Left hepatic duct Right hepatic duct Cystic duct Gallbladder: Body Fundus Neck Common hepatic duct F Common Liver bile duct The gallbladder stores and concentrates bile, and releases it into the two-way cystic duct when it is needed by the small intestine. Chapter Review Chemical digestion in the small intestine cannot occur without the help of the liver and pancreas. The liver produces bile and delivers it to the common hepatic duct. Bile contains bile salts and phospholipids, which emulsify large lipid globules into tiny lipid droplets, a necessary step in lipid digestion and absorption. The gallbladder stores and concentrates bile, releasing it when it is needed by the small intestine. The pancreas produces the enzyme- and bicarbonate-rich pancreatic juice and delivers it to the small intestine through ducts. Pancreatic juice buffers the acidic gastric juice in chyme, inactivates pepsin from the stomach, and enables the optimal functioning of digestive enzymes in the small intestine. Interactive Link Questions Exercise: Problem: Watch this video to see the structure of the liver and how this structure supports the functions of the liver, including the processing of nutrients, toxins, and wastes. At rest, about 1500 mL of blood per minute flow through the liver. What percentage of this blood flow comes from the hepatic portal system? Solution: Answers may vary. Review Questions Exercise: Problem: Which of these statements about bile is true? a. About 500 mL is secreted daily. b. Its main function is the denaturation of proteins. c. It is synthesized in the gallbladder. d. Bile salts are recycled. Solution: D Exercise: Problem: Pancreatic juice a. deactivates bile. b. is secreted by pancreatic islet cells. c. buffers chyme. d. is released into the cystic duct. Solution: C Critical Thinking Questions Exercise: Problem: Why does the pancreas secrete some enzymes in their inactive forms, and where are these enzymes activated? Solution: The pancreas secretes protein-digesting enzymes in their inactive forms. If secreted in their active forms, they would self-digest the pancreas. These enzymes are activated in the duodenum. Exercise: Problem: Describe the location of hepatocytes in the liver and how this arrangement enhances their function. Solution: The hepatocytes are the main cell type of the liver. They process, store, and release nutrients into the blood. Radiating out from the central vein, they are tightly packed around the hepatic sinusoids, allowing the hepatocytes easy access to the blood flowing through the sinusoids. Glossary accessory duct (also, duct of Santorini) duct that runs from the pancreas into the duodenum acinus cluster of glandular epithelial cells in the pancreas that secretes pancreatic juice in the pancreas bile alkaline solution produced by the liver and important for the emulsification of lipids bile canaliculus small duct between hepatocytes that collects bile bilirubin main bile pigment, which is responsible for the brown color of feces central vein vein that receives blood from hepatic sinusoids common bile duct structure formed by the union of the common hepatic duct and the gallbladder’s cystic duct common hepatic duct duct formed by the merger of the two hepatic ducts cystic duct duct through which bile drains and enters the gallbladder enterohepatic circulation recycling mechanism that conserves bile salts enteropeptidase intestinal brush-border enzyme that activates trypsinogen to trypsin gallbladder accessory digestive organ that stores and concentrates bile hepatic artery artery that supplies oxygenated blood to the liver hepatic lobule hexagonal-shaped structure composed of hepatocytes that radiate outward from a central vein hepatic portal vein vein that supplies deoxygenated nutrient-rich blood to the liver hepatic sinusoid blood capillaries between rows of hepatocytes that receive blood from the hepatic portal vein and the branches of the hepatic artery hepatic vein vein that drains into the inferior vena cava hepatocytes major functional cells of the liver liver largest gland in the body whose main digestive function is the production of bile pancreas accessory digestive organ that secretes pancreatic juice pancreatic juice secretion of the pancreas containing digestive enzymes and bicarbonate porta hepatis “gateway to the liver” where the hepatic artery and hepatic portal vein enter the liver portal triad bile duct, hepatic artery branch, and hepatic portal vein branch reticuloendothelial cell (also, Kupffer cell) phagocyte in hepatic sinusoids that filters out material from venous blood from the alimentary canal Anatomy of the Lymphatic and Immune Systems By the end of this section, you will be able to: e Describe the structure and function of the lymphatic tissue (lymph fluid, vessels, ducts, and organs) e Describe the structure and function of the primary and secondary lymphatic organs e Discuss the cells of the immune system, how they function, and their relationship with the lymphatic system The immune system is the complex collection of cells and organs that destroys or neutralizes pathogens that would otherwise cause disease or death. The lymphatic system, for most people, is associated with the immune system to such a degree that the two systems are virtually indistinguishable. The lymphatic system is the system of vessels, cells, and organs that carries excess fluids to the bloodstream and filters pathogens from the blood. The swelling of lymph nodes during an infection and the transport of lymphocytes via the lymphatic vessels are but two examples of the many connections between these critical organ systems. Functions of the Lymphatic System A major function of the lymphatic system is to drain body fluids and return them to the bloodstream. Blood pressure causes leakage of fluid from the capillaries, resulting in the accumulation of fluid in the interstitial space— that is, spaces between individual cells in the tissues. In humans, 20 liters of plasma is released into the interstitial space of the tissues each day due to capillary filtration. Once this filtrate is out of the bloodstream and in the tissue spaces, it is referred to as interstitial fluid. Of this, 17 liters is reabsorbed directly by the blood vessels. But what happens to the remaining three liters? This is where the lymphatic system comes into play. It drains the excess fluid and empties it back into the bloodstream via a series of vessels, trunks, and ducts. Lymph is the term used to describe interstitial fluid once it has entered the lymphatic system. When the lymphatic system is damaged in some way, such as by being blocked by cancer cells or destroyed by injury, protein-rich interstitial fluid accumulates (sometimes “backs up” from the lymph vessels) in the tissue spaces. This inappropriate accumulation of fluid referred to as lymphedema may lead to serious medical consequences. As the vertebrate immune system evolved, the network of lymphatic vessels became convenient avenues for transporting the cells of the immune system. Additionally, the transport of dietary lipids and fat-soluble vitamins absorbed in the gut uses this system. Cells of the immune system not only use lymphatic vessels to make their way from interstitial spaces back into the circulation, but they also use lymph nodes as major staging areas for the development of critical immune responses. A lymph node is one of the small, bean-shaped organs located throughout the lymphatic system. Note: openstax COLLEGE” Visit this website for an overview of the lymphatic system. What are the three main components of the lymphatic system? Structure of the Lymphatic System The lymphatic vessels begin as open-ended capillaries, which feed into larger and larger lymphatic vessels, and eventually empty into the bloodstream by a series of ducts. Along the way, the lymph travels through the lymph nodes, which are commonly found near the groin, armpits, neck, chest, and abdomen. Humans have about 500—600 lymph nodes throughout the body ([link]). Anatomy of the Lymphatic System Adenoid Tonsil J Ah Lymph nodes _ j 4\ Thymus | Lymphatic | vessel ; 7 Thymus Right lymphatic duct, entering vein Tissue cell Spleen Interstitial fluid capillary ot Lymphatic capillary Masses of lymphocytes and macrophages Bone marrow Lymph node Lymphatic vessels in the arms and legs convey lymph to the larger lymphatic vessels in the torso. A major distinction between the lymphatic and cardiovascular systems in humans is that lymph is not actively pumped by the heart, but is forced through the vessels by the movements of the body, the contraction of skeletal muscles during body movements, and breathing. One-way valves (semi-lunar valves) in lymphatic vessels keep the lymph moving toward the heart. Lymph flows from the lymphatic capillaries, through lymphatic vessels, and then is dumped into the circulatory system via the lymphatic ducts located at the junction of the jugular and subclavian veins in the neck. Lymphatic Capillaries Lymphatic capillaries, also called the terminal lymphatics, are vessels where interstitial fluid enters the lymphatic system to become lymph fluid. Located in almost every tissue in the body, these vessels are interlaced among the arterioles and venules of the circulatory system in the soft connective tissues of the body ({link]). Exceptions are the central nervous system, bone marrow, bones, teeth, and the cornea of the eye, which do not contain lymph vessels. Lymphatic Capillaries Lymph capillaries in the tissue spaces Lymph capillary. Gy x Collagen fiber Arteriole Interstitial fluid Lymph (interstitial fluid) x Lymphatic vessel Tissue fluid Endothelial “flaps” Lymph vessel endothelial cells prevention valve Lymphatic capillaries are interlaced with the arterioles and venules of the cardiovascular system. Collagen fibers anchor a lymphatic capillary in the tissue (inset). Interstitial fluid slips through spaces between the overlapping endothelial cells that compose the lymphatic capillary. Lymphatic capillaries are formed by a one cell-thick layer of endothelial cells and represent the open end of the system, allowing interstitial fluid to flow into them via overlapping cells (see [link]). When interstitial pressure is low, the endothelial flaps close to prevent “backflow.” As interstitial pressure increases, the spaces between the cells open up, allowing the fluid to enter. Entry of fluid into lymphatic capillaries is also enabled by the collagen filaments that anchor the capillaries to surrounding structures. As interstitial pressure increases, the filaments pull on the endothelial cell flaps, opening up them even further to allow easy entry of fluid. In the small intestine, lymphatic capillaries called lacteals are critical for the transport of dietary lipids and lipid-soluble vitamins to the bloodstream. In the small intestine, dietary triglycerides combine with other lipids and proteins, and enter the lacteals to form a milky fluid called chyle. The chyle then travels through the lymphatic system, eventually entering the bloodstream. Larger Lymphatic Vessels, Trunks, and Ducts The lymphatic capillaries empty into larger lymphatic vessels, which are similar to veins in terms of their three-tunic structure and the presence of valves. These one-way valves are located fairly close to one another, and each one causes a bulge in the lymphatic vessel, giving the vessels a beaded appearance (see [link]). The superficial and deep lymphatics eventually merge to form larger lymphatic vessels known as lymphatic trunks. On the right side of the body, the right sides of the head, thorax, and right upper limb drain lymph fluid into the right subclavian vein via the right lymphatic duct ([link]). On the left side of the body, the remaining portions of the body drain into the larger thoracic duct, which drains into the left subclavian vein. The thoracic duct itself begins just beneath the diaphragm in the cisterna chyli, a sac- like chamber that receives lymph from the lower abdomen, pelvis, and lower limbs by way of the left and right lumbar trunks and the intestinal trunk. Major Trunks and Ducts of the Lymphatic System Right lymphatic Right internal Left internal Thoracic duct duct jugular vein jugular vein drains into subclavian vein Left subclavian vein Right subclavian vein Thoracic duct Cisterna chyli of Drained by right thoracic duct lymphatic duct Drained by thoracic duct The thoracic duct drains a much larger portion of the body than does the right lymphatic duct. The overall drainage system of the body is asymmetrical (see [link]). The right lymphatic duct receives lymph from only the upper right side of the body. The lymph from the rest of the body enters the bloodstream through the thoracic duct via all the remaining lymphatic trunks. In general, lymphatic vessels of the subcutaneous tissues of the skin, that is, the superficial lymphatics, follow the same routes as veins, whereas the deep lymphatic vessels of the viscera generally follow the paths of arteries. The Organization of Immune Function The immune system is a collection of barriers, cells, and soluble proteins that interact and communicate with each other in extraordinarily complex ways. The modern model of immune function is organized into three phases based on the timing of their effects. The three temporal phases consist of the following: ¢ Barrier defenses such as the skin and mucous membranes, which act instantaneously to prevent pathogenic invasion into the body tissues e The rapid but nonspecific innate immune response, which consists of a variety of specialized cells and soluble factors e The slower but more specific and effective adaptive immune response, which involves many cell types and soluble factors, but is primarily controlled by white blood cells (leukocytes) known as lymphocytes, which help control immune responses The cells of the blood, including all those involved in the immune response, arise in the bone marrow via various differentiation pathways from hematopoietic stem cells ({link]). In contrast with embryonic stem cells, hematopoietic stem cells are present throughout adulthood and allow for the continuous differentiation of blood cells to replace those lost to age or function. These cells can be divided into three classes based on function: e Phagocytic cells, which ingest pathogens to destroy them e Lymphocytes, which specifically coordinate the activities of adaptive immunity ¢ Cells containing cytoplasmic granules, which help mediate immune responses against parasites and intracellular pathogens such as viruses Hematopoietic System of the Bone Marrow After division some cells remain stem cells. Multipotent hematopoietic stem cell (hemocytoblast) a e , The remaining cell goes down one of two paths @ depending on the chemical signals received. e Myeloid stem cell Lymphoid stem cell Megakaryoblast Proerythroblast Myeloblast Monoblast Lymphoblast @ Reticulocyte =i oo yy rc ae Le Small lymphocyte ork A 2 : —_— arge granular Mm @ ovr YY | — “ Megakaryocyte Erythrocyte Basophil Neutrophil Eosinophil Monocyte sy) @& T lymphocyte —_B lymphocyte b) p) Ad) BY) 7 7X SS Platelets 7) => o ° Plasma cell Macrophage All the cells of the immune response as well as of the blood arise by differentiation from hematopoietic stem cells. Platelets are cell fragments involved in the clotting of blood. Lymphocytes: B Cells, T Cells, Plasma Cells, and Natural Killer Cells As stated above, lymphocytes are the primary cells of adaptive immune responses ({link]). The two basic types of lymphocytes, B cells and T cells, are identical morphologically with a large central nucleus surrounded by a thin layer of cytoplasm. They are distinguished from each other by their surface protein markers as well as by the molecules they secrete. While B cells mature in red bone marrow and T cells mature in the thymus, they both initially develop from bone marrow. T cells migrate from bone marrow to the thymus gland where they further mature. B cells and T cells are found in many parts of the body, circulating in the bloodstream and lymph, and residing in secondary lymphoid organs, including the spleen and lymph nodes, which will be described later in this section. The human body contains approximately 10'* lymphocytes. B Cells B cells are immune cells that function primarily by producing antibodies. An antibody is any of the group of proteins that binds specifically to pathogen-associated molecules known as antigens. An antigen is a chemical structure on the surface of a pathogen that binds to T or B lymphocyte antigen receptors. Once activated by binding to antigen, B cells differentiate into cells that secrete a soluble form of their surface antibodies. These activated B cells are known as plasma cells. T Cells The T cell, on the other hand, does not secrete antibody but performs a variety of functions in the adaptive immune response. Different T cell types have the ability to either secrete soluble factors that communicate with other cells of the adaptive immune response or destroy cells infected with intracellular pathogens. The roles of T and B lymphocytes in the adaptive immune response will be discussed further in this chapter. Plasma Cells Another type of lymphocyte of importance is the plasma cell. A plasma cell is a B cell that has differentiated in response to antigen binding, and has thereby gained the ability to secrete soluble antibodies. These cells differ in morphology from standard B and T cells in that they contain a large amount of cytoplasm packed with the protein-synthesizing machinery known as rough endoplasmic reticulum. Natural Killer Cells A fourth important lymphocyte is the natural killer cell, a participant in the innate immune response. A natural killer cell (NK) is a circulating blood cell that contains cytotoxic (cell-killing) granules in its extensive cytoplasm. It shares this mechanism with the cytotoxic T cells of the adaptive immune response. NK cells are among the body’s first lines of defense against viruses and certain types of cancer. Lymphocytes Type of lymphocyte Primary function B lymphocyte Generates diverse antibodies T lymphocyte Secretes chemical messengers Plasma cell Secretes antibodies NK cell Destroys virally infected cells Note: Visit this website to learn about the many different cell types in the immune system and their very specialized jobs. What is the role of the dendritic cell in an HIV infection? Primary Lymphoid Organs and Lymphocyte Development Understanding the differentiation and development of B and T cells is critical to the understanding of the adaptive immune response. It is through this process that the body (ideally) learns to destroy only pathogens and leaves the body’s own cells relatively intact. The primary lymphoid organs are the bone marrow and thymus gland. The lymphoid organs are where lymphocytes mature, proliferate, and are selected, which enables them to attack pathogens without harming the cells of the body. Bone Marrow In the embryo, blood cells are made in the yolk sac. As development proceeds, this function is taken over by the spleen, lymph nodes, and liver. Later, the bone marrow takes over most hematopoietic functions, although the final stages of the differentiation of some cells may take place in other organs. The red bone marrow is a loose collection of cells where hematopoiesis occurs, and the yellow bone marrow is a site of energy storage, which consists largely of fat cells ({link]). The B cell undergoes nearly all of its development in the red bone marrow, whereas the immature T cell, called a thymocyte, leaves the bone marrow and matures largely in the thymus gland. Bone Marrow Red bone marrow fills the head of the femur, and a spot of yellow bone marrow is visible in the center. The white reference bar is 1 cm. Thymus The thymus gland is a bilobed organ found in the space between the sternum and the aorta of the heart ({link]). Connective tissue holds the lobes closely together but also separates them and forms a capsule. Location, Structure, and Histology of the Thymus Cortex Trabeculae Fibrous capsule Right lymphatic duct, entering vein Lymph nodes Cortical epithelial cell Thymocytes _Trabecula Heart FONG oy Hai eee —}— Fibrous Oma \ Wows ke capsule 3 5, e: . Cortex t <9 Spleen A a Yo a Medulla Dendritic cell », ry. t Macrophage Blood vessel Medullary epithelial cell The thymus lies above the heart. The trabeculae and lobules, including the darkly staining cortex and the lighter staining medulla of each lobule, are clearly visible in the light micrograph of the thymus of a newborn. LM x 100. (Micrograph provided by the Regents of the University of Michigan Medical School © 2012) View the University of Michigan WebScope to explore the tissue sample in greater detail. The connective tissue capsule further divides the thymus into lobules via extensions called trabeculae. The outer region of the organ is known as the cortex and contains large numbers of thymocytes with some epithelial cells, macrophages, and dendritic cells (two types of phagocytic cells that are derived from monocytes). The cortex is densely packed so it stains more intensely than the rest of the thymus (see [link]). The medulla, where thymocytes migrate before leaving the thymus, contains a less dense collection of thymocytes, epithelial cells, and dendritic cells. Note: Aging and the... Immune System By the year 2050, 25 percent of the population of the United States will be 60 years of age or older. The CDC estimates that 80 percent of those 60 years and older have one or more chronic disease associated with deficiencies of the immune systems. This loss of immune function with age is called immunosenescence. To treat this growing population, medical professionals must better understand the aging process. One major cause of age-related immune deficiencies is thymic involution, the shrinking of the thymus gland that begins at birth, at a rate of about three percent tissue loss per year, and continues until 35-45 years of age, when the rate declines to about one percent loss per year for the rest of one’s life. At that pace, the total loss of thymic epithelial tissue and thymocytes would occur at about 120 years of age. Thus, this age is a theoretical limit to a healthy human lifespan. Thymic involution has been observed in all vertebrate species that have a thymus gland. Animal studies have shown that transplanted thymic grafts between inbred strains of mice involuted according to the age of the donor and not of the recipient, implying the process is genetically programmed. There is evidence that the thymic microenvironment, so vital to the development of naive T cells, loses thymic epithelial cells according to the decreasing expression of the FOXNI1 gene with age. It is also known that thymic involution can be altered by hormone levels. Sex hormones such as estrogen and testosterone enhance involution, and the hormonal changes in pregnant women cause a temporary thymic involution that reverses itself, when the size of the thymus and its hormone levels return to normal, usually after lactation ceases. What does all this tell us? Can we reverse immunosenescence, or at least slow it down? The potential is there for using thymic transplants from younger donors to keep thymic output of naive T cells high. Gene therapies that target gene expression are also seen as future possibilities. The more we learn through immunosenescence research, the more opportunities there will be to develop therapies, even though these therapies will likely take decades to develop. The ultimate goal is for everyone to live and be healthy longer, but there may be limits to immortality imposed by our genes and hormones. Secondary Lymphoid Organs and their Roles in Active Immune Responses Lymphocytes develop and mature in the primary lymphoid organs, but they mount immune responses from the secondary lymphoid organs. A naive lymphocyte is one that has left the primary organ and entered a secondary lymphoid organ. Naive lymphocytes are fully functional immunologically, but have yet to encounter an antigen to respond to. In addition to circulating in the blood and lymph, lymphocytes concentrate in secondary lymphoid organs, which include the lymph nodes, spleen, and lymphoid nodules. All of these tissues have many features in common, including the following: e The presence of lymphoid follicles, the sites of the formation of lymphocytes, with specific B cell-rich and T cell-rich areas e An internal structure of reticular fibers with associated fixed macrophages ¢ Germinal centers, which are the sites of rapidly dividing and differentiating B lymphocytes e Specialized post-capillary vessels known as high endothelial venules; the cells lining these venules are thicker and more columnar than normal endothelial cells, which allow cells from the blood to directly enter these tissues Lymph Nodes Lymph nodes function to remove debris and pathogens from the lymph, and are thus sometimes referred to as the “filters of the lymph” ({link]). Any bacteria that infect the interstitial fluid are taken up by the lymphatic capillaries and transported to a regional lymph node. Dendritic cells and macrophages within this organ internalize and kill many of the pathogens that pass through, thereby removing them from the body. The lymph node is also the site of adaptive immune responses mediated by T cells, B cells, and accessory Cells of the adaptive immune system. Like the thymus, the bean- shaped lymph nodes are surrounded by a tough capsule of connective tissue and are separated into compartments by trabeculae, the extensions of the capsule. In addition to the structure provided by the capsule and trabeculae, the structural support of the lymph node is provided by a series of reticular fibers laid down by fibroblasts. Structure and Histology of a Lymph Node Efferent lymphatic Connective tissue Cortex vessels capsule Ofc Z . J ie : Rive. fad | 2 Prats fe 2, ~~ Roh . ‘ ey 3 : 7 ‘ 3 ~ i - Connective tissue capsule Subcapsular Subcapsular sinus Afferent lymphatic vessels Lymph nodes are masses of lymphatic tissue located along the larger lymph vessels. The micrograph of the lymph nodes shows a germinal center, which consists of rapidly dividing B cells surrounded by a layer of T cells and other accessory cells. LM x 128. (Micrograph provided by the Regents of the University of Michigan Medical School © 2012) Note: [=] [a ro = pense COLLEGE . rg Ott lt View the University of Michigan WebScope to explore the tissue sample in greater detail. The major routes into the lymph node are via afferent lymphatic vessels (see [link]). Cells and lymph fluid that leave the lymph node may do so by another set of vessels known as the efferent lymphatic vessels. Lymph enters the lymph node via the subcapsular sinus, which is occupied by dendritic cells, macrophages, and reticular fibers. Within the cortex of the lymph node are lymphoid follicles, which consist of germinal centers of rapidly dividing B cells surrounded by a layer of T cells and other accessory cells. As the lymph continues to flow through the node, it enters the medulla, which consists of medullary cords of B cells and plasma cells, and the medullary sinuses where the lymph collects before leaving the node via the efferent lymphatic vessels. Spleen In addition to the lymph nodes, the spleen is a major secondary lymphoid organ ({link]). It is about 12 cm (5 in) long and is attached to the lateral border of the stomach via the gastrosplenic ligament. The spleen is a fragile organ without a strong capsule, and is dark red due to its extensive vascularization. The spleen is sometimes called the “filter of the blood” because of its extensive vascularization and the presence of macrophages and dendritic cells that remove microbes and other materials from the blood, including dying red blood cells. The spleen also functions as the location of immune responses to blood-borne pathogens. Spleen (a) Cross section of the spleen Hilum Trabecula Splenic vein Diaphragm White pulp Arteriole Venule Trabecula Marginal zone Central artery or arteriole = Germinal center Arterial capillaries =< = 2S 2 at Bp 4 Venous sinus (a) The spleen is attached to the stomach. (b) A micrograph of spleen tissue shows the germinal center. The marginal zone is the region between the red pulp and white pulp, which sequesters particulate antigens from the circulation and presents these antigens to lymphocytes in the white pulp. EM x 660. (Micrograph provided by the Regents of the University of Michigan Medical School © 2012) The spleen is also divided by trabeculae of connective tissue, and within each splenic nodule is an area of red pulp, consisting of mostly red blood cells, and white pulp, which resembles the lymphoid follicles of the lymph nodes. Upon entering the spleen, the splenic artery splits into several arterioles (surrounded by white pulp) and eventually into sinusoids. Blood from the capillaries subsequently collects in the venous sinuses and leaves via the splenic vein. The red pulp consists of reticular fibers with fixed macrophages attached, free macrophages, and all of the other cells typical of the blood, including some lymphocytes. The white pulp surrounds a central arteriole and consists of germinal centers of dividing B cells surrounded by T cells and accessory cells, including macrophages and dendritic cells. Thus, the red pulp primarily functions as a filtration system of the blood, using cells of the relatively nonspecific immune response, and white pulp is where adaptive T and B cell responses are mounted. Lymphoid Nodules The other lymphoid tissues, the lymphoid nodules, have a simpler architecture than the spleen and lymph nodes in that they consist of a dense cluster of lymphocytes without a surrounding fibrous capsule. These nodules are located in the respiratory and digestive tracts, areas routinely exposed to environmental pathogens. Tonsils are lymphoid nodules located along the inner surface of the pharynx and are important in developing immunity to oral pathogens ([link]). The tonsil located at the back of the throat, the pharyngeal tonsil, is sometimes referred to as the adenoid when swollen. Such swelling is an indication of an active immune response to infection. Histologically, tonsils do not contain a complete capsule, and the epithelial layer invaginates deeply into the interior of the tonsil to form tonsillar crypts. These structures, which accumulate all sorts of materials taken into the body through eating and breathing, actually “encourage” pathogens to penetrate deep into the tonsillar tissues where they are acted upon by numerous lymphoid follicles and eliminated. This seems to be the major function of tonsils—to help children’s bodies recognize, destroy, and develop immunity to common environmental pathogens so that they will be protected in their later lives. Tonsils are often removed in those children who have recurring throat infections, especially those involving the palatine tonsils on either side of the throat, whose swelling may interfere with their breathing and/or swallowing. Locations and Histology of the Tonsils (a) Locations of the tonsils Brain Palatine Sphenoidal sinus tonsil Palatine Sphenoid bone bone Tongue ie in Mandible Nasopharynx Hyoid Trachea Esophagus Palatine Hard palate tonsil Soft palate Uvula Lingual tonsil Palatine tonsils (swollen due to infection) Epiglottis Tongue (b) Histology of palatine tonsil Crypt Stratified squamous epithelium Germinal centers (a) The pharyngeal tonsil is located on the roof of the posterior superior wall of the nasopharynx. The palatine tonsils lay on each side of the pharynx. (b) A micrograph shows the palatine tonsil tissue. LM x 40. (Micrograph provided by the Regents of the University of Michigan Medical School © 2012) Note: — mess OPenstax COLLEGE Poo - fi ae View the University of Michigan WebScope to explore the tissue sample in greater detail. Mucosa-associated lymphoid tissue (MALT) consists of an aggregate of lymphoid follicles directly associated with the mucous membrane epithelia. MALT makes up dome-shaped structures found underlying the mucosa of the gastrointestinal tract, breast tissue, lungs, and eyes. Peyer’s patches, a type of MALT in the small intestine, are especially important for immune responses against ingested substances ({link]). Peyer’s patches contain specialized endothelial cells called M (or microfold) cells that sample material from the intestinal lumen and transport it to nearby follicles so that adaptive immune responses to potential pathogens can be mounted. Mucosa-associated Lymphoid Tissue (MALT) Nodule Peyer's patches LM ~x 40. (Micrograph provided by the Regents of the University of Michigan Medical School © 2012) Bronchus-associated lymphoid tissue (BALT) consists of lymphoid follicular structures with an overlying epithelial layer found along the bifurcations of the bronchi, and between bronchi and arteries. They also have the typically less-organized structure of other lymphoid nodules. These tissues, in addition to the tonsils, are effective against inhaled pathogens. Chapter Review The lymphatic system is a series of vessels, ducts, and trunks that remove interstitial fluid from the tissues and return it the blood. The lymphatics are also used to transport dietary lipids and cells of the immune system. Cells of the immune system all come from the hematopoietic system of the bone marrow. Primary lymphoid organs, the bone marrow and thymus gland, are the locations where lymphocytes of the adaptive immune system proliferate and mature. Secondary lymphoid organs are site in which mature lymphocytes congregate to mount immune responses. Many immune system cells use the lymphatic and circulatory systems for transport throughout the body to search for and then protect against pathogens. Interactive Link Questions Exercise: Problem: Visit this website for an overview of the lymphatic system. What are the three main components of the lymphatic system? Solution: The three main components are the lymph vessels, the lymph nodes, and the lymph. Exercise: Problem: Visit this website to learn about the many different cell types in the immune system and their very specialized jobs. What is the role of the dendritic cell in infection by HIV? Solution: The dendritic cell transports the virus to a lymph node. Review Questions Exercise: Problem: Which of the following cells is phagocytic? a. plasma cell b. macrophage c. B cell d. NK cell Solution: B Exercise: Problem: Which structure allows lymph from the lower right limb to enter the bloodstream? a. thoracic duct b. right lymphatic duct c. right lymphatic trunk d. left lymphatic trunk Solution: A Exercise: Problem: Which of the following cells is important in the innate immune response? a. B cells b. T cells c. macrophages d. plasma cells Solution: c Exercise: Problem: Which of the following cells would be most active in early, antiviral immune responses the first time one is exposed to pathogen? a. macrophage b. T cell c. neutrophil d. natural killer cell Solution: D Exercise: Problem: Which of the lymphoid nodules is most likely to see food antigens first? a. tonsils b. Peyer’s patches c. bronchus-associated lymphoid tissue d. mucosa-associated lymphoid tissue Solution: A Critical Thinking Questions Exercise: Problem: Describe the flow of lymph from its origins in interstitial fluid to its emptying into the venous bloodstream. Solution: The lymph enters through lymphatic capillaries, and then into larger lymphatic vessels. The lymph can only go in one direction due to valves in the vessels. The larger lymphatics merge to form trunks that enter into the blood via lymphatic ducts. Glossary adaptive immune response relatively slow but very specific and effective immune response controlled by lymphocytes afferent lymphatic vessels lead into a lymph node antibody antigen-specific protein secreted by plasma cells; immunoglobulin antigen molecule recognized by the receptors of B and T lymphocytes barrier defenses antipathogen defenses deriving from a barrier that physically prevents pathogens from entering the body to establish an infection B cells lymphocytes that act by differentiating into an antibody-secreting plasma cell bone marrow tissue found inside bones; the site of all blood cell differentiation and maturation of B lymphocytes bronchus-associated lymphoid tissue (BALT) lymphoid nodule associated with the respiratory tract chyle lipid-rich lymph inside the lymphatic capillaries of the small intestine cisterna chyli bag-like vessel that forms the beginning of the thoracic duct efferent lymphatic vessels lead out of a lymph node germinal centers clusters of rapidly proliferating B cells found in secondary lymphoid tissues high endothelial venules vessels containing unique endothelial cells specialized to allow migration of lymphocytes from the blood to the lymph node immune system series of barriers, cells, and soluble mediators that combine to response to infections of the body with pathogenic organisms innate immune response rapid but relatively nonspecific immune response lymph fluid contained within the lymphatic system lymph node one of the bean-shaped organs found associated with the lymphatic vessels lymphatic capillaries smallest of the lymphatic vessels and the origin of lymph flow lymphatic system network of lymphatic vessels, lymph nodes, and ducts that carries lymph from the tissues and back to the bloodstream. lymphatic trunks large lymphatics that collect lymph from smaller lymphatic vessels and empties into the blood via lymphatic ducts lymphocytes white blood cells characterized by a large nucleus and small rim of cytoplasm lymphoid nodules unencapsulated patches of lymphoid tissue found throughout the body mucosa-associated lymphoid tissue (MALT) lymphoid nodule associated with the mucosa naive lymphocyte mature B or T cell that has not yet encountered antigen for the first time natural killer cell (NK) cytotoxic lymphocyte of innate immune response plasma cell differentiated B cell that is actively secreting antibody primary lymphoid organ site where lymphocytes mature and proliferate; red bone marrow and thymus gland right lymphatic duct drains lymph fluid from the upper right side of body into the right subclavian vein secondary lymphoid organs sites where lymphocytes mount adaptive immune responses; examples include lymph nodes and spleen spleen secondary lymphoid organ that filters pathogens from the blood (white pulp) and removes degenerating or damaged blood cells (red pulp) T cell lymphocyte that acts by secreting molecules that regulate the immune system or by causing the destruction of foreign cells, viruses, and cancer cells thoracic duct large duct that drains lymph from the lower limbs, left thorax, left upper limb, and the left side of the head thymocyte immature T cell found in the thymus thymus primary lymphoid organ; where T lymphocytes proliferate and mature tonsils lymphoid nodules associated with the nasopharynx Gross Anatomy of Urine Transport By the end of this section, you will be able to: e Identify the ureters, urinary bladder, and urethra, as well as their location, structure, histology, and function e Compare and contrast male and female urethras e Describe the micturition reflex e Describe voluntary and involuntary neural control of micturition Rather than start with urine formation, this section will start with urine excretion. Urine is a fluid of variable composition that requires specialized structures to remove it from the body safely and efficiently. Blood is filtered, and the filtrate is transformed into urine at a relatively constant rate throughout the day. This processed liquid is stored until a convenient time for excretion. All structures involved in the transport and storage of the urine are large enough to be visible to the naked eye. This transport and storage system not only stores the waste, but it protects the tissues from damage due to the wide range of pH and osmolarity of the urine, prevents infection by foreign organisms, and for the male, provides reproductive functions. Urethra The urethra transports urine from the bladder to the outside of the body for disposal. The urethra is the only urologic organ that shows any significant anatomic difference between males and females; all other urine transport structures are identical ([link]). Female and Male Urethras The urethra transports urine from the bladder to the outside of the body. This image shows (a) a female urethra and (b) a male urethra. Urinary bladder Pubic bone Ureter Ureter Seminal vesicle Uterus —~_ Ductus Urinary \A nN ~ bladder a . Pubic bone sas Feu bi | i Clitoris ee 7 Vagina tactig ji: Prostate gland Rectum Anus The urethra in both males and females begins inferior and central to the two ureteral openings forming the three points of a triangular-shaped area at the base of the bladder called the trigone (Greek tri- = “triangle” and the root of the word “trigonometry”). The urethra tracks posterior and inferior to the pubic symphysis (see [link ]a). In both males and females, the proximal urethra is lined by transitional epithelium, whereas the terminal portion is a nonkeratinized, stratified squamous epithelium. In the male, pseudostratified columnar epithelium lines the urethra between these two cell types. Voiding is regulated by an involuntary autonomic nervous system-controlled internal urinary sphincter, consisting of smooth muscle and voluntary skeletal muscle that forms the external urinary sphincter below it. Female Urethra The external urethral orifice is embedded in the anterior vaginal wall inferior to the clitoris, superior to the vaginal opening (introitus), and medial to the labia minora. Its short length, about 4 cm, is less of a barrier to fecal bacteria than the longer male urethra and the best explanation for the greater incidence of UTI in women. Voluntary control of the external urethral sphincter is a function of the pudendal nerve. It arises in the sacral region of the spinal cord, traveling via the S2—S4 nerves of the sacral plexus. Male Urethra The male urethra passes through the prostate gland immediately inferior to the bladder before passing below the pubic symphysis (see [link]b). The length of the male urethra varies between men but averages 20 cm in length. It is divided into four regions: the preprostatic urethra, the prostatic urethra, the membranous urethra, and the spongy or penile urethra. The preprostatic urethra is very short and incorporated into the bladder wall. The prostatic urethra passes through the prostate gland. During sexual intercourse, it receives sperm via the ejaculatory ducts and secretions from the seminal vesicles. Paired Cowper’s glands (bulbourethral glands) produce and secrete mucus into the urethra to buffer urethral pH during sexual stimulation. The mucus neutralizes the usually acidic environment and lubricates the urethra, decreasing the resistance to ejaculation. The membranous urethra passes through the deep muscles of the perineum, where it is invested by the overlying urethral sphincters. The spongy urethra exits at the tip (external urethral orifice) of the penis after passing through the corpus spongiosum. Mucous glands are found along much of the length of the urethra and protect the urethra from extremes of urine pH. Innervation is the same in both males and females. Bladder The urinary bladder collects urine from both ureters ({link]). The bladder lies anterior to the uterus in females, posterior to the pubic bone and anterior to the rectum. During late pregnancy, its capacity is reduced due to compression by the enlarging uterus, resulting in increased frequency of urination. In males, the anatomy is similar, minus the uterus, and with the addition of the prostate inferior to the bladder. The bladder is partially retroperitoneal (outside the peritoneal cavity) with its peritoneal-covered “dome” projecting into the abdomen when the bladder is distended with urine. Bladder Ureter Peritoneum Detrusor muscle Ureteral , Transitional epithelium openings Lamina propria Submucosa Internal urethral sphincter | External urethral =a sphincter (a) (b) (a) Anterior cross section of the bladder. (b) The detrusor muscle of the bladder (source: monkey tissue) LM x 448. (Micrograph provided by the Regents of the University of Michigan Medical School © 2012) View the University of Michigan WebScope to explore the tissue sample in greater detail. The bladder is a highly distensible organ comprised of irregular crisscrossing bands of smooth muscle collectively called the detrusor muscle. The interior surface is made of transitional cellular epithelium that is structurally suited for the large volume fluctuations of the bladder. When empty, it resembles columnar epithelia, but when stretched, it “transitions” (hence the name) to a squamous appearance (see [link]). Volumes in adults can range from nearly zero to 500-600 mL. The detrusor muscle contracts with significant force in the young. The bladder’s strength diminishes with age, but voluntary contractions of abdominal skeletal muscles can increase intra-abdominal pressure to promote more forceful bladder emptying. Such voluntary contraction is also used in forceful defecation and childbirth. Micturition Reflex Micturition is a less-often used, but proper term for urination or voiding. It results from an interplay of involuntary and voluntary actions by the internal and external urethral sphincters. When bladder volume reaches about 150 mL, an urge to void is sensed but is easily overridden. Voluntary control of urination relies on consciously preventing relaxation of the external urethral sphincter to maintain urinary continence. As the bladder fills, subsequent urges become harder to ignore. Ultimately, voluntary constraint fails with resulting incontinence, which will occur as bladder volume approaches 300 to 400 mL. Normal micturition is a result of stretch receptors in the bladder wall that transmit nerve impulses to the sacral region of the spinal cord to generate a spinal reflex. The resulting parasympathetic neural outflow causes contraction of the detrusor muscle and relaxation of the involuntary internal urethral sphincter. At the same time, the spinal cord inhibits somatic motor neurons, resulting in the relaxation of the skeletal muscle of the external urethral sphincter. The micturition reflex is active in infants but with maturity, children learn to override the reflex by asserting external sphincter control, thereby delaying voiding (potty training). This reflex may be preserved even in the face of spinal cord injury that results in paraplegia or quadriplegia. However, relaxation of the external sphincter may not be possible in all cases, and therefore, periodic catheterization may be necessary for bladder emptying. Nerves involved in the control of urination include the hypogastric, pelvic, and pudendal ([link]). Voluntary micturition requires an intact spinal cord and functional pudendal nerve arising from the sacral micturition center. Since the external urinary sphincter is voluntary skeletal muscle, actions by cholinergic neurons maintain contraction (and thereby continence) during filling of the bladder. At the same time, sympathetic nervous activity via the hypogastric nerves suppresses contraction of the detrusor muscle. With further bladder stretch, afferent signals traveling over sacral pelvic nerves activate parasympathetic neurons. This activates efferent neurons to release acetylcholine at the neuromuscular junctions, producing detrusor contraction and bladder emptying. Nerves Innervating the Urinary System Sacrum Uterus Urinary bladder Pubic bone Sphincter Clitoris Pudendal nerve Labium minora Anus Ureters The kidneys and ureters are completely retroperitoneal, and the bladder has a peritoneal covering only over the dome. As urine is formed, it drains into the calyces of the kidney, which merge to form the funnel-shaped renal pelvis in the hilum of each kidney. The renal pelvis narrows to become the ureter of each kidney. As urine passes through the ureter, it does not passively drain into the bladder but rather is propelled by waves of peristalsis. As the ureters enter the pelvis, they sweep laterally, hugging the pelvic walls. As they approach the bladder, they turn medially and pierce the bladder wall obliquely. This is important because it creates an one-way valve (a physiological sphincter rather than an anatomical sphincter) that allows urine into the bladder but prevents reflux of urine from the bladder back into the ureter. Children born lacking this oblique course of the ureter through the bladder wall are susceptible to “vesicoureteral reflux,” which dramatically increases their risk of serious UTI. Pregnancy also increases the likelihood of reflux and UTI. The ureters are approximately 30 cm long. The inner mucosa is lined with transitional epithelium ([link]) and scattered goblet cells that secrete protective mucus. The muscular layer of the ureter consists of longitudinal and circular smooth muscles that create the peristaltic contractions to move the urine into the bladder without the aid of gravity. Finally, a loose adventitial layer composed of collagen and fat anchors the ureters between the parietal peritoneum and the posterior abdominal wall. Ureter Peristaltic contractions help to move urine through the lumen with contributions from fluid pressure and gravity. LM x 128. (Micrograph provided by the Regents of the University of Michigan Medical School © 2012) Chapter Review The urethra is the only urinary structure that differs significantly between males and females. This is due to the dual role of the male urethra in transporting both urine and semen. The urethra arises from the trigone area at the base of the bladder. Urination is controlled by an involuntary internal sphincter of smooth muscle and a voluntary external sphincter of skeletal muscle. The shorter female urethra contributes to the higher incidence of bladder infections in females. The male urethra receives secretions from the prostate gland, Cowper’s gland, and seminal vesicles as well as sperm. The bladder is largely retroperitoneal and can hold up to 500-600 mL urine. Micturition is the process of voiding the urine and involves both involuntary and voluntary actions. Voluntary control of micturition requires a mature and intact sacral micturition center. It also requires an intact spinal cord. Loss of control of micturition is called incontinence and results in voiding when the bladder contains about 250 mL urine. The ureters are retroperitoneal and lead from the renal pelvis of the kidney to the trigone area at the base of the bladder. A thick muscular wall consisting of longitudinal and circular smooth muscle helps move urine toward the bladder by way of peristaltic contractions. Review Questions Exercise: Problem: Peristaltic contractions occur in the a. urethra b. bladder c. ureters d. urethra, bladder, and ureters Solution: C Exercise: Problem: Somatic motor neurons must be to relax the external urethral sphincter to allow urination. a. stimulated b. inhibited Solution: B Exercise: Problem: Which part of the urinary system is not completely retroperitoneal? a. kidneys b. ureters c. bladder d. nephrons Solution: C Critical Thinking Questions Exercise: Problem: Why are females more likely to contract bladder infections than males? Solution: The longer urethra of males means bacteria must travel farther to the bladder to cause an infection. Exercise: Problem: Describe how forceful urination is accomplished. Solution: Forceful urination is accomplished by contraction of abdominal muscles. Glossary anatomical sphincter smooth or skeletal muscle surrounding the lumen of a vessel or hollow organ that can restrict flow when contracted detrusor muscle smooth muscle in the bladder wall; fibers run in all directions to reduce the size of the organ when emptying it of urine external urinary sphincter skeletal muscle; must be relaxed consciously to void urine internal urinary sphincter smooth muscle at the juncture of the bladder and urethra; relaxes as the bladder fills to allow urine into the urethra incontinence loss of ability to control micturition micturition also called urination or voiding physiological sphincter sphincter consisting of circular smooth muscle indistinguishable from adjacent muscle but possessing differential innervations, permitting its function as a sphincter; structurally weak retroperitoneal outside the peritoneal cavity; in the case of the kidney and ureters, between the parietal peritoneum and the abdominal wall sacral micturition center group of neurons in the sacral region of the spinal cord that controls urination; acts reflexively unless its action is modified by higher brain centers to allow voluntary urination trigone area at the base of the bladder marked by the two ureters in the posterior—lateral aspect and the urethral orifice in the anterior aspect oriented like points on a triangle urethra transports urine from the bladder to the outside environment Gross Anatomy of the Kidney By the end of this section, you will be able to: ¢ Describe the external structure of the kidney, including its location, support structures, and covering ¢ Identify the major internal divisions and structures of the kidney ¢ Identify the major blood vessels associated with the kidney and trace the path of blood through the kidney e Compare and contrast the cortical and juxtamedullary nephrons e Name structures found in the cortex and medulla e Describe the physiological characteristics of the cortex and medulla The kidneys lie on either side of the spine in the retroperitoneal space between the parietal peritoneum and the posterior abdominal wall, well protected by muscle, fat, and ribs. They are roughly the size of your fist, and the male kidney is typically a bit larger than the female kidney. The kidneys are well vascularized, receiving about 25 percent of the cardiac output at rest. Note: — mess OPenstax COLLEGE There have never been sufficient kidney donations to provide a kidney to each person needing one. Watch this video to learn about the TED (Technology, Entertainment, Design) Conference held in March 2011. In this video, Dr. Anthony Atala discusses a cutting-edge technique in which anew kidney is “printed.” The successful utilization of this technology is still several years in the future, but imagine a time when you can print a replacement organ or tissue on demand. External Anatomy The left kidney is located at about the T12 to L3 vertebrae, whereas the right is lower due to slight displacement by the liver. Upper portions of the kidneys are somewhat protected by the eleventh and twelfth ribs ((link]). Each kidney weighs about 125-175 g in males and 115-155 g in females. They are about 11—14 cm in length, 6 cm wide, and 4 cm thick, and are directly covered by a fibrous capsule composed of dense, irregular connective tissue that helps to hold their shape and protect them. This capsule is covered by a shock-absorbing layer of adipose tissue called the renal fat pad, which in turn is encompassed by a tough renal fascia. The fascia and, to a lesser extent, the overlying peritoneum serve to firmly anchor the kidneys to the posterior abdominal wall in a retroperitoneal position. Kidneys Liver Kidney 12th rib Ureter The kidneys are slightly protected by the ribs and are surrounded by fat for protection (not shown). On the superior aspect of each kidney is the adrenal gland. The adrenal cortex directly influences renal function through the production of the hormone aldosterone to stimulate sodium reabsorption. Internal Anatomy A frontal section through the kidney reveals an outer region called the renal cortex and an inner region called the medulla (({link]). The renal columns are connective tissue extensions that radiate downward from the cortex through the medulla to separate the most characteristic features of the medulla, the renal pyramids and renal papillae. The papillae are bundles of collecting ducts that transport urine made by nephrons to the calyces of the kidney for excretion. The renal columns also serve to divide the kidney into 6-8 lobes and provide a supportive framework for vessels that enter and exit the cortex. The pyramids and renal columns taken together constitute the kidney lobes. Left Kidney Cortical ——_ jie blood vessels _ Arcuate y) blood vessels | Interlobar ——_—_ ane | blood vessels Renal vein i) —_—— Major calyx Minor calyx aanel Renal pelvis Renal ena hilum nerve Pyramid Renal artery Papilla Medulla Renal column Ureter rt Capsule saute Renal Hilum The renal hilum is the entry and exit site for structures servicing the kidneys: vessels, nerves, lymphatics, and ureters. The medial-facing hila are tucked into the sweeping convex outline of the cortex. Emerging from the hilum is the renal pelvis, which is formed from the major and minor calyxes in the kidney. The smooth muscle in the renal pelvis funnels urine via peristalsis into the ureter. The renal arteries form directly from the descending aorta, whereas the renal veins return cleansed blood directly to the inferior vena cava. The artery, vein, and renal pelvis are arranged in an anterior-to-posterior order. Nephrons and Vessels The renal artery first divides into segmental arteries, followed by further branching to form interlobar arteries that pass through the renal columns to reach the cortex ([link]). The interlobar arteries, in turn, branch into arcuate arteries, cortical radiate arteries, and then into afferent arterioles. The afferent arterioles service about 1.3 million nephrons in each kidney. Blood Flow in the Kidney Peritubular capillaries Efferent arteriole Glomerulus Afferent arteriole Cortical radiate artery Arcuate artery Ee ~ Interlobar artery Segmental Interlobar vein Renal artery —— Renal vein Nephrons are the “functional units” of the kidney; they cleanse the blood and balance the constituents of the circulation. The afferent arterioles form a tuft of high-pressure capillaries about 200 pm in diameter, the glomerulus. The rest of the nephron consists of a continuous sophisticated tubule whose proximal end surrounds the glomerulus in an intimate embrace—this is Bowman’s capsule. The glomerulus and Bowman’s capsule together form the renal corpuscle. As mentioned earlier, these glomerular capillaries filter the blood based on particle size. After passing through the renal corpuscle, the capillaries form a second arteriole, the efferent arteriole ({link]). These will next form a capillary network around the more distal portions of the nephron tubule, the peritubular capillaries and vasa recta, before returning to the venous system. As the glomerular filtrate progresses through the nephron, these capillary networks recover most of the solutes and water, and return them to the circulation. Since a capillary bed (the glomerulus) drains into a vessel that in turn forms a second capillary bed, the definition of a portal system is met. This is the only portal system in which an arteriole is found between the first and second capillary beds. (Portal systems also link the hypothalamus to the anterior pituitary, and the blood vessels of the digestive viscera to the liver.) Blood Flow in the Nephron Glomerular capsule Efferent arteriole Afferent arteriole Proximal convoluted tubule Interlobular artery el Loop of A, the nephron Peritubular capillary network Urine flows into renal papilla The two capillary beds are clearly shown in this figure. The efferent arteriole is the connecting vessel between the glomerulus and the peritubular capillaries and vasa recta. Note: a] Visit this link to view an interactive tutorial of the flow of blood through the kidney. Cortex In a dissected kidney, it is easy to identify the cortex; it appears lighter in color compared to the rest of the kidney. All of the renal corpuscles as well as both the proximal convoluted tubules (PCTs) and distal convoluted tubules are found here. Some nephrons have a short loop of Henle that does not dip beyond the cortex. These nephrons are called cortical nephrons. About 15 percent of nephrons have long loops of Henle that extend deep into the medulla and are called juxtamedullary nephrons. Chapter Review As noted previously, the structure of the kidney is divided into two principle regions—the peripheral rim of cortex and the central medulla. The two kidneys receive about 25 percent of cardiac output. They are protected in the retroperitoneal space by the renal fat pad and overlying ribs and muscle. Ureters, blood vessels, lymph vessels, and nerves enter and leave at the renal hilum. The renal arteries arise directly from the aorta, and the renal veins drain directly into the inferior vena cava. Kidney function is derived from the actions of about 1.3 million nephrons per kidney; these are the “functional units.” A capillary bed, the glomerulus, filters blood and the filtrate is captured by Bowman’s capsule. A portal system is formed when the blood flows through a second capillary bed surrounding the proximal and distal convoluted tubules and the loop of Henle. Most water and solutes are recovered by this second capillary bed. This filtrate is processed and finally gathered by collecting ducts that drain into the minor calyces, which merge to form major calyces; the filtrate then proceeds to the renal pelvis and finally the ureters. Review Questions Exercise: Problem: The renal pyramids are separated from each other by extensions of the renal cortex called a. renal medulla b. minor calyces c. medullary cortices d. renal columns Solution: D Exercise: Problem: The primary structure found within the medulla is the a. loop of Henle b. minor calyces c. portal system d. ureter Solution: A Exercise: Problem:The right kidney is slightly lower because a. it is displaced by the liver b. it is displace by the heart c. it is slightly smaller d. it needs protection of the lower ribs Solution: A Critical Thinking Questions Exercise: Problem: What anatomical structures provide protection to the kidney? Solution: Retroperitoneal anchoring, renal fat pads, and ribs provide protection to the kidney. Exercise: Problem: How does the renal portal system differ from the hypothalamo— hypophyseal and digestive portal systems? Solution: The renal portal system has an artery between the first and second capillary bed. The others have a vein. Exercise: Problem: Name the structures found in the renal hilum. Solution: The structures found in the renal hilum are arteries, veins, ureters, lymphatics, and nerves. Glossary Bowman’s capsule cup-shaped sack lined by a simple squamous epithelium (parietal surface) and specialized cells called podocytes (visceral surface) that participate in the filtration process; receives the filtrate which then passes on to the PCTs calyces cup-like structures receiving urine from the collecting ducts where it passes on to the renal pelvis and ureter cortical nephrons nephrons with loops of Henle that do not extend into the renal medulla distal convoluted tubules portions of the nephron distal to the loop of Henle that receive hyposmotic filtrate from the loop of Henle and empty into collecting ducts efferent arteriole arteriole carrying blood from the glomerulus to the capillary beds around the convoluted tubules and loop of Henle; portion of the portal system glomerulus tuft of capillaries surrounded by Bowman’s capsule; filters the blood based on size juxtamedullary nephrons nephrons adjacent to the border of the cortex and medulla with loops of Henle that extend into the renal medulla loop of Henle descending and ascending portions between the proximal and distal convoluted tubules; those of cortical nephrons do not extend into the medulla, whereas those of juxtamedullary nephrons do extend into the medulla nephrons functional units of the kidney that carry out all filtration and modification to produce urine; consist of renal corpuscles, proximal and distal convoluted tubules, and descending and ascending loops of Henle; drain into collecting ducts medulla inner region of kidney containing the renal pyramids peritubular capillaries second capillary bed of the renal portal system; surround the proximal and distal convoluted tubules; associated with the vasa recta proximal convoluted tubules (PCTs) tortuous tubules receiving filtrate from Bowman’s capsule; most active part of the nephron in reabsorption and secretion renal columns extensions of the renal cortex into the renal medulla; separates the renal pyramids; contains blood vessels and connective tissues renal corpuscle consists of the glomerulus and Bowman’s capsule renal cortex outer part of kidney containing all of the nephrons; some nephrons have loops of Henle extending into the medulla renal fat pad adipose tissue between the renal fascia and the renal capsule that provides protective cushioning to the kidney renal hilum recessed medial area of the kidney through which the renal artery, renal vein, ureters, lymphatics, and nerves pass renal papillae medullary area of the renal pyramids where collecting ducts empty urine into the minor calyces renal pyramids six to eight cone-shaped tissues in the medulla of the kidney containing collecting ducts and the loops of Henle of juxtamedullary nephrons vasa recta branches of the efferent arterioles that parallel the course of the loops of Henle and are continuous with the peritubular capillaries; with the glomerulus, form a portal system Microscopic Anatomy of the Kidney By the end of this section, you will be able to: e Distinguish the histological differences between the renal cortex and medulla e Describe the structure of the filtration membrane e Identify the major structures and subdivisions of the renal corpuscles, renal tubules, and renal capillaries e Discuss the function of the peritubular capillaries and vasa recta e Identify the location of the juxtaglomerular apparatus and describe the cells that line it e Describe the histology of the proximal convoluted tubule, loop of Henle, distal convoluted tubule, and collecting ducts The renal structures that conduct the essential work of the kidney cannot be seen by the naked eye. Only a light or electron microscope can reveal these structures. Even then, serial sections and computer reconstruction are necessary to give us a comprehensive view of the functional anatomy of the nephron and its associated blood vessels. Nephrons: The Functional Unit Nephrons take a simple filtrate of the blood and modify it into urine. Many changes take place in the different parts of the nephron before urine is created for disposal. The term forming urine will be used hereafter to describe the filtrate as it is modified into true urine. The principle task of the nephron population is to balance the plasma to homeostatic set points and excrete potential toxins in the urine. They do this by accomplishing three principle functions—filtration, reabsorption, and secretion. They also have additional secondary functions that exert control in three areas: blood pressure (via production of renin), red blood cell production (via the hormone EPO), and calcium absorption (via conversion of calcidiol into calcitriol, the active form of vitamin D). Renal Corpuscle As discussed earlier, the renal corpuscle consists of a tuft of capillaries called the glomerulus that is largely surrounded by Bowman’s (glomerular) capsule. The glomerulus is a high-pressure capillary bed between afferent and efferent arterioles. Bowman’s capsule surrounds the glomerulus to form a lumen, and captures and directs this filtrate to the PCT. The outermost part of Bowman’s capsule, the parietal layer, is a simple squamous epithelium. It transitions onto the glomerular capillaries in an intimate embrace to form the visceral layer of the capsule. Here, the cells are not squamous, but uniquely shaped cells (podocytes) extending finger-like arms (pedicels) to cover the glomerular capillaries ((link]). These projections interdigitate to form filtration slits, leaving small gaps between the digits to form a sieve. As blood passes through the glomerulus, 10 to 20 percent of the plasma filters between these sieve-like fingers to be captured by Bowman’s capsule and funneled to the PCT. Where the fenestrae (windows) in the glomerular capillaries match the spaces between the podocyte “fingers,” the only thing separating the capillary lumen and the lumen of Bowman’s capsule is their shared basement membrane ([link]). These three features comprise what is known as the filtration membrane. This membrane permits very rapid movement of filtrate from capillary to capsule though pores that are only 70 nm in diameter. Podocytes Cell Filtration bodies slits Capillary (b) Podocytes interdigitate with structures called pedicels and filter substances in a way similar to fenestrations. In (a), the large cell body can be seen at the top right comer, with branches extending from the cell body. The smallest finger-like extensions are the pedicels. Pedicels on one podocyte always interdigitate with the pedicels of another podocyte. (b) This capillary has three podocytes wrapped around it. Fenestrated Capillary Basement lle ae Endothelium Fenestrations Fenestrations allow many substances to diffuse from the blood based primarily on size. The fenestrations prevent filtration of blood cells or large proteins, but allow most other constituents through. These substances cross readily if they are less than 4 nm in size and most pass freely up to 8 nm in size. An additional factor affecting the ability of substances to cross this barrier is their electric charge. The proteins associated with these pores are negatively charged, so they tend to repel negatively charged substances and allow positively charged substances to pass more readily. The basement membrane prevents filtration of medium-to-large proteins such as globulins. There are also mesangial cells in the filtration membrane that can contract to help regulate the rate of filtration of the glomerulus. Overall, filtration is regulated by fenestrations in capillary endothelial cells, podocytes with filtration slits, membrane charge, and the basement membrane between capillary cells. The result is the creation of a filtrate that does not contain cells or large proteins, and has a slight predominance of positively charged substances. Lying just outside Bowman’s capsule and the glomerulus is the juxtaglomerular apparatus (JGA) ([link]). At the juncture where the afferent and efferent arterioles enter and leave Bowman’s capsule, the initial part of the distal convoluted tubule (DCT) comes into direct contact with the arterioles. The wall of the DCT at that point forms a part of the JGA known as the macula densa. This cluster of cuboidal epithelial cells monitors the fluid composition of fluid flowing through the DCT. In response to the concentration of Na” in the fluid flowing past them, these cells release paracrine signals. They also have a single, nonmotile cilium that responds to the rate of fluid movement in the tubule. The paracrine signals released in response to changes in flow rate and Na* concentration are adenosine triphosphate (ATP) and adenosine. Juxtaglomerular Apparatus and Glomerulus (a) The JGA allows specialized cells to monitor the composition of the fluid in the DCT and adjust the glomerular filtration rate. (b) This micrograph shows the glomerulus and surrounding structures. LM x 1540. (Micrograph provided by the Regents of University of Michigan Medical School © 2012) Podocyte ; Macula densa Proximal convoluted tubule Brush border Juxtaglomerular Distal cells Proximal convoluted convoluted tubule Renal tubule nerve Basement membrane (a) (b) Afferent arteriole A second cell type in this apparatus is the juxtaglomerular cell. This is a modified, smooth muscle cell lining the afferent arteriole that can contract or relax in response to ATP or adenosine released by the macula densa. Such contraction and relaxation regulate blood flow to the glomerulus. If the osmolarity of the filtrate is too high (hyperosmotic), the juxtaglomerular cells will contract, decreasing the glomerular filtration rate (GFR) so less plasma is filtered, leading to less urine formation and greater retention of fluid. This will ultimately decrease blood osmolarity toward the physiologic norm. If the osmolarity of the filtrate is too low, the juxtaglomerular cells will relax, increasing the GFR and enhancing the loss of water to the urine, causing blood osmolarity to rise. In other words, when osmolarity goes up, filtration and urine formation decrease and water is retained. When osmolarity goes down, filtration and urine formation increase and water is lost by way of the urine. The net result of these opposing actions is to keep the rate of filtration relatively constant. A second function of the macula densa cells is to regulate renin release from the juxtaglomerular cells of the afferent arteriole ((link]). Active renin is a protein comprised of 304 amino acids that cleaves several amino acids from angiotensinogen to produce angiotensin I. Angiotensin I is not biologically active until converted to angiotensin II by angiotensin-converting enzyme (ACE) from the lungs. Angiotensin IT is a systemic vasoconstrictor that helps to regulate blood pressure by increasing it. Angiotensin II also stimulates the release of the steroid hormone aldosterone from the adrenal cortex. Aldosterone stimulates Na* reabsorption by the kidney, which also results in water retention and increased blood pressure. Conversion of Angiotensin I to Angiotensin II Macula densa senses low fluid flow or low Na* concentration Juxtaglomerular Angiotensin-converting cells secrete renin enzyme (ACE) in pulmonary blood Widespread vasoconstriction Kidney releases enzyme renin into blood Adrenal cortex to secrete Enzyme Angiotensin | 3 Aldosterone reaction ersten Angiotensin II Stimulates Liver releases angiotensinogen into blood ADH causes aquaporins to move to the collecting duct plasma membrane, which increases water reabsorption >| Aldosterone stimulates Nat uptake on the apical cell membrane in the distal convoluted tubule and collecting ducts The enzyme renin converts the pro-enzyme angiotensin I; the lung- derived enzyme ACE converts angiotensin I into active angiotensin i. Proximal Convoluted Tubule (PCT) Filtered fluid collected by Bowman’s capsule enters into the PCT. It is called convoluted due to its tortuous path. Simple cuboidal cells form this tubule with prominent microvilli on the luminal surface, forming a brush border. These microvilli create a large surface area to maximize the absorption and secretion of solutes (Na*, Cl’, glucose, etc.), the most essential function of this portion of the nephron. These cells actively transport ions across their membranes, so they possess a high concentration of mitochondria in order to produce sufficient ATP. Loop of Henle The descending and ascending portions of the loop of Henle (sometimes referred to as the nephron loop) are, of course, just continuations of the same tubule. They run adjacent and parallel to each other after having made a hairpin turn at the deepest point of their descent. The descending loop of Henle consists of an initial short, thick portion and long, thin portion, whereas the ascending loop consists of an initial short, thin portion followed by a long, thick portion. The descending thick portion consists of simple cuboidal epithelium similar to that of the PCT. The descending and ascending thin portions consists of simple squamous epithelium. As you will see later, these are important differences, since different portions of the loop have different permeabilities for solutes and water. The ascending thick portion consists of simple cuboidal epithelium similar to the DCT. Distal Convoluted Tubule (DCT) The DCT, like the PCT, is very tortuous and formed by simple cuboidal epithelium, but it is shorter than the PCT. These cells are not as active as those in the PCT; thus, there are fewer microvilli on the apical surface. However, these cells must also pump ions against their concentration gradient, so you will find of large numbers of mitochondria, although fewer than in the PCT. Collecting Ducts The collecting ducts are continuous with the nephron but not technically part of it. In fact, each duct collects filtrate from several nephrons for final modification. Collecting ducts merge as they descend deeper in the medulla to form about 30 terminal ducts, which empty at a papilla. They are lined with simple squamous epithelium with receptors for ADH. When stimulated by ADH, these cells will insert aquaporin channel proteins into their membranes, which as their name suggests, allow water to pass from the duct lumen through the cells and into the interstitial spaces to be recovered by the vasa recta. This process allows for the recovery of large amounts of water from the filtrate back into the blood. In the absence of ADH, these channels are not inserted, resulting in the excretion of water in the form of dilute urine. Most, if not all, cells of the body contain aquaporin molecules, whose channels are so small that only water can pass. At least 10 types of aquaporins are known in humans, and six of those are found in the kidney. The function of all aquaporins is to allow the movement of water across the lipid-rich, hydrophobic cell membrane ([link]). Aquaporin Water Channel Water channel Cell membrane Positive charges inside the channel prevent the leakage of electrolytes across the cell membrane, while allowing water to move due to osmosis. Chapter Review The functional unit of the kidney, the nephron, consists of the renal corpuscle, PCT, loop of Henle, and DCT. Cortical nephrons have short loops of Henle, whereas juxtamedullary nephrons have long loops of Henle extending into the medulla. About 15 percent of nephrons are juxtamedullary. The glomerulus is a capillary bed that filters blood principally based on particle size. The filtrate is captured by Bowman’s capsule and directed to the PCT. A filtration membrane is formed by the fused basement membranes of the podocytes and the capillary endothelial cells that they embrace. Contractile mesangial cells further perform a role in regulating the rate at which the blood is filtered. Specialized cells in the JGA produce paracrine signals to regulate blood flow and filtration rates of the glomerulus. Other JGA cells produce the enzyme renin, which plays a central role in blood pressure regulation. The filtrate enters the PCT where absorption and secretion of several substances occur. The descending and ascending limbs of the loop of Henle consist of thick and thin segments. Absorption and secretion continue in the DCT but to a lesser extent than in the PCT. Each collecting duct collects forming urine from several nephrons and responds to the posterior pituitary hormone ADH by inserting aquaporin water channels into the cell membrane to fine tune water recovery. Review Questions Exercise: Problem:Blood filtrate is captured in the lumen of the a. glomerulus b. Bowman’s capsule c. calyces d. renal papillae Solution: B Exercise: Problem: What are the names of the capillaries following the efferent arteriole? a. arcuate and medullary b. interlobar and interlobular c. peritubular and vasa recta d. peritubular and medullary Solution: C Exercise: Problem:The functional unit of the kidney is called a. the renal hilus b. the renal corpuscle c. the nephron d. Bowman’s capsule Solution: C Critical Thinking Questions Exercise: Problem: Which structures make up the renal corpuscle? Solution: The structures that make up the renal corpuscle are the glomerulus, Bowman’s capsule, and PCT. Exercise: Problem: What are the major structures comprising the filtration membrane? Solution: The major structures comprising the filtration membrane are fenestrations and podocyte fenestra, fused basement membrane, and filtration slits. Glossary angiotensin-converting enzyme (ACE) enzyme produced by the lungs that catalyzes the reaction of inactive angiotensin I into active angiotensin II angiotensin I protein produced by the enzymatic action of renin on angiotensinogen; inactive precursor of angiotensin II angiotensin II protein produced by the enzymatic action of ACE on inactive angiotensin I; actively causes vasoconstriction and stimulates aldosterone release by the adrenal cortex angiotensinogen inactive protein in the circulation produced by the liver; precursor of angiotensin I; must be modified by the enzymes renin and ACE to be activated aquaporin protein-forming water channels through the lipid bilayer of the cell; allows water to cross; activation in the collecting ducts is under the control of ADH brush border formed by microvilli on the surface of certain cuboidal cells; in the kidney it is found in the PCT; increases surface area for absorption in the kidney fenestrations small windows through a cell, allowing rapid filtration based on size; formed in such a way as to allow substances to cross through a cell without mixing with cell contents filtration slits formed by pedicels of podocytes; substances filter between the pedicels based on size forming urine filtrate undergoing modifications through secretion and reabsorption before true urine is produced juxtaglomerular apparatus (JGA) located at the juncture of the DCT and the afferent and efferent arterioles of the glomerulus; plays a role in the regulation of renal blood flow and GFR juxtaglomerular cell modified smooth muscle cells of the afferent arteriole; secretes renin in response to a drop in blood pressure macula densa cells found in the part of the DCT forming the JGA; sense Na* concentration in the forming urine mesangial contractile cells found in the glomerulus; can contract or relax to regulate filtration rate pedicels finger-like projections of podocytes surrounding glomerular capillaries; interdigitate to form a filtration membrane podocytes cells forming finger-like processes; form the visceral layer of Bowman’s capsule; pedicels of the podocytes interdigitate to form a filtration membrane renin enzyme produced by juxtaglomerular cells in response to decreased blood pressure or sympathetic nervous activity; catalyzes the conversion of angiotensinogen into angiotensin I An Overview of the Endocrine System By the end of this section, you will be able to: e Distinguish the types of intercellular communication, their importance, mechanisms, and effects e Identify the major organs and tissues of the endocrine system and their location in the body Communication is a process in which a sender transmits signals to one or more receivers to control and coordinate actions. In the human body, two major organ systems participate in relatively “long distance” communication: the nervous system and the endocrine system. Together, these two systems are primarily responsible for maintaining homeostasis in the body. Neural and Endocrine Signaling The nervous system uses two types of intercellular communication— electrical and chemical signaling—either by the direct action of an electrical potential, or in the latter case, through the action of chemical neurotransmitters such as serotonin or norepinephrine. Neurotransmitters act locally and rapidly. When an electrical signal in the form of an action potential arrives at the synaptic terminal, they diffuse across the synaptic cleft (the gap between a sending neuron and a receiving neuron or muscle cell). Once the neurotransmitters interact (bind) with receptors on the receiving (post-synaptic) cell, the receptor stimulation is transduced into a response such as continued electrical signaling or modification of cellular response. The target cell responds within milliseconds of receiving the chemical “message”; this response then ceases very quickly once the neural signaling ends. In this way, neural communication enables body functions that involve quick, brief actions, such as movement, sensation, and cognition.In contrast, the endocrine system uses just one method of communication: chemical signaling. These signals are sent by the endocrine organs, which secrete chemicals—the hormone— into the extracellular fluid. Hormones are transported primarily via the bloodstream throughout the body, where they bind to receptors on target cells, inducing a characteristic response. As a result, endocrine signaling requires more time than neural signaling to prompt a response in target cells, though the precise amount of time varies with different hormones. For example, the hormones released when you are confronted with a dangerous or frightening situation, called the fight-or-flight response, occur by the release of adrenal hormones —epinephrine and norepinephrine—within seconds. In contrast, it may take up to 48 hours for target cells to respond to certain reproductive hormones. fli: fm) x Fr = openstax COLLEGE” 1 Visit this link to watch an animation of the events that occur when a hormone binds to a cell membrane receptor. What is the secondary messenger made by adenylyl cyclase during the activation of liver cells by epinephrine? In addition, endocrine signaling is typically less specific than neural signaling. The same hormone may play a role in a variety of different physiological processes depending on the target cells involved. For example, the hormone oxytocin promotes uterine contractions in women in labor. It is also important in breastfeeding, and may be involved in the sexual response and in feelings of emotional attachment in both males and females. In general, the nervous system involves quick responses to rapid changes in the external environment, and the endocrine system is usually slower acting —taking care of the internal environment of the body, maintaining homeostasis, and controlling reproduction ({link]). So how does the fight- or-flight response that was mentioned earlier happen so quickly if hormones are usually slower acting? It is because the two systems are connected. It is the fast action of the nervous system in response to the danger in the environment that stimulates the adrenal glands to secrete their hormones. As a result, the nervous system can cause rapid endocrine responses to keep up with sudden changes in both the external and internal environments when necessary. Endocrine and Nervous Systems Endocrine system Siena Chemical mechanism(s) Primary chemical : Hormones signal Distance traveled Long or short Response time Fast or slow Environment targeted Internal Structures of the Endocrine System Nervous system Chemical/electrical Neurotransmitters Always short Always fast Internal and external The endocrine system consists of cells, tissues, and organs that secrete hormones as a primary or secondary function. The endocrine gland is the major player in this system. The primary function of these ductless glands is to secrete their hormones directly into the surrounding fluid. The interstitial fluid and the blood vessels then transport the hormones throughout the body. The endocrine system includes the pituitary, thyroid, parathyroid, adrenal, and pineal glands ({link]). Some of these glands have both endocrine and non-endocrine functions. For example, the pancreas contains cells that function in digestion as well as cells that secrete the hormones insulin and glucagon, which regulate blood glucose levels. The hypothalamus, thymus, heart, kidneys, stomach, small intestine, liver, skin, female ovaries, and male testes are other organs that contain cells with endocrine function. Moreover, adipose tissue has long been known to produce hormones, and recent research has revealed that even bone tissue has endocrine functions. Endocrine System Pineal gland Thalamus Pituitary gland Thyroid cartilage of the larynx Thyroid gland Thymus Parathyroid glands (on posterior side of thyroid) Trachea Adrenal glands Pancreas Uterus Ovaries (female) Testes (male) Endocrine glands and cells are located throughout the body and play an important role in homeostasis. The ductless endocrine glands are not to be confused with the body’s exocrine system, whose glands release their secretions through ducts. Examples of exocrine glands include the sebaceous and sweat glands of the skin. As just noted, the pancreas also has an exocrine function: most of its cells secrete pancreatic juice through the pancreatic and accessory ducts to the lumen of the small intestine. Other Types of Chemical Signaling In endocrine signaling, hormones secreted into the extracellular fluid diffuse into the blood or lymph, and can then travel great distances throughout the body. In contrast, autocrine signaling takes place within the same cell. An autocrine (auto- = “self”) is a chemical that elicits a response in the same cell that secreted it. Interleukin-1, or IL-1, is a signaling molecule that plays an important role in inflammatory response. The cells that secrete IL-1 have receptors on their cell surface that bind these molecules, resulting in autocrine signaling. Local intercellular communication is the province of the paracrine, also called a paracrine factor, which is a chemical that induces a response in neighboring cells. Although paracrines may enter the bloodstream, their concentration is generally too low to elicit a response from distant tissues. A familiar example to those with asthma is histamine, a paracrine that is released by immune cells in the bronchial tree. Histamine causes the smooth muscle cells of the bronchi to constrict, narrowing the airways. Another example is the neurotransmitters of the nervous system, which act only locally within the synaptic cleft. Note: Career Connections Endocrinologist Endocrinology is a specialty in the field of medicine that focuses on the treatment of endocrine system disorders. Endocrinologists—medical doctors who specialize in this field—are experts in treating diseases associated with hormonal systems, ranging from thyroid disease to diabetes mellitus. Endocrine surgeons treat endocrine disease through the removal, or resection, of the affected endocrine gland. Patients who are referred to endocrinologists may have signs and symptoms or blood test results that suggest excessive or impaired functioning of an endocrine gland or endocrine cells. The endocrinologist may order additional blood tests to determine whether the patient’s hormonal levels are abnormal, or they may stimulate or suppress the function of the suspect endocrine gland and then have blood taken for analysis. Treatment varies according to the diagnosis. Some endocrine disorders, such as type 2 diabetes, may respond to lifestyle changes such as modest weight loss, adoption of a healthy diet, and regular physical activity. Other disorders may require medication, such as hormone replacement, and routine monitoring by the endocrinologist. These include disorders of the pituitary gland that can affect growth and disorders of the thyroid gland that can result in a variety of metabolic problems. Some patients experience health problems as a result of the normal decline in hormones that can accompany aging. These patients can consult with an endocrinologist to weigh the risks and benefits of hormone replacement therapy intended to boost their natural levels of reproductive hormones. In addition to treating patients, endocrinologists may be involved in research to improve the understanding of endocrine system disorders and develop new treatments for these diseases. Chapter Review The endocrine system consists of cells, tissues, and organs that secrete hormones critical to homeostasis. The body coordinates its functions through two major types of communication: neural and endocrine. Neural communication includes both electrical and chemical signaling between neurons and target cells. Endocrine communication involves chemical signaling via the release of hormones into the extracellular fluid. From there, hormones diffuse into the bloodstream and may travel to distant body regions, where they elicit a response in target cells. Endocrine glands are ductless glands that secrete hormones. Many organs of the body with other primary functions—such as the heart, stomach, and kidneys—also have hormone-secreting cells. Interactive Link Questions Exercise: Problem: Visit this link to watch an animation of the events that occur when a hormone binds to a cell membrane receptor. What is the secondary messenger made by adenylyl cyclase during the activation of liver cells by epinephrine? Solution: cAMP Review Questions Exercise: Problem:Endocrine glands a. secrete hormones that travel through a duct to the target organs b. release neurotransmitters into the synaptic cleft c. secrete chemical messengers that travel in the bloodstream d. include sebaceous glands and sweat glands Solution: G Exercise: Problem: Chemical signaling that affects neighboring cells is called a. autocrine b. paracrine c. endocrine d. neuron Solution: B Critical Thinking Questions Exercise: Problem: Describe several main differences in the communication methods used by the endocrine system and the nervous system. Solution: The endocrine system uses chemical signals called hormones to convey information from one part of the body to a distant part of the body. Hormones are released from the endocrine cell into the extracellular environment, but then travel in the bloodstream to target tissues. This communication and response can take seconds to days. In contrast, neurons transmit electrical signals along their axons. At the axon terminal, the electrical signal prompts the release of a chemical signal called a neurotransmitter that carries the message across the synaptic cleft to elicit a response in the neighboring cell. This method of communication is nearly instantaneous, of very brief duration, and is highly specific. Exercise: Problem:Compare and contrast endocrine and exocrine glands. Solution: Endocrine glands are ductless. They release their secretion into the surrounding fluid, from which it enters the bloodstream or lymph to travel to distant cells. Moreover, the secretions of endocrine glands are hormones. Exocrine glands release their secretions through a duct that delivers the secretion to the target location. Moreover, the secretions of exocrine glands are not hormones, but compounds that have an immediate physiologic function. For example, pancreatic juice contains enzymes that help digest food. Exercise: Problem: True or false: Neurotransmitters are a special class of paracrines. Explain your answer. Solution: True. Neurotransmitters can be classified as paracrines because, upon their release from a neuron’s axon terminals, they travel across a microscopically small cleft to exert their effect on a nearby neuron or muscle cell. Glossary autocrine chemical signal that elicits a response in the same cell that secreted it endocrine gland tissue or organ that secretes hormones into the blood and lymph without ducts such that they may be transported to organs distant from the site of secretion endocrine system cells, tissues, and organs that secrete hormones as a primary or secondary function and play an integral role in normal bodily processes exocrine system cells, tissues, and organs that secrete substances directly to target tissues via glandular ducts hormone secretion of an endocrine organ that travels via the bloodstream or lymphatics to induce a response in target cells or tissues in another part of the body paracrine chemical signal that elicits a response in neighboring cells; also called paracrine factor The Pituitary Gland and Hypothalamus By the end of this section, you will be able to: e Explain the interrelationships of the anatomy and functions of the hypothalamus and the posterior and anterior lobes of the pituitary gland e Identify the two hormones released from the posterior pituitary, their target cells, and their principal actions e Identify the six hormones produced by the anterior lobe of the pituitary gland, their target cells, their principal actions, and their regulation by the hypothalamus The hypothalamus-—pituitary complex can be thought of as the “command center” of the endocrine system. This complex secretes several hormones that directly produce responses in target tissues, as well as hormones that regulate the synthesis and secretion of hormones of other glands. In addition, the hypothalamus-pituitary complex coordinates the messages of the endocrine and nervous systems. In many cases, a stimulus received by the nervous system must pass through the hypothalamus-pituitary complex to be translated into hormones that can initiate a response. The hypothalamus is a structure of the diencephalon of the brain located anterior and inferior to the thalamus ({link]). It has both neural and endocrine functions, producing and secreting many hormones. In addition, the hypothalamus is anatomically and functionally related to the pituitary gland (or hypophysis), a bean-sized organ suspended from it by a stem called the infundibulum (or pituitary stalk). The pituitary gland is cradled within the sellaturcica of the sphenoid bone of the skull. It consists of two lobes that arise from distinct parts of embryonic tissue: the posterior pituitary (neurohypophysis) is neural tissue, whereas the anterior pituitary (also known as the adenohypophysis) is glandular tissue that develops from the primitive digestive tract. The hormones secreted by the posterior and anterior pituitary, and the intermediate zone between the lobes are summarized in [link]. Hypothalamus—Pituitary Complex Thalamus Hypothalamus Infundibulum Anterior pituitary Posterior pituitary Se NE The hypothalamus region lies inferior and anterior to the thalamus. It connects to the pituitary gland by the stalk-like infundibulum. The pituitary gland consists of an anterior and posterior lobe, with each lobe secreting different hormones in response to signals from the hypothalamus. Pituitary Hormones Pituitary Associated Chemical lobe hormones class Effect Promotes Growth hormone Anterior (GH) Protein growth of body tissues Pituitary Hormones Pituitary lobe Anterior Anterior Anterior Anterior Anterior Posterior Associated hormones Prolactin (PRL) Thyroid-stimulating hormone (TSH) Adrenocorticotropic hormone (ACTH) Follicle-stimulating hormone (FSH) Luteinizing hormone (LH) Antidiuretic hormone (ADH) Chemical class Peptide Glycoprotein Peptide Glycoprotein Glycoprotein Peptide Effect Promotes milk production from mammary glands Stimulates thyroid hormone release from thyroid Stimulates hormone release by adrenal cortex Stimulates gamete production in gonads Stimulates androgen production by gonads Stimulates water reabsorption by kidneys Pituitary Hormones Pituitary Associated Chemical lobe hormones class Posterior Oxytocin Peptide Melanocyte- Intermediate stimulating Peptide zone hormone Posterior Pituitary Effect Stimulates uterine contractions during childbirth Stimulates melanin formation in melanocytes The posterior pituitary is actually an extension of the neurons of the paraventricular and supraoptic nuclei of the hypothalamus. The cell bodies of these regions rest in the hypothalamus, but their axons descend as the hypothalamic—hypophyseal tract within the infundibulum, and end in axon terminals that comprise the posterior pituitary ([Link]). Posterior Pituitary Neurosecretory cells of Neurosecretory cells of paraventricular nucleus supraoptic nucleus ADH release Hypothalamus Infundibulum Hypothalamohypophyseal tract Posterior pituitary Pituitary Anterior pituitary gland Capillary plexus Oitielease ADH release Neurosecretory cells in the hypothalamus release oxytocin (OT) or ADH into the posterior lobe of the pituitary gland. These hormones are stored or released into the blood via the capillary plexus. The posterior pituitary gland does not produce hormones, but rather stores and secretes hormones produced by the hypothalamus. The paraventricular nuclei produce the hormone oxytocin, whereas the supraoptic nuclei produce ADH. These hormones travel along the axons into storage sites in the axon terminals of the posterior pituitary. In response to signals from the same hypothalamic neurons, the hormones are released from the axon terminals into the bloodstream. Oxytocin When fetal development is complete, the peptide-derived hormone oxytocin (tocia- = “childbirth”) stimulates uterine contractions and dilation of the cervix. Throughout most of pregnancy, oxytocin hormone receptors are not expressed at high levels in the uterus. Toward the end of pregnancy, the synthesis of oxytocin receptors in the uterus increases, and the smooth muscle cells of the uterus become more sensitive to its effects. Oxytocin is continually released throughout childbirth through a positive feedback mechanism. As noted earlier, oxytocin prompts uterine contractions that push the fetal head toward the cervix. In response, cervical stretching stimulates additional oxytocin to be synthesized by the hypothalamus and released from the pituitary. This increases the intensity and effectiveness of uterine contractions and prompts additional dilation of the cervix. The feedback loop continues until birth. Although the mother’s high blood levels of oxytocin begin to decrease immediately following birth, oxytocin continues to play a role in maternal and newborn health. First, oxytocin is necessary for the milk ejection reflex (commonly referred to as “let-down”) in breastfeeding women. As the newborn begins suckling, sensory receptors in the nipples transmit signals to the hypothalamus. In response, oxytocin is secreted and released into the bloodstream. Within seconds, cells in the mother’s milk ducts contract, ejecting milk into the infant’s mouth. Secondly, in both males and females, oxytocin is thought to contribute to parent-newborn bonding, known as attachment. Oxytocin is also thought to be involved in feelings of love and closeness, as well as in the sexual response. Antidiuretic Hormone (ADH) The solute concentration of the blood, or blood osmolarity, may change in response to the consumption of certain foods and fluids, as well as in response to disease, injury, medications, or other factors. Blood osmolarity is constantly monitored by osmoreceptors—specialized cells within the hypothalamus that are particularly sensitive to the concentration of sodium ions and other solutes. In response to high blood osmolarity, which can occur during dehydration or following a very salty meal, the osmoreceptors signal the posterior pituitary to release antidiuretic hormone (ADH). The target cells of ADH are located in the tubular cells of the kidneys. Its effect is to increase epithelial permeability to water, allowing increased water reabsorption. The more water reabsorbed from the filtrate, the greater the amount of water that is returned to the blood and the less that is excreted in the urine. A greater concentration of water results in a reduced concentration of solutes. ADH is also known as vasopressin because, in very high concentrations, it causes constriction of blood vessels, which increases blood pressure by increasing peripheral resistance. The release of ADH is controlled by a negative feedback loop. As blood osmolarity decreases, the hypothalamic osmoreceptors sense the change and prompt a corresponding decrease in the secretion of ADH. As a result, less water is reabsorbed from the urine filtrate. Interestingly, drugs can affect the secretion of ADH. For example, alcohol consumption inhibits the release of ADH, resulting in increased urine production that can eventually lead to dehydration and a hangover. A disease called diabetes insipidus is characterized by chronic underproduction of ADH that causes chronic dehydration. Because little ADH is produced and secreted, not enough water is reabsorbed by the kidneys. Although patients feel thirsty, and increase their fluid consumption, this doesn’t effectively decrease the solute concentration in their blood because ADH levels are not high enough to trigger water reabsorption in the kidneys. Electrolyte imbalances can occur in severe cases of diabetes insipidus. Anterior Pituitary The anterior pituitary originates from the digestive tract in the embryo and migrates toward the brain during fetal development. There are three regions: the pars distalis is the most anterior, the pars intermedia is adjacent to the posterior pituitary, and the pars tuberalis is a slender “tube” that wraps the infundibulum. Recall that the posterior pituitary does not synthesize hormones, but merely stores them. In contrast, the anterior pituitary does manufacture hormones. However, the secretion of hormones from the anterior pituitary is regulated by two classes of hormones. These hormones—secreted by the hypothalamus—are the releasing hormones that stimulate the secretion of hormones from the anterior pituitary and the inhibiting hormones that inhibit secretion. Hypothalamic hormones are secreted by neurons, but enter the anterior pituitary through blood vessels ([{link]). Within the infundibulum is a bridge of capillaries that connects the hypothalamus to the anterior pituitary. This network, called the hypophyseal portal system, allows hypothalamic hormones to be transported to the anterior pituitary without first entering the systemic circulation. The system originates from the superior hypophyseal artery, which branches off the carotid arteries and transports blood to the hypothalamus. The branches of the superior hypophyseal artery form the hypophyseal portal system (see [link]). Hypothalamic releasing and inhibiting hormones travel through a primary capillary plexus to the portal veins, which carry them into the anterior pituitary. Hormones produced by the anterior pituitary (in response to releasing hormones) enter a secondary capillary plexus, and from there drain into the circulation. Anterior Pituitary @) Hypothalamus releases hormone Superior Hypothalamus hypophyseal Neurosecretory cells Infundibulum : WSs Primary capillary Hypophyseal y plexus of hypophyseal portal veins r portal system Posterior pituitary Anterior Pituitary gland Secondary capillary plexus of hypophyseal portal system @) Anterior pituitary @) Hypothalamus hormone stimulates hormone pituitary to release hormones The anterior pituitary manufactures seven hormones. The hypothalamus produces separate hormones that stimulate or inhibit hormone production in the anterior pituitary. Hormones from the hypothalamus reach the anterior pituitary via the hypophyseal portal system. The anterior pituitary produces seven hormones. These are the growth hormone (GH), thyroid-stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), follicle-stimulating hormone (FSH), luteinizing hormone (LH), beta endorphin, and prolactin. Of the hormones of the anterior pituitary, TSH, ACTH, FSH, and LH are collectively referred to as tropic hormones (trope- = “turning”) because they turn on or off the function of other endocrine glands. Growth Hormone The endocrine system regulates the growth of the human body, protein synthesis, and cellular replication. A major hormone involved in this process is growth hormone (GH), also called somatotropin—a protein hormone produced and secreted by the anterior pituitary gland. Its primary function is anabolic; it promotes protein synthesis and tissue building through direct and indirect mechanisms ([link]). GH levels are controlled by the release of GHRH and GHIH (also known as somatostatin) from the hypothalamus. Hormonal Regulation of Growth 1) Release of growth hormone: GHRH release 3) Inhibition of growth hormone: GHIH release ¢ Hypothalamus releases * High IGF-1 levels perceived by growth hormone-releasing hypothalamus hormone (GHRH) ra fy * Growth hormone-—inhibiting ¢ GHRH stimulates the anterior we YY hormone (GHIH) is released to pituitary to release growth YY inhibit GH release hormone (GH) * GHIH inhibits GH release in the anterior pituitary GH release Sa GH }Gase 2a) Glucose-sparing effect: 2b) Growth effects: 2c) Diabetogenic effect: ¢ Stimulates adipose cells ¢ Increases uptake of amino * GH stimulates liver to to break down stored fat, acids from the blood break down glycogen fueling growth effects ¢ Enhances cellular proliferation into glucose, fueling and reduces apoptosis growth effects Targets: Liver releases IGF-1, further Adipose cells stimulating growth effects a oe” Bone cells Muscle cells IGF-1 release , Nervous system ’ cells Immune system cells Growth hormone (GH) directly accelerates the rate of protein synthesis in skeletal muscle and bones. Insulin-like growth factor 1 (IGF-1) is activated by growth hormone and indirectly supports the formation of new proteins in muscle cells and bone. A glucose-sparing effect occurs when GH stimulates lipolysis, or the breakdown of adipose tissue, releasing fatty acids into the blood. As a result, many tissues switch from glucose to fatty acids as their main energy source, which means that less glucose is taken up from the bloodstream. GH also initiates the diabetogenic effect in which GH stimulates the liver to break down glycogen to glucose, which is then deposited into the blood. The name “diabetogenic” is derived from the similarity in elevated blood glucose levels observed between individuals with untreated diabetes mellitus and individuals experiencing GH excess. Blood glucose levels rise as the result of a combination of glucose-sparing and diabetogenic effects. GH indirectly mediates growth and protein synthesis by triggering the liver and other tissues to produce a group of proteins called insulin-like growth factors (IGFs). These proteins enhance cellular proliferation and inhibit apoptosis, or programmed cell death. IGFs stimulate cells to increase their uptake of amino acids from the blood for protein synthesis. Skeletal muscle and cartilage cells are particularly sensitive to stimulation from IGFs. Dysfunction of the endocrine system’s control of growth can result in several disorders. For example, gigantism is a disorder in children that is caused by the secretion of abnormally large amounts of GH, resulting in excessive growth. A similar condition in adults is acromegaly, a disorder that results in the growth of bones in the face, hands, and feet in response to excessive levels of GH in individuals who have stopped growing. Abnormally low levels of GH in children can cause growth impairment—a disorder called pituitary dwarfism (also known as growth hormone deficiency). Thyroid-Stimulating Hormone The activity of the thyroid gland is regulated by thyroid-stimulating hormone (TSH), also called thyrotropin. TSH is released from the anterior pituitary in response to thyrotropin-releasing hormone (TRH) from the hypothalamus. As discussed shortly, it triggers the secretion of thyroid hormones by the thyroid gland. In a classic negative feedback loop, elevated levels of thyroid hormones in the bloodstream then trigger a drop in production of TRH and subsequently SEL. Adrenocorticotropic Hormone The adrenocorticotropic hormone (ACTH), also called corticotropin, stimulates the adrenal cortex (the more superficial “bark” of the adrenal glands) to secrete corticosteroid hormones such as cortisol. ACTH come from a precursor molecule known as pro-opiomelanotropin (POMC) which produces several biologically active molecules when cleaved, including ACTH, melanocyte-stimulating hormone, and the brain opioid peptides known as endorphins. The release of ACTH is regulated by the corticotropin-releasing hormone (CRH) from the hypothalamus in response to normal physiologic rhythms. A variety of stressors can also influence its release, and the role of ACTH in the stress response is discussed later in this chapter. Follicle-Stimulating Hormone and Luteinizing Hormone The endocrine glands secrete a variety of hormones that control the development and regulation of the reproductive system (these glands include the anterior pituitary, the adrenal cortex, and the gonads—the testes in males and the ovaries in females). Much of the development of the reproductive system occurs during puberty and is marked by the development of sex-specific characteristics in both male and female adolescents. Puberty is initiated by gonadotropin-releasing hormone (GnRH), a hormone produced and secreted by the hypothalamus. GnRH stimulates the anterior pituitary to secrete gonadotropins—hormones that regulate the function of the gonads. The levels of GnRH are regulated through a negative feedback loop; high levels of reproductive hormones inhibit the release of GnRH. Throughout life, gonadotropins regulate reproductive function and, in the case of women, the onset and cessation of reproductive capacity. The gonadotropins include two glycoprotein hormones: follicle-stimulating hormone (FSH) stimulates the production and maturation of sex cells, or gametes, including ova in women and sperm in men. FSH also promotes follicular growth; these follicles then release estrogens in the female ovaries. Luteinizing hormone (LH) triggers ovulation in women, as well as the production of estrogens and progesterone by the ovaries. LH stimulates production of testosterone by the male testes. Prolactin As its name implies, prolactin (PRL) promotes lactation (milk production) in women. During pregnancy, it contributes to development of the mammary glands, and after birth, it stimulates the mammary glands to produce breast milk. However, the effects of prolactin depend heavily upon the permissive effects of estrogens, progesterone, and other hormones. And as noted earlier, the let-down of milk occurs in response to stimulation from oxytocin. In a non-pregnant woman, prolactin secretion is inhibited by prolactin- inhibiting hormone (PIH), which is actually the neurotransmitter dopamine, and is released from neurons in the hypothalamus. Only during pregnancy do prolactin levels rise in response to prolactin-releasing hormone (PRH) from the hypothalamus. Intermediate Pituitary: Melanocyte-Stimulating Hormone The cells in the zone between the pituitary lobes secrete a hormone known as melanocyte-stimulating hormone (MSH) that is formed by cleavage of the pro- opiomelanocortin (POMC) precursor protein. Local production of MSH in the skin is responsible for melanin production in response to UV light exposure. The role of MSH made by the pituitary is more complicated. For instance, people with lighter skin generally have the same amount of MSH as people with darker skin. Nevertheless, this hormone is capable of darkening of the skin by inducing melanin production in the skin’s melanocytes. Women also show increased MSH production during pregnancy; in combination with estrogens, it can lead to darker skin pigmentation, especially the skin of the areolas and labia minora. [link] is a summary of the pituitary hormones and their principal effects. Major Pituitary Hormones Posterior Pituitary Hormones Releasing hormone Pituitary (hypothalamus) hormone Target Effects ADH Stores ————» Kidneys, ——_ Water balance ADH sweat glands, circulatory system - OT ——+» Female ——~ Triggers uterine reproductive contractions during system childbirth Anterior Pituitary Hormones Releasing hormone Pituitary (hypothalamus) hormone Target Effects GnRH = ——e» LH ——+* Reproductive ———® Stimulates production system of sex hormones by gonads GnRH ——» FSH ——® Ffeproductive ———» Stimulates production system of sperm and eggs TRH —> TSH — Thyroid gland ———* Stimulates the release of thyroid hormone (TH). TH regulates metabolism. PRH —> +~=PRL — Mammary ——_ Promotes milk (inhibited glands production by PIH) GHRH mY GH — Liver,bone, ——— Induces targets to (inhibited muscles produce insulin-like by GHIH) growth factors (IGF). IGFs stimulate body growth and a higher metabolic rate. CRH ——» ACTH ——+* Adrenal ——> Induces targets to glands produce glucocorticoids, which regulate metabolism and the stress response Major pituitary hormones and their target organs. Note: Sch Visit this link to watch an animation showing the role of the hypothalamus and the pituitary gland. Which hormone is released by the pituitary to stimulate the thyroid gland? Chapter Review The hypothalamus—pituitary complex is located in the diencephalon of the brain. The hypothalamus and the pituitary gland are connected by a structure called the infundibulum, which contains vasculature and nerve axons. The pituitary gland is divided into two distinct structures with different embryonic origins. The posterior lobe houses the axon terminals of hypothalamic neurons. It stores and releases into the bloodstream two hypothalamic hormones: oxytocin and antidiuretic hormone (ADH). The anterior lobe is connected to the hypothalamus by vasculature in the infundibulum and produces and secretes six hormones. Their secretion is regulated, however, by releasing and inhibiting hormones from the hypothalamus. The six anterior pituitary hormones are: growth hormone (GH), thyroid-stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), follicle-stimulating hormone (FSH), luteinizing hormone (LH), and prolactin (PRL). Interactive Link Questions Exercise: Problem: Visit this link to watch an animation showing the role of the hypothalamus and the pituitary gland. Which hormone is released by the pituitary to stimulate the thyroid gland? Solution: Thyroid-stimulating hormone. Review Questions Exercise: Problem: The hypothalamus is functionally and anatomically connected to the posterior pituitary lobe by a bridge of a. blood vessels b. nerve axons c. cartilage d. bone Solution: B Exercise: Problem: Which of the following is an anterior pituitary hormone? a. ADH b. oxytocin c. TSH d. cortisol Solution: C Exercise: Problem:How many hormones are produced by the posterior pituitary? a. O ans DMN Solution: A Exercise: Problem: Which of the following hormones contributes to the regulation of the body’s fluid and electrolyte balance? a. adrenocorticotropic hormone b. antidiuretic hormone c. luteinizing hormone d. all of the above Solution: B Critical Thinking Questions Exercise: Problem: Compare and contrast the anatomical relationship of the anterior and posterior lobes of the pituitary gland to the hypothalamus. Solution: The anterior lobe of the pituitary gland is connected to the hypothalamus by vasculature, which allows regulating hormones from the hypothalamus to travel to the anterior pituitary. In contrast, the posterior lobe is connected to the hypothalamus by a bridge of nerve axons called the hypothalamic—hypophyseal tract, along which the hypothalamus sends hormones produced by hypothalamic nerve cell bodies to the posterior pituitary for storage and release into the circulation. Exercise: Problem: Name the target tissues for prolactin. Solution: The mammary glands are the target tissues for prolactin. Glossary acromegaly disorder in adults caused when abnormally high levels of GH trigger growth of bones in the face, hands, and feet adrenocorticotropic hormone (ACTH) anterior pituitary hormone that stimulates the adrenal cortex to secrete corticosteroid hormones (also called corticotropin) antidiuretic hormone (ADH) hypothalamic hormone that is stored by the posterior pituitary and that signals the kidneys to reabsorb water follicle-stimulating hormone (FSH) anterior pituitary hormone that stimulates the production and maturation of sex cells gigantism disorder in children caused when abnormally high levels of GH prompt excessive growth gonadotropins hormones that regulate the function of the gonads growth hormone (GH) anterior pituitary hormone that promotes tissue building and influences nutrient metabolism (also called somatotropin) hypophyseal portal system network of blood vessels that enables hypothalamic hormones to travel into the anterior lobe of the pituitary without entering the systemic circulation hypothalamus region of the diencephalon inferior to the thalamus that functions in neural and endocrine signaling infundibulum stalk containing vasculature and neural tissue that connects the pituitary gland to the hypothalamus (also called the pituitary stalk) insulin-like growth factors (IGF) protein that enhances cellular proliferation, inhibits apoptosis, and stimulates the cellular uptake of amino acids for protein synthesis luteinizing hormone (LH) anterior pituitary hormone that triggers ovulation and the production of ovarian hormones in females, and the production of testosterone in males osmoreceptor hypothalamic sensory receptor that is stimulated by changes in solute concentration (osmotic pressure) in the blood oxytocin hypothalamic hormone stored in the posterior pituitary gland and important in stimulating uterine contractions in labor, milk ejection during breastfeeding, and feelings of attachment (also produced in males) pituitary dwarfism disorder in children caused when abnormally low levels of GH result in growth retardation pituitary gland bean-sized organ suspended from the hypothalamus that produces, stores, and secretes hormones in response to hypothalamic stimulation (also called hypophysis) prolactin (PRL) anterior pituitary hormone that promotes development of the mammary glands and the production of breast milk thyroid-stimulating hormone (TSH) anterior pituitary hormone that triggers secretion of thyroid hormones by the thyroid gland (also called thyrotropin) The Thyroid Gland By the end of this section, you will be able to: ¢ Describe the location and anatomy of the thyroid gland e Discuss the synthesis of triiodothyronine and thyroxine e Explain the role of thyroid hormones in the regulation of basal metabolism e Identify the hormone produced by the parafollicular cells of the thyroid A butterfly-shaped organ, the thyroid gland is located anterior to the trachea, just inferior to the larynx ([link]). The medial region, called the isthmus, is flanked by wing-shaped left and right lobes. Each of the thyroid lobes are embedded with parathyroid glands, primarily on their posterior surfaces. The tissue of the thyroid gland is composed mostly of thyroid follicles. The follicles are made up of a central cavity filled with a sticky fluid called colloid. Surrounded by a wall of epithelial follicle cells, the colloid is the center of thyroid hormone production, and that production is dependent on the hormones’ essential and unique component: iodine. Thyroid Gland Hyoid bone Thyroid cartilage Superior thyroid artery Isthmus of the thyroid Common carotid arteries Trachea Hyoid bone Thyroid cartilage Cricoid cartilage Right parathyroid Left parathyroid glands glands Left inferior thyroid artery From left subclavian _, * From right subclavian artery artery b) Posterior view Right inferior thyroid artery Parafollicular cell Colloid-containing follicle Follicle cells (cuboidal epithelium) c) Thyroid follicle cells The thyroid gland is located in the neck where it wraps around the trachea. (a) Anterior view of the thyroid gland. (b) Posterior view of the thyroid gland. (c) The glandular tissue is composed primarily of thyroid follicles. The larger parafollicular cells often appear within the matrix of follicle cells. LM x 1332. (Micrograph provided by the Regents of University of Michigan Medical School © 2012) Synthesis and Release of Thyroid Hormones Hormones are produced in the colloid when atoms of the mineral iodine attach to a glycoprotein, called thyroglobulin, that is secreted into the colloid by the follicle cells. The following steps outline the hormones’ assembly: 1. Binding of TSH to its receptors in the follicle cells of the thyroid gland causes the cells to actively transport iodide ions (I-) across their cell membrane, from the bloodstream into the cytosol. As a result, the concentration of iodide ions “trapped” in the follicular cells is many times higher than the concentration in the bloodstream. 2. Iodide ions then move to the lumen of the follicle cells that border the colloid. There, the ions undergo oxidation (their negatively charged electrons are removed). The oxidation of two iodide ions (2 I") results in iodine (Ip), which passes through the follicle cell membrane into the colloid. 3. In the colloid, peroxidase enzymes link the iodine to the tyrosine amino acids in thyroglobulin to produce two intermediaries: a tyrosine attached to one iodine and a tyrosine attached to two iodines. When one of each of these intermediaries is linked by covalent bonds, the resulting compound is triiodothyronine (T3), a thyroid hormone with three iodines. Much more commonly, two copies of the second intermediary bond, forming tetraiodothyronine, also known as thyroxine (T,), a thyroid hormone with four iodines. These hormones remain in the colloid center of the thyroid follicles until TSH stimulates endocytosis of colloid back into the follicle cells. There, lysosomal enzymes break apart the thyroglobulin colloid, releasing free T3 and Ty, which diffuse across the follicle cell membrane and enter the bloodstream. In the bloodstream, less than one percent of the circulating T3 and T, remains unbound. This free T3 and T, can cross the lipid bilayer of cell membranes and be taken up by cells. The remaining 99 percent of circulating T3 and Ty is bound to specialized transport proteins called thyroxine-binding globulins (TBGs), to albumin, or to other plasma proteins. This “packaging” prevents their free diffusion into body cells. When blood levels of T3 and Ty begin to decline, bound T3 and Ty are released from these plasma proteins and readily cross the membrane of target cells. T3 is more potent than Ty, and many cells convert T, to T3 through the removal of an iodine atom. Regulation of TH Synthesis The release of T3 and T, from the thyroid gland is regulated by thyroid- stimulating hormone (TSH). As shown in [link], low blood levels of T3 and T, stimulate the release of thyrotropin-releasing hormone (TRH) from the hypothalamus, which triggers secretion of TSH from the anterior pituitary. In turn, TSH stimulates the thyroid gland to secrete T3 and Ty. The levels of TRH, TSH, T3, and Ty are regulated by a negative feedback system in which increasing levels of T3 and T, decrease the production and secretion or TSH, Classic Negative Feedback Loop 1) Metabolic rate and/or T; and T,4 concentration in blood... Low? * Hypothalamus releases TRH. : This triggers TSH release by High? the pituitary. * Hypothalamus stops TRH 4) Negative feedback: <—— release ¢ Elevated T3 and T, levels inhibit release of TRH and TSH —— ¢ Anterior pituitary stops TSH release Thyroid follicle T, release T, release 3) Effects of T, and T, release: ¢ Increased basal metabolic 2) Effects of TSH release: rate of body cells * Triggers release of T3 and T4 * Rise in body temperature by thyroid follicle cells (calorigenic effect) A classic negative feedback loop controls the regulation of thyroid hormone levels. Functions of Thyroid Hormones The thyroid hormones, T3 and Ty, are often referred to as metabolic hormones because their levels influence the body’s basal metabolic rate, the amount of energy used by the body at rest. When T3 and Ty bind to intracellular receptors located on the mitochondria, they cause an increase in nutrient breakdown and the use of oxygen to produce ATP. In addition, T3 and T, initiate the transcription of genes involved in glucose oxidation. Although these mechanisms prompt cells to produce more ATP, the process is inefficient, and an abnormally increased level of heat is released as a byproduct of these reactions. This so-called calorigenic effect (calor- = “heat”) raises body temperature. Adequate levels of thyroid hormones are also required for protein synthesis and for fetal and childhood tissue development and growth. They are especially critical for normal development of the nervous system both in utero and in early childhood, and they continue to support neurological function in adults. As noted earlier, these thyroid hormones have a complex interrelationship with reproductive hormones, and deficiencies can influence libido, fertility, and other aspects of reproductive function. Finally, thyroid hormones increase the body’s sensitivity to catecholamines (epinephrine and norepinephrine) from the adrenal medulla by upregulation of receptors in the blood vessels. When levels of T3 and T, hormones are excessive, this effect accelerates the heart rate, strengthens the heartbeat, and increases blood pressure. Because thyroid hormones regulate metabolism, heat production, protein synthesis, and many other body functions, thyroid disorders can have severe and widespread consequences. Note: Disorders of the... Endocrine System: Iodine Deficiency, Hypothyroidism, and Hyperthyroidism As discussed above, dietary iodine is required for the synthesis of T3 and Ty. But for much of the world’s population, foods do not provide adequate levels of this mineral, because the amount varies according to the level in the soil in which the food was grown, as well as the irrigation and fertilizers used. Marine fish and shrimp tend to have high levels because they concentrate iodine from seawater, but many people in landlocked regions lack access to seafood. Thus, the primary source of dietary iodine in many countries is iodized salt. Fortification of salt with iodine began in the United States in 1924, and international efforts to iodize salt in the world’s poorest nations continue today. Dietary iodine deficiency can result in the impaired ability to synthesize T3 and Ty, leading to a variety of severe disorders. When T3 and T,4 cannot be produced, TSH is secreted in increasing amounts. As a result of this hyperstimulation, thyroglobulin accumulates in the thyroid gland follicles, increasing their deposits of colloid. The accumulation of colloid increases the overall size of the thyroid gland, a condition called a goiter ([link]). A goiter is only a visible indication of the deficiency. Other iodine deficiency disorders include impaired growth and development, decreased fertility, and prenatal and infant death. Moreover, iodine deficiency is the primary cause of preventable mental retardation worldwide. Neonatal hypothyroidism (cretinism) is characterized by cognitive deficits, short stature, and sometimes deafness and muteness in children and adults born to mothers who were iodine-deficient during pregnancy. Goiter (credit: “Almazi”/Wikimedia Commons) In areas of the world with access to iodized salt, dietary deficiency is rare. Instead, inflammation of the thyroid gland is the more common cause of low blood levels of thyroid hormones. Called hypothyroidism, the condition is characterized by a low metabolic rate, weight gain, cold extremities, constipation, reduced libido, menstrual irregularities, and reduced mental activity. In contrast, hyperthyroidism—an abnormally elevated blood level of thyroid hormones—is often caused by a pituitary or thyroid tumor. In Graves’ disease, the hyperthyroid state results from an autoimmune reaction in which antibodies overstimulate the follicle cells of the thyroid gland. Hyperthyroidism can lead to an increased metabolic rate, excessive body heat and sweating, diarrhea, weight loss, tremors, and increased heart rate. The person’s eyes may bulge (called exophthalmos) as antibodies produce inflammation in the soft tissues of the orbits. The person may also develop a goiter. Calcitonin The thyroid gland also secretes a hormone called calcitonin that is produced by the parafollicular cells (also called C cells) that stud the tissue between distinct follicles. Calcitonin is released in response to a rise in blood calcium levels. It appears to have a function in decreasing blood calcium concentrations by: e Inhibiting the activity of osteoclasts, bone cells that release calcium into the circulation by degrading bone matrix e Increasing osteoblastic activity e Decreasing calcium absorption in the intestines e Increasing calcium loss in the urine However, these functions are usually not significant in maintaining calcium homeostasis, so the importance of calcitonin is not entirely understood. Pharmaceutical preparations of calcitonin are sometimes prescribed to reduce osteoclast activity in people with osteoporosis and to reduce the degradation of cartilage in people with osteoarthritis. The hormones secreted by thyroid are summarized in [link]. Thyroid Hormones Chemical Associated hormones class Effect Thyroxine (Ta), melee Stimulate basal triiodothyronine (T3) metabolic rate 2+ Calcitonin Peptide Reduces blood Ca levels Of course, calcium is critical for many other biological processes. It is a second messenger in many signaling pathways, and is essential for muscle contraction, nerve impulse transmission, and blood clotting. Given these roles, it is not surprising that blood calcium levels are tightly regulated by the endocrine system. The organs involved in the regulation are the parathyroid glands. Chapter Review The thyroid gland is a butterfly-shaped organ located in the neck anterior to the trachea. Its hormones regulate basal metabolism, oxygen use, nutrient metabolism, the production of ATP, and calcium homeostasis. They also contribute to protein synthesis and the normal growth and development of body tissues, including maturation of the nervous system, and they increase the body’s sensitivity to catecholamines. The thyroid hormones triiodothyronine (T3) and thyroxine (T4) are produced and secreted by the thyroid gland in response to thyroid-stimulating hormone (TSH) from the anterior pituitary. Synthesis of the amino acid—derived T3 and T, hormones requires iodine. Insufficient amounts of iodine in the diet can lead to goiter, cretinism, and many other disorders. Review Questions Exercise: Problem: Which of the following statements about the thyroid gland is true? a. It is located anterior to the trachea and inferior to the larynx. b. The parathyroid glands are embedded within it. c. It manufactures three hormones. d. all of the above Solution: D Exercise: Problem: The secretion of thyroid hormones is controlled by a. TSH from the hypothalamus b. TSH from the anterior pituitary c. thyroxine from the anterior pituitary d. thyroglobulin from the thyroid’s parafollicular cells Solution: B Exercise: Problem:The development of a goiter indicates that a. the anterior pituitary is abnormally enlarged b. there is hypertrophy of the thyroid’s follicle cells c. there is an excessive accumulation of colloid in the thyroid follicles d. the anterior pituitary is secreting excessive growth hormone Solution: C Exercise: Problem: Iodide ions cross from the bloodstream into follicle cells via a. simple diffusion b. facilitated diffusion c. active transport d. osmosis Solution: C Critical Thinking Questions Exercise: Problem: Explain why maternal iodine deficiency might lead to neurological impairment in the fetus. Solution: Iodine deficiency in a pregnant woman would also deprive the fetus. Iodine is required for the synthesis of thyroid hormones, which contribute to fetal growth and development, including maturation of the nervous system. Insufficient amounts would impair these functions. Exercise: Problem: Define hyperthyroidism and explain why one of its symptoms is weight loss. Solution: Hyperthyroidism is an abnormally elevated blood level of thyroid hormones due to an overproduction of T3 and T,. An individual with hyperthyroidism is likely to lose weight because one of the primary roles of thyroid hormones is to increase the body’s basal metabolic rate, increasing the breakdown of nutrients and the production of ATP. Glossary calcitonin peptide hormone produced and secreted by the parafollicular cells (C cells) of the thyroid gland that functions to decrease blood calcium levels colloid viscous fluid in the central cavity of thyroid follicles, containing the glycoprotein thyroglobulin goiter enlargement of the thyroid gland either as a result of iodine deficiency or hyperthyroidism hyperthyroidism clinically abnormal, elevated level of thyroid hormone in the blood; characterized by an increased metabolic rate, excess body heat, sweating, diarrhea, weight loss, and increased heart rate hypothyroidism clinically abnormal, low level of thyroid hormone in the blood; characterized by low metabolic rate, weight gain, cold extremities, constipation, and reduced mental activity neonatal hypothyroidism condition characterized by cognitive deficits, short stature, and other signs and symptoms in people born to women who were iodine- deficient during pregnancy thyroid gland large endocrine gland responsible for the synthesis of thyroid hormones thyroxine (also, tetraiodothyronine, T4) amino acid—derived thyroid hormone that is more abundant but less potent than T3 and often converted to T3 by target cells triiodothyronine (also, T3) amino acid—derived thyroid hormone that is less abundant but more potent than T, The Parathyroid Glands By the end of this section, you will be able to: ¢ Describe the location and structure of the parathyroid glands e Describe the hormonal control of blood calcium levels e Discuss the physiological response of parathyroid dysfunction The parathyroid glands are tiny, round structures usually found embedded in the posterior surface of the thyroid gland ([link]). A thick connective tissue capsule separates the glands from the thyroid tissue. Most people have four parathyroid glands, but occasionally there are more in tissues of the neck or chest. The function of one type of parathyroid cells, the oxyphil cells, is not clear. The primary functional cells of the parathyroid glands are the chief cells. These epithelial cells produce and secrete the parathyroid hormone (PTH), the major hormone involved in the regulation of blood calcium levels. Parathyroid Glands pa Hyoid bone oa Ly Thyroid y| 4 cartilage Oxyphil cells Cricoid i Blood vessel cartilage - Wit P 7 ; y: OT ed. Oe = 1 Parathyroid parathyroid parathyroid glands glands a) Thyroid gland, posterior view b) Micrograph of parathyroid tissue The small parathyroid glands are embedded in the posterior surface of the thyroid gland. LM x 760. (Micrograph provided by the Regents of University of Michigan Medical School © 2012) Note: mss" OPENStax COLLEGE F : ca [aS View the University of Michigan WebScope to explore the tissue sample in greater detail. The parathyroid glands produce and secrete PTH, a peptide hormone, in response to low blood calcium levels ({link]). PTH secretion causes the release of calcium from the bones by stimulating osteoclasts, which secrete enzymes that degrade bone and release calcium into the interstitial fluid. PTH also inhibits osteoblasts, the cells involved in bone deposition, thereby sparing blood calcium. PTH causes increased reabsorption of calcium (and magnesium) in the kidney tubules from the urine filtrate. In addition, PTH initiates the production of the steroid hormone calcitriol (also known as 1,25-dihydroxyvitamin D), which is the active form of vitamin Ds, in the kidneys. Calcitriol then stimulates increased absorption of dietary calcium by the intestines. A negative feedback loop regulates the levels of PTH, with rising blood calcium levels inhibiting further release of PTH. Parathyroid Hormone in Maintaining Blood Calcium Homeostasis \ 2s Osteoclasts Ss : 1) Blood calcium Compact bone concentration drops 6) Effects of calcitonin on bone: + Stimulates osteoblasts + Inhibits osteoclasts + Calcium is removed from blood and used to build bone 2) Release of PTH: 5) Calcitonin release: + Chief cells of the parathyroid gland release parathyroid hormone (PTH). + High concentrations of calcium stimulate parafollicular cells in the thyroid to release calcitonin. i 4 C ¥ rr 4 2 TA EAT vy om 4) Blood calcium levels increase 3a) Effects of PTH on bone: 3b) Effects of PTH on kidneys: 3c) Effects of calcitriol on intestine: + Inhibits osteoblasts + PTH stimulates kidney tubule cells to + Stimulates intestines to absorb + Stimulates osteoclasts recover waste calcium from the urine. calcium from digesting food + Bone is broken down, releasing + PTH stimulates kidney tubule cells calcium ions into bloodstream to release calcitriol. i at Intestinal lumen Kidney _ Interstitial Urine tubule cells fluid + Food — . Intestinal cells Compact / a A bone = | Intestinal IF - : connective tissue Osteoclasts = | with blood supply PT Osteoblasts Parathyroid hormone increases blood calcium levels when they drop too low. Conversely, calcitonin, which is released from the thyroid gland, decreases blood calcium levels when they become too high. These two mechanisms constantly maintain blood calcium concentration at homeostasis. Abnormally high activity of the parathyroid gland can cause hyperparathyroidism, a disorder caused by an overproduction of PTH that results in excessive calcium reabsorption from bone. Hyperparathyroidism can significantly decrease bone density, leading to spontaneous fractures or deformities. As blood calcium levels rise, cell membrane permeability to sodium is decreased, and the responsiveness of the nervous system is reduced. At the same time, calcium deposits may collect in the body’s tissues and organs, impairing their functioning. In contrast, abnormally low blood calcium levels may be caused by parathyroid hormone deficiency, called hypoparathyroidism, which may develop following injury or surgery involving the thyroid gland. Low blood calcium increases membrane permeability to sodium, resulting in muscle twitching, cramping, spasms, or convulsions. Severe deficits can paralyze muscles, including those involved in breathing, and can be fatal. When blood calcium levels are high, calcitonin is produced and secreted by the parafollicular cells of the thyroid gland. As discussed earlier, calcitonin inhibits the activity of osteoclasts, reduces the absorption of dietary calcium in the intestine, and signals the kidneys to reabsorb less calcium, resulting in larger amounts of calcium excreted in the urine. Chapter Review Calcium is required for a variety of important physiologic processes, including neuromuscular functioning; thus, blood calcium levels are closely regulated. The parathyroid glands are small structures located on the posterior thyroid gland that produce parathyroid hormone (PTH), which regulates blood calcium levels. Low blood calcium levels cause the production and secretion of PTH. In contrast, elevated blood calcium levels inhibit secretion of PTH and trigger secretion of the thyroid hormone calcitonin. Underproduction of PTH can result in hypoparathyroidism. In contrast, overproduction of PTH can result in hyperparathyroidism. Review Questions Exercise: Problem: When blood calcium levels are low, PTH stimulates a. urinary excretion of calcium by the kidneys b. a reduction in calcium absorption from the intestines c. the activity of osteoblasts d. the activity of osteoclasts Solution: D Exercise: Problem: Which of the following can result from hyperparathyroidism? a. increased bone deposition b. fractures c. convulsions d. all of the above Solution: B Critical Thinking Questions Exercise: Problem: Describe the role of negative feedback in the function of the parathyroid gland. Solution: The production and secretion of PTH is regulated by a negative feedback loop. Low blood calcium levels initiate the production and secretion of PTH. PTH increases bone resorption, calcium absorption from the intestines, and calcium reabsorption by the kidneys. As a result, blood calcium levels begin to rise. This, in turn, inhibits the further production and secretion of PTH. Exercise: Problem: Explain why someone with a parathyroid gland tumor might develop kidney stones. Solution: A parathyroid gland tumor can prompt hypersecretion of PTH. This can raise blood calcium levels so excessively that calcium deposits begin to accumulate throughout the body, including in the kidney tubules, where they are referred to as kidney stones. Glossary hyperparathyroidism disorder caused by overproduction of PTH that results in abnormally elevated blood calcium hypoparathyroidism disorder caused by underproduction of PTH that results in abnormally low blood calcium parathyroid glands small, round glands embedded in the posterior thyroid gland that produce parathyroid hormone (PTH) parathyroid hormone (PTH) peptide hormone produced and secreted by the parathyroid glands in response to low blood calcium levels The Adrenal Glands By the end of this section, you will be able to: ¢ Describe the location and structure of the adrenal glands e Identify the hormones produced by the adrenal cortex and adrenal medulla, and summarize their target cells and effects The adrenal glands are wedges of glandular and neuroendocrine tissue adhering to the top of the kidneys by a fibrous capsule ({link]). The adrenal glands have a rich blood supply and experience one of the highest rates of blood flow in the body. They are served by several arteries branching off the aorta, including the suprarenal and renal arteries. Blood flows to each adrenal gland at the adrenal cortex and then drains into the adrenal medulla. Adrenal hormones are released into the circulation via the left and right suprarenal veins. Adrenal Glands Connective tissue | capsule Tissue area Hormones released Examples ‘+ Zona glomerulosa ——*> Mineralcorticoids eee f eh ‘ Se hs cortex) (regulate mineral 4 pes - oe 4} Aldosterone balance) Zona fasciculata ———® Glucocorticoids Cortisol (adrenal cortex) (regulate glucose Corticosterone metabolism) Cortisone Zona reticularis ———- Androgens A a Dehydroepian- (adrenal cortex) (stimulate rap sabe masculinization) Adrenal medulla ——— Stress hormones Epinephrine (stimulate Norepinephrine sympathetic ANS) Adrenal gland Superior surface of kidney Both adrenal glands sit atop the kidneys and are composed of an outer cortex and an inner medulla, all surrounded by a connective tissue capsule. The cortex can be subdivided into additional zones, all of which produce different types of hormones. LM x 204. (Micrograph provided by the Regents of University of Michigan Medical School © 2012) Note: od es —_s mss’ OPENStax COLLEGE — , 7 View the University of Michigan WebScope to explore the tissue sample in greater detail. The adrenal gland consists of an outer cortex of glandular tissue and an inner medulla of nervous tissue. The cortex itself is divided into three zones: the zona glomerulosa, the zona fasciculata, and the zona reticularis. Each region secretes its own set of hormones. The adrenal cortex, as a component of the hypothalamic-pituitary-adrenal (HPA) axis, secretes steroid hormones important for the regulation of the long-term stress response, blood pressure and blood volume, nutrient uptake and storage, fluid and electrolyte balance, and inflammation. The HPA axis involves the stimulation of hormone release of adrenocorticotropic hormone (ACTH) from the pituitary by the hypothalamus. ACTH then stimulates the adrenal cortex to produce the hormone cortisol. This pathway will be discussed in more detail below. The adrenal medulla is neuroendocrine tissue composed of postganglionic sympathetic nervous system (SNS) neurons. It is really an extension of the autonomic nervous system, which regulates homeostasis in the body. The sympathomedullary (SAM) pathway involves the stimulation of the medulla by impulses from the hypothalamus via neurons from the thoracic spinal cord. The medulla is stimulated to secrete the amine hormones epinephrine and norepinephrine. One of the major functions of the adrenal gland is to respond to stress. Stress can be either physical or psychological or both. Physical stresses include exposing the body to injury, walking outside in cold and wet conditions without a coat on, or malnutrition. Psychological stresses include the perception of a physical threat, a fight with a loved one, or just a bad day at school. The body responds in different ways to short-term stress and long-term stress following a pattern known as the general adaptation syndrome (GAS). Stage one of GAS is called the alarm reaction. This is short-term stress, the fight-or-flight response, mediated by the hormones epinephrine and norepinephrine from the adrenal medulla via the SAM pathway. Their function is to prepare the body for extreme physical exertion. Once this stress is relieved, the body quickly returns to normal. The section on the adrenal medulla covers this response in more detail. If the stress is not soon relieved, the body adapts to the stress in the second stage called the stage of resistance. If a person is starving for example, the body may send signals to the gastrointestinal tract to maximize the absorption of nutrients from food. If the stress continues for a longer term however, the body responds with symptoms quite different than the fight-or-flight response. During the stage of exhaustion, individuals may begin to suffer depression, the suppression of their immune response, severe fatigue, or even a fatal heart attack. These symptoms are mediated by the hormones of the adrenal cortex, especially cortisol, released as a result of signals from the HPA axis. Adrenal hormones also have several non-stress-related functions, including the increase of blood sodium and glucose levels, which will be described in detail below. Adrenal Cortex The adrenal cortex consists of multiple layers of lipid-storing cells that occur in three structurally distinct regions. Each of these regions produces different hormones. Note: . or mss’ OPENStAX COLLEGE Visit this link to view an animation describing the location and function of the adrenal glands. Which hormone produced by the adrenal glands is responsible for the mobilization of energy stores? Hormones of the Zona Glomerulosa The most superficial region of the adrenal cortex is the zona glomerulosa, which produces a group of hormones collectively referred to as mineralocorticoids because of their effect on body minerals, especially sodium and potassium. These hormones are essential for fluid and electrolyte balance. Aldosterone is the major mineralocorticoid. It is important in the regulation of the concentration of sodium and potassium ions in urine, sweat, and saliva. For example, it is released in response to elevated blood K™, low blood Na‘, low blood pressure, or low blood volume. In response, aldosterone increases the excretion of K” and the retention of Na‘, which in turn increases blood volume and blood pressure. Its secretion is prompted when CRH from the hypothalamus triggers ACTH release from the anterior pituitary. Aldosterone is also a key component of the renin-angiotensin-aldosterone system (RAAS) in which specialized cells of the kidneys secrete the enzyme renin in response to low blood volume or low blood pressure. Renin then catalyzes the conversion of the blood protein angiotensinogen, produced by the liver, to the hormone angiotensin I. Angiotensin I is converted in the lungs to angiotensin II by angiotensin-converting enzyme (ACE). Angiotensin II has three major functions: 1. Initiating vasoconstriction of the arterioles, decreasing blood flow 2. Stimulating kidney tubules to reabsorb NaCl and water, increasing blood volume 3. Signaling the adrenal cortex to secrete aldosterone, the effects of which further contribute to fluid retention, restoring blood pressure and blood volume For individuals with hypertension, or high blood pressure, drugs are available that block the production of angiotensin IT. These drugs, known as ACE inhibitors, block the ACE enzyme from converting angiotensin I to angiotensin II, thus mitigating the latter’s ability to increase blood pressure. Hormones of the Zona Fasciculata The intermediate region of the adrenal cortex is the zona fasciculata, named as such because the cells form small fascicles (bundles) separated by tiny blood vessels. The cells of the zona fasciculata produce hormones called glucocorticoids because of their role in glucose metabolism. The most important of these is cortisol, some of which the liver converts to cortisone. A glucocorticoid produced in much smaller amounts is corticosterone. In response to long-term stressors, the hypothalamus secretes CRH, which in turn triggers the release of ACTH by the anterior pituitary. ACTH triggers the release of the glucocorticoids. Their overall effect is to inhibit tissue building while stimulating the breakdown of stored nutrients to maintain adequate fuel supplies. In conditions of long-term stress, for example, cortisol promotes the catabolism of glycogen to glucose, the catabolism of stored triglycerides into fatty acids and glycerol, and the catabolism of muscle proteins into amino acids. These raw materials can then be used to synthesize additional glucose and ketones for use as body fuels. The hippocampus, which is part of the temporal lobe of the cerebral cortices and important in memory formation, is highly sensitive to stress levels because of its many glucocorticoid receptors. You are probably familiar with prescription and over-the-counter medications containing glucocorticoids, such as cortisone injections into inflamed joints, prednisone tablets and steroid-based inhalers used to manage severe asthma, and hydrocortisone creams applied to relieve itchy skin rashes. These drugs reflect another role of cortisol—the downregulation of the immune system, which inhibits the inflammatory response. Hormones of the Zona Reticularis The deepest region of the adrenal cortex is the zona reticularis, which produces small amounts of a class of steroid sex hormones called androgens. During puberty and most of adulthood, androgens are produced in the gonads. The androgens produced in the zona reticularis supplement the gonadal androgens. They are produced in response to ACTH from the anterior pituitary and are converted in the tissues to testosterone or estrogens. In adult women, they may contribute to the sex drive, but their function in adult men is not well understood. In post-menopausal women, as the functions of the ovaries decline, the main source of estrogens becomes the androgens produced by the zona reticularis. Adrenal Medulla As noted earlier, the adrenal cortex releases glucocorticoids in response to long-term stress such as severe illness. In contrast, the adrenal medulla releases its hormones in response to acute, short-term stress mediated by the sympathetic nervous system (SNS). The medullary tissue is composed of unique postganglionic SNS neurons called chromaffin cells, which are large and irregularly shaped, and produce the neurotransmitters epinephrine (also called adrenaline) and norepinephrine (or noradrenaline). Epinephrine is produced in greater quantities—approximately a 4 to 1 ratio with norepinephrine—and is the more powerful hormone. Because the chromaffin cells release epinephrine and norepinephrine into the systemic circulation, where they travel widely and exert effects on distant cells, they are considered hormones. Derived from the amino acid tyrosine, they are chemically classified as catecholamines. The secretion of medullary epinephrine and norepinephrine is controlled by a neural pathway that originates from the hypothalamus in response to danger or stress (the SAM pathway). Both epinephrine and norepinephrine signal the liver and skeletal muscle cells to convert glycogen into glucose, resulting in increased blood glucose levels. These hormones increase the heart rate, pulse, and blood pressure to prepare the body to fight the perceived threat or flee from it. In addition, the pathway dilates the airways, raising blood oxygen levels. It also prompts vasodilation, further increasing the oxygenation of important organs such as the lungs, brain, heart, and skeletal muscle. At the same time, it triggers vasoconstriction to blood vessels serving less essential organs such as the gastrointestinal tract, kidneys, and skin, and downregulates some components of the immune system. Other effects include a dry mouth, loss of appetite, pupil dilation, and a loss of peripheral vision. The major hormones of the adrenal glands are summarized in [link]. Hormones of the Adrenal Glands Adrenal Associated Chemical gland hormones class Effect Adrenal iaeiaiane Sirota Dice’ blood cortex Na’ levels Adrenal Cortisol, ; Increase blood corticosterone, Steroid cortex ; glucose levels cortisone Stimulate fight- Amine or-flight response Adrenal Epinephrine, medulla norepinephrine Disorders Involving the Adrenal Glands Several disorders are caused by the dysregulation of the hormones produced by the adrenal glands. For example, Cushing’s disease is a disorder characterized by high blood glucose levels and the accumulation of lipid deposits on the face and neck. It is caused by hypersecretion of cortisol. The most common source of Cushing’s disease is a pituitary tumor that secretes cortisol or ACTH in abnormally high amounts. Other common signs of Cushing’s disease include the development of a moon-shaped face, a buffalo hump on the back of the neck, rapid weight gain, and hair loss. Chronically elevated glucose levels are also associated with an elevated risk of developing type 2 diabetes. In addition to hyperglycemia, chronically elevated glucocorticoids compromise immunity, resistance to infection, and memory, and can result in rapid weight gain and hair loss. In contrast, the hyposecretion of corticosteroids can result in Addison’s disease, a rare disorder that causes low blood glucose levels and low blood sodium levels. The signs and symptoms of Addison’s disease are vague and are typical of other disorders as well, making diagnosis difficult. They may include general weakness, abdominal pain, weight loss, nausea, vomiting, sweating, and cravings for salty food. Chapter Review The adrenal glands, located superior to each kidney, consist of two regions: the adrenal cortex and adrenal medulla. The adrenal cortex—the outer layer of the gland—produces mineralocorticoids, glucocorticoids, and androgens. The adrenal medulla at the core of the gland produces epinephrine and norepinephrine. The adrenal glands mediate a short-term stress response and a long-term stress response. A perceived threat results in the secretion of epinephrine and norepinephrine from the adrenal medulla, which mediate the fight-or- flight response. The long-term stress response is mediated by the secretion of CRH from the hypothalamus, which triggers ACTH, which in turn stimulates the secretion of corticosteroids from the adrenal cortex. The mineralocorticoids, chiefly aldosterone, cause sodium and fluid retention, which increases blood volume and blood pressure. Interactive Link Questions Exercise: Problem: Visit this link to view an animation describing the location and function of the adrenal glands. Which hormone produced by the adrenal glands is responsible for mobilization of energy stores? Solution: Cortisol. Review Questions Exercise: Problem:The adrenal glands are attached superiorly to which organ? a. thyroid b. liver c. kidneys d. hypothalamus Solution: C Exercise: Problem: What secretory cell type is found in the adrenal medulla? a. chromaffin cells b. neuroglial cells c. follicle cells d. oxyphil cells Solution: A Exercise: Problem: Cushing’s disease is a disorder caused by a. abnormally low levels of cortisol b. abnormally high levels of cortisol c. abnormally low levels of aldosterone d. abnormally high levels of aldosterone Solution: B Exercise: Problem: Which of the following responses s not part of the fight-or-flight response? a. pupil dilation b. increased oxygen supply to the lungs c. suppressed digestion d. reduced mental activity Solution: D Critical Thinking Questions Exercise: Problem: What are the three regions of the adrenal cortex and what hormones do they produce? Solution: The outer region is the zona glomerulosa, which produces mineralocorticoids such as aldosterone; the next region is the zona fasciculata, which produces glucocorticoids such as cortisol; the inner region is the zona reticularis, which produces androgens. Exercise: Problem: If innervation to the adrenal medulla were disrupted, what would be the physiological outcome? Solution: Damage to the innervation of the adrenal medulla would prevent the adrenal glands from responding to the hypothalamus during the fight- or-flight response. Therefore, the response would be reduced. Exercise: Problem: Compare and contrast the short-term and long-term stress response. Solution: The short-term stress response involves the hormones epinephrine and norepinephrine, which work to increase the oxygen supply to organs important for extreme muscular action such as the brain, lungs, and muscles. In the long-term stress response, the hormone cortisol is involved in catabolism of glycogen stores, proteins, and triglycerides, glucose and ketone synthesis, and downregulation of the immune system. Glossary adrenal cortex outer region of the adrenal glands consisting of multiple layers of epithelial cells and capillary networks that produces mineralocorticoids and glucocorticoids adrenal glands endocrine glands located at the top of each kidney that are important for the regulation of the stress response, blood pressure and blood volume, water homeostasis, and electrolyte levels adrenal medulla inner layer of the adrenal glands that plays an important role in the stress response by producing epinephrine and norepinephrine angiotensin-converting enzyme the enzyme that converts angiotensin I to angiotensin IT alarm reaction the short-term stress, or the fight-or-flight response, of stage one of the general adaptation syndrome mediated by the hormones epinephrine and norepinephrine aldosterone hormone produced and secreted by the adrenal cortex that stimulates sodium and fluid retention and increases blood volume and blood pressure chromaffin neuroendocrine cells of the adrenal medulla cortisol glucocorticoid important in gluconeogenesis, the catabolism of glycogen, and downregulation of the immune system epinephrine primary and most potent catecholamine hormone secreted by the adrenal medulla in response to short-term stress; also called adrenaline general adaptation syndrome (GAS) the human body’s three-stage response pattern to short- and long-term stress glucocorticoids hormones produced by the zona fasciculata of the adrenal cortex that influence glucose metabolism mineralocorticoids hormones produced by the zona glomerulosa cells of the adrenal cortex that influence fluid and electrolyte balance norepinephrine secondary catecholamine hormone secreted by the adrenal medulla in response to short-term stress; also called noradrenaline stage of exhaustion stage three of the general adaptation syndrome; the body’s long-term response to stress mediated by the hormones of the adrenal cortex stage of resistance stage two of the general adaptation syndrome; the body’s continued response to stress after stage one diminishes zona fasciculata intermediate region of the adrenal cortex that produce hormones called glucocorticoids zona glomerulosa most superficial region of the adrenal cortex, which produces the hormones collectively referred to as mineralocorticoids zona reticularis deepest region of the adrenal cortex, which produces the steroid sex hormones called androgens The Pineal Gland By the end of this section, you will be able to: e Describe the location and structure of the pineal gland e Discuss the function of melatonin Recall that the hypothalamus, part of the diencephalon of the brain, sits inferior and somewhat anterior to the thalamus. Inferior but somewhat posterior to the thalamus is the pineal gland, a tiny endocrine gland whose functions are not entirely clear. The pinealocyte cells that make up the pineal gland are known to produce and secrete the amine hormone melatonin, which is derived from serotonin. The secretion of melatonin varies according to the level of light received from the environment. When photons of light stimulate the retinas of the eyes, a nerve impulse is sent to a region of the hypothalamus called the suprachiasmatic nucleus (SCN), which is important in regulating biological rhythms. From the SCN, the nerve signal is carried to the spinal cord and eventually to the pineal gland, where the production of melatonin is inhibited. As a result, blood levels of melatonin fall, promoting wakefulness. In contrast, as light levels decline—such as during the evening —melatonin production increases, boosting blood levels and causing drowsiness. Note: openstax COLLEGE” : 2 Visit this link to view an animation describing the function of the hormone melatonin. What should you avoid doing in the middle of your sleep cycle that would lower melatonin? The secretion of melatonin may influence the body’s circadian rhythms, the dark-light fluctuations that affect not only sleepiness and wakefulness, but also appetite and body temperature. Interestingly, children have higher melatonin levels than adults, which may prevent the release of gonadotropins from the anterior pituitary, thereby inhibiting the onset of puberty. Finally, an antioxidant role of melatonin is the subject of current research. Jet lag occurs when a person travels across several time zones and feels sleepy during the day or wakeful at night. Traveling across multiple time zones significantly disturbs the light-dark cycle regulated by melatonin. It can take up to several days for melatonin synthesis to adjust to the light- dark patterns in the new environment, resulting in jet lag. Some air travelers take melatonin supplements to induce sleep. Chapter Review The pineal gland is an endocrine structure of the diencephalon of the brain, and is located inferior and posterior to the thalamus. It is made up of pinealocytes. These cells produce and secrete the hormone melatonin in response to low light levels. High blood levels of melatonin induce drowsiness. Jet lag, caused by traveling across several time zones, occurs because melatonin synthesis takes several days to readjust to the light-dark patterns in the new environment. Interactive Link Questions Exercise: Problem: Visit this link to view an animation describing the function of the hormone melatonin. What should you avoid doing in the middle of your sleep cycle that would lower melatonin? Solution: Turning on the lights. Review Questions Exercise: Problem: What cells secrete melatonin? a. melanocytes b. pinealocytes c. suprachiasmatic nucleus cells d. retinal cells Solution: B Exercise: Problem: The production of melatonin is inhibited by a. declining levels of light b. exposure to bright light c. the secretion of serotonin d. the activity of pinealocytes Solution: B Critical Thinking Questions Exercise: Problem: Seasonal affective disorder (SAD) is a mood disorder characterized by, among other symptoms, increased appetite, sluggishness, and increased sleepiness. It occurs most commonly during the winter months, especially in regions with long winter nights. Propose a role for melatonin in SAD and a possible non-drug therapy. Solution: SAD is thought to occur in part because low levels and duration of sunlight allow excessive and prolonged secretion of melatonin. Light therapy—daytime exposure to very bright lighting—is one common therapy. Exercise: Problem: Retinitis pigmentosa (RP) is a disease that causes deterioration of the retinas of the eyes. Describe the impact RP would have on melatonin levels. Solution: The retina is important for melatonin production because it senses light. Bright light inhibits the production of melatonin, whereas low light levels promote the production of melatonin. Therefore, deterioration of the retinas would most likely disturb the sleep-wake pattern because melatonin production would be elevated. Glossary melatonin amino acid—derived hormone that is secreted in response to low light and causes drowsiness pineal gland endocrine gland that secretes melatonin, which is important in regulating the sleep-wake cycle pinealocyte cell of the pineal gland that produces and secretes the hormone melatonin Gonadal and Placental Hormones By the end of this section, you will be able to: e Identify the most important hormones produced by the testes and ovaries e Name the hormones produced by the placenta and state their functions This section briefly discusses the hormonal role of the gonads—the male testes and female ovaries—which produce the sex cells (sperm and ova) and secrete the gonadal hormones. The roles of the gonadotropins released from the anterior pituitary (FSH and LH) were discussed earlier. The primary hormone produced by the male testes is testosterone, a steroid hormone important in the development of the male reproductive system, the maturation of sperm cells, and the development of male secondary sex characteristics such as a deepened voice, body hair, and increased muscle mass. Interestingly, testosterone is also produced in the female ovaries, but at a much reduced level. In addition, the testes produce the peptide hormone inhibin, which inhibits the secretion of FSH from the anterior pituitary gland. FSH stimulates spermatogenesis. The primary hormones produced by the ovaries are estrogens, which include estradiol, estriol, and estrone. Estrogens play an important role in a larger number of physiological processes, including the development of the female reproductive system, regulation of the menstrual cycle, the development of female secondary sex characteristics such as increased adipose tissue and the development of breast tissue, and the maintenance of pregnancy. Another significant ovarian hormone is progesterone, which contributes to regulation of the menstrual cycle and is important in preparing the body for pregnancy as well as maintaining pregnancy. In addition, the granulosa cells of the ovarian follicles produce inhibin, which —as in males—inhibits the secretion of FSH.During the initial stages of pregnancy, an organ called the placenta develops within the uterus. The placenta supplies oxygen and nutrients to the fetus, excretes waste products, and produces and secretes estrogens and progesterone. The placenta produces human chorionic gonadotropin (hCG) as well. The hCG hormone promotes progesterone synthesis and reduces the mother’s immune function to protect the fetus from immune rejection. It also secretes human placental lactogen (hPL), which plays a role in preparing the breasts for lactation, and relaxin, which is thought to help soften and widen the pubic symphysis in preparation for childbirth. The hormones controlling reproduction are summarized in [link]. Reproductive Hormones Associated Gonad hormones Testes Testosterone Testes Inhibin Estrogens Ovaries and progesterone Human Placenta chorionic gonadotropin Chemical class Steroid Protein Steroid Protein Effect Stimulates development of male secondary sex characteristics and sperm production Inhibits FSH release from pituitary Stimulate development of female secondary sex characteristics and prepare the body for childbirth Promotes progesterone synthesis during pregnancy and inhibits immune response against fetus Note: Everyday Connections Anabolic Steroids The endocrine system can be exploited for illegal or unethical purposes. A prominent example of this is the use of steroid drugs by professional athletes. Commonly used for performance enhancement, anabolic steroids are synthetic versions of the male sex hormone, testosterone. By boosting natural levels of this hormone, athletes experience increased muscle mass. Synthetic versions of human growth hormone are also used to build muscle mass. The use of performance-enhancing drugs is banned by all major collegiate and professional sports organizations in the United States because they impart an unfair advantage to athletes who take them. In addition, the drugs can cause significant and dangerous side effects. For example, anabolic steroid use can increase cholesterol levels, raise blood pressure, and damage the liver. Altered testosterone levels (both too low or too high) have been implicated in causing structural damage to the heart, and increasing the risk for cardiac arrhythmias, heart attacks, congestive heart failure, and sudden death. Paradoxically, steroids can have a feminizing effect in males, including shriveled testicles and enlarged breast tissue. In females, their use can cause masculinizing effects such as an enlarged clitoris and growth of facial hair. In both sexes, their use can promote increased aggression (commonly known as “roid-rage”), depression, sleep disturbances, severe acne, and infertility. Chapter Review The male and female reproductive system is regulated by follicle- stimulating hormone (FSH) and luteinizing hormone (LH) produced by the anterior lobe of the pituitary gland in response to gonadotropin-releasing hormone (GnRH) from the hypothalamus. In males, FSH stimulates sperm maturation, which is inhibited by the hormone inhibin. The steroid hormone testosterone, a type of androgen, is released in response to LH and is responsible for the maturation and maintenance of the male reproductive system, as well as the development of male secondary sex characteristics. In females, FSH promotes egg maturation and LH signals the secretion of the female sex hormones, the estrogens and progesterone. Both of these hormones are important in the development and maintenance of the female reproductive system, as well as maintaining pregnancy. The placenta develops during early pregnancy, and secretes several hormones important for maintaining the pregnancy. Review Questions Exercise: Problem:The gonads produce what class of hormones? a. amine hormones b. peptide hormones c. steroid hormones d. catecholamines Solution: G Exercise: Problem: The production of FSH by the anterior pituitary is reduced by which hormone? a. estrogens b. progesterone c. relaxin d. inhibin Solution: D Exercise: Problem: The function of the placental hormone human placental lactogen (hPL) is to a. prepare the breasts for lactation b. nourish the placenta c. regulate the menstrual cycle d. all of the above Solution: A Critical Thinking Questions Exercise: Problem:Compare and contrast the role of estrogens and progesterone. Solution: Both estrogens and progesterone are steroid hormones produced by the ovaries that help regulate the menstrual cycle. Estrogens play an important role in the development of the female reproductive tract and secondary sex characteristics. They also help maintain pregnancy. Progesterone prepares the body for pregnancy and helps maintain pregnancy. Exercise: Problem: Describe the role of placental secretion of relaxin in preparation for childbirth. Solution: Relaxin produced by the placenta is thought to soften and widen the pubic symphysis. This increases the size of the pelvic outlet, the birth canal through which the fetus passes during vaginal childbirth. Glossary estrogens class of predominantly female sex hormones important for the development and growth of the female reproductive tract, secondary sex characteristics, the female reproductive cycle, and the maintenance of pregnancy inhibin hormone secreted by the male and female gonads that inhibits FSH production by the anterior pituitary progesterone predominantly female sex hormone important in regulating the female reproductive cycle and the maintenance of pregnancy testosterone steroid hormone secreted by the male testes and important in the maturation of sperm cells, growth and development of the male reproductive system, and the development of male secondary sex characteristics The Endocrine Pancreas By the end of this section, you will be able to: e Describe the location and structure of the pancreas, and the morphology and function of the pancreatic islets ¢ Compare and contrast the functions of insulin and glucagon The pancreas is a long, slender organ, most of which is located posterior to the bottom half of the stomach ([link]). Although it is primarily an exocrine gland, secreting a variety of digestive enzymes, the pancreas has an endocrine function. Its pancreatic islets—clusters of cells formerly known as the islets of Langerhans—secrete the hormones glucagon, insulin, somatostatin, and pancreatic polypeptide (PP). Pancreas Splenic artery Pancreatic hormones: ¢ Insulin ¢ Glucagon Spleen Pancreatic islets Bile duct (from gall bladder) Common bile duct Duodenum of small intestine Acinar cells secrete digestive enzymes Pancreatic duct } ~~ Exocrine acinus The pancreatic exocrine function involves the acinar cells secreting digestive enzymes that are transported into the small intestine by the pancreatic duct. Its endocrine function involves the secretion of insulin (produced by beta cells) and glucagon (produced by alpha cells) within the pancreatic islets. These two hormones regulate the rate of glucose metabolism in the body. The micrograph reveals pancreatic islets. LM x 760. (Micrograph provided by the Regents of University of Michigan Medical School © 2012) Note: | [=] 1 - - mss’ OPENStax COLLEGE pot Data View the University of Michigan WebScope to explore the tissue sample in greater detail. Cells and Secretions of the Pancreatic Islets The pancreatic islets each contain four varieties of cells: The alpha cell produces the hormone glucagon and makes up approximately 20 percent of each islet. Glucagon plays an important role in blood glucose regulation; low blood glucose levels stimulate its release. The beta cell produces the hormone insulin and makes up approximately 75 percent of each islet. Elevated blood glucose levels stimulate the release of insulin. The delta cell accounts for four percent of the islet cells and secretes the peptide hormone somatostatin. Recall that somatostatin is also released by the hypothalamus (as GHIH), and the stomach and intestines also secrete it. An inhibiting hormone, pancreatic somatostatin inhibits the release of both glucagon and insulin. The PP cell accounts for about one percent of islet cells and secretes the pancreatic polypeptide hormone. It is thought to play a role in appetite, as well as in the regulation of pancreatic exocrine and endocrine secretions. Pancreatic polypeptide released following a meal may reduce further food consumption; however, it is also released in response to fasting. Regulation of Blood Glucose Levels by Insulin and Glucagon Glucose is required for cellular respiration and is the preferred fuel for all body cells. The body derives glucose from the breakdown of the carbohydrate-containing foods and drinks we consume. Glucose not immediately taken up by cells for fuel can be stored by the liver and muscles as glycogen, or converted to triglycerides and stored in the adipose tissue. Hormones regulate both the storage and the utilization of glucose as required. Receptors located in the pancreas sense blood glucose levels, and subsequently the pancreatic cells secrete glucagon or insulin to maintain normal levels. Glucagon Receptors in the pancreas can sense the decline in blood glucose levels, such as during periods of fasting or during prolonged labor or exercise ({link]). In response, the alpha cells of the pancreas secrete the hormone glucagon, which has several effects: e It stimulates the liver to convert its stores of glycogen back into glucose. This response is known as glycogenolysis. The glucose is then released into the circulation for use by body cells. e It stimulates the liver to take up amino acids from the blood and convert them into glucose. This response is known as gluconeogenesis. e It stimulates lipolysis, the breakdown of stored triglycerides into free fatty acids and glycerol. Some of the free glycerol released into the bloodstream travels to the liver, which converts it into glucose. This is also a form of gluconeogenesis. Taken together, these actions increase blood glucose levels. The activity of glucagon is regulated through a negative feedback mechanism; rising blood glucose levels inhibit further glucagon production and secretion. Homeostatic Regulation of Blood Glucose Levels Insulin release: Insulin effects: ¢ Beta cells of pancreas release insulin * Triggers body cells to take up glucose from the blood and utilize it in cellular respiration Splenic artery ¢ Inhibits glycogenolysis — glucose is removed from the blood and stored as glycogen in the liver ¢ Inhibits gluconeogenesis Rough ER - amino acids and free glycerol are NOT converted to glucose in the ER Smooth ER Blood glucose concentration decreases Hyperglycemia (elevated blood glucose) Hypoglycemia (low blood glucose) yh START: Homeostasis (70-110 mg/dL) ee Blood glucose concentration increases Glucagon effects: Inhibits body cells from taking up glucose from the blood and Glucagon release: utilizing it in cellular respiration ¢ Alpha cells of pancreas | | nee ee Splenic artery * Stimulates glycogenolysis — glycogen in the liver is broken down into glucose and released into the blood ¢ Stimulates gluconeogenesis — amino acids and free glycerol Rough ER are converted to glucose in the ER and released into the blood Smooth ER Blood glucose concentration is tightly maintained between 70 mg/dL and 110 mg/dL. If blood glucose concentration rises above this range, insulin is released, which stimulates body cells to remove glucose from the blood. If blood glucose concentration drops below this range, glucagon is released, which stimulates body cells to release glucose into the blood. Insulin The primary function of insulin is to facilitate the uptake of glucose into body cells. Red blood cells, as well as cells of the brain, liver, kidneys, and the lining of the small intestine, do not have insulin receptors on their cell membranes and do not require insulin for glucose uptake. Although all other body cells do require insulin if they are to take glucose from the bloodstream, skeletal muscle cells and adipose cells are the primary targets of insulin. The presence of food in the intestine triggers the release of gastrointestinal tract hormones such as glucose-dependent insulinotropic peptide (previously known as gastric inhibitory peptide). This is in turn the initial trigger for insulin production and secretion by the beta cells of the pancreas. Once nutrient absorption occurs, the resulting surge in blood glucose levels further stimulates insulin secretion. Precisely how insulin facilitates glucose uptake is not entirely clear. However, insulin appears to activate a tyrosine kinase receptor, triggering the phosphorylation of many substrates within the cell. These multiple biochemical reactions converge to support the movement of intracellular vesicles containing facilitative glucose transporters to the cell membrane. In the absence of insulin, these transport proteins are normally recycled slowly between the cell membrane and cell interior. Insulin triggers the rapid movement of a pool of glucose transporter vesicles to the cell membrane, where they fuse and expose the glucose transporters to the extracellular fluid. The transporters then move glucose by facilitated diffusion into the cell interior. Visit this link to view an animation describing the location and function of the pancreas. What goes wrong in the function of insulin in type 2 diabetes? Insulin also reduces blood glucose levels by stimulating glycolysis, the metabolism of glucose for generation of ATP. Moreover, it stimulates the liver to convert excess glucose into glycogen for storage, and it inhibits enzymes involved in glycogenolysis and gluconeogenesis. Finally, insulin promotes triglyceride and protein synthesis. The secretion of insulin is regulated through a negative feedback mechanism. As blood glucose levels decrease, further insulin release is inhibited. The pancreatic hormones are summarized in [link]. Hormones of the Pancreas Chemical Associated hormones class Effect Insulin (beta cells) Protein Reduces blood glucose levels Hormones of the Pancreas Chemical Associated hormones class Effect Glucagon (alpha cells) Protein Tacreases DiOOG eileos: levels Somatostatin (delta Peavieia Inhibits insulin and cells) glucagon release Palicrealle/poly pepude Protein Role in appetite (PP cells) Note: Disorders of the... Endocrine System: Diabetes Mellitus Dysfunction of insulin production and secretion, as well as the target cells’ responsiveness to insulin, can lead to a condition called diabetes mellitus. An increasingly common disease, diabetes mellitus has been diagnosed in more than 18 million adults in the United States, and more than 200,000 children. It is estimated that up to 7 million more adults have the condition but have not been diagnosed. In addition, approximately 79 million people in the US are estimated to have pre-diabetes, a condition in which blood glucose levels are abnormally high, but not yet high enough to be classified as diabetes. There are two main forms of diabetes mellitus. Type 1 diabetes is an autoimmune disease affecting the beta cells of the pancreas. Certain genes are recognized to increase susceptibility. The beta cells of people with type 1 diabetes do not produce insulin; thus, synthetic insulin must be administered by injection or infusion. This form of diabetes accounts for less than five percent of all diabetes cases. Type 2 diabetes accounts for approximately 95 percent of all cases. It is acquired, and lifestyle factors such as poor diet, inactivity, and the presence of pre-diabetes greatly increase a person’s risk. About 80 to 90 percent of people with type 2 diabetes are overweight or obese. In type 2 diabetes, cells become resistant to the effects of insulin. In response, the pancreas increases its insulin secretion, but over time, the beta cells become exhausted. In many cases, type 2 diabetes can be reversed by moderate weight loss, regular physical activity, and consumption of a healthy diet; however, if blood glucose levels cannot be controlled, the diabetic will eventually require insulin. Two of the early manifestations of diabetes are excessive urination and excessive thirst. They demonstrate how the out-of-control levels of glucose in the blood affect kidney function. The kidneys are responsible for filtering glucose from the blood. Excessive blood glucose draws water into the urine, and as a result the person eliminates an abnormally large quantity of sweet urine. The use of body water to dilute the urine leaves the body dehydrated, and so the person is unusually and continually thirsty. The person may also experience persistent hunger because the body cells are unable to access the glucose in the bloodstream. Over time, persistently high levels of glucose in the blood injure tissues throughout the body, especially those of the blood vessels and nerves. Inflammation and injury of the lining of arteries lead to atherosclerosis and an increased risk of heart attack and stroke. Damage to the microscopic blood vessels of the kidney impairs kidney function and can lead to kidney failure. Damage to blood vessels that serve the eyes can lead to blindness. Blood vessel damage also reduces circulation to the limbs, whereas nerve damage leads to a loss of sensation, called neuropathy, particularly in the hands and feet. Together, these changes increase the risk of injury, infection, and tissue death (necrosis), contributing to a high rate of toe, foot, and lower leg amputations in people with diabetes. Uncontrolled diabetes can also lead to a dangerous form of metabolic acidosis called ketoacidosis. Deprived of glucose, cells increasingly rely on fat stores for fuel. However, in a glucose-deficient state, the liver is forced to use an alternative lipid metabolism pathway that results in the increased production of ketone bodies (or ketones), which are acidic. The build-up of ketones in the blood causes ketoacidosis, which—if left untreated—may lead to a life-threatening “diabetic coma.” Together, these complications make diabetes the seventh leading cause of death in the United States. Diabetes is diagnosed when lab tests reveal that blood glucose levels are higher than normal, a condition called hyperglycemia. The treatment of diabetes depends on the type, the severity of the condition, and the ability of the patient to make lifestyle changes. As noted earlier, moderate weight loss, regular physical activity, and consumption of a healthful diet can reduce blood glucose levels. Some patients with type 2 diabetes may be unable to control their disease with these lifestyle changes, and will require medication. Historically, the first-line treatment of type 2 diabetes was insulin. Research advances have resulted in alternative options, including medications that enhance pancreatic function. Note: [=] [= — meee, OPENStAX COLLEGE Aor = Visit this link to view an animation describing the role of insulin and the pancreas in diabetes. Chapter Review The pancreas has both exocrine and endocrine functions. The pancreatic islet cell types include alpha cells, which produce glucagon; beta cells, which produce insulin; delta cells, which produce somatostatin; and PP cells, which produce pancreatic polypeptide. Insulin and glucagon are involved in the regulation of glucose metabolism. Insulin is produced by the beta cells in response to high blood glucose levels. It enhances glucose uptake and utilization by target cells, as well as the storage of excess glucose for later use. Dysfunction of the production of insulin or target cell resistance to the effects of insulin causes diabetes mellitus, a disorder characterized by high blood glucose levels. The hormone glucagon is produced and secreted by the alpha cells of the pancreas in response to low blood glucose levels. Glucagon stimulates mechanisms that increase blood glucose levels, such as the catabolism of glycogen into glucose. Interactive Link Questions Exercise: Problem: Visit this link to view an animation describing the location and function of the pancreas. What goes wrong in the function of insulin in type 2 diabetes? Solution: Insulin is overproduced. Review Questions Exercise: Problem: If an autoimmune disorder targets the alpha cells, production of which hormone would be directly affected? a. somatostatin b. pancreatic polypeptide c. insulin d. glucagon Solution: D Exercise: Problem: Which of the following statements about insulin is true? a. Insulin acts as a transport protein, carrying glucose across the cell membrane. b. Insulin facilitates the movement of intracellular glucose transporters to the cell membrane. c. Insulin stimulates the breakdown of stored glycogen into glucose. d. Insulin stimulates the kidneys to reabsorb glucose into the bloodstream. Solution: B Critical Thinking Questions Exercise: Problem: What would be the physiological consequence of a disease that destroyed the beta cells of the pancreas? Solution: The beta cells produce the hormone insulin, which is important in the regulation of blood glucose levels. All insulin-dependent cells of the body require insulin in order to take up glucose from the bloodstream. Destruction of the beta cells would result in an inability to produce and secrete insulin, leading to abnormally high blood glucose levels and the disease called type 1 diabetes mellitus. Exercise: Problem: Why is foot care extremely important for people with diabetes mellitus? Solution: Excessive blood glucose levels damage the blood vessels and nerves of the body’s extremities, increasing the risk for injury, infection, and tissue death. Loss of sensation to the feet means that a diabetic patient will not be able to feel foot trauma, such as from ill-fitting shoes. Even minor injuries commonly lead to infection, which , can progress to tissue death without proper care, requiring amputation. Glossary alpha cell pancreatic islet cell type that produces the hormone glucagon beta cell pancreatic islet cell type that produces the hormone insulin delta cell minor cell type in the pancreas that secretes the hormone somatostatin diabetes mellitus condition caused by destruction or dysfunction of the beta cells of the pancreas or cellular resistance to insulin that results in abnormally high blood glucose levels glucagon pancreatic hormone that stimulates the catabolism of glycogen to glucose, thereby increasing blood glucose levels hyperglycemia abnormally high blood glucose levels insulin pancreatic hormone that enhances the cellular uptake and utilization of glucose, thereby decreasing blood glucose levels pancreas organ with both exocrine and endocrine functions located posterior to the stomach that is important for digestion and the regulation of blood glucose pancreatic islets specialized clusters of pancreatic cells that have endocrine functions; also called islets of Langerhans PP cell minor cell type in the pancreas that secretes the hormone pancreatic polypeptide Organs with Secondary Endocrine Functions By the end of this section, you will be able to: e Identify the organs with a secondary endocrine function, the hormone they produce, and its effects In your study of anatomy and physiology, you have already encountered a few of the many organs of the body that have secondary endocrine functions. Here, you will learn about the hormone-producing activities of the heart, gastrointestinal tract, kidneys, skeleton, adipose tissue, skin, and thymus. Heart When the body experiences an increase in blood volume or pressure, the cells of the heart’s atrial wall stretch. In response, specialized cells in the wall of the atria produce and secrete the peptide hormone atrial natriuretic peptide (ANP). ANP signals the kidneys to reduce sodium reabsorption, thereby decreasing the amount of water reabsorbed from the urine filtrate and reducing blood volume. Other actions of ANP include the inhibition of renin secretion and the initiation of the renin-angiotensin-aldosterone system (RAAS) and vasodilation. Therefore, ANP aids in decreasing blood pressure, blood volume, and blood sodium levels. Gastrointestinal Tract The endocrine cells of the GI tract are located in the mucosa of the stomach and small intestine. Some of these hormones are secreted in response to eating a meal and aid in digestion. An example of a hormone secreted by the stomach cells is gastrin, a peptide hormone secreted in response to stomach distention that stimulates the release of hydrochloric acid. Secretin is a peptide hormone secreted by the small intestine as acidic chyme (partially digested food and fluid) moves from the stomach. It stimulates the release of bicarbonate from the pancreas, which buffers the acidic chyme, and inhibits the further secretion of hydrochloric acid by the stomach. Cholecystokinin (CCK) is another peptide hormone released from the small intestine. It promotes the secretion of pancreatic enzymes and the release of bile from the gallbladder, both of which facilitate digestion. Other hormones produced by the intestinal cells aid in glucose metabolism, such as by stimulating the pancreatic beta cells to secrete insulin, reducing glucagon secretion from the alpha cells, or enhancing cellular sensitivity to insulin. Kidneys The kidneys participate in several complex endocrine pathways and produce certain hormones. A decline in blood flow to the kidneys stimulates them to release the enzyme renin, triggering the renin- angiotensin-aldosterone (RAAS) system, and stimulating the reabsorption of sodium and water. The reabsorption increases blood flow and blood pressure. The kidneys also play a role in regulating blood calcium levels through the production of calcitriol from vitamin D3, which is released in response to the secretion of parathyroid hormone (PTH). In addition, the kidneys produce the hormone erythropoietin (EPO) in response to low oxygen levels. EPO stimulates the production of red blood cells (erythrocytes) in the bone marrow, thereby increasing oxygen delivery to tissues. You may have heard of EPO as a performance-enhancing drug (in a synthetic form). Skeleton Although bone has long been recognized as a target for hormones, only recently have researchers recognized that the skeleton itself produces at least two hormones. Fibroblast growth factor 23 (FGF23) is produced by bone cells in response to increased blood levels of vitamin D3 or phosphate. It triggers the kidneys to inhibit the formation of calcitriol from vitamin D3 and to increase phosphorus excretion. Osteocalcin, produced by osteoblasts, stimulates the pancreatic beta cells to increase insulin production. It also acts on peripheral tissues to increase their sensitivity to insulin and their utilization of glucose. Adipose Tissue Adipose tissue produces and secretes several hormones involved in lipid metabolism and storage. One important example is leptin, a protein manufactured by adipose cells that circulates in amounts directly proportional to levels of body fat. Leptin is released in response to food consumption and acts by binding to brain neurons involved in energy intake and expenditure. Binding of leptin produces a feeling of satiety after a meal, thereby reducing appetite. It also appears that the binding of leptin to brain receptors triggers the sympathetic nervous system to regulate bone metabolism, increasing deposition of cortical bone. Adiponectin—another hormone synthesized by adipose cells—appears to reduce cellular insulin resistance and to protect blood vessels from inflammation and atherosclerosis. Its levels are lower in people who are obese, and rise following weight loss. Skin The skin functions as an endocrine organ in the production of the inactive form of vitamin D3, cholecalciferol. When cholesterol present in the epidermis is exposed to ultraviolet radiation, it is converted to cholecalciferol, which then enters the blood. In the liver, cholecalciferol is converted to an intermediate that travels to the kidneys and is further converted to calcitriol, the active form of vitamin D3. Vitamin D is important in a variety of physiological processes, including intestinal calcium absorption and immune system function. In some studies, low levels of vitamin D have been associated with increased risks of cancer, severe asthma, and multiple sclerosis. Vitamin D deficiency in children causes rickets, and in adults, osteomalacia—both of which are characterized by bone deterioration. Thymus The thymus is an organ of the immune system that is larger and more active during infancy and early childhood, and begins to atrophy as we age. Its endocrine function is the production of a group of hormones called thymosins that contribute to the development and differentiation of T lymphocytes, which are immune cells. Although the role of thymosins is not yet well understood, it is clear that they contribute to the immune response. Thymosins have been found in tissues other than the thymus and have a wide variety of functions, so the thymosins cannot be strictly categorized as thymic hormones. Liver The liver is responsible for secreting at least four important hormones or hormone precursors: insulin-like growth factor (somatomedin), angiotensinogen, thrombopoetin, and hepcidin. Insulin-like growth factor-1 is the immediate stimulus for growth in the body, especially of the bones. Angiotensinogen is the precursor to angiotensin, mentioned earlier, which increases blood pressure. Thrombopoetin stimulates the production of the blood’s platelets. Hepcidins block the release of iron from cells in the body, helping to regulate iron homeostasis in our body fluids. The major hormones of these other organs are summarized in [Link]. Organs with Secondary Endocrine Functions and Their Major Hormones Organ Major hormones Effects Reduces blood Heart Atrial natriuretic volume, blood peptide (ANP) pressure, and Na* concentration Gastrointestinal Gastrin, secretin, and a digestion oad ae and buffering of tract cholecystokinin stomach acids Organs with Secondary Endocrine Functions and Their Major Hormones Organ Gastrointestinal tract Kidneys Kidneys Kidneys Skeleton Skeleton Adipose tissue Adipose tissue Major hormones Glucose-dependent insulinotropic peptide (GIP) and glucagon- like peptide 1 (GLP- 1) Renin Calcitriol Erythropoietin FGF23 Osteocalcin Leptin Adiponectin Effects Stimulate beta cells of the pancreas to release insulin Stimulates release of aldosterone Aids in the absorption of Ca** Triggers the formation of red blood cells in the bone marrow Inhibits production of calcitriol and increases phosphate excretion Increases insulin production Promotes satiety signals in the brain Reduces insulin resistance Organs with Secondary Endocrine Functions and Their Major Hormones Organ Skin Thymus (and other organs) Liver Liver Liver Liver Chapter Review Major hormones Cholecalciferol Thymosins Insulin-like growth factor-1 Angiotensinogen Thrombopoetin Hepcidin Effects Modified to form vitamin D Among other things, aids in the development of T lymphocytes of the immune system Stimulates bodily growth Raises blood pressure Causes increase in platelets Blocks release of iron into body fluids Some organs have a secondary endocrine function. For example, the walls of the atria of the heart produce the hormone atrial natriuretic peptide (ANP), the gastrointestinal tract produces the hormones gastrin, secretin, and cholecystokinin, which aid in digestion, and the kidneys produce erythropoietin (EPO), which stimulates the formation of red blood cells. Even bone, adipose tissue, and the skin have secondary endocrine functions. Review Questions Exercise: Problem:The walls of the atria produce which hormone? a. cholecystokinin b. atrial natriuretic peptide c. renin d. calcitriol Solution: B Exercise: Problem:The end result of the RAAS is to a. reduce blood volume b. increase blood glucose c. reduce blood pressure d. increase blood pressure Solution: D Exercise: Problem: Athletes may take synthetic EPO to boost their a. blood calcium levels b. secretion of growth hormone c. blood oxygen levels d. muscle mass Solution: C Exercise: Problem: Hormones produced by the thymus play a role in the a. development of T cells b. preparation of the body for childbirth c. regulation of appetite d. release of hydrochloric acid in the stomach Solution: A Critical Thinking Questions Exercise: Problem:Summarize the role of GI tract hormones following a meal. Solution: The presence of food in the GI tract stimulates the release of hormones that aid in digestion. For example, gastrin is secreted in response to stomach distention and causes the release of hydrochloric acid in the stomach. Secretin is secreted when acidic chyme enters the small intestine, and stimulates the release of pancreatic bicarbonate. In the presence of fat and protein in the duodenum, CCK stimulates the release of pancreatic digestive enzymes and bile from the gallbladder. Other GI tract hormones aid in glucose metabolism and other functions. Exercise: Problem: Compare and contrast the thymus gland in infancy and adulthood. Solution: The thymus gland is important for the development and maturation of T cells. During infancy and early childhood, the thymus gland is large and very active, as the immune system is still developing. During adulthood, the thymus gland atrophies because the immune system is already developed. Glossary atrial natriuretic peptide (ANP) peptide hormone produced by the walls of the atria in response to high blood pressure, blood volume, or blood sodium that reduces the reabsorption of sodium and water in the kidneys and promotes vasodilation erythropoietin (EPO) protein hormone secreted in response to low oxygen levels that triggers the bone marrow to produce red blood cells leptin protein hormone secreted by adipose tissues in response to food consumption that promotes satiety thymosins hormones produced and secreted by the thymus that play an important role in the development and differentiation of T cells thymus organ that is involved in the development and maturation of T-cells and is particularly active during infancy and childhood Male Anatomy By the end of this section, you will be able to: e Describe the structure and function of the organs of the male reproductive system ¢ Describe the structure and function of the sperm cell e Explain the events during spermatogenesis that produce haploid sperm from diploid cells e Identify the importance of testosterone in male reproductive function Unique for its role in human reproduction, a gamete is a specialized sex cell carrying 23 chromosomes—one half the number in body cells. At fertilization, the chromosomes in one male gamete, called a sperm (or spermatozoon), combine with the chromosomes in one female gamete, called an oocyte. The function of the male reproductive system ((link]) is to produce sperm and transfer them to the female reproductive tract. The paired testes are a crucial component in this process, as they produce both sperm and androgens, the hormones that support male reproductive physiology. In humans, the most important male androgen is testosterone. Several accessory organs and ducts aid the process of sperm maturation and transport the sperm and other seminal components to the penis, which delivers sperm to the female reproductive tract. In this section, we examine each of these different structures, and discuss the process of sperm production and transport. Male Reproductive System (a) Uncircumcised penis (b) Circumcised penis Penis Prepuce (foreskin) (c) Male Reproductive System: lateral view i Ductus (vas) deferens Suspensory ligament of penis Ampulla of ductus deferens a : aml | a) | Seminal vesicle Pubic symphysis ME; if Ejaculatory duct Prostatic urethra Deep muscles of perineum Bulbourethral gland ie Muscles of perineum Corpus cavernosum —— } surrounding anus = Membranous urethra Spongy urethra Testis Epididymis Corpus spongiosum Scrotum External urethral opening The structures of the male reproductive system include the testes, the epididymides, the penis, and the ducts and glands that produce and carry semen. Sperm exit the scrotum through the ductus deferens, which is bundled in the spermatic cord. The seminal vesicles and prostate gland add fluids to the sperm to create semen. Scrotum The testes are located in a skin-covered, highly pigmented, muscular sack called the scrotum that extends from the body behind the penis (see [Link]). This location is important in sperm production, which occurs within the testes, and proceeds more efficiently when the testes are kept 2 to 4°C below core body temperature. The dartos muscle makes up the subcutaneous muscle layer of the scrotum ({link]). It continues internally to make up the scrotal septum, a wall that divides the scrotum into two compartments, each housing one testis. Descending from the internal oblique muscle of the abdominal wall are the two cremaster muscles, which cover each testis like a muscular net. By contracting simultaneously, the dartos and cremaster muscles can elevate the testes in cold weather (or water), moving the testes closer to the body and decreasing the surface area of the scrotum to retain heat. Alternatively, as the environmental temperature increases, the scrotum relaxes, moving the testes farther from the body core and increasing scrotal surface area, which promotes heat loss. Externally, the scrotum has a raised medial thickening on the surface called the raphae. The Scrotum and Testes External view of scrotum Muscle layer Deep tissues Ductus Spermatic deferens cord Testicular artery Autonomic nerve Lymphatic vessel |> Testis Y Cremaster muscles muscles This anterior view shows the structures of the scrotum and testes. Testes The testes (singular = testis) are the male gonads—that is, the male reproductive organs. They produce both sperm and androgens, such as testosterone, and are active throughout the reproductive lifespan of the male. Paired ovals, the testes are each approximately 4 to 5 cm in length and are housed within the scrotum (see [link]). They are surrounded by two distinct layers of protective connective tissue ({link]). The outer tunica vaginalis is a serous membrane that has both a parietal and a thin visceral layer. Beneath the tunica vaginalis is the tunica albuginea, a tough, white, dense connective tissue layer covering the testis itself. Not only does the tunica albuginea cover the outside of the testis, it also invaginates to form septa that divide the testis into 300 to 400 structures called lobules. Within the lobules, sperm develop in structures called seminiferous tubules. During the seventh month of the developmental period of a male fetus, each testis moves through the abdominal musculature to descend into the scrotal cavity. This is called the “descent of the testis.” Cryptorchidism is the clinical term used when one or both of the testes fail to descend into the scrotum prior to birth. Anatomy of the Testis } Into inguinal canal Spermatic cord __a Cremaster muscle Efferent Tunica vaginalis ductule Body of Head of» epididymis / epididymis Ductus | Seminiferous deferens tubule lobules ' nC = Septa (tunica Rete w4 FP BOD oars albuginea) testis ——s * “4 Tunica albuginea Straight tubule Tail of epididymis This sagittal view shows the seminiferous tubules, the site of sperm production. Formed sperm are transferred to the epididymis, where they mature. They leave the epididymis during an ejaculation via the ductus deferens. The tightly coiled seminiferous tubules form the bulk of each testis. They are composed of developing sperm cells surrounding a lumen, the hollow center of the tubule, where formed sperm are released into the duct system of the testis. Specifically, from the lumens of the seminiferous tubules, sperm move into the straight tubules (or tubuli recti), and from there into a fine meshwork of tubules called the rete testes. Sperm leave the rete testes, and the testis itself, through the 15 to 20 efferent ductules that cross the tunica albuginea. Inside the seminiferous tubules are six different cell types. These include supporting cells called sustentacular cells, as well as five types of developing sperm cells called germ cells. Germ cell development progresses from the basement membrane—at the perimeter of the tubule— toward the lumen. Let’s look more closely at these cell types. Sertoli Cells Surrounding all stages of the developing sperm cells are elongate, branching Sertoli cells. Sertoli cells are a type of supporting cell called a sustentacular cell, or sustentocyte, that are typically found in epithelial tissue. Sertoli cells secrete signaling molecules that promote sperm production and can control whether germ cells live or die. They extend physically around the germ cells from the peripheral basement membrane of the seminiferous tubules to the lumen. Tight junctions between these sustentacular cells create the blood—testis barrier, which keeps bloodborne substances from reaching the germ cells and, at the same time, keeps surface antigens on developing germ cells from escaping into the bloodstream and prompting an autoimmune response. Germ Cells The least mature cells, the spermatogonia (singular = spermatogonium), line the basement membrane inside the tubule. Spermatogonia are the stem cells of the testis, which means that they are still able to differentiate into a variety of different cell types throughout adulthood. Spermatogonia divide to produce primary and secondary spermatocytes, then spermatids, which finally produce formed sperm. The process that begins with spermatogonia and concludes with the production of sperm is called spermatogenesis. Spermatogenesis As just noted, spermatogenesis occurs in the seminiferous tubules that form the bulk of each testis (see [link]). The process begins at puberty, after which time sperm are produced constantly throughout a man’s life. One production cycle, from spermatogonia through formed sperm, takes approximately 64 days. A new cycle starts approximately every 16 days, although this timing is not synchronous across the seminiferous tubules. Sperm counts—the total number of sperm a man produces—slowly decline after age 35, and some studies suggest that smoking can lower sperm counts irrespective of age. The process of spermatogenesis begins with mitosis of the diploid spermatogonia ([link]). Because these cells are diploid (2n), they each have a complete copy of the father’s genetic material, or 46 chromosomes. However, mature gametes are haploid (1n), containing 23 chromosomes— meaning that daughter cells of spermatogonia must undergo a second cellular division through the process of meiosis. Spermatogenesis (a) Spermatogenesis foe '2n ) | 2n ) Primary spermatocyte BU interstitial =— Lymphatic capillary \)S Sertoli }| (sustentacular) cell a Early G spermatids be S ig