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