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Anatomy & Physiology Lesson 6 THE LYMPHATIC SYSTEM The lymphatic system is the body system that is responsible for carrying out immune responses. It consists of lymph (a fluid) which flows within lymphatic vessels (lymphatics), the lymph nodes, spleen, thymus gland, and red bone marrow (where stem cells which develop into lymphocytes are found). Lymph and interstitial fluid are essentially the same—after the fluid flows out of the interstitial spaces into the lymphatics it is called lymph. Lymphatic tissue is a form of specialized reticular CT that contains large numbers of lymphocytes. FUNCTIONS OF THE LYMPHATIC SYSTEM Draining interstitial fluid—Lymphatic vessels drain tissue spaces of excess interstitial fluid. Transporting dietary lipids—Lymphatic vessels carry lipids and lipid-soluble vitamins (A, D, E, and K) absorbed by the GI tract to the blood. Protecting against invasion—Lymphatic tissue carries out immune responses. LYMPHATIC VESSELS AND CIRCULATION Lymphatic vessels start as lymphatic capillaries, which are small, close-ended vessels found in intercellular spaces throughout the body (except in avascular tissues, the CNS, splenic pulp, and bone marrow). Lymphatic capillaries converge to form larger lymphatic vessels. The walls of lymphatic capillaries are specially designed to allow the flow of fluid into them, but not out—the endothelial cells that compose the capillary walls overlap slightly, creating one-way valves. Anchoring filaments attach to capillary walls. When edema is present, they open the spaces between cells even further, allow more fluid to flow into the capillaries. LYMPHATIC VESSELS AND CIRCULATION Lymphatic vessels are similar to veins, but have more valves and thinner walls. Lymphatic vessels periodically flow through lymphatic tissue structures called lymph nodes. In the skin, lymphatic vessels lie in subcutaneous tissue and usually follow veins. In the viscera, lymphatic vessels tend to follow arteries, forming plexuses around them. Fluid from blood is constantly seeping out of veins and into interstitial spaces. This fluid enters lymphatic capillaries to form lymph. The lymph then flows into lymphatic vessels and through a series of lymph nodes. After passing the most proximal lymph node in a chain, lymphatic vessels unite to form lymph trunks, which connect to either the right lymphatic duct or the thoracic duct (left lymphatic duct). The lymphatic ducts drain into the jugular veins, returning the fluid back into the circulatory system. LYMPHATIC VESSELS AND CIRCULATION Lymph flow is maintained primarily by pressure from surrounding skeletal muscle contractions. Breathing action also helps with lymphatic flow, creating a pressure gradient between the abdominal region (high pressure) and the thoracic region (low pressure). Also, every time a lymphatic vessel distends, the smooth muscle in its walls contracts, help to move lymph from one segment of the vessel to the next. Lymphatic vessels contain many one-way valves, which prevent the lymph from flowing backward during muscular relaxation or between breaths. LYMPHATIC VESSELS AND CIRCULATION Lymphatic Capillary LYMPHATIC TISSUES The primary lymphatic organs are called such because they produce B and T cells, the lymphocytes that carry out immune responses. These are: The major secondary lymphatic organs are: Red bone marrow—B cells and pre-T cells. Thymus gland—T cells. Lymph nodes Spleen Lymphatic nodules are also included among the secondary lymphatic organs, although they are not technically organs, since they are not encapsulated. THYMUS GLAND The bilobed thymus gland is located in the mediastinum, posterior to the sternum. Each lobe is divided into lobules. The lobules consist of an outer cortex (composed of tightly packed lymphocytes, epithelial cells, and macrophages) and the inner medulla (mostly epithelial cells and scattered lymphocytes). Pre-T cells migrate from red bone marrow to thymus, where they proliferate and mature into T cells. T-helper cells assist B cells in producing antibodies. T-suppressor cells prevent B cells from producing antibodies. The epithelial cells produce thymic hormones, which apparently help with T cell maturation. The thymus gland is large in infants, reaches its maximum size at about 10-12 years of age, then begins to atrophy after puberty. Most T cells arise before puberty, but some continue to mature throughout life. LYMPH NODES Lymph nodes are bean-shaped structures located along the length of lymphatic vessels throughout the body (often concentrated in specific areas). They range from 1 to 25 mm (1 inch) in length. Lymphocytes are produced and stored within the nodes. Lymph passes through a lymph node in one direction, entering through afferent lymphatic vessels and leaving through efferent lymphatic vessels. Lymph nodes filter foreign substances from lymph as it moves back toward the bloodstream. These substances are trapped by reticular fibers within the nodes, where macrophages destroy some substances through phagocytosis and lymphocytes destroy others by immune responses. SPLEEN Measuring 12 cm in length, the spleen is the largest mass of lymphatic tissue in the body. It is located in the left hypochondriac region, between the stomach and diaphragm, lateral to the liver. Lymph is not filtered in the spleen, since it does not have any afferent lymphatic vessels. The spleen does have an artery, vein, and efferent lymphatic vessel, however. It is encapsulated by dense connective tissue and the stroma (framework) is composed of trabeculae, reticular fibers, and fibroblasts. SPLEEN The parenchyma (functional part) of the spleen is composed of two kinds of tissue: White pulp Red pulp White pulp is lymphatic tissue—mostly lymphocytes (B cells) arranged around central arteries. White pulp functions in immunity, proliferating B cells into antibody-producing plasma cells. Red pulp is composed of blood-filled venous sinuses interspersed with thin plates of tissue called splenic (Billroth’s) cords. Splenic cords contain red blood cells, macrophages, lymphocytes, plasma cells, and granulocytes. Red pulp conducts the main functions of the spleen: phagocytosis of bacteria and worn-out or damaged red blood cells and platelets. The fetal spleen also paticipates in blood cell formation. LYMPHATIC NODULES Lymphatic nodules are unencapsulated concentrations of lymphatic tissue. They are usually small, solitary, and scattered throughout the lamina propria of the mucous membranes lining the GI tract, respiratory airways, urinary tract, and reproductive tract. This type of lymphatic tissue is referred to as mucosa-associated lymphoid tissue (MALT). Some lymphatic nodules occur in multiple, large aggregations in certain parts of the body. These include the five tonsils, Peyer’s patches in the ileum of the small intestine, and in the appendix. The tonsils are strategically located to participate in immune responses against foreign substances that are inhaled or ingested. T cells destroy intruders directly, while B cells develop into antibody-secreting plasma cells that destroy foreign substances. IMMUNITY Resistance is the ability to ward off disease through our defenses. Lack of resistance is called susceptibility. Resistance can be broadly classified into two groups: Nonspecific resistance—mechanisms that provide general protection against a broad range of pathogens (skin and mucous membrane barriers, antimicrobial chemicals, phagocytosis, inflammation, and fever). Immunity—involves activation of specific lymphocytes that combat a particular pathogen or foreign substance. The lymphatic system is responsible for immunity. IMMUNITY Immunity is the ability of the body to defend itself against specific invading agents, such as bacteria, toxins, viruses, and foreign tissues. Antigens are substances that are recognized as foreign by the immune system and provoke immune responses. Two properties distinguish immunity from nonspecific defenses. Specificity for particular antigens, which includes the ability to distinguish self from non-self molecules. Memory for most previously encountered antigens so that a second encounter stimulates and even more rapid and vigorous response. IMMUNITY Immunity consists of two kind of immune responses, both triggered by antigens: Cell-mediated (cellular) immune (CMI) responses Antibody-mediated (humoral) immune (AMI) responses Antibodies (Abs) or immunoglobulins are proteins that bind to and inactivate a particular antigen. CMI responses are particularly good at dealing with intracellar pathogens (fungi, parasites, viruses), some cancer cells, and foreign tissue transplants. AMI responses are more effective against antigens dissolved in body fluids and extracellular pathogens (primarily bacteria that are found in body fluids but usually do not enter body cells). Many pathogens provoke both types of immune responses. CELL-MEDIATED IMMUNE RESPONSES Begins with activation of a few T cells (lymphocytes) by a particular antigen. An activated T cell undergoes proliferation and differentiation into a clone of effector cells, a group of identical cells capable of recognizing the same antigen and carrying out some aspect of the immune attack. Ultimately, the immune response results in the elimination of the intruding antigen. ANTIBODY-MEDIATED IMMUNE RESPONSES The body not only contains millions of different T cells, but also millions of different B cells, each capable of responding to a specific antigen. While T cells leave the lymphatic tissue to seek out and destroy an antigen, B cells remain in the lymph nodes, spleen, or other lymphatic tissue. In the presence of a foreign antigen, B cells become activated, differentiating into plasma cells that secrete specific antibodies. The antibodies then circulate in the lymph and blood until they reach the site of invasion, where they proceed to destroy the antigen. ANTIBODIES An antibody binds to the antigen that triggered its production like a key fits into a specific lock. Antibodies belong to a group of glycoproteins called globulins, thus the name immunoglobulins. Most antibodies are composed of four polypeptide chains. Most antibodies have two antigen binding sites, which allows an antibody to bind to two different sites on an antigen. Five types of antibodies are: IgG, IgA, IgM, IgD, and IgE. ANTIBODIES IgG—Most abundant. About 75% of antibodies in blood. Found in blood, lymph, and the intestines. Act by enhancing phagocytosis, neutralizing toxins, and triggering the complement system. Only class of antibodies that cross the placental barrier, providing immune protection for newborns. IgA—About 15% of blood antibodies. Found mostly in sweat, tears, saliva, mucous, milk, and GI secretions. Some found in blood and lymph. Levels decrease during stress, lowering resistance. Provide localized protection on mucous membranes against bacteria and viruses. IgM—5-10% of antibodies in blood. First antibodies excreted by plasma cells after an initial exposure to an antigen. Found in blood and lymph. Activate complement and cause agglutination and lysis of microbes. IgD—<1% of blood antibodies. Found in blood, lymph, and on the surfaces of B cells as antigen receptors. Involved in B cell activation. IgE—<0.1% of antibodies in blood. Located on mast cells and basophils. Involved in allergic and hypersensitivity reactions. Provide protection against parasitic worms. THE NERVOUS SYSTEM The nervous system is the body’s control center (closely seconded by the endocrine system) and communication network. The nervous system serves three basic functions: Sensory function—senses changes within the body and external environment. Integrative function—analyzes sensory information, stores some aspects, and makes decisions regarding apropriate responses. Motor function—may respond to stimuli by initiating muscular contractions or glandular secretions. THE NERVOUS SYSTEM The two principal divisions of the nervous system are: The central nervous system (CNS)—consists of the brain and spinal cord. All neurological impulses are either sent or received by the CNS. The peripheral nervous system (PNS)—consists of cranial nerves and spinal nerves and is connected to sensory receptors, muscles, glands in peripheral parts of the body. Conveys impulses into our out of the CNS. Sensory or afferent neurons provide input to the CNS from various parts of the body. Motor or efferent neurons transmit output from the CNS to muscles and glands. THE NERVOUS SYSTEM The PNS can further be divided into: The somatic nervous system (SNS)— conveys sensory information from cutaneous and special sense receptors in the head, body walls, and limbs to the CNS. Conducts motor impulses to our skeletal muscles. This portion of the SNS is voluntary. The autonomic nervous system (ANS)— conveys sensory information from the viscera to the CNS. Conducts impulses from the CNS to smooth muscle, cardiac muscle, and glands. The ANS is involuntary since it is not normally under our conscious control. THE NERVOUS SYSTEM The ANS can be subclassified, once again, into the sympathetic division and the parasympathetic division. Both divisions provide directions to the viscera, but they are constructed differently and use different neurotransmitters when conveying impulses to their target organs. These two subdivisions generally have opposing actions. For example, sympathetic neurons speed the heartbeat, while parasympathetic neurons slow it down. Sympathetic processes often involve energy expenditure, while parasympathetic processes restore and conserve body energy. THE NERVOUS SYSTEM The nervous system is composed of two basic cell types: Neurons and neuroglia or glial cells. Neurons are responsible for conducting energy (impulses) from one part of the body to another. Neurons have a cell body, dendrites (which carry information toward the cell) and an axon (which conveys information away from the cell body). Neuroglia outnumber neurons 5 to 10 times. Neuroglia perform functions of support and protection. There are several types of neuroglia cells. NEUROGLIA (CNS) Astrocytes—Star-shaped cells with many processes found in the CNS. Participate in metabolizing neurotransmitters and maintain the proper balance of K+ for nerve impulse generation. Participate in brain development by assisting migration of neurons. Help to form the blood-brain barrier. Provide a link between blood vessels and neurons. Oligodendrocytes—Smaller than astrocyts with fewer processes. Round or oval cell body. Form a supporting network around neurons in the CNS. Produce myelin sheath around axons of CNS neurons. Each oligodendrocyte wraps myelin around several axons. NEUROGLIA (CNS) Microglia—Small cells with few processes. Derived from monocytes. Phagocytic cells that engulf and destroy microbes and cellular debris in the CNS. May migrate to injured nervous tissue. Ependymal cells—Epithelial cells that are arranged in a single layer and may range from cuboidal to columnar in shape. Many are ciliated. Line the ventricles of the brain (spaces filled with cerebrospinal fluid) and the central canal of the spinal cord. Form cerebrospinal fluid (CSF) and assist in the circulation of CSF. NEUROGLIA (PNS) Neurolemmocytes (Schwann cells)— Flattened cells arranged around axons in the PNS. Each cell produces part of the myelin sheath around a single axon of a PNS neuron. Satellite cells—Flattened cells arranged around the cell bodies of neurons in ganglia (collections of neuronal cell bodies in the PNS). Support neurons in PNS ganglia. MYELINATION The axons of most neurons are surrounded by a multilayered lipid and protein covering called a myelin sheath. The sheath insulates the axon, increasing the rate of impulse conduction. Axons with a myelin sheath are said to be myelinated, while those without are unmyelinated. Oligodendrocytes produce myelin sheaths in the CNS and Neurolemmocytes do the same in the PNS. MYELINATED AND UNMYELINATED AXONS MYELINATED NEURON ANATOMY NERVES A nerve is a bundle of motor and sensory neurons, together with CT and blood vessels. There are 43 major nerves. They arise in the CNS. Nerve impulses are conducted from one neuron to another across synapses—tiny gaps between the axon of one neuron and the dendrites of an adjacent neuron. Chemicals called nerve transmitter substances or neurotransmitters transmit the information between nerves. Cold slows these impulse transmissions while heat speeds them up. THE SPINAL CORD The spinal cord is a roughly cylindrical column of nervous tissue that is 16 to 18 inches in length and ¾ inch in diameter. It is enclosed and protected by the spinal vertebrae and extends from the base of the brain to the second lumbar vertebra, in adults. It has two obvious enlargements: The cervical enlargement extends from C4 to T1 and is the location from which nerves serving the upper limbs arise. The lumbar enlargement extend from T9 to T12 and is the location from which the nerves serving the lower limbs arise. Two grooves divide the spinal cord into right and left halves: The anterior median fissure and the posterior median sulcus. THE SPINAL CORD 31 pairs of spinal nerves emerge from the spinal cord. The spinal nerves are the communication paths between the spinal cord and most of the body. Each spinal nerve is connected to the spinal cord by two roots: The posterior or dorsal (sensory) root contains sensory nerve fibers that convey impulses from the periphery to the spinal cord. Each posterior root also has a swelling called the posterior or dorsal (sensory) root ganglion, which contains the sensory nerve cell bodies. The anterior or ventral (motor) root contains motor neuron axons and conducts impulses from the spinal cord to the periphery. THE SPINAL CORD The spinal cord is composed of collections of neurons and bundles of nerve fibers. When viewed in cross-section, the spinal cord is composed of gray matter (shaped like a butterfly or a sideways “H”) surrounded by three columns of white matter. Gray matter consists primarily of neuronal cell bodies, neuroglia, unmyelinated axons, and dendrites of association and motor neurons. White matter is composed of bundles of myelinated axons and motor and sensory neurons. THE SPINAL CORD The spinal cord performs to principal functions: 1) The white matter tracts propagate nerve impulses. Along these “highways,” sensory impulses travels from the periphery to the brain and motor impulses flow from the brain to the periphery. 2) The gray matter of the spinal cord receives and integrates incoming and outgoing information. As one aspect of this function, it can sometimes bypass the usual impulse transmissions to the brain, allowing for and controlling simple reflexes itself. Both of these functions are essential to the maintenance of homeostasis. THE SPINAL CORD Meninges are protective CT coverings that surround the spinal cord (and brain). There are three layers of meninges: The dura mater is the outermost layer. The arachnoid forms the middle layer. Between the arachnoid and the dura mater is a thin subdural space with contains interstitial fluid. The deepest layer is the pia mater. Between the pia mater and the arachnoid is the subarachnoid space, which is filled with cerebrospinal fluid. The spinal cord is also protected by a cushion of fat and CT located in the epidural space between the dura mater and the vertebral canal. THE SPINAL CORD CRANIAL NERVES 12 pairs of nerves emerge from the underside of the brain. These nerves supply the muscles and sensory organs of the head. In addition, the vagus nerve (cranial nerve X) supplies the digestive organs, heart, and air passages in the lungs. Some of these nerves supply sensory information only, some are primarily responsible for motor impulses (with some sensory aspects, as well), and some mix both sensory and motor functions. SPINAL NERVES The 31 pairs of spinal nerves are named and numbered for the region and level of the spinal cord from which they emerge. The first cervical pair emerges from between the occipital bone and the atlas. All other spinal nerves leave the spinal cord through channels between adjacent vertebrae. There are 8 pairs of cervical nerves, 12 pairs of thoracic nerves, 5 pairs of lumbar nerves, 5 pairs of sacral nerves, and 1 pair of coccygeal nerves. Each pair of nerves is responsible for the control of certain parts of the body. SPINAL NERVES A simplified summary of the areas of influence of the various spinal nerves is as follows: C1-C5—skin and muscles of the head, neck, and upper part of the shoulders. C5-C8 & T1—upper extremities and shoulder regions. T2—under the arms and the back of the arms. T3 & T6—intercostal muscles and skin of the anterior and lateral chest. T7-T11—intercostal muscles, abdominal muscles and overlying skin, deep back muscles, and skin of the dorsal thorax. L1-L4—anterolateral abdominal wall, external genitals, and part of the upper legs. L4-L5 & S1-S4—buttocks, perineum, and lower extremeties, including the legs and feet. THE BRAIN The brain is one of the largest organs in the body, averaging about 3 pounds in weight. It possesses about 100 billion neurons and 1000 billion neuroglia. The brain can be divided into four principal parts: the brain stem, cerebellum, diencephalon, and cerebrum. THE BRAIN The brain stem is continuous with the spinal cord and consists of the medulla oblongata, pons, and midbrain. Posterior to the brain stem is the cerebellum. Superior to the brain stem is the diencephalon, which consists primarily of the thalamus and hypothalamus. The cerebrum covers the diencephalon and fills most of the cranium. It is divided into right and left halves called hemispheres.. THE BRAIN THE BRAIN The brain is protected by the cranial bones and cranial meninges. The cranial meninges surround the brain, are continuous with the spinal meninges, have the same basic structure and bear the same names. The outer layer is called the dura mater, the middle layer the arachnoid, and the inner layer the pia mater. Extensions of the dura mater separate the two hemispheres of the cerebrum, the two hemispheres of the cerebellum, and the cerebrum from the cerebellum. CEREBROSPINAL FLUID (CSF) The brain and spinal cord are nourished and protected from chemical or physical injury by cerebrospinal fluid (CSF). CSF circulates continuously through the subarachnoid space around the brain and spinal cord, and through cavities within the brain. The four CSF-filled cavities in the brain are called ventricles. The two larger lateral ventricles are located in the two hemispheres of the cerebrum, the narrow third ventricle is situated at the midline, superior to the hypothalamus and between the right and left halves of the thalamus, and the fourth ventricle lies between the brain stem and the cerebellum. CEREBROSPINAL FLUID (CSF) The CNS contains a total of about 3-5 ounces of CSF. CSF is a clear, colorless liquid that contains glucose, proteins, lactic acid, urea, cations, anions, and some white blood cells. It is constantly being produced from blood by specialized brain cells. CSF contributes to homeostasis in three main ways: Mechanical protection—serves as a shock absorber and allows the brain to “float” in the cranial cavity. Chemical protection—provides and optimal chemical environment for accurate neuronal signaling. Circulation—is a medium for exchange of nutrients and waste products between the blood and nervous tissue. FUNCTIONS OF PARTS OF THE BRAIN Brain stem: Medulla oblongata—Relays motor and sensory impulses between other parts of the brain and the spinal cord. Functions in consciousness and arousal. Helps to regulate heartbeat, breathing, and blood vessel diameter. Coordinates swallowing, vomiting, coughing, sneezing, and hiccupping. Contains nuclei of origin for cranial nerves VII, IX, X, XI, and XII. Pons—Relays impulses from one side of the cerebellum to the other and between the medulla and midbrain. Helps to control breathing. Contains nuclei of origin for cranial nerves V, VI, VII, and VIII. FUNCTIONS OF PARTS OF THE BRAIN Brain stem (cont): Midbrain—Relays motor impulses from the cerebral cortex to the pons and sensory impulses from the spinal cord to the thalamus. Coordinates movements of the eyeballs in response to visual and other stimuli and the head and trunk in response to auditory stimuli. Contributes to movement control. Contains nuclei of origin for cranial nerves III and IV. Cerebellum Compares intended movements with actual movements to smooth and coordinate complex, skilled movements. Regulates posture and balance. FUNCTIONS OF PARTS OF THE BRAIN Diencephalon Epithalamus—Contains pineal gland, which secretes melatonin, habenular nuclei, and the choroid plexus of the third ventricle. Thalamus—Relays all sensory input to the cerebral cortex. Provides crude appreciation of touch, pressure, pain, and temperature. Involved in motor actions, arousal, emotions, memory, cognition, and awareness. Subthalamus—Helps to control muscle movement. Hypothalamus—Controls and integrates activities of the ANS and pituitary gland. Regulates emotional and behavioral patterns and diurnal rhythms. Controls body temperature, eating, and drinking behavior. Helps maintain waking and establishes sleep patterns. FUNCTIONS OF PARTS OF THE BRAIN Cerebrum Sensory areas of the cerebral cortex interpret sensory impulses, motor areas control muscular movements, and associational areas function in emotional and intellectual processes. Basal ganglia coordinate gross, automatic muscle movements and regulate muscle tone. Limbic system functions in emotional aspects of behavior related to survival. The cerebral cortex is the superficial layer of the cerebrum, composed of gray matter. It is 2-4 mm thick and contains billions of neurons. BRAIN LATERALIZATION Although the brain may appear bilaterally symmetrical, it actual has structural and functional differences between the hemispheres. The left hemisphere recieves sensory input from and controls the right side of the body, while the right hemisphere receives sensory input from and controls the left side of the body. Other important functional differences (in most people) are as follows: Left hemisphere—Control of: muscles on right side of body, spoken and written language, numerical and scientific skills, reasoning. Right hemisphere—Control of: muscles on left side of body, musical and artistic awareness, space and pattern perception, insight, imagination, generating mental images to compare spacial relationships. STRUCTURES OF THE EYE