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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 the...

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 receive only sympathetic innervation—sweat glands, arrector pili muscles attached to hair follicles in the skin, the kidneys, the spleen, most blood vessels, and the adrenal medullae (see Figure 19.4). In these structures there is no opposition from the parasympathetic division; increases and decreases in sympathetic activity are responsible for the changes. Sympathetic Responses During physical or emotional stress, the sympathetic division dominates the parasympathetic division. High sympathetic activity favors body functions that can support vigorous physical activity and rapid production of ATP. At the same time, the sympathetic division decreases body functions that favor the storage of energy. Physical exertion and a variety of emotions—such as fear, embarrassment, or rage—stimulate the sympathetic division. Visualizing body changes that occur during “E situations” such as exercise, emergency, excitement, and embarrassment, will help you remember most of the sympathetic responses. Activation of the sympathetic division and release of hormones by the adrenal medullae set in motion a series of physiological responses collectively called the fight-or-flight response, which includes the following effects (many of which were experienced by the student in the introduction to this chapter): 1. The pupils of the eyes dilate. 2. Heart rate, force of heart contraction, and blood pressure increase. 3. The airways dilate, allowing faster movement of air into and out of the lungs. 4. Blood vessels that supply organs involved in exercise or fighting off danger—skeletal muscles, cardiac muscle, liver, and adipose tissue—dilate, allowing greater blood flow through these tissues. 5. Liver cells perform glycogenolysis (breakdown of glycogen to glucose), and adipose tissue cells perform lipolysis (breakdown of triglycerides to fatty acids and glycerol). 6. Release of glucose by the liver increases blood glucose level. 7. Processes that are not essential for meeting the stressful situation are inhibited. For example, the blood vessels that supply the kidneys and gastrointestinal tract constrict, which decreases blood flow through these tissues. The result is a slowing of urine formation and digestive activities. The effects of sympathetic stimulation are longer lasting and more widespread than the effects of parasympathetic stimulation for three reasons: (1) Sympathetic postganglionic axons diverge more extensively; as a result, many tissues are activated simultaneously. (2) AChE quickly inactivates ACh, but NE lingers in the synaptic cleft for a longer period. (3) The secretion of epinephrine and NE into the blood from the adrenal medulla (as hormones) intensifies and prolongs the responses caused by NE released as a neurotransmitter from sympathetic postganglionic axons. These CLINICAL CONNECTION | Drugs and Receptor Selectivity A large variety of drugs and natural products can selectively activate or block specific cholinergic or adrenergic receptors. An agonist (agon⫽a contest) is a substance that binds to and activates a receptor, in the process mimicking the effect of a natural neurotransmitter or hormone. Phenylephrine, an adrenergic agonist at ␣1 receptors, is a common ingredient in cold and sinus medications. Because it constricts blood vessels in the nasal mucosa, phenylephrine reduces production of mucus, thus relieving nasal congestion. An antagonist (anti-⫽against) is a substance that binds to and blocks a receptor, thereby preventing a natural neurotransmitter or hormone from exerting its effect. For example, atropine, which blocks muscarinic ACh receptors, dilates the pupils, reduces glandular secretions, and relaxes smooth muscle in the gastrointestinal tract. It is used to dilate the pupils during eye examinations, in the treatment of smooth muscle disorders such as iritis and intestinal hypermotility, and as an antidote for chemical warfare agents that inactivate AChE. Propranolol (Inderal®) often is prescribed for patients with hypertension (high blood pressure). It is a nonselective beta blocker, meaning it binds to all types of beta receptors and prevents their activation by epinephrine and norepinephrine. The desired effects of propranolol are due to its blockade of ␤1 receptors—namely, decreased heart rate and force of contraction and a consequent decrease in blood pressure. Undesired effects due to blockade of ␤2 receptors may include hypoglycemia (low blood glucose), resulting from decreased glycogen breakdown and decreased gluconeogenesis (the conversion of a noncarbohydrate into glucose in the liver), and mild bronchoconstriction (narrowing of the airways). If these side effects pose a threat to the patient, a selective ␤1 blocker that binds only to specific beta receptors, such as metoprolol (Lopressor®), can be prescribed. • 689 Normal pupil Dilated pupil Atropine, an antagonist, blocks muscarinic ACh receptors and dilates the pupils