CH 17 Autonomic Nervous System

A Summary of the Parasympathetic Division

- The parasympathetic division includes visceral motor nuclei in the brainstem associated with four cranial nerves (III, VII, IX, and X). Autonomic nuclei lie in the lateral portions of the anterior horns in sacral segments S2-S4. - The ganglionic neurons are located in terminal or intramural ganglia. - The parasympathetic division innervates structures in the head and organs in the thoracic and abdominopelvic cavities. - All parasympathetic neurons are cholinergic. Release of ACh by preganglionic neurons stimulates nicotinic receptors on ganglionic neurons, and the effect is always excitatory. The release of ACh at neuroeffector junctions stimulates muscarinic receptors, and the effects may be either excitatory or inhibitory, depending on the nature of the enzymes activated when ACh binds to the receptor. - The effects of parasympathetic stimulation are brief and restricted to specific organs and sites.

A Summary of the Sympathetic Division

- The sympathetic division of the ANS includes two sympathetic chains resembling a string of beads, one on each side of the vertebral column; three collateral ganglia anterior to the spinal column; and two adrenal medullae. - Preganglionic fibers are short because the ganglia are close to the spinal cord. The postganglionic fibers are longer and extend a considerable distance before reaching their target organs. (In the case of the adrenal medullae, very short axons from modified ganglionic neurons end at capillaries that carry their secretions to the bloodstream.) - The sympathetic division shows extensive divergence; a single preganglionic fiber may innervate as many as 32 ganglionic neurons in several different ganglia. As a result, a single sympathetic motor neuron inside the CNS controls a variety of peripheral effectors and produced a complex and coordinated response. - All preganglionic neurons release ACh at their synapses with ganglionic neurons. Most of the postganglionic fibers release norepinephrine, but a few release ACh. - The effector response depends on the function of the plasma membrane receptor activated when epinephrine or norepinephrine binds to either alpha or beta receptors.

Parasympathetic Activation and Neurotransmitter Release

All preganglionic and postganglionic fibers of the parasympathetic division release ACh at their synapses and neuroeffector junctions. Parasympathetic neuroeffector junctions are small, with narrow synaptic clefts. The effects of stimulation are short lived because most of the ACh released is inactivated by acetylcholinesterase (AChE) within the synapse. Any ACh diffusing into the surrounding tissues is deactivated by AChE. As a result, the effects of parasympathetic stimulation are quite localized and last a few seconds at most.

Anatomy of Dual Innervation

Although the parasympathetic and sympathetic branches of the ANS exit the CNS from different regions, parasympathetic and sympathetic fibers are often found within the same peripheral ganglia or plexus. In the head, parasympathetic postganglionic fibers from the ciliary, pterygopalatine, submandibular, and otic ganglia accompany the cranial nerves to their peripheral destinations. Sympathetic innervation reaches the same structures by traveling directly from the superior cervical ganglia of the sympathetic chain. In the thoracic and abdominopelvic cavities, the sympathetic postganglionic fibers intermix with parasympathetic preganglionic fibers at a series of plexuses. These are the cardiac plexus, pulmonary plexus, esophageal plexus, celiac plexus, inferior mesenteric plexus, and hypogastric plexus. Nerves leaving these plexuses travel with the blood vessels and lymphatic supplying visceral organs. Autonomic fibers entering the thoracic cavity intersect at the *cardiac plexus* and the *pulmonary plexus*. These plexuses contain both sympathetic fibers innervating the heart and parasympathetic fibers innervating the heart and lungs. The *esophageal plexus* contains descending branches of the vagus nerve and splanchnic nerves leaving the sympathetic chain ganglia on each side. Parasympathetic preganglionic fibers of the vagus nerve follow the esophagus as it enters the abdominopelvic cavity. There the parasympathetic fibers join the network of the *celiac plexus*. The celiac plexus and an associated smaler plexus, the *inferior mesenteric plexus*, innervate viscera within the abdominal cavity. The *hypogastric plexus* contains the parasympathetic outflow of the pelvic nerves, sympathetic postganglionic fibers from the inferior mesenteric ganglion, and dacral splanchnic nerves from the sympathetic chain. The hypogastric plexus innervates the digestive, urinary, and reproductive organs of the pelvic cavity.

Relationship between the Sympathetic and Parasympathetic Divisions

Most organs innervated by the autonomic nervous system are innervated by both the sympathetic and parasympathetic branches. Typically, one division will increase activity of the organ, and the other will decrease the organ's activity. The sympathetic division has a widespread impact, reaching visceral organs and tissues throughout the body. The parasympathetic division modifies the activity of structures innervated by specific cranial nerves and pelvic nerves. This includes the visceral organs within the thoracic and abdominopelvic cavities. Although some of these organs are innervated by only one autonomic division (sympathetic or parasympathetic), most vital organs receive *dual innervation* - that is, they are innervated by both the sympathetic and parasympathetic divisions. Where dual innervation exists, the two divisions often have opposite, or antagonistic, effects. Dual innervation is most common in the digestive tract, the heart, and the lungs. For example, sympathetic stimulation decreases digestive tract motility, while parasympathetic stimulation increases its motility.

Intro

Our conscious thoughts, plans, and actions are only a tiny fraction of the activities of the nervous system. When all consciousness is eliminated, such as when we sleep, vital homeostatic processes continue virtually unchanged. Longer, deeper states of unconsciousness are not more dangerous, as long as nourishment is provided. People who have suffered severe brain injuries have survived in a come for decades. Survival is possible under such conditions because the *autonomic nervous system (ANS)* makes routine adjustments in physiological systems. The ANS regulates body temperature and coordinates cardiovascular, respiratory, digestive, excretory, and reproductive functions.

Collateral Ganglia

Splanchnic nerves carry visceral efferent motor fibers and visceral afferent sensory fibers. Postganglionic neurons within the collateral ganglia send postganglionic fibers to viscera within the abdominal and pelvic cavities. The abdominopelvic viscera receive sympathetic innervation by sympathetic preganglionic fibers that synapse in separate collateral ganglia. These fibers pass through the sympathetic chain without synapsing. They form the paired *splanchnic nerves* (SPLANK-nik), which lie in the posterior wall of the abdominal cavity. They originate as paired ganglia, but the two usually fuse.

Sympathetic Activation and Neurotransmitter Release

Sympathetic preganglionic fibers release acetylcholine (ACh) at cholinergic sypanses. Postganglionic fibers release norepinephrine (NE) at adrenergic, neuroeffector junctions. The ACh released by cholinergic, preganglionic neurons during sympathetic activation always stimulates the ganglionic neurons. This leads to postganglionic fibers releasing norepinephrine (NE) at neuroeffector junctions. These neuroeffector junctions are adrenergic, sympathetic terminals. These symapthetic division also contains a small but significant number of ganglionic neurons that release ACh, rather than NE, at their neuroeffector junctions. For example, ACh is released at sympathetic neuroeffector junctions in the body wall, in the skin, and within skeletal muscles. This type of synaptic junction forms an extensive branching network rather than ending in a single axon terminal (as seen in skeletal neuromuscluar junction). Each branch resembles a string of beads, and each bead, or *varicosity*, is packed with mitochondria and neurotransmitter vesicles. These varicosities pass along or near the surfaces of many effector cells. A single axon may supply 20,000 varicosities, which can effect dozens of surrounding cells. Receptor proteins are scattered across most plasma membranes, and there are no specialized postsynaptic membranes. The effects caused by the neurotransmitter released at a varicosity last for only a few seconds between the neurotransmitter is reabsorbed, broken down by enzymes, or removed by diffusion into the blood stream. In contrast, the effects of the epinephrine and noepinephrine secreted by the adrenal medullae last must longer because (1) the bloodstream does not contain the enzymes required to break down epinephrine or norepinephrine, and (2) most tissues contain relatively low concentrations of these enzymes. As a result, stimulation of the adrenal medulla causes widespread effects that continue for a relatively long time. For example, tissue concentrations of epinephrine may remain elevated for as long as 30 seconds, and the effects may last for several minutes.

A Comparison of the Somatic and Autonomic Nervous Systems

The autonomic nervous sytem (ANS) innervates visceral effectors, while the somatic nervous system (SNS) has lower motor neurons that innervate skeletal muscles. Like the SNS, the ANS has afferent and efferent neurons. Also like the SNS, the afferent sensory information of the ANS is processed in the CNS, and then efferent impulses are sent to effector organs. However, in the ANS, the afferent pathways originate in visceral receptors, and the efferent pathways connect visceral effector organs, such as smooth muscle and glands. In addition to the difference in receptor and effector organ location, the ANS, composed of the sympathetic and parasympathetic divisions, differs from the SNS in arrangement of the efferent neurons. In the ANS, the axon of a visceral motor neuron within the CNS innervates a second neuron located in a peripheral ganglion. This second neuron innervates the peripheral effector. Visceral motor neurons in the CNS send short, myelinated axons, called *preganglionic fibers*, to synapse on a group of neurons located within a *ganglion* located outside the CNS. Axons leaving the ganglia are relatively long and are unmyelinated. These axons are called *postganglionic fibers* because they carry impulses away from the ganglion. Postganglionic fibers innervate peripheral tissues and organs, such as cardiac and smooth muscle, adipose tissue, and glands.

Adrenal Medulla

The cells of the adrenal medulla secrete epinephrine and norepinephrine following stimulation by pympathetic prganglionic neurons. Some preganglionic fibers originating between T5 and T8 pass through the sympathetic chain and the celiac ganglion without synapsing and proceed to the *adrenal medulla.* There they synapse on modified neurons that perform an endocrine function. When stimulated, these modified neurons release the neurotransmitters epinephrine (E) and norepinephrine (NE) into an extensive network of capillaries. The neurotransmitters function as hormones, exerting their effects in other regions of the body. Epinephrine, also called adrenaline, accounts for 75-80% of the secretory output; the rest is norepinephrine (noradrenaline). The circulating blood distributes these hormones throughout the body, changing the metabolic activities of many different cells. In general, the effects resemble those produced by the stimulation of sympathetic postganglionic fibers. They differ, however, it two ways: (1) Cells not innervated by symapthetic postganglionic fibers are affected by circulating levels of epinephrine and norepinephrine only if they possess receptors for these molecules; and (2) the effects last much longer than those produced by direct sympathetic innervation, because the released hormones continue to diffuse out of the circulating blood for an extended period.

Plasma Membrane Receptors and Sympathetic Function

The effects of sympathetic stimulation result from the interaction between epinephrine or norepinephrine and plasma membrane receptors. There are two classes of sympathetic receptors sensitive to epinephrine and norepinephrine: *alpha receptors* and *beta receptors*. Each of these classes of receptors has two or three subtypes. The diversity of receptors and the varying combinations found on the plasma membranes account for the wide variety in target organs responses to sympathetic stimulation. In general, epinephrine stimulates both classes of receptors, while norepinephrine primarily stimulates alpha receptors.

General Functions of the Parasympathetic Division

The following is a partial listing of the major effects produced by the parasympathetic division: - Constriction of the pupils, which restricts the amount of light entering the eyes and aids in focusing on nearby objects. - Secretion by digestive glands, including salivary glands, gastric glands, duodenal and other intestinal glands, the pancreas, and the liver. - Secretion of the hormones promoting nutrient absorption by peripheral cells. - Increased smooth muscle activity along the digestive tract. - Stimulation and coordination of defecation. - Contraction of the urinary bladder during urination. - Constriction of the respiratory passageways. - Reduction in heart rate and force of contraction. - Sexual arousal and stimulation of sexual glands in both sexes. These functions center on relaxation, food processing, and energy absorption. Stimulation of the parasympathetic division leads to an increase in the nutrient content within the blood. Cells throughout the body respond to this increase by absorbing nutrients and using them to support growth and other anabolic activities.

The Parasympathetic Division

The parasympathetic division of the ANS operates through a series of interconnected neurons. Efferent parasympathetic neurons originate from cranial nerves III, VII, IX, and X and sacral spinal nerves S2-S4 and synapse with neurons within or near the innervated organ. The parasympathetic division of the ANS consists of the following: - Preganglionic neurons located in the brainstem and in sacral segments of the spinal cord. The mesencephalon (midbrain), pons, and medulla oblongata contain autonomic nuclei associated with cranial nerves III, VII, IX, and X. In the sacral segments of the spinal cord, the autonomic nuclei lie in spinal segments S2-S4. - Ganglionic neurons located in peripheral ganglia within or adjacent to the target organs. Preganglionic fibers of the parasympathetic division do not diverge as extensively as do those of the sympathetic division. A typical preganglionic fiber synapses on 6-8 ganglionic neurons. These neurons are all located in the same ganglion, and their postganglionic fibers include the same target organ. The ganglion may be a terminal ganglion (near the target organs) or an intramural ganglion (within the tissues of the target organ). As a result, the effects of parasympathetic stimulation are more specified and localized than those of the sympathetic division.

Organization and Anatomy of the Parasympathetic Division

The parasympathetic division originates from cranial nerves and sacral spinal nerves. Parasympathetic preganglionic fibers leave the brain in cranial nerves III (oculomotor), VII (facial), IX (glossopharyngeal), and X (vagus). The fibers in N III, N VII, and N IX control visceral structures in the head. These preganglionic fibers synapse in the *ciliary*, *pterygopalatine*, *submandibular*, and *otic ganglia*. Short postganglionic fibers continue to their peripheral targets. The vagus nerve (X) provides preganglionic parasympathetic innervation to intramural ganglia within viscera in the thoracic and abdominopelvic cavities, traveling as far as the last segments of the large intestine. The vagus nerve alone provides roughly 75% of all parasympathetic outflow. The sacral parasympathetic outflow does not join the ventral rami of the spinal nerves. Instead, the preganglionic fibers form distinct *pelvic nerves* that innervate intramural ganglia in the kidney and urinary bladder, the terminal portions of the large intestine, and the sex organs.

Plasma Membrane Receptors and Response

The parasympathetic division uses the same neurotransmitter, ACh, at all of its synapses (neuron-to-neuron) and neuromuscular or neuroglandular junctions. Two types of ACh receptors are found on postsynaptic plasma membranes: 1) *Nicotinic receptors* are on the surfaces of all ganglionic neurons of both the parasympathetic and sympathetic divisions, as well as at neuromuscular synapses of the somatic nervous system. Exposure to ACh always causes excitation of the ganglionic neuron or muscle fiber through the opening of chemically gated Na+ channels in the postsynaptic membrane. 2) *Muscarinic receptors* are found at all cholinergic neuromuscular or neuroglandular junctions in the parasympathetic division, as well as at the few cholinergic neuroeffector junctions in the sympathetic division. Stimulation of muscarinic receptors produces longer-lasting effects than does stimulation of nicotinic receptors. The response, which reflects the activation or inactivation of specific enzymes, may be either excitatory or inhibitory. The names nicotinic and muscarinic indicate the chemical compounds that stimulate these receptor sites. Nicotinic receptors bind nicotine, a powerful component of tobacco smoke. Muscarinic receptors are stimulated by muscarine, a toxin produced by some poisonous mushrooms.

Sympathetic Chain Ganglia

The preganglionic neurons of the sympathetic division form synapses with ganglionic neurons within the sympathetic trunk. The sympathetic trunk is found on each side of the vertebral column. Each sympathetic chain ganglion has 3 cervical, 11-12 thoracic, 2-5 lumbar, and 4-5 sacral sympathetic ganglia and 1 coccygeal sympathetic ganglion. Numbers may vary because adjacent ganglia may fuse. For example, the coccygeal ganglia from both sides usually fuse to form a single median ganglion, the ganglion impar, while the inferior cervical and first thoracic ganglia from both sides occasionally fuse to form a stellate ganglion. *Preganglionic sympathetic neurons are only found in segments T1-L2 of the spinal cord, and the spinal nerves of these segments have both white rami communicantes (preganglionic fibers) and gray rami communicantes (postganglionic fibers).* The neurons in the cervical, inferior lumbar, and sacral sympathetic chain ganglia are innervated by preganglionic fibers extending along the length of the chain. In turn, these chain ganglia provide postganglionic fibers, through the gray rami, to the cervical, lumbar, and sacral spinal nerves. *Every spinal nerve along the entire length of the spinal cord has a pair of gray rami communicantes carrying sympathetic postganglionic fibers.* About 8% of the axons in each spinal nerve are sympathetic postganglionic fibers. The dorsal and ventral rami of the spinal nerves provide extensive sympathetic innervation to structures in the body wall an dlimbs. In the head, postganglionic fibers leaving the cervical sympathetic ganglia supply the regions and structures innervated by cranial nerves III, VII, IX, and X.

Anatomy of the Collateral Ganglia

The splanchnic nerves (greater, lesser, lumber, and sacral) innervate three collateral ganglia. Preganglionic fibers from the seven inferior thoracic segments end at the *celiac ganglion* and the *superior mesenteric ganglion*. These ganglia are located within an extensive, weblike network of nerve fibers termed the autonomic plexus. Preganglionic fibers from the lumbar segments form the splanchnic nerves that end at the *inferior mesenteric ganglion*. The sacral splanchnic nerves end in the hypogastric plexus, an autonomic network supplying pelvic organs and the external genitalia. *The Celiac Ganglion:* located at the base of the celiac trunk. Postganglionic fibers from the celiac ganglion innervate the stomach, duodenum, liver, gallbladder, pancreas, spleen, and kidney. The celiac ganglion varies considerably in appearance and often consist of a pair of interconnected masses of gray matter. *The Superior Mesenteric Ganglion:* located as the base of the superior mesenteric artery. Postganglionic fibers from the superior mesenteric ganglion innvervate the small intestine and the initial segments of the large intestine. *The Inferior Mesenteric Ganglion:* located at the base of the inferior mesenteric artery. Postganglionic fibers from this ganglion innervate the terminal portions of the large intestine, the kidney and the bladder, and the sex organs.

Sympathetic and Parasympathetic Subdivisions of the ANS

The sympathetic division is most active during times of exertion, stress, and sexual climax, often termed the "fight-or-flight" reaction. The parasympathetic division is most active during sexual arousal and periods of "rest-and-digest." The two divisions of the ANS often have opposing effects; if the sympathetic division causes excitation, the parasympathetic division causes inhibition. However, this is not always the case because (1) the two divisions may work independently, with some structures innervated y only one division, and (2) the two divisions may work together, each controlling one stage of a complex process. In general, the parasympathetic division is most active under resting conditions, while the sympathetic division is most active during times of excitation, stress, or emergency. The sympathetic division is also termed the thoracolumbar division, and the parasympathetic division is called the craniosacral division. The autonomic nervous system also includes a third division - the *enteric nervous system (ENS)*. The enteric nervous system is an extensive network of neurons located within the walls of the digestive tract. Although the ENS is affected by the sympathetic and parasympathetic divisions, many complex visceral reflexes are initiated and coordinated locally, without instruction from the CNS. We will discuss the ENS in a later chapter.

Effects of Sympathetic Stimulation

The sympathetic division of the ANS changes tissue and organ activities by releasing norepinephrine at peripheral synapses and by releasing epinephrine and norepinephrine from the adrenal medulla. Symapthetic motor fibers innervating specific effectors, such as smooth muscle fibers in blood vessels of the skin, are activated in reflexes not involving other peripheral effectors. In a crisis, however, the entire division responds. This event, called *sympathetic activation*, affects peripheral tissues and alters CNS activity. Sympathetic centers in the hypothalamus control symapthetic activation. When symapthetic activation occurs, we experience the following: - Increased alertness, through stimulation of the reticular activating system, causing us to feel "on edge." - A feeling of energy and euphoria, often associated with a disregard for danger and temporary insensitivity to painful stimuli. - Increased activity in the cardiovacular and respiratory centers of the pons and medulla oblongata, leading to increased heart rate and contraction strength, elevations in blood pressure, breathing rate, and depth of respiration. - A general elevation in muscle tone through stimulation of the extrapyramidal system, so that we look tense and may even begin to shiver. - The mobilization of energy reserves through the accelerated breakdown of glycogen in muscle and liver cells and the release of lipids in adipose tissues. These changes, coupled with the peripheral changes already discussed, complete the preparations necessary for us to handle stressful and potentially dangerous situations. We will now consider the cellular basis for the general effects of sympathetic activation on peripheral organs.

The Sympathetic Division

The sympathetic division of the ANS operates through a series of interconnected neurons. Efferent sympathetic neurons originate from thoracic and lumbar spinal nerves and synapse with neurons in the PNS at a series of sympathetic ganglia. Preganglionic neurons are only located between segments T1 and L2 of the spinal cord. The cell bodies of these neurons occupy the lateral horns of the spinal cord between T1 and L2, and their axons enter the ventral roots of those segments. The ganglionic neurons are in three locations: 1) *Sympathetic chain ganglia* are on both sides of the vertebral column. Postganglionic fibers exiting these ganglia innervate effector organs in the body wall, head and neck, limbs, and inside the thoracic cavity. 2) *Collateral ganglia* are anterior to the vertebral column. Postganglionic fibers exiting these ganglia innervate effector organs in the abdominopelvic cavity. 3) Specialized sympathetic neurons are located in the interior of the adrenal gland, known as the *adrenal medulla*. The adrenal medulla is a modified sympathetic ganglion. These ganglionic neurons have very short axons. When stimulated, they release neurotransmitters into the bloodstream for distribution throughout the body as hormones.

Visceral Reflexes

Visceral reflexes play an important role in regulating and coordinating the activities of various organs in the digestive system. All autonomic, visceral reflexes are polysynaptic, with at least one synapse in the CNS and another in an autonomic nervous system ganglion. *Visceral reflexes* are autonomic reflexes initiated in the viscera. They provide automatic motor responses that can be modified, facilitated, or inhibited by higher centers, especially those of the hypothalamus. For example, when a light is shone in one of your eyes, a visceral reflex constricts the pupils of both eyes. In darkness, your pupils dilate. The motor nuclei directing pupillary constriction or dilation are also controlled by hypothalamic centers concerned with emotional states. For example, when you are queasy or nauseated, your pupils constrict; when you are sexually aroused, your pupils dilate. All visceral reflexes are polysynpatic (having more than one synapse). Each *visceral reflex arc* is made up of a receptor, a sensory nerve, a processing enter (one or more interneurons) in the CNS, and two visceral motor neurons (preganglionic and ganglionic). Afferent (sensory) nerves deliver information to the CNS along spinal nerves, cranial nerves, and the autonomic nerves innervating peripheral effectors. Visceral reflexes are either long reflexes or short reflexes. *Long reflexes* of the autonomic nervous system resemble the polysynaptic reflexes introduced in Chapter 14. Visceral sensory neurons deliver information to the CNS by the dorsal roots of spinal nerves, within the sensory branches of cranial nerves, and within the autonomic nerves innervating visceral effectors. The processing steps involve interneurons within the CNS. The motor neurons of these long reflexes are located within the brainstem or spinal cord. The ANS carries the motor commands to the appropriate visceral effectors after a synapse within a peripheral autonomic ganglion. *Short reflexes* bypass the CNS entirely. They involve sensory neurons and interneurons with neuronal somas located within autonomic ganglia. The interneurons synapse on ganglionic neurons, and the motor commands are distributed by postganglionic fibers. Short reflexes control very simple motor responses with localized effects. In general, short reflexes control patterns of activity in one part of a target organ, while long reflexes coordinate the activities of the entire organ. In most organs, long reflexes are most important in regulating visceral activities, but this is not the case with the digestive tract and its associated glands. Here, short reflexes provide most of the control and coordination for normal function, and the neurons involved form the *enteric nervous system*. Parasympathetic innervation by visceral motor neurons can stimulate and coordinate various digestive activities, but the enteric nervous system can control digestive activities independent of the central nervous system.