Chapter 19: The Autonomic Nervous System PDF

JWCL299_c19_674-696.qxd 5/27/10 19 11:36 PM Page 674 THE AUTONOMIC NERVOUS SYSTEM I N T R O D U C T I O N It is the end of the semester, you have studied diligently for your anatomy final, and now it is time to take the exam. As you enter the crowded lecture hall and take a seat, you sense tens...

JWCL299_c19_674-696.qxd 5/27/10 19 11:36 PM Page 674 THE AUTONOMIC NERVOUS SYSTEM I N T R O D U C T I O N It is the end of the semester, you have studied diligently for your anatomy final, and now it is time to take the exam. As you enter the crowded lecture hall and take a seat, you sense tension in the room as the other students nervously chatter about last-minute details they think will be important to know for the test. Suddenly you feel your heart race with excitement— or is that apprehension? You notice that your mouth becomes somewhat dry, and you break out in a cold sweat. You also notice that your breathing is a little bit faster and deeper. As you wait for the professor to pass out the test, these symptoms become more and more pronounced. Finally the test arrives at your desk. As you slowly flip through the exam to get a feel for the questions being asked, you recognize that you can answer them all with confidence. What a relief! Your symptoms begin to disappear as you focus on transferring your knowledge from your brain to the paper. Most of the effects just described fall under the control of the autonomic nervous system. As you learned in Chapter 16, the autonomic nervous system (ANS) (aw⬘-to- -NOM-ik; auto⫽self; -nomic⫽law) is a division of the peripheral nervous system (PNS). The word autonomic is based on the Latin words for self and law because the ANS was originally believed to be self-governing. The ANS consists of (1) autonomic sensory neurons in visceral organs and in blood vessels that convey information to (2) integrating centers in the central nervous system (CNS), (3) autonomic motor neurons that propagate from the CNS to various effector tissues to regulate the activity of smooth muscle, cardiac muscle, and many glands, and (4) the enteric division, a specialized network of nerves and ganglia forming an independent nerve network within the wall of the gastrointestinal (GI) tract. Functionally, the ANS usually operates without conscious control. However, centers in the hypothalamus and brain stem do regulate ANS reflexes, so it is not completely self-governing. ? 674 Did you ever wonder how some blood pressure medications exert their effects through the autonomic nervous system? JWCL299_c19_674-696.qxd 5/27/10 11:36 PM Page 675 CONTENTS AT A GLANCE 19.1 Comparison of Somatic and Autonomic Nervous Systems 675 • Somatic Nervous System 675 • Autonomic Nervous System 675 • Comparison of Somatic and Autonomic Motor Neurons 676 19.2 Anatomy of Autonomic Motor Pathways 677 • Understanding Autonomic Motor Pathways 677 • Shared Anatomical Components of an Autonomic Motor Pathway 679 19.3 Structure of the Sympathetic Division 681 • Sympathetic Preganglionic Neurons 681 • Sympathetic Ganglia and Postganglionic Neurons 684 19.5 Structure of the Enteric Division 687 19.6 ANS Neurotransmitters and Receptors 687 • Cholinergic Neurons and Receptors 687 • Adrenergic Neurons and Receptors 688 19.7 Functions of the ANS 688 • Sympathetic Responses 689 • Parasympathetic Responses 690 19.8 Integration and Control of Autonomic Functions 692 • Autonomic Reflexes 692 • Autonomic Control by Higher Centers 692 Key Medical Terms Associated with the Autonomic Nervous System 693 19.4 Structure of the Parasympathetic Division 685 • Parasympathetic Preganglionic Neurons 685 • Parasympathetic Ganglia and Postganglionic Neurons 685 In this chapter, we compare structural and functional Chapter 16. Then we discuss the anatomy of the ANS and features of the autonomic nervous system with those of compare the organization and actions of its two major the somatic nervous system, which was introduced in parts, the sympathetic and parasympathetic divisions. • 19.1 COMPARISON OF SOMATIC AND AUTONOMIC NERVOUS SYSTEMS the degree of stretch in the walls of organs or blood vessels. These sensory signals are not consciously perceived most of the time, although intense activation may produce conscious sensations. Two examples of perceived visceral sensations are sensations of pain from damaged viscera and angina pectoris (chest pain) from inadequate blood flow to the heart. Some sensations monitored by somatic sensory (Chapter 20) and special sensory neurons (Chapter 21) also influence the ANS. For example, pain can produce dramatic changes in some autonomic activities. Autonomic motor neurons regulate visceral activities by either increasing (exciting) or decreasing (inhibiting) activities in their effector tissues, which are cardiac muscle, smooth muscle, and glands. Changes in the diameter of the pupils, dilation and constriction of blood vessels, and adjustment of the rate and force of the heartbeat are examples of autonomic motor responses. Unlike skeletal muscle, tissues innervated by the ANS often function to some extent even if their nerve supply is damaged. For example, the heart continues to beat when it is removed for transplantation. Single-unit smooth muscle, like that found in the lining of the gastrointestinal tract, contracts rhythmically on its own, and glands produce some secretions in the absence of ANS control. Most autonomic responses cannot be consciously altered or suppressed to any great degree. You probably cannot voluntarily slow your heartbeat to half its normal rate. For this reason, some autonomic responses are the basis for polygraph (“lie detector”) tests. Nevertheless, practitioners of yoga or other techniques of meditation may learn how to regulate at least some of their autonomic activities through long practice. (Biofeedback, in which monitoring devices display information about a body function such as heart rate or blood pressure, enhances the ability to learn such conscious control.) Signals from the general somatic and special senses, acting via the limbic system, also influence responses of autonomic motor neurons. For example, seeing a bike about to hit you, hearing the squealing brakes of a nearby car as you cross the street, or being grabbed from behind by an attacker would increase the rate and force of your heartbeat. OBJECTIVE • Compare the structures and functions of the somatic and autonomic nervous systems. Somatic Nervous System As you learned in Chapter 16, the somatic nervous system includes both sensory and motor neurons. Sensory neurons convey input from receptors for somatic senses (pain, thermal, tactile, and proprioceptive sensations; see Chapter 20) and from receptors for the special senses (vision, audition, gustation, olfaction, and equilibrium; see Chapter 21). Normally, all of these sensations are consciously perceived. Somatic motor neurons innervate skeletal muscles—the effectors of the somatic nervous system—and produce both reflexive and voluntary movements of the musculoskeletal system. When a somatic motor neuron stimulates a skeletal muscle, the muscle contracts; the effect always is excitation. If somatic motor neurons cease to stimulate a muscle, the result is a paralyzed, limp muscle that has no muscle tone. In addition, even though we are generally not conscious of breathing, the muscles that generate respiratory movements are skeletal muscles controlled by somatic motor neurons. If the respiratory motor neurons become inactive, breathing stops. A few skeletal muscles, such as those in the middle ear, are controlled by reflexes and cannot be contracted voluntarily. Autonomic Nervous System The main input to the ANS comes from autonomic (visceral) sensory neurons. Mostly, these neurons are associated with interoceptors (receptors inside the body), such as chemoreceptors that monitor blood CO2 level, and mechanoreceptors that detect 675 JWCL299_c19_674-696.qxd 676 6/19/10 3:23 PM Page 676 CHAPTER 19 • THE AUTONOMIC NERVOUS SYSTEM Comparison of Somatic and Autonomic Motor Neurons has its cell body in the CNS; its myelinated axon extends from the CNS to an autonomic ganglion. (Recall that a ganglion is a collection of neuronal cell bodies outside the CNS). The cell body of the second neuron (the postganglionic neuron) is in that autonomic ganglion; its unmyelinated axon extends directly from the ganglion to the effector (smooth muscle, cardiac muscle, or a gland). In some autonomic pathways, the preganglionic neuron extends to specialized cells of the adrenal medullae (inner portions of the adrenal glands) called chromaffin Recall from Chapter 10 that the axon of a single, myelinated somatic motor neuron extends from the central nervous system (CNS) all the way to the skeletal muscle fibers in its motor unit (Figure 19.1a). By contrast, most autonomic motor pathways consist of two motor neurons in series, one following the other (Figure 19.1b). The first neuron (the preganglionic neuron) Figure 19.1 Motor neuron pathways in the (a) somatic nervous system and (b) autonomic nervous system (ANS). Note that autonomic motor neurons release either acetylcholine (ACh) or norepinephrine (NE); somatic motor neurons release ACh. Somatic nervous system stimulation always excites its effectors (skeletal muscle fibers); stimulation by the autonomic nervous system either excites or inhibits visceral effectors. Somatic motor neuron (myelinated) ACh Spinal cord Effector: skeletal muscle (a) Somatic nervous system NE Autonomic motor neurons ACh Spinal cord Sympathetic preganglionic neuron (myelinated) Autonomic ganglion Sympathetic postganglionic neuron (unmyelinated) Effectors: glands, cardiac muscle (in the heart), and smooth muscle (mainly in the walls of blood vessels) Adrenal cortex Adrenal medulla Chromaffin cell ACh Spinal cord Sympathetic preganglionic neuron (myelinated) Distributed in the blood throughout the body Adrenal medulla ACh Spinal cord Parasympathetic preganglionic neuron (myelinated) Autonomic ganglion Parasympathetic postganglionic neuron (unmyelinated) (b) Autonomic nervous system What does dual innervation mean? Epinephrine and NE ACh Effectors: glands, cardiac muscle (in the heart), and smooth muscle (mainly in the organs of the gastrointestinal and respiratory tracts) JWCL299_c19_674-696.qxd 5/27/10 11:36 PM Page 677 19.2 ANATOMY OF AUTONOMIC MOTOR PATHWAYS 677 TABLE 19.1 Comparison of the Somatic and Autonomic Nervous Systems SOMATIC NERVOUS SYSTEM AUTONOMIC NERVOUS SYSTEM Sensory input Special senses and somatic senses Mainly from interoceptors; some from special senses and somatic senses Control of motor output Voluntary control from cerebral cortex, with contributions from basal nuclei, cerebellum, spinal cord Involuntary control from limbic system, hypothalamus, brain stem, and spinal cord; limited control from brain stem and cerebral cortex Motor neuron pathway One-neuron pathway: Somatic motor neurons extending from CNS synapse directly with effector Usually two-neuron pathway: Preganglionic CNS neurons n an autonomic ganglion n postganglionic neurons n a visceral effector OR Preganglionic CNS neurons n chromaffin cells of adrenal medullae n postganglionic neurons n effector Neurotransmitters and hormones All somatic motor neurons release ACh All preganglionic axons release acetylcholine (ACh); most sympathetic postganglionic neurons release norepinephrine (NE); those to most sweat glands release ACh; all parasympathetic postganglionic neurons release ACh; adrenal medullae release epinephrine and norepinephrine Effectors Skeletal muscle Smooth muscle, cardiac muscle, and glands Responses Contraction of skeletal muscle Contraction or relaxation of smooth muscle; increased or decreased rate and force of contraction of cardiac muscle; increased or decreased secretions of glands cells. These cells develop from the same embryonic cells that give rise to the autonomic ganglia. All somatic motor neurons release only acetylcholine (ACh) as their neurotransmitter; autonomic motor neurons release either ACh or norepinephrine (NE). Unlike somatic output (motor), the output part of the ANS has two divisions: the sympathetic division and the parasympathetic division. Most organs have dual innervation, that is, they receive impulses from both sympathetic and parasympathetic neurons. In some organs, nerve impulses from one division of the ANS stimulate the organ to increase its activity (excitation), and impulses from the other division decrease the organ’s activity (inhibition). For example, an increased rate of nerve impulses from the sympathetic division increases heart rate; an increased rate of nerve impulses from the parasympathetic division decreases heart rate. The majority of the output of the sympathetic division, often called the fight-or-flight division, is directed at the smooth muscle of blood vessels. Sympathetic activities result in increased alertness and enhanced metabolic activities in order to prepare the body for an emergency situation. Responses to such situations, which may occur during physical activity or emotional stress, include a rapid heart rate, faster breathing rate, dilation of the pupils, dry mouth, sweaty but cool skin, dilation of blood vessels to organs involved in combating stress (such as the heart and skeletal muscles), constriction of blood vessels to organs not involved in combating stress (for example, the gastrointestinal tract and kidneys), and the release of glucose from the liver. The parasympathetic division is often referred to as the restand-digest division because its activities conserve and restore body energy during times of rest or digesting a meal; the majority of its output is directed to the smooth muscle and glandular tissue of the gastrointestinal and respiratory tracts. The parasympathetic division conserves energy and replenishes nutrient stores. Although both the sympathetic and parasympathetic divisions are concerned with maintaining homeostasis, they do so in dramatically different ways. Table 19.1 summarizes the comparisons of the somatic and autonomic nervous systems presented in this section. CHECKPOINT 1. Why is the autonomic nervous system so named? 2. What are the main input and output components of the autonomic nervous system? 19.2 ANATOMY OF AUTONOMIC MOTOR PATHWAYS OBJECTIVES • Outline the development of the autonomic motor pathways and explain how it relates to their adult anatomy. • Compare preganglionic and postganglionic neurons of the autonomic nervous system. • Describe the anatomy of the autonomic ganglia. Understanding Autonomic Motor Pathways When studying the autonomic motor pathways, the student of anatomy is confronted with many questions: 1. Why are there two distinct divisions—sympathetic and parasympathetic? 2. Why is there a series of two motor neurons from the CNS to an effector organ? 3. Why is the sympathetic system more widely distributed in the body than the parasympathetic system? 4. Why does the parasympathetic system originate in the cranial and sacral regions of the CNS, while the sympathetic system originates from the thoracic and lumbar regions of the CNS? JWCL299_c19_674-696.qxd 678 6/19/10 3:24 PM Page 678 CHAPTER 19 • THE AUTONOMIC NERVOUS SYSTEM and is a key player in the formation of the autonomic motor neuron pathways. 5. Why is there no autonomic output from the cervical region and from lower lumbar and upper sacral regions? The answers to these questions are seldom clarified and yet they are the true keys to understanding the anatomy of the autonomic pathways. To help answer these questions we must briefly review the development of the nervous system and enhance it with a few important details. As you learned in Chapter 4, development of the nervous system begins in the third week of gestation with a thickening of the ectoderm called the neural plate (Figure 19.2). During the process of neurulation the plate folds inward and forms a longitudinal groove, the neural groove. The raised edges of the neural plate are called neural folds. As development continues, the neural folds increase in height and meet to form a tube called the neural tube. This tube becomes the central nervous system. During this folding process, a mass of tissue from the edge of the fold, the neural crest, migrates between the neural tube and the skin ectoderm (Figure 19.2b). The neural crest tissue plays a prominent role in the formation of the peripheral nervous system Migration of the Neural Crest Tissue Some of the neural crest tissue cells give rise to the cell bodies of the dorsal root and cranial ganglia of all the somatic and visceral sensory neurons in the body. However, other neural crest cells migrate toward the developing smooth muscle of blood vessels and the gut tube. These migrating cells will develop into postganglionic neurons, the second of the two motor neurons of the autonomic motor pathway. As these cells migrate they are followed by axons that grow from cells in the ventrolateral part of the neural tube. These ventrolateral tube cells become preganglionic neurons, the first motor neurons in the autonomic motor pathway, and eventually form synapses with the migrating crest cells (Figure 19.2c). The migrating crest cells form two distinct populations of developing neurons. One population migrates into the cranial and caudal ends of the gut tube, as these are the initial developing Figure 19.2 Development of autonomic motor pathways. (a) Neurulation and (b) migration of the neural crest cells. The nervous system begins developing in the third week from a thickening of ectoderm called the neural plate. Future neural crest Neural plate Ectoderm 1. Notochord Endoderm HEAD END Mesoderm Neural plate Neural folds Neural groove 1. Neural crest Neural folds Ectoderm Somite 2. 2. Neural tube 3. Notochord Neural groove Endoderm Cut edge of amnion Neural crest Neural tube TAIL END Somite 3. (a) Dorsal view Notochord Endoderm (b) Transverse sections Ectoderm JWCL299_c19_674-696.qxd 6/19/10 3:24 PM Page 679 19.2 ANATOMY OF AUTONOMIC MOTOR PATHWAYS regions of the gut tube, and the second takes up positions near the yolk sac in the central region of the embryo around developing blood vessels. Because the gut tube develops more rapidly at its two ends (the pharynx and the cloaca), the initial neuronal relationship between the neural tube cell (first motor neuron), the migrating crest cell (second motor neuron), and smooth muscle and gland cells in the developing gut wall (autonomic effectors) arise in the cranial and sacral regions of the developing central nervous system. As the gut tube completes its development from the two ends, this neuronal relationship is carried toward the midgut. This neuronal pattern establishes the craniosacral outflow to the gut tube and establishes the parasympathetic division of the autonomic nervous system. As major blood vessels begin to emerge around the developing yolk sac in the central region of the embryo, the migrating neural crest cells in the thoracic and upper lumbar regions establish connections with the developing smooth muscle cells in the walls of the blood vessels. From this central region of the embryo the initial neuronal relationship between the neural tube cell (first motor neuron), the migrating crest cell (second motor neuron), and smooth muscle cells in the developing vessels (autonomic effectors) arise in the thoracic and lumbar regions of the developing central nervous system. This neuronal pattern establishes the thoracolumbar outflow to the smooth muscle of the cardiovascular system and becomes the anatomy of the sympathetic division of the autonomic system. With this knowledge of embryonic development we can answer the question about the division of the ANS into sympathetic and parasympathetic divisions. These two distinct divisions emerged to control the two distinct populations of developing smooth muscle—gut tube smooth muscle and cardiovascular smooth muscle. The migration of the crest cells, which were Cranial parasympathetic preganglionic neurons in neural tube Parasympathetic postganglionic neurons (migrating crest cells) Sympathetic postganglionic neurons (migrating crest cells) Thoracolumbar sympathetic preganglionic neurons in neural tube Parasympathetic postganglionic neurons (migrating crest cells) Sacral parasympathetic preganglionic neurons in neural tube (c) Relationship of neural tube neurons and neural crest neurons What is the origin of the neural crest? 679 pursued by neural tube axons, explains the two-neuron pathway in motor control. The reason that the sympathetic division is more widespread than the parasympathetic division is that the sympathetic division controls blood vessels that are distributed throughout the body, and the parasympathetic distribution is limited to the gut tube and its derivatives. The migration of the two separate populations of crest cells clarifies why sympathetic control comes from the thoracolumbar regions of the CNS and parasympathetic control comes from the craniosacral regions. The final question we raised is why there are gaps in the autonomic output. The answer is limb development. The lack of autonomic output from the cervical and lower lumbar–upper sacral regions is a result of the massive dominance of skeletal muscle innervation into the developing limbs, which displaces the autonomic output from these regions of the CNS. In other words, the somatic motor neurons muscle the autonomic neurons out of the way as the limbs develop. Shared Anatomical Components of an Autonomic Motor Pathway As a result of their development, both the sympathetic and parasympathetic divisions share certain features, while other aspects of their anatomy are unique. We will first describe the features common to both divisions, and then we will explore each of the two divisions in greater detail. Motor Neurons and Autonomic Ganglia As you learned in Section 19.1, each ANS pathway has two motor neurons (see Figure 19.1b). The cell body of the preganglionic neuron (the first neuron in the pathway) is in the brain or spinal cord, and its axon exits the CNS as part of a cranial or spinal nerve. The axon of a preganglionic neuron is a smalldiameter, myelinated fiber that usually extends to an autonomic ganglion, an aggregate of migrated neural crest cells. The autonomic ganglia may be divided into three general groups: Two groups are components of the sympathetic division (the sympathetic ganglia), and one is a component of the parasympathetic division (the parasympathetic ganglia). The preganglionic neuron synapses with a postganglionic neuron (the second neuron in the pathway) within the autonomic ganglion (see Figure 19.1b). Notice that, because of its origin from migrating neural crest tissue, the postganglionic neuron lies entirely outside the CNS in the PNS. Its cell body and dendrites are located in the autonomic ganglion, and its axon is a smalldiameter, unmyelinated type C fiber that terminates in a visceral effector. Unlike other neurons, postganglionic autonomic fibers do not end in a single terminal swelling like a synaptic knob or end plate. The terminal branches of autonomic fibers contain numerous swellings, called varicosities, which simultaneously release neurotransmitter over a large area of the innervated organ. This extensive release of neurotransmitter and the greater number of postganglionic neurons means that entire organs, rather than discrete cells, are typically influenced by autonomic activity. Autonomic Plexuses In the thorax, abdomen, and pelvis, axons of preganglionic neurons of both the sympathetic and parasympathetic divisions form tangled networks called autonomic plexuses, many of which lie along major arteries. The autonomic plexuses also may contain autonomic ganglia (groups of cell bodies for the postganglionic neurons in the plexuses) and axons of autonomic sensory neurons. JWCL299_c19_674-696.qxd 680 5/29/10 7:57 PM Page 680 CHAPTER 19 • THE AUTONOMIC NERVOUS SYSTEM Figure 19.3 Autonomic plexuses in the thorax, abdomen, and pelvis. An autonomic plexus is a network of sympathetic and parasympathetic axons that sometimes also includes autonomic sensory axons and sympathetic ganglia. Trachea Right vagus (X) nerve Left vagus (X) nerve Arch of aorta CARDIAC PLEXUS PULMONARY PLEXUS Right primary bronchus Esophagus Right sympathetic trunk ganglion Thoracic aorta Greater splanchnic nerve ESOPHAGEAL PLEXUS Lesser splanchnic nerve Inferior vena cava (cut) Diaphragm Celiac trunk (artery) AORTICORENAL GANGLION CELIAC GANGLION AND PLEXUS Right kidney SUPERIOR MESENTERIC GANGLION AND PLEXUS Superior mesenteric artery RENAL GANGLION AND RENAL PLEXUS INFERIOR MESENTERIC GANGLION AND PLEXUS Inferior mesenteric artery Right sympathetic trunk ganglion HYPOGASTRIC PLEXUS (a) Anterior view JWCL299_c19_674-696.qxd 5/29/10 7:57 PM Page 681 19.3 STRUCTURE OF THE SYMPATHETIC DIVISION The major plexuses in the thorax are the cardiac plexus, which supplies the heart, and the pulmonary plexus, which supplies the bronchial tree (Figure 19.3; see also Figure 19.4). The abdomen and pelvis also contain major autonomic plexuses that often are named after the artery along which they are distributed (Figure 19.3). The celiac (solar) plexus, the largest autonomic plexus, surrounds the celiac and superior mesenteric arteries. It contains two large celiac ganglia, two aorticorenal ganglia, and a dense network of autonomic axons and is distributed to the liver, gallbladder, stomach, pancreas, spleen, kidneys, medullae (inner regions) of the adrenal glands, testes, and ovaries. The superior mesenteric plexus contains the superior mesenteric ganglion and supplies the small and large intestine. The inferior mesenteric plexus contains the inferior mesenteric ganglion, which innervates the large intestine. The hypogastric plexus supplies pelvic viscera. The renal plexuses contain the renal ganglion and supply the renal arteries within the kidneys and the ureters. With this understanding of the development and the basic anatomical features shared by the sympathetic and parasympathetic divisions of the autonomic motor pathways, we are now ready to explore the two divisions in greater detail. CHECKPOINT 3. Describe the migrations of neural crest tissue in the early development of the autonomic motor pathways. 4. Describe the shared anatomical features of the autonomic motor pathways. 19.3 STRUCTURE OF THE SYMPATHETIC DIVISION OBJECTIVES • Explain the central nervous system origin of the sympathetic division. • Describe the location of the sympathetic ganglia. • List the synapses between the preganglionic and postganglionic motor neurons of the sympathetic division and the different pathways of the postganglionic neurons to their effector organs. Sympathetic Preganglionic Neurons In the sympathetic division, the preganglionic neurons have their cell bodies in the lateral horns of the gray matter in the 12 thoracic segments and the first two or three lumbar segments of the spinal cord (Figure 19.4). For this reason, the sympathetic division is also called the thoracolumbar division (thor⬘-a-ko- LUM-bar), and the axons of the sympathetic preganglionic neurons are known as the thoracolumbar outflow. The preganglionic axons leave the spinal cord along with the somatic motor neurons via the anterior rootlets of the spinal nerve. After exiting via the spinal nerve trunk through the intervertebral foramina, the myelinated preganglionic sympathetic axons pass into the anterior root of a spinal nerve and enter a short pathway called a white ramus (RA-mus) before passing to the nearest Anterior ramus of spinal nerve Ramus communicans Right sympathetic chain Right sympathetic trunk ganglion Greater splanchnic nerve Rib (cut) Lesser splanchnic nerve Diaphragm Vagal and sympathetic splanchnic plexus to adrenal gland CELIAC GANGLION AORTICORENAL GANGLION SUPERIOR MESENTERIC GANGLION Kidney Aorta (b) Anterior view Which is the largest autonomic plexus? 681 JWCL299_c19_674-696.qxd 682 6/19/10 3:24 PM Page 682 CHAPTER 19 • THE AUTONOMIC NERVOUS SYSTEM Figure 19.4 The sympathetic division of the autonomic nervous system. Solid lines represent preganglionic axons; dashed lines represent postganglionic axons. Although the innervated structures are shown for only one side of the body for diagrammatic purposes, the sympathetic division actually innervates tissues and organs on both sides. Cell bodies of sympathetic preganglionic neurons are located in the lateral horns of gray matter in the 12 thoracic and first two lumbar segments of the spinal cord. SYMPATHETIC DIVISION (thoracolumbar) Key: Distributed primarily to smooth muscle of blood vessels of these organs: Preganglionic neurons Postganglionic neurons Pineal gland Eye Lacrimal gland Brain Mucous membrane of nose and palate Sublingual and submandibular glands Parotid gland Heart Spinal cord Atrial muscle fibers SA/AV nodes C1 C2 Ventricular muscle fibers Superior cervical ganglion C3 Trachea Cardiac plexus C4 C5 C6 C7 C8 Middle cervical ganglion Bronchi Inferior cervical ganglion Lungs Pulmonary plexus T1 Skin T2 T3 Greater splanchnic nerve T4 T5 Hair follicle smooth muscle Blood vessels (each sympathetic trunk innervates the skin and viscera) T7 T8 T9 T10 T11 T12 L1 L2 L3 L4 L5 Sympathetic trunk ganglia (on both sides) Stomach Celiac ganglion Transverse colon Aorticorenal ganglion T6 Sweat gland Liver, gallbladder, and bile ducts S1 S2 S3 S4 S5 Coccygeal (fused together) Spleen Pancreas Small intestine Lesser splanchnic nerve Least splanchnic nerve Ascending colon Sigmoid colon Adrenal gland Superior mesenteric ganglion Descending colon Rectum Kidney Renal ganglion Ureter Lumbar splanchnic Inferior nerve mesenteric ganglion Prevertebral ganglia Urinary bladder Which division, sympathetic or parasympathetic, has longer preganglionic axons? Why? External genitals Uterus JWCL299_c19_674-696.qxd 6/19/10 3:24 PM Page 683 19.3 STRUCTURE OF THE SYMPATHETIC DIVISION sympathetic trunk ganglion on the same side (Figure 19.5). Collectively, the white rami are called the white rami communicantes (ko- -mu- -ni-KAN-te- z; singular is ramus communicans). The “white” in their name indicates that they contain myelinated axons. Only the thoracic and first two or three lumbar nerves have white rami communicantes, because these thoracolumbar output levels are the only levels from which sympathetic preganglionic motor neurons (the myelinated neurons of the autonomic motor pathway) leave the spinal cord (as a result of the development pattern discussed previously). The white rami communicantes connect the anterior ramus of the spinal nerve with the ganglia of the sympathetic trunk. As preganglionic axons extend from a white ramus communicans into the sympathetic trunk ganglion, they give off several axon collaterals (branches). These collaterals terminate and synapse in several ways (Figure 19.5): 683 Some synapse in the first ganglion at the level of entry. Others pass up or down the sympathetic trunk for a variable distance to form the sympathetic chains, the fibers on which the ganglia are strung. Some preganglionic axons pass through the sympathetic trunk without terminating in it. Beyond the trunk, they form nerves known as splanchnic nerves (SPLANK-nik; see Figure 19.4), which extend to and terminate in the outlying prevertebral ganglia. These ganglia, formed by neural crest cells that migrated toward the major blood vessels, supply the organs that arise from the abdominal portion of the gut tube. 1 2 3 A single sympathetic preganglionic fiber has many axon collaterals (branches) and may synapse with 20 or more postganglionic neurons. This pattern of projection is an example of divergence Figure 19.5 Connections between ganglia and postganglionic neurons in the sympathetic division of the ANS. Also illustrated are the gray and white rami communicantes. See also Figure 17.5a. Sympathetic ganglia lie in two chains on either side of the vertebral column (sympathetic trunk ganglia) and near large abdominal arteries anterior to the vertebral column (prevertebral ganglia). Posterior horn Posterior ramus of spinal nerve Posterior root Anterior ramus of spinal nerve Posterior root ganglion 2 Skin Lateral horn Spinal nerve Anterior horn Spinal cord Anterior root 1 Sympathetic trunk ganglion Gray ramus communicans 3 2 White ramus communicans Prevertebral ganglion (celiac ganglion) Visceral effector: primarily blood vessels of intestines Preganglionic neuron Postganglionic neurons What substance gives the white rami their white appearance? Anterior view To visceral effectors: smooth muscle of blood vessels, arrector pili muscles, sweat glands of skin JWCL299_c19_674-696.qxd 684 5/27/10 11:36 PM Page 684 CHAPTER 19 • THE AUTONOMIC NERVOUS SYSTEM (see Chapter 16) and helps explain why many sympathetic responses affect almost the entire body simultaneously. Sympathetic Ganglia and Postganglionic Neurons The sympathetic ganglia are the sites of synapses between sympathetic preganglionic and postganglionic neurons and contain the postganglionic neuron cell bodies. There are two groups of sympathetic ganglia—the sympathetic trunk ganglia and prevertebral ganglia. Sympathetic Trunk Ganglia Sympathetic trunk ganglia (also called vertebral chain ganglia or paravertebral ganglia) lie in a vertical row on either side of the vertebral column. The position of the sympathetic trunk ganglia is established in the embryo as blood vessels branch from the aorta into each segment of the developing embryonic trunk. (Recall that the sympathetic pathways are following blood vessels during development and establish positions along these branches of the aorta.) These ganglia extend from the base of the skull to the coccyx (Figure 19.4). The paired sympathetic trunk ganglia are arranged anterior and lateral to the vertebral column, one on either side. Typically, there are 3 cervical, 11 or 12 thoracic, 4 or 5 lumbar, and 4 or 5 sacral sympathetic trunk ganglia, and 1 coccygeal ganglion. The right and left coccygeal ganglia are fused together and usually lie at the midline. The sympathetic trunk ganglia extend inferiorly from the neck, chest, and abdomen to the coccyx (recall, these were sites of migration of neural crest cells to locations near segmental vessels arising from the embryonic aorta); however, they receive preganglionic axons only from the thoracic and lumbar segments of the spinal cord (see Figure 19.4). Postganglionic neurons arising from the sympathetic trunk ganglia do one of the following (see Figure 19.4): 1. From all the ganglia of the sympathetic chain they return via gray communicating rami to the anterior ramus of a spinal nerve where they are distributed to blood vessels, sweat glands, and arrector pili muscles in the body wall. 2. From cervical sympathetic chain ganglia they exit into nerve branches that supply the heart or that follow blood vessels into the head, neck, and shoulder region. 3. From upper thoracic, lower abdominal, and pelvic sympathetic trunk ganglia they exit the trunk in nerves that enter plexuses that follow blood vessels of those regions. The cervical portion of each sympathetic trunk ganglion is located in the neck and is subdivided into superior, middle, and inferior ganglia (see Figure 19.4). Postganglionic neurons leaving the superior cervical ganglion serve the head and heart. They are distributed primarily to blood vessels in the head, but also innervate sweat glands, smooth muscle of the eye, lacrimal glands, nasal mucosa, salivary glands (submandibular, sublingual, and parotid), and the heart. Gray rami communicantes (described shortly) from the superior cervical ganglion also pass to the upper two to four cervical spinal nerves, through which they supply blood vessels, sweat glands, and arrector pili muscles in the occipital region of the head and in the neck. Postganglionic neurons leaving the middle cervical ganglion and inferior cervical ganglion innervate the heart and blood vessels of the neck and shoulder. The thoracic portion of each sympathetic trunk ganglion lies anterior to the necks of the corresponding ribs. This region of the sympathetic trunk receives most of the sympathetic preganglionic axons, and its postganglionic neurons innervate the thoracic blood vessels, heart, lungs, and bronchial tree. In the skin, these neurons also innervate blood vessels, sweat glands, and arrector pili muscles of hair follicles. The lumbar portion of each sympathetic trunk ganglion lies lateral to the corresponding lumbar vertebrae. The sacral region of the sympathetic trunk ganglion lies in the pelvic cavity on the medial side of the anterior sacral foramina. Unmyelinated postganglionic axons from the lumbar and sacral sympathetic trunk ganglia enter a short pathway called a gray ramus and then merge with a spinal nerve or join the hypogastric plexus via direct visceral branches. The gray rami communicantes are structures containing the postganglionic axons that connect the ganglia of the various portions of the sympathetic trunk ganglion to spinal nerves (Figure 19.5). The axons of postganglionic neurons in the gray rami are unmyelinated. Gray rami communicantes outnumber the white rami because there is a gray ramus leading to each of the 31 pairs of spinal nerves that carries sympathetic output to the smooth muscle and glands of the body wall and limbs, primarily the smooth muscle of blood vessels. Prevertebral Ganglia As noted earlier, some preganglionic neurons pass through the sympathetic trunk ganglia and chain and exit ventrally as the splanchnic nerves. These nerves lead the second group of sympathetic ganglia, the prevertebral (collateral ) ganglia, which lies anterior to the vertebral column and close to the large abdominal arteries that supply the derivatives of the embryonic gut. Postganglionic axons leaving the prevertebral ganglia follow the course of various arteries to abdominal and pelvic visceral effectors. There are four major prevertebral ganglia (Figure 19.4; see also Figure 19.3a): (1) The celiac ganglion (SE -le- -ak) is on either side of the celiac artery just inferior to the diaphragm; (2) the superior mesenteric ganglion (MEZ-en-ter⬘-ik) is near the beginning of the superior mesenteric artery in the upper abdomen; (3) the inferior mesenteric ganglion is near the beginning of the inferior mesenteric artery in the middle of the abdomen; (4) the aorticorenal ganglion (a--or⬘-ti-ko- -RE-nal) is near the renal artery as it branches from the aorta. Splanchnic nerves from the thoracic area form synapses with postganglionic cell bodies in the celiac ganglion. Preganglionic axons from the fifth through ninth or tenth thoracic ganglia (T5–T9 or T10) form the greater splanchnic nerve, which pierces the diaphragm and enters the celiac ganglion of the celiac plexus. From there, postganglionic neurons follow and innervate blood vessels to the stomach, spleen, liver, kidneys, and small intestine. Preganglionic axons from the tenth and eleventh thoracic ganglia (T10–T11) form the lesser splanchnic nerve, which pierces the diaphragm and passes through the celiac plexus to enter the aorticorenal ganglion and superior mesenteric ganglion of the superior mesenteric plexus. Postganglionic neurons from the superior mesenteric ganglion follow and innervate blood vessels of the small intestine and proximal colon. The least or lowest splanchnic nerve, which is not always present, is formed by preganglionic axons from the twelfth thoracic ganglia (T12) or a branch of the lesser splanchnic nerve. It passes through the diaphragm and enters the renal plexus near the kidney. Postganglionic neurons from the renal plexus supply kidney arterioles and the ureter. JWCL299_c19_674-696.qxd 5/27/10 11:36 PM Page 685 19.4 STRUCTURE OF THE PARASYMPATHETIC DIVISION Preganglionic axons that form the lumbar splanchnic nerve from the first through fourth lumbar ganglia (L1–L4) enter the inferior mesenteric plexus and terminate in the inferior mesenteric ganglion, where they synapse with postganglionic neurons. Axons of postganglionic neurons extend through the hypogastric plexus and principally supply blood vessels of the distal colon and rectum, urinary bladder, and genital organs. Sympathetic preganglionic neurons also extend to the adrenal medullae (me-DUL-e- ). Developmentally, the adrenal medullae and sympathetic ganglia are derived from the same tissue, the neural crest (see Figure 19.2). The adrenal medullae arise from migrating neural crest cells that develop into chromaffin cells, which are developmentally similar to sympathetic postganglionic neurons. Rather than extending to another organ, however, these cells release hormones into the blood. Upon stimulation by sympathetic preganglionic neurons, the adrenal medullae release a mixture of hormones—about 80 percent epinephrine, 20 percent norepinephrine, and a trace amount of dopamine. These hormones circulate throughout the body and intensify responses elicited by sympathetic postganglionic neurons. CHECKPOINT 5. Why is the sympathetic division called the thoracolumbar division even though its ganglia extend from the cervical region to the sacral region? 6. List the organs served by each sympathetic and parasympathetic ganglion. 7. Where are sympathetic trunk ganglia and prevertebral ganglia located? 19.4 STRUCTURE OF THE PARASYMPATHETIC DIVISION OBJECTIVES • Explain the central nervous system origin of the parasympathetic division. • Describe the location of the sympathetic ganglia. Parasympathetic Preganglionic Neurons Cell bodies of preganglionic neurons of the parasympathetic division are located in the nuclei of four cranial nerves in the brain stem (III, VII, IX, and X) and in the lateral gray horns of the second through fourth sacral segments of the spinal cord (Figure 19.6). (This results from the development we discussed previously.) Hence, the parasympathetic division is also known as the craniosacral division (kra-⬘-ne- -o- -SA-kral), and the axons of the parasympathetic preganglionic neurons are referred to as the craniosacral outflow. Their axons emerge as part of a cranial nerve or as part of the anterior root of a sacral spinal nerve. The cranial parasympathetic outflow consists of preganglionic axons that extend from the brain stem in four cranial nerves. The sacral parasympathetic outflow consists of preganglionic axons in anterior roots of the second through fourth sacral nerves. The preganglionic axons of both the cranial and sacral outflows end in terminal ganglia, where they synapse with postganglionic neurons. The cranial outflow has five components: four pairs of ganglia and the plexuses associated with the vagus (X) nerve. The four pairs of cranial parasympathetic ganglia innervate structures in the head and are located close to the organs they innervate (Figure 19.6). Preganglionic axons that leave the brain as part of the 685 vagus (X) nerves carry nearly 80 percent of the total craniosacral outflow. Vagal axons extend to many terminal ganglia in the thorax and abdomen. As the vagus nerve passes through the thorax, it sends axons to the heart and to the airways of the lungs. In the abdomen, it supplies the liver, gallbladder, stomach, pancreas, small intestine, and part of the large intestine. The sacral parasympathetic outflow consists of preganglionic axons from the anterior roots of the second through fourth sacral nerves (S2–S4), which form the pelvic splanchnic nerves (Figure 19.6). These nerves synapse with parasympathetic postganglionic neurons located in terminal ganglia in the walls of the innervated viscera. From the ganglia, parasympathetic postganglionic axons innervate smooth muscle and glands in the walls of the colon, ureters, urinary bladder, and reproductive organs. Because the axons of parasympathetic preganglionic neurons extend from the CNS to a terminal ganglion in an innervated organ, they are longer than most of the axons of sympathetic preganglionic neurons. Parasympathetic Ganglia and Postganglionic Neurons The parasympathetic ganglia are the sites of synapses between parasympathetic preganglionic and postganglionic neurons, and contain the postganglionic neuron cell bodies. Parasympathetic ganglia are often referred to as terminal ganglia (neural crest cells that migrated into the developing gut wall) because most of these ganglia are located close to or actually within the wall of a visceral organ (the preganglionic neurons terminate at the organ). Most terminal ganglia do not have individual names. Only the terminal ganglia in the head have specific names (Figure 19.6): 1. The ciliary ganglia lie lateral to each optic (II) nerve near the posterior aspect of the orbit. Preganglionic axons pass with the oculomotor (III) nerves to the ciliary ganglia. Postganglionic axons from the ciliary ganglia innervate smooth muscle fibers in the eyeball. 2. The pterygopalatine ganglia (ter⬘-i-go- -PAL-a-tı- n) are located lateral to the sphenopalatine foramen in the pterygopalatine fossa, between the sphenoid and palatine bones. They receive preganglionic axons from the facial (VII) nerve and send postganglionic axons to the nasal mucosa, palate, pharynx, and lacrimal glands. 3. The submandibular ganglia are found near the ducts of the submandibular salivary glands. They receive preganglionic axons from the facial nerves and send postganglionic axons to the submandibular and sublingual salivary glands. 4. The otic ganglia are situated just inferior to each foramen ovale. They receive preganglionic axons from the glossopharyngeal (IX) nerves and send postganglionic axons to the parotid salivary glands. In a parasympathetic ganglion, the presynaptic neuron usually synapses with only four or five postsynaptic neurons, all of which supply a single visceral effector. Thus, parasympathetic responses can be localized to a single effector. Because the terminal ganglia are close to or in the walls of their visceral effectors, postganglionic parasympathetic axons are very short. CHECKPOINT 8. Name the organs served by each parasympathetic ganglion. 9. Where are the pterygopalatine ganglia located, and what type of ganglia are they? JWCL299_c19_674-696.qxd 686 6/19/10 3:24 PM Page 686 CHAPTER 19 • THE AUTONOMIC NERVOUS SYSTEM Figure 19.6 The parasympathetic division of the autonomic nervous system. Solid lines represent preganglionic axons; dashed lines represent postganglionic axons. Although the innervated structures are shown for only one side of the body for diagrammatic purposes, the parasympathetic division actually innervates tissues and organs on both sides. Cell bodies of parasympathetic preganglionic neurons are located in brain stem nuclei and in the lateral horns of gray matter in the second through fourth sacral segments of the spinal cord. Key: PARASYMPATHETIC DIVISION (craniosacral) Preganglionic neurons Postganglionic neurons Distributed primarily to smooth muscle and glands of these organs: Terminal ganglia CN III Eye Brain CN VII Spinal cord Ciliary ganglion Lacrimal gland Mucous membrane of nose and palate Parotid gland Sublingual and submandibular glands Pterygopalatine ganglion Atrial muscle fibers Heart SA/AV nodes C1 C2 CN IX Submandibular ganglion C3 Larynx Trachea C4 Bronchi CN X C5 Otic ganglion C6 Lungs C7 C8 T1 Liver, gallbladder, and bile ducts T2 T3 T4 T5 T6 Stomach Pancreas Transverse colon T7 T8 T9 Ascending colon T10 T11 Small intestine Descending colon Sigmoid colon Rectum T12 L1 L2 L3 L4 L5 Ureter Pelvic splanchnic nerves S1 S2 S3 S4 S5 Coccygeal Urinary bladder Which ganglia are associated with the parasympathetic division? Sympathetic division? External genitals Uterus JWCL299_c19_674-696.qxd 5/27/10 11:36 PM Page 687 19.6 ANS NEUROTRANSMITTERS AND RECEPTORS 19.5 STRUCTURE OF THE ENTERIC DIVISION OBJECTIVES • Describe the relationship of the enteric division to the sympathetic and parasympathetic divisions of the autonomic nervous system. • Explain how the enteric division of the autonomic nervous system is different from other parts of the peripheral nervous system. It is important to realize that the gastrointestinal tract, like the surface of the body, forms an extensive area of contact with the environment. Although this environment is inside the body, it is still considered part of the external environment. Just as the surface of the body must respond to important environmental stimuli in order to function properly, the surface of the gastrointestinal tract must respond to surrounding stimuli to generate proper homeostatic controls. In fact, these responses and controls are so important that the gastrointestinal tract has its own nervous system with intrinsic input, processing, and output. This division can and does function independently of central nervous system activity, but can also receive controlling input from the central nervous system. The enteric division (en-TER-ik) of the autonomic nervous system is the specialized network of nerves and ganglia forming a complex, integrated neuronal network within the wall of the gastrointestinal tract, pancreas, and gallbladder. This incredible nerve network contains in the neighborhood of 100 million neurons, approximately the same number as the spinal cord, and is capable of continued function without input from the central nervous system. The enteric network of nerves and ganglia contains sensory neurons capable of monitoring tension in the intestinal wall and assessing the composition of the intestinal contents. These sensory neurons relay their input signals to interneurons within the enteric ganglia. The interneurons establish an integrative network that processes the incoming signals and generates regulatory output signals to motor neurons throughout plexuses within the wall of the digestive organs. The motor neurons carry the output signals to the smooth muscle and glands of the gastrointestinal tract, as well as the smooth muscle of blood vessels, to exert control over its motility, secretory activities, and blood supply. Most of the nerve fibers that innervate the digestive organs arise from two plexuses within the enteric system. The largest, the myenteric plexus (mı--en-TER-ik), is positioned between the outer longitudinal and circular muscle layers from the upper esophagus to the anus. The myenteric plexus communicates extensively with a somewhat smaller plexus, the submucosal plexus, which occupies the gut wall between the circular muscle layer and the muscularis mucosae (see Section 24.2) and runs from the stomach to the anus. Neurons emerge from the ganglia of these two plexuses to form smaller plexuses around blood vessels and within the muscle layers and mucosa of the gut wall. It is this system of nerves that makes possible the normal motility and secretory functions of the gastrointestinal tract. CHECKPOINT 10. How does the enteric division differ from the sympathetic and parasympathetic divisions of the autonomic nervous system? 687 CLINICAL CONNECTION | Autonomic Dysreflexia Autonomic dysreflexia (dis-rē-FLEKS-sē-a) is an exaggerated response of the sympathetic division of the ANS that occurs in about 85 percent of individuals with spinal cord injury at or above the level of T6. The condition occurs due to interruption of the control of ANS neurons by higher centers. When certain sensory impulses are unable to ascend the spinal cord, such as those resulting from stretching of a full urinary bladder, mass stimulation of the sympathetic nerves below the level of injury occurs. Among the effects of increased sympathetic activity is severe vasoconstriction, which elevates blood pressure. In response, the cardiovascular center in the medulla oblongata (1) increases parasympathetic output via the vagus nerve, which decreases heart rate, and (2) decreases sympathetic output, which causes dilation of blood vessels above the level of the injury. Autonomic dysreflexia is characterized by a pounding headache; severe high blood pressure (hypertension); flushed, warm skin with profuse sweating above the injury level; pale, cold, and dry skin below the injury level; and anxiety. It is an emergency condition that requires immediate intervention. If untreated, autonomic dysreflexia can cause seizures, stroke, or heart attack. • 19.6 ANS NEUROTRANSMITTERS AND RECEPTORS OBJECTIVE • Describe the neurotransmitters and receptors involved in autonomic responses. Autonomic neurons are classified based on the neurotransmitter they produce and release. The receptors for the neurotransmitters are integral membrane proteins located in the plasma membrane of the postsynaptic neuron or effector cell. Cholinergic Neurons and Receptors Cholinergic neurons (ko-⬘-lin-ER-jik) release the neurotransmitter acetylcholine (Ach) (as⬘-e- -til-KO-le- n). (Remember: acetylcholine⫽cholinergic.) In the ANS, the cholinergic neurons include (1) all sympathetic and parasympathetic preganglionic neurons, (2) sympathetic postganglionic neurons that innervate most sweat glands, and (3) all parasympathetic postganglionic neurons (Figure 19.7). ACh is stored in synaptic vesicles and released by exocytosis. It then diffuses across the synaptic cleft and binds with specific cholinergic receptors, integral membrane proteins in the postsynaptic plasma membrane. The two types of cholinergic receptors, both of which bind ACh, are nicotinic receptors and muscarinic receptors. Nicotinic receptors (nik-o- -TIN-ik) are present in the plasma membranes of dendrites and cell bodies of sympathetic and parasympathetic postganglionic neurons (Figure 19.7a, b), and in the motor end plate at the neuromuscular junction. They are so named because nicotine mimics the action of ACh by binding to these receptors. (Nicotine, a natural substance in tobacco leaves, is not normally present in the bodies of nonsmokers.) Muscarinic receptors (mus⬘-ka-RIN-ik) are present in the plasma membranes of all effectors innervated by parasympathetic postganglionic axons (smooth muscle, cardiac muscle, and glands). Most sweat glands, which receive their innervation from cholinergic sympathetic postganglionic neurons, possess JWCL299_c19_674-696.qxd 688 5/27/10 11:36 PM Page 688 CHAPTER 19 • THE AUTONOMIC NERVOUS SYSTEM Figure 19.7 Cholinergic neurons and adrenergic neurons in the sympathetic and parasympathetic divisions. Cholinergic neurons release acetylcholine; adrenergic neurons release norepinephrine. Cholinergic and adrenergic receptors are integral membrane proteins located in the plasma membrane of a postsynaptic neuron or an effector cell. Most sympathetic postganglionic neurons are adrenergic; other autonomic neurons are cholinergic. Nicotinic receptors Effector cell Adrenergic receptor ACh NE Preganglionic neuron Ganglion Postganglionic neuron (a) Sympathetic division–innervation to most effector tissues Muscarinic receptor Nicotinic receptors ACh ACh Cell of sweat gland (b) Sympathetic division–innervation to most sweat glands Nicotinic receptors ACh Muscarinic receptor Effector cell ACh (c) Parasympathetic division times causes depolarization (excitation) and sometimes causes hyperpolarization (inhibition), depending on which particular cell bears the muscarinic receptors. For example, binding of ACh to muscarinic receptors inhibits (relaxes) smooth muscle sphincters in the gastrointestinal tract. By contrast, ACh excites smooth muscle fibers in the circular muscles of the iris of the eye, causing them to contract. Because acetylcholine is quickly inactivated by the enzyme acetylcholinesterase (AChE), effects triggered by cholinergic neurons are brief. Adrenergic Neurons and Receptors In the ANS, adrenergic neurons (ad⬘-ren-ER-jik) release norepinephrine (nor⬘-ep-i-NEF-rin) or NE, also known as noradrenalin (Figure 19.7a). (Remember: adrenergic⫽noradrenalin.) Most sympathetic postganglionic neurons are adrenergic. Like ACh, NE is synthesized and stored in synaptic vesicles and released by exocytosis. Molecules of NE diffuse across the synaptic cleft and bind to specific adrenergic receptors on the postsynaptic membrane, causing either excitation or inhibition of the effector cell. Adrenergic receptors bind both NE and epinephrine, a hormone with actions similar to NE. As noted previously, NE is released as a neurotransmitter by sympathetic postganglionic neurons. In addition, both epinephrine and NE are released as hormones into the blood by the chromaffin cells of the adrenal medullae. The two main types of adrenergic receptors are alpha (␣) receptors and beta (␤) receptors, which are found on visceral effectors innervated by most sympathetic postganglionic axons. These receptors are further classified into subtypes—␣1, ␣2, ␤1, ␤2, and ␤3—based on the specific responses they elicit and by their selective binding of drugs that activate or block them. Although there are some exceptions, activation of ␣1 and ␤1 receptors generally produces excitation, in contrast to activation of ␣2 and ␤2 receptors, which causes inhibition of effector tissues. ␤3 receptors are present only on cells of brown adipose tissue, where their activation causes thermogenesis (heat production). Cells of most effectors contain either ␣ or ␤ receptors; some visceral effector cells contain both. NE stimulates alpha receptors more strongly than beta receptors; epinephrine is a potent stimulator of both alpha and beta receptors. The activity of NE at a synapse is terminated when (1) the NE is taken up by the axon that released it or (2) when NE is enzymatically inactivated by either catechol-O-methyltransferase (COMT) (kat-e-ko- l⬘-o- -meth-il-TRANS-fer-a- s) or monoamine oxidase (MAO) (mon-o- -AM-e- n OK-si-da- s⬘). NE lingers in the synaptic cleft for a longer time than ACh. Thus, effects triggered by adrenergic neurons typically are longer lasting than those triggered by cholinergic neurons. CHECKPOINT Which neurons are cholinergic and possess nicotinic ACh receptors? What type of receptors for ACh do their effector tissues possess? 11. Why are cholinergic and adrenergic neurons so named? 12. What substances bind to adrenergic receptors? 19.7 FUNCTIONS OF THE ANS muscarinic receptors (see Figure 19.7). These receptors are also named for a substance that does not naturally occur in the human body; a mushroom poison called muscarine mimics the actions of ACh by binding to muscarinic receptors. Activation of nicotinic receptors by ACh always causes depolarization and thus excitation of the postsynaptic cell, which can be a postganglionic neuron, an autonomic effector, or a skeletal muscle fiber. Activation of muscarinic receptors by ACh some- OBJECTIVES • Describe the major responses of the body to stimulation by the sympathetic division of the ANS. • Explain the reactions of the body to stimulation by the parasympathetic division. As noted earlier in the chapter, most body organs are innervated by both divisions of the ANS, which typically work in opposition to JWCL299_c19_674-696.qxd 6/19/10 3:24 PM Page 689 19.7 FUNCTIONS OF THE ANS one another. The balance between sympathetic and parasympathetic activity is regulated by the hypothalamus. The hypothalamus typically increases sympathetic activity at the same time it decreases parasympathetic activity, and vice versa. As you learned in Section 19.6, the two divisions affect body organs differently because of the different neurotransmitters released by their postganglionic neurons and the different adrenergic and cholinergic receptors on the cells of their effector organs. A few structures receiv