Autonomic Nervous System

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Autonomic Nervous System - page 1
Seeley−Stephens−Tate: Anatomy and Physiology, Sixth Edition III. Integration and Control Systems 16. Autonomic Nervous System © The McGraw−Hill Companies, 2004 Autonomic Nervous System Light photomicrograph from a section of the small intestine, showing the nerve cells of the enteric plexus. These nerve cells regulate the contraction of smooth muscle and the secretion of glands within the intestinal wall. 16 C H A P T E R Part 3 Integration and Control Systems During a picnic on a sunny spring day, it is easy to concentrate on the deli- cious food and the pleasant surround- ings. Maintenance of homeostasis, however, requires no conscious thought. The autonomic nervous system (ANS) helps to keep body temperature at a constant level by controlling the activity of sweat glands and the amount of blood flowing through the skin. The ANS helps to regulate the complex activities necessary for the digestion of food. The movement of absorbed nutrients to tissues is possible because the ANS controls heart rate, which helps to maintain the blood pressure necessary to deliver blood to tissues. Without the ANS, all of the activities necessary to maintain homeosta- sis would be overwhelming. A functional knowledge of the ANS enables you to predict general re- sponses to a variety of stimuli, explain responses to changes in environmental conditions, comprehend symptoms that result from abnormal autonomic func- tions, and understand how drugs affect the ANS. This chapter examines the au- tonomic nervous system by contrasting the somatic and autonomic nervous systems (548); describing the anatomy of the autonomic nervous system (549), the physiology of the autonomic nervous system (555), and the regulation of the autonomic nervous system (559); and examining functional generalizations about the autonomic nervous system (562).
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Autonomic Nervous System - page 2
Seeley−Stephens−Tate: Anatomy and Physiology, Sixth Edition III. Integration and Control Systems 16. Autonomic Nervous System © The McGraw−Hill Companies, 2004 548 Part 3 Integration and Control System Contrasting the Somatic and Autonomic Nervous Systems Objective Compare the structural and functional differences between the somatic and autonomic nervous systems. The peripheral nervous system (PNS) is composed of sen- sory and motor neurons. Sensory neurons carry action potentials from the periphery to the central nervous system (CNS), and mo- tor neurons carry action potentials from the CNS to the periphery. Motor neurons are either somatic motor neurons, which innervate skeletal muscle, or autonomic motor neurons, which innervate smooth muscle, cardiac muscle, and glands. Although axons of autonomic, somatic, and sensory neurons are in the same nerves, the proportion varies from nerve to nerve. For example, nerves innervating smooth muscle, cardiac muscle, and glands consist primarily of autonomic neurons; and nerves in- nervating skeletal muscles consist primarily of somatic neurons. Some cranial nerves such as the olfactory, optic, and vestibulo- cochlear nerves are composed entirely of sensory neurons. The cell bodies of somatic motor neurons are in the CNS, and their axons extend from the CNS to skeletal muscle (figure 16.1a). The ANS, on the other hand, has two neurons in a series ex- tending between the CNS and the organs innervated (figure 16.1b). The first neurons of the series are called preganglionic neurons. Their cell bodies are located within either the brainstem or the spinal cord, and their axons extend to autonomic ganglia located outside the CNS. The autonomic ganglia contain the cell bodies of the second neurons of the series, which are called postganglionic neurons. The preganglionic neurons synapse with the postgan- glionic neurons in the autonomic ganglia. The axons of the post- ganglionic neurons extend to effector organs, where they synapse with their target tissues. Many movements controlled by the somatic nervous system are conscious, whereas ANS functions are unconsciously con- trolled. The effect of somatic motor neurons on skeletal muscle is always excitatory, but the effect of the ANS on target tissues can be excitatory or inhibitory. For example, after a meal, the ANS can stimulate stomach activities, but during exercise, the ANS can in- hibit those activities. Table 16.1 summarizes the differences be- tween the somatic nervous system and the ANS. Sensory neurons are not classified as somatic or autonomic. These neurons propagate action potentials from sensory receptors to the CNS and can provide information for reflexes mediated through the somatic nervous system or the ANS. For example, stimulation of pain receptors can initiate somatic reflexes such as the withdrawal reflex and autonomic reflexes such as an increase in heart rate. Although some sensory neurons primarily affect so- matic functions and others primarily influence autonomic func- tions, functional overlap makes attempts to classify sensory neurons as either somatic or autonomic meaningless. Spinal nerve Somatic motor neuron Spinal cord Skeletal muscle (a) Spinal nerve Autonomic ganglion Spinal cord Preganglionic neuron Postganglionic neuron Effector organ (e.g., smooth muscle of colon) (b) Figure 16.1 Organization of Somatic and Autonomic Nervous System Neurons (a) The cell body of the somatic neuron is in the CNS, and its axon extends to the skeletal muscle. (b) The cell body of the preganglionic neuron is in the CNS, and its axon extends to the autonomic ganglion and synapses with the postganglionic neuron. The postganglionic neuron extends to and synapses with its effector organ.
Autonomic Nervous System - page 3
Seeley−Stephens−Tate: Anatomy and Physiology, Sixth Edition III. Integration and Control Systems 16. Autonomic Nervous System © The McGraw−Hill Companies, 2004 Chapter 16 Autonomic Nervous System 549 Table 16.1 Comparison of the Somatic and Autonomic Nervous Systems Features Target tissues Regulation Response to stimulation Neuron arrangement Somatic Nervous System Skeletal muscle Controls all conscious and unconscious movements of skeletal muscle Skeletal muscle contracts One neuron extends from the central nervous system (CNS) to skeletal muscle Autonomic Nervous System Smooth muscle, cardiac muscle, and glands Unconscious regulation, although influenced by conscious mental functions Target tissues are stimulated or inhibited Two neurons in series; the preganglionic neuron extends from the CNS to an autonomic ganglion, and the postganglionic neuron extends from the autonomic ganglion to the target tissue Preganglionic neuron cell bodies are in autonomic nuclei of the cranial nerves and in the lateral part of the spinal cord; postganglionic neuron cell bodies are in autonomic ganglia Two synapses; first is in the autonomic ganglia; second is at the target tissue Preganglionic axons are myelinated; postganglionic axons are unmyelinated Acetylcholine is released by preganglionic neurons; either acetylcholine or norepinephrine is released by postganglionic neurons In autonomic ganglia, receptor molecules for acetylcholine are nicotinic; in target tissues, receptor molecules for acetylcholine are muscarinic, whereas receptor molecules for norepinephrine are either α- or β-adrenergic Neuron cell body location Neuron cell bodies are in motor nuclei of the cranial nerves and in the ventral horn of the spinal cord One synapse between the somatic motor neuron and the skeletal muscle Myelinated Acetylcholine Number of synapses Axon sheaths Neurotransmitter substance Receptor molecules Receptor molecules for acetylcholine are nicotinic 1. Contrast the somatic nervous system with the ANS for each of the following: a. the number of neurons between the CNS and effector organ b. the location of neuron cell bodies c. the structures each innervates d. inhibitory or excitatory effects e. conscious or unconscious control 2. Why are sensory neurons not classified as somatic or autonomic? 3. Define the terms preganglionic neuron, postganglionic neuron, and autonomic ganglia. The enteric nervous system is a complex network of neuron cell bodies and axons within the wall of the digestive tract. An im- portant part of this network is sympathetic and parasympathetic neurons. For this reason, the enteric nervous system is considered to be part of the ANS. Sympathetic Division Cell bodies of sympathetic preganglionic neurons are in the lateral horns of the spinal cord gray matter between the first thoracic (T1) and the second lumbar (L2) segments (figure 16.2). Because of the location of the preganglionic cell bodies, the sympathetic division is sometimes called the thoracolumbar division. The axons of the preganglionic neurons pass through the ventral roots of spinal nerves T1–L2, course through the spinal nerves for a short dis- tance, leave these nerves, and project to autonomic ganglia on ei- ther side of the vertebral column behind the parietal pleura. These ganglia are called sympathetic chain ganglia, because they are connected to one another and form a chain, or paravertebral gan- glia, because they are located along both sides of the vertebral col- umn. Only the ganglia from T1–L2 receive preganglionic axons from the spinal cord, although the sympathetic chain extends into the cervical and sacral regions so that one pair of ganglia is associ- ated with nearly every pair of spinal nerves. The cervical ganglia usually fuse during fetal development so only two or three pairs ex- ist in the adult. The axons of preganglionic neurons are small in diameter and myelinated. The short connection between a spinal nerve and a sympathetic chain ganglion through which the preganglionic Anatomy of the Autonomic Nervous System Objectives Compare the structural differences between the sympathetic and parasympathetic divisions. Describe the structure of the enteric nervous system. Describe how sympathetic and parasympathetic axons are distributed to organs. The ANS is subdivided into the sympathetic and the parasym- pathetic divisions and the enteric (en-ter ik; bowels) nervous system (ENS). The sympathetic and parasympathetic divisions differ structurally in (1) the location of their preganglionic neuron cell bod- ies within the CNS and (2) the location of their autonomic ganglia.
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Seeley−Stephens−Tate: Anatomy and Physiology, Sixth Edition III. Integration and Control Systems 16. Autonomic Nervous System © The McGraw−Hill Companies, 2004 550 Part 3 Integration and Control System Preganglionic neuron Postganglionic neuron Preganglionic cell body in lateral horn of gray matter Preganglionic neuron to sympathetic chain ganglion T1 Postganglionic neurons Preganglionic neuron to collateral ganglion L2 Postganglionic neurons Collateral ganglia Sympathetic chain ganglia Figure 16.2 Sympathetic Division The location of sympathetic preganglionic (solid blue) and postganglionic (dotted blue) neurons. The preganglionic cell bodies are in the lateral gray matter of the thoracic and lumbar parts of the spinal cord. The cell bodies of the postganglionic neurons are within the sympathetic chain ganglia or within collateral ganglia. axons pass is called a white ramus communicans (ra mı s ko- ¯ ˘ ˘ mu ni-kans; pl., rami communicantes, ra mı ko-mu-ni-kan tez) ¯ ¯ ¯ ˘ ¯ ¯ because of the whitish color of the myelinated axons (figure 16.3). Sympathetic axons exit the sympathetic chain ganglia by the following four routes: 1. Spinal nerves (figure 16.3a). Preganglionic axons synapse with postganglionic neurons in sympathetic chain ganglia at the same level that the preganglionic axons enter the sympathetic chain. Alternatively, preganglionic axons pass either superiorly or inferiorly through one or more ganglia and synapse with postganglionic neurons in a sympathetic chain ganglion at a different level. Axons of the postgan- glionic neurons pass through a gray ramus communicans and reenter a spinal nerve. Postganglionic axons are not myelinated, thereby giving the gray ramus communicans its grayish color. The postganglionic axons then project through the spinal nerve to the organs they innervate. 2. Sympathetic nerves (figure 16.3b). Preganglionic axons enter the sympathetic chain and synapse in a sympathetic chain ganglion at the same or a different level with postganglionic neurons. The postganglionic axons leaving the sympathetic chain ganglion form sympathetic nerves. 3. Splanchnic (splangk nik) nerves (figure 16.3c). Some preganglionic axons enter sympathetic chain ganglia and, without synapsing, exit at the same or a different level to form splanchnic nerves. Those preganglionic axons extend to collateral, or prevertebral, ganglia, where they synapse with postganglionic neurons. Axons of the postganglionic neurons leave the collateral ganglia through small nerves that extend to target organs. 4. Innervation to the adrenal gland (figure 16.3d). The splanchnic nerve innervation to the adrenal glands is different from other ANS nerves because it consists of only preganglionic neurons. Axons of the preganglionic neurons do not synapse in sympathetic chain ganglia or in collateral ganglia. Instead, the axons pass through those ganglia and synapse with cells in the adrenal medulla. The adrenal medulla (me-dool a) is the inner portion of the adrenal ˘ gland and consists of specialized cells derived during embryonic development from neural crest cells (see figure 13.13), which are the same population of cells that give rise to the postganglionic cells of the ANS. Adrenal medullary cells are round in shape, have no axons or dendrites, and are divided into two groups. About 80% of the cells secrete epinephrine (ep i-nef rin), also called adrenaline (a-dren a-lin), and about 20% secrete norepinephrine ˘ ˘ (nor ep-i-nef rin), also called noradrenaline (nor-a-dren a- ¯ ¯ ˘ ˘ lin). Stimulation of these cells by preganglionic axons causes the release of epinephrine and norepinephrine. These substances circulate in the blood and affect all tissues having receptors to which they can bind. The general response to epinephrine and norepinephrine released from the adrenal medulla is to prepare the individual for physical activity. Secretions of the adrenal medulla are considered hormones because they are released into the general circulation and travel some distance to the tissues in which they have their effect (see chapters 17 and 18). Parasympathetic Division Parasympathetic preganglionic neurons are located both supe- rior and inferior to the thoracic and lumbar regions of the spinal cord where sympathetic preganglionic neurons are found. The cell bodies of parasympathetic preganglionic neurons are either within cranial nerve nuclei in the brainstem or within the lateral parts of the gray matter in the sacral region of the spinal cord from S2–S4 (figure 16.4). For that reason, the parasympathetic division is sometimes called the craniosacral (kra ne-o ¯ kral) ¯ ¯ ¯-sa ˘ division. Axons of the parasympathetic preganglionic neurons from the brain are in cranial nerves III, VII, IX, and X; and from the spinal cord in pelvic nerves. The preganglionic axons course through these nerves to terminal ganglia where they synapse with postganglionic neurons. The axons of the postganglionic neurons extend relatively short distances from the terminal ganglia to the target organs. The terminal ganglia are either near or embedded
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Seeley−Stephens−Tate: Anatomy and Physiology, Sixth Edition III. Integration and Control Systems 16. Autonomic Nervous System © The McGraw−Hill Companies, 2004 Chapter 16 Autonomic Nervous System 551 Dorsal root ganglion Preganglionic neuron Postganglionic neuron Gray ramus communicans White ramus communicans Spinal nerve Sympathetic chain ganglion (a) Preganglionic neuron Postganglionic neuron Ventral root Preganglionic neuron White ramus communicans Sympathetic nerves Postganglionic neuron Heart (b) Gray ramus communicans White ramus communicans White ramus communicans Preganglionic neuron Splanchnic nerve Preganglionic neuron Collateral ganglion Postganglionic neuron Sympathetic chain ganglion Adrenal gland Collateral ganglion Viscera (c) (d) Figure 16.3 Routes Taken by Sympathetic Axons (a) Preganglionic axons enter a sympathetic chain ganglion through a white ramus communicans. Some axons synapse with a postganglionic neuron at the level of entry; others ascend or descend to other levels before synapsing. Postganglionic axons exit the sympathetic chain ganglia through gray rami communicantes and enter spinal nerves. (b) Like part (a), except that postganglionic axons exit through a sympathetic nerve (only an ascending axon is illustrated). (c) Preganglionic neurons do not synapse in the sympathetic chain ganglia but exit in splanchnic nerves and extend to collateral ganglia, where they synapse with postganglionic neurons. (d ) Like part (c), except that preganglionic axons extend to the adrenal medulla, where they synapse. There are no postganglionic neurons.
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Seeley−Stephens−Tate: Anatomy and Physiology, Sixth Edition III. Integration and Control Systems 16. Autonomic Nervous System © The McGraw−Hill Companies, 2004 552 Part 3 Integration and Control System Preganglionic neuron Postganglionic neuron within the walls of the organs innervated by the parasympathetic neurons. Many of the parasympathetic ganglia are small in size, but some, such as those in the wall of the digestive tract, are large. Table 16.2 summarizes the structural differences between the sympathetic and parasympathetic divisions. 4. For both the sympathetic and parasympathetic divisions, state (a) the locations of their preganglionic neuron cell bodies and (b) the names and locations of their ganglia. 5. What types of axon (preganglionic or postganglionic, myelinated or unmyelinated) are found in white and gray rami communicantes? 6. Where do preganglionic neurons synapse with postganglionic neurons that are found in spinal and sympathetic nerves? 7. Where do preganglionic axons that form splanchnic nerves (except those to the adrenal gland) synapse with postganglionic neurons? 8. What is unusual about the splanchnic nerve innervation to the adrenal gland? What do the specialized cells of the adrenal medulla secrete, and what is the effect of these substances? Midbrain Cranial nerves Pons Medulla Brainstem Postganglionic neurons Terminal ganglia Preganglionic neurons Enteric Nervous System Sacral region of spinal cord (S2–S4) Pelvic nerves Figure 16.4 Parasympathetic Division The location of parasympathetic preganglionic (solid red) and postganglionic (dotted red ) neurons. The preganglionic neuron cell bodies are in the brainstem and the lateral gray matter of the sacral part of the spinal cord, and the postganglionic neuron cell bodies are within terminal ganglia. The enteric nervous system consists of nerve plexuses within the wall of the digestive tract (see figure 24.2). The plexuses have con- tributions from three sources: (1) sensory neurons that connect the digestive tract to the CNS, (2) ANS motor neurons that connect the CNS to the digestive tract, and (3) enteric neurons, which are con- fined to the enteric plexuses. The CNS is capable of monitoring the digestive tract through sensory neurons and controlling its smooth muscle and glands through ANS motor neurons. There are several major types of enteric neurons: (1) Enteric sensory neurons can detect changes in the chemical composition of the contents of the digestive tract or detect stretch of the digestive tract wall. (2) Enteric motor neurons can stimulate or inhibit smooth muscle contraction and gland secretion. (3) Enteric Table 16.2 Comparison of the Sympathetic and Parasympathetic Divisions Features Location of preganglionic cell body Outflow from the CNS Sympathetic Division Lateral horns of spinal cord gray matter (T1–L2) Spinal nerves Sympathetic nerves Splanchnic nerves Sympathetic chain ganglia along spinal cord for spinal and sympathetic nerves; collateral ganglia for splanchnic nerves Many (much divergence) Parasympathetic Division Brainstem and lateral parts of spinal gray matter (S2–S4) Cranial nerves Pelvic nerves Terminal ganglia near or on effector organ Ganglia Number of postganglionic neurons for each preganglionic neuron Relative length of neurons Few (less divergence) Short preganglionic Long postganglionic Long preganglionic Short postganglionic
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Seeley−Stephens−Tate: Anatomy and Physiology, Sixth Edition III. Integration and Control Systems 16. Autonomic Nervous System © The McGraw−Hill Companies, 2004 Chapter 16 Autonomic Nervous System 553 interneurons connect enteric sensory and motor neurons to each other. Although the enteric neurons are capable of controlling the activities of the digestive tract completely independently of the CNS, normally the two systems work together. P R E D I C T Would the ANS ganglia found in the enteric plexus be chain ganglia, collateral ganglia, or terminal ganglia? What type (preganglionic or postganglionic) of sympathetic and parasympathetic axons contribute to the enteric plexus? The Distribution of Autonomic Nerve Fibers Sympathetic Division Sympathetic axons pass from the sympathetic chain ganglia to their target tissues through spinal, sympathetic, and splanchnic nerves. The sympathetic and splanchnic nerves can join auto- nomic nerve plexuses, which are complex, interconnected neural networks formed by neurons of the sympathetic and parasympa- thetic divisions. In addition, the axons of sensory neurons con- tribute to these plexuses. The autonomic nerve plexuses typically are named accord- ing to organs they supply or to blood vessels along which they are found. For example, the cardiac plexus supplies the heart and the thoracic aortic plexus is found along the thoracic aorta. Plexuses following the route of blood vessels is a major means by which autonomic axons are distributed throughout the body. The major means by which sympathetic axons reach organs include the following: 1. Spinal nerves. From all levels of the sympathetic chain, some postganglionic axons project through gray rami communicates to spinal nerves. The axons extend to the same structures innervated by the spinal nerves and supply sweat glands in the skin, smooth muscle in skeletal and skin blood vessels, and the smooth muscle of the arrector pili. See figure 12.14 for the distribution of spinal nerves to the skin. 2. Head and neck nerve plexuses. Most of the sympathetic nerve supply to the head and neck is derived from the superior cervical ganglion of the sympathetic chain (figure 16.5). Postganglionic axons of sympathetic nerves form plexuses that extend superiorly to the head and inferiorly to the neck. The plexuses give off branches to supply sweat glands in the skin, smooth muscle in skeletal and skin blood vessels, and the smooth muscle of the arrector pili. Axons from the plexuses also join branches of the trigeminal nerves (cranial nerve V) to supply the skin of the face, the salivary glands, the iris, and the ciliary muscles of the eye. 3. Thoracic nerve plexuses. The sympathetic supply for organs of the thorax is mainly derived from the cervical and upper five thoracic sympathetic chain ganglia. Postganglionic axons in sympathetic nerves contribute to the cardiac plexus, supplying the heart, the pulmonary plexus, supplying the lungs, and other thoracic plexuses (see figure 16.5). 4. Abdominopelvic nerve plexuses. Sympathetic chain ganglia from T5 and below mainly supply the abdominopelvic organs. The preganglionic axons of splanchnic nerves synapse with postganglionic neurons in the collateral ganglia of abdominopelvic nerve plexuses. Postganglionic axons from the collateral ganglia innervate smooth muscle and glands in the abdominopelvic organs. There are several abdominopelvic nerve plexuses (see figure 16.5). The celiac (se le-ak) plexus has two large celiac ganglia and other ¯ ¯ smaller ganglia. It supplies the diaphragm, stomach, spleen, liver, gallbladder, adrenal glands, kidneys, testes, and ovaries. The superior mesenteric (mez-en-ter ik) plexus includes the superior mesenteric ganglion and supplies the pancreas, small intestine, ascending colon, and the transverse colon. The inferior mesenteric plexus includes the inferior mesenteric ganglion and supplies the transverse colon to the rectum. The hypogastric plexuses supply the descending colon to the rectum, the urinary bladder, and reproductive organs in the pelvis. Parasympathetic Division Parasympathetic outflow is through cranial and sacral nerves. Branches of these nerves either supply organs or join nerve plexuses to be distributed to organs. The major means by which parasympathetic axons reach organs include the following: 1. Cranial nerves supplying the head and neck. Three pairs of cranial nerves have parasympathetic preganglionic axons that extend to terminal ganglia in the head. Postganglionic neurons from the terminal ganglia supply nearby structures. The parasympathetic cranial nerves, their terminal ganglia, and the structures innervated are (see figure 16.5 and table 14.1): a. The oculomotor (III) nerve, through the ciliary (sil e-ar-e) ganglion, supplies the ciliary muscles and the ¯ ¯ iris of the eye. b. The facial (VII) nerve, through the pterygopalatine (ter i-go-pal a-tı n) ganglion, supplies the lacrimal gland ¯ ˘ ¯ and mucosal glands of the nasal cavity and palate. The facial nerve, through the submandibular ganglion, also supplies the submandibular and sublingual salivary glands. c. The glossopharyngeal (IX) nerve, through the otic (o tik) ganglion, supplies the parotid salivary gland. ¯ 2. The vagus nerve and thoracic nerve plexuses. Although cranial nerve X, the vagus nerve, has somatic motor and sensory functions in the head and neck, its parasympathetic distribution is to the thorax and abdomen. Preganglionic axons extend through the vagus nerves to the thorax, where they pass through branches of the vagus nerves to contribute to the cardiac plexus, which supplies the heart, and the pulmonary plexus, which supplies the lungs. The vagus nerves continue down the esophagus, and give off branches to form the esophageal plexus. 3. Abdominal nerve plexuses. After the esophageal plexus passes through the diaphragm, some of the vagal preganglionic
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Seeley−Stephens−Tate: Anatomy and Physiology, Sixth Edition III. Integration and Control Systems 16. Autonomic Nervous System © The McGraw−Hill Companies, 2004 554 Part 3 Integration and Control System Facial nerve Glossopharyngeal nerve Internal carotid plexus Superior cervical sympathetic chain ganglion Sympathetic nerves Cervicothoracic ganglion Oculomotor nerve Ciliary ganglion Pterygopalatine ganglion Otic ganglion Submandibular ganglion Vagus nerve Pulmonary plexus Cardiac plexus Sympathetic nerves Fifth thoracic sympathetic chain ganglion Greater splanchnic nerve Spinal nerve White ramus communicans Gray ramus communicans Lesser splanchnic nerve Kidney Second lumbar sympathetic chain ganglion Lumbar splanchnic nerves Stomach Celiac ganglion and plexus Superior mesenteric ganglion and plexus Aorta and abdominal aortic plexus Small intestine Inferior mesenteric ganglion and plexus Superior hypogastric plexus Colon Inferior hypogastric plexus Urinary bladder Prostate gland Esophagus and esophageal plexus Heart Aorta and thoracic aortic plexus Sacral splanchnic nerves Pelvic nerves Sacral plexus Rectum Sympathetic Parasympathetic Figure 16.5 Distribution of Autonomic Nerve Fibers Sympathetic supply: (1) spinal nerves to limbs and body, (2) head and neck by sympathetic nerves from the superior cervical chain ganglia, (3) thoracic organs by sympathetic nerves from the cervical and thoracic chain ganglia (to T5) supplying thoracic nerve plexuses, and (4) abdominopelvic nerves by splanchnic nerves from chain ganglia below T5 supplying abdominopelvic nerve plexuses. Parasympathetic supply: (1) head and neck by cranial nerves and their ganglia, (2) thoracic organs by vagus nerves supplying thoracic plexuses, (3) abdominal organs by vagus nerves supplying abdominal nerve plexuses, and (4) pelvic nerves from S2–S4.
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Seeley−Stephens−Tate: Anatomy and Physiology, Sixth Edition III. Integration and Control Systems 16. Autonomic Nervous System © The McGraw−Hill Companies, 2004 Chapter 16 Autonomic Nervous System 555 axons supply terminal ganglia in the wall of the stomach, while others contribute to the celiac and superior mesenteric plexuses. Through these plexuses, the preganglionic axons supply terminal ganglia in the walls of the gallbladder, biliary ducts, pancreas, small intestine, ascending colon, and the transverse colon. 4. Pelvic nerves and pelvic nerve plexuses. Parasympathetic preganglionic axons whose cell bodies are in the S2–S4 region of the spinal cord pass to the ventral rami of spinal nerves and enter the pelvic nerves. The pelvic nerves supply the transverse colon to the rectum, and they also contribute to the hypogastric plexus. The hypogastric plexus and its derivatives supply the lower colon, rectum, urinary bladder, and organs of the reproductive system in the pelvis. Physiology of the Autonomic Nervous System Objective Describe the major neurotransmitters and receptors of the ANS. Neurotransmitters Sympathetic and parasympathetic nerve endings secrete one of two neurotransmitters. If the neuron secretes acetylcholine, it is a cholinergic (kol-in-er jik) neuron, and if it secretes norepi- nephrine, it is an adrenergic (ad-re-ner jik) neuron. All pregan- ˘ glionic neurons of the sympathetic and parasympathetic divisions and all postganglionic neurons of the parasympathetic division are cholinergic. Almost all postganglionic neurons of the sympathetic division are adrenergic, but a few postganglionic neurons that innervate thermoregulatory sweat glands are cholinergic (figure 16.6). In recent years, substances in addition to the regular neuro- transmitters have been extracted from ANS neurons. These sub- stances include nitric oxide; fatty acids, such as prostaglandins; peptides, such as gastrin, somatostatin, cholecystokinin, vasoactive intestinal peptide, enkephalins, and substance P; and monoamines, such as dopamine, serotonin, and histamine. The specific role that many of these compounds play in the regulation of the ANS is un- clear, but they appear to function as either neurotransmitters or neuromodulator substances (see chapter 11). Sensory Neurons in Autonomic Nerve Plexuses Although not strictly part of the ANS, the axons of sensory neu- rons run alongside ANS axons within ANS nerves and plexuses. Some of these sensory neurons are part of reflex arcs regulating or- gan activities. Sensory neurons also transmit pain and pressure sensations from organs to the CNS. The cell bodies of these sensory neurons are found in the dorsal root ganglia and in the sensory ganglia of certain cranial nerves, which are swellings on the nerves close to their attachment to the brain. Effects of Spinal Cord Injury on ANS Functions Spinal cord injury can damage nerve tracts and interrupt control of autonomic neurons by ANS centers in the brain. For the parasympathetic division, effector organs innervated through the sacral region of the spinal cord are affected, but most effector organs still have normal parasympathetic function because they are innervated by the vagus nerve. For the sympathetic division, brain control of sympathetic neurons is lost below the site of the injury. The higher the level of injury, the greater the number of body parts affected. Receptors Receptors for acetylcholine and norepinephrine are located in the plasma membrane of certain cells (table 16.3). The combination of neurotransmitter and receptor functions as a signal to cells, caus- ing them to respond. Depending on the type of cell, the response can be excitatory or inhibitory. 9. Where is the enteric nervous system located? Describe the types of neurons found in it. 10. Define autonomic nerve plexuses. How are they typically named? 11. Describe the four major ways by which sympathetic axons pass from sympathetic chain ganglia to reach organs. Name four thoracic and four abdominopelvic autonomic nerve plexuses. 12. List the four major means by which parasympathetic axons reach organs. List the cranial nerves and ganglia that supply the head and neck. What cranial nerve supplies the thoracic and abdominal nerve plexuses? To what plexus do pelvic nerves contribute? P R E D I C T Starting in the small intestine and ending with the ganglia where their cell bodies are located, trace the route for sensory axons passing alongside sympathetic axons. Name all of the plexuses, nerves, ganglia, etc. that the sensory axon passes through. Also trace the route for sensory neurons passing alongside parasympathetic axons. Cholinergic Receptors Receptors to which acetylcholine binds are called cholinergic receptors. They have two major structurally different forms. Nicotinic (nik-o ¯-tin ik) receptors also bind to nicotine, an alka- loid substance found in tobacco; and muscarinic (mu ˘- ˘s-ka rin ik) receptors also bind to muscarine, an alkaloid extracted from some poisonous mushrooms. Although nicotine and mus- carine are not naturally in the human body, they demonstrate differences in the two classes of cholinergic receptors. Nicotine binds to nicotinic receptors but not to muscarinic receptors, whereas muscarine binds to muscarinic receptors but not to nicotinic receptors. On the other hand, nicotinic and muscarinic receptors are very similar because acetylcholine binds to and ac- tivates both types of receptors. The membranes of all postganglionic neurons in autonomic ganglia and the membranes of skeletal muscle cells have nicotinic receptors. The membranes of effector cells that respond to acetyl- choline released from postganglionic neurons have muscarinic receptors.
Autonomic Nervous System - page 10
Seeley−Stephens−Tate: Anatomy and Physiology, Sixth Edition III. Integration and Control Systems 16. Autonomic Nervous System © The McGraw−Hill Companies, 2004 556 Part 3 Integration and Control System Sympathetic division Most target tissues innervated by the sympathetic division have adrenergic receptors. When norepinephrine (NE) binds to adrenergic receptors, some target tissues are stimulated, and others are inhibited. For example, smooth muscle cells in blood vessels are stimulated to constrict, and stomach glands are inhibited. Cell of target tissue Location of nicotinic receptors Location of adrenergic receptors ACh released NE released Preganglionic neuron Postganglionic neuron Sympathetic division Some sympathetic target tissues, such as sweat glands, have muscarinic receptors, which respond to acetylcholine (ACh). Stimulation of sweat glands results in increased sweat production. Location of muscarinic receptors Location of nicotinic receptors ACh released ACh released Preganglionic neuron Postganglionic neuron Cell of target tissue Parasympathetic division All parasympathetic target tissues have muscarinic receptors. The general response to ACh is excitatory, but some target tissues, such as the heart, are inhibited. Location of nicotinic receptors Cell of target tissue Location of muscarinic receptors ACh released Preganglionic neuron ACh released Postganglionic neuron Figure 16.6 Location of ANS Receptors Nicotinic receptors are on the cell bodies of both sympathetic and parasympathetic postganglionic cells in the autonomic ganglia. Abbreviations: NE, norepinephrine; ACh, acetylcholine. P R E D I C T Would structures innervated by the sympathetic division or the parasympathetic division be affected after the consumption of nicotine? After the consumption of muscarine? Explain. Adrenergic Receptors Norepinephrine or epinephrine can bind to adrenergic receptors. Norepinephrine that is released from adrenergic postganglionic neurons of the sympathetic division (see figure 16.6) diffuses across the synapse and binds to receptor molecules within the plasma membranes of effector organs. Epinephrine and norepinephrine re- leased from the adrenal glands and carried to effector organs by the blood can also bind to adrenergic receptors. The response of cells to norepinephrine or epinephrine binding to adrenergic receptors is mediated through G proteins (see chapters 3 and 17). Adrenergic receptors are subdivided into two major cate- gories: alpha ( ) receptors and beta ( ) receptors, each of which has subtypes. The main subtypes for alpha receptors are 1 - and 2 -adrenergic receptors and for beta receptors are 1 - and Acetylcholine binding to nicotinic receptors has an excita- tory effect because it results in the direct opening of Na channels and the production of action potentials. When acetylcholine binds to muscarinic receptors, the cell’s response is mediated through G proteins (see chapters 3 and 17). The response is either excitatory or inhibitory, depending on the target tissue in which the receptors are found. For example, acetylcholine binds to muscarinic recep- tors in cardiac muscle, thereby reducing heart rate; and acetyl- choline binds to muscarinic receptors in smooth muscle cells of the stomach, thus increasing its rate of contraction.
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