The Autonomic Nervous System

The autonomic nervous system is a part of the peripheral nervous system that functions to regulate the basic visceral (organ) processes needed for the maintenance of normal bodily functions. It operates independently of voluntary control, although certain events, such as emotional stress, fear, sexual excitement, and alterations in the sleep-wakefulness cycle, change the level of autonomic activity.

The autonomic system is usually defined as a motor system that innervates three major types of tissue: cardiac muscle, smooth muscle, and glands. However, this definition needs to be expanded to encompass the fact that it also relays visceral sensory information into the central nervous system and processes it in such a way as to make alterations in the activity of specific autonomic motor outflows, such as those that control the heart, blood vessels, and other visceral organs. It also causes the release of certain hormones involved in energy metabolism (e.g., insulin, glucagon, epinephrine) or cardiovascular functions (e.g., renin, vasopressin). These integrated responses maintain the normal internal environment of the body in an equilibrium state called homeostasis.

The autonomic system consists of two major divisions: the sympathetic nervous system and the parasympathetic nervous system. These often function in antagonistic ways. The motor outflow of both systems is formed by two serially connected sets of neurons. The first set, called preganglionic neurons, originates in the brain stem or the spinal cord, and the second set, called ganglion cells or postganglionic neurons, lies outside the central nervous system in collections of nerve cells called autonomic ganglia. Parasympathetic ganglia tend to lie close to or within the organs or tissues that their neurons innervate, whereas sympathetic ganglia lie at a more distant site from their target organs. Both systems have associated sensory fibers that send feedback information into the central nervous system regarding the functional condition of target tissues. To view a figure depicting the difference in function between the sympathetic and parasympathetic nervous system click here.

A third division of the autonomic system, termed the enteric nervous system, consists of a collection of neurons embedded within the wall of the entire gastrointestinal tract and its derivatives. This system controls gastrointestinal motility and secretions.

Sympathetic preganglionic neurons originate in the lateral horns of the 12 thoracic and the first 2 or 3 lumbar segments of the spinal cord. (For this reason the sympathetic system is sometimes referred to as the thoracolumbar outflow. The diagram to the left depicts this.) The axons of these neurons exit the spinal cord in the ventral roots and then synapse on either sympathetic ganglion cells or specialized cells in the adrenal gland called chromaffin cells.

Sympathetic ganglia can be divided into two major groups, paravertebral and prevertebral (or preaortic), on the basis of their location within the body. Paravertebral ganglia generally lie on each side of the vertebrae and are connected to form the sympathetic chain or trunk. There are usually 21 or 22 pairs of these ganglia: 3 in the cervical region, 10 to 11 in the thoracic region, 4 in the lumbar region, 4 in the sacral region, and a single, unpaired ganglion lying in front of the coccyx called the ganglion impar. The three cervical sympathetic ganglia are the superior cervical ganglion, the middle cervical ganglion, and the cervicothoracic ganglion (also called the stellate ganglion). The superior ganglion innervates viscera of the head; the middle and stellate ganglia innervate viscera of the neck, thorax (i.e., the bronchi and heart), and upper limb. The thoracic sympathetic ganglia innervate the trunk region, and the lumbar and sacral sympathetic ganglia innervate the pelvic floor and lower limb. All the paravertebral ganglia provide sympathetic innervation to blood vessels in muscle and skin, arrector pili muscles attached to hairs, and sweat glands.

The three preaortic ganglia are the celiac, superior mesenteric, and inferior mesenteric. Lying on the anterior surface of the aorta, they provide axons that are distributed with the three major gastrointestinal arteries arising from the aorta. The three ganglia retain a pattern of innervation that originates in the embryo. Thus, the celiac ganglion innervates structures derived from the embryonic foregut, including the stomach, liver, pancreas, duodenum, and the first part of the small intestine; the superior mesenteric ganglion innervates the small intestine, which is derived from the embryonic midgut; and the inferior mesenteric ganglion innervates embryonic hindgut derivatives, which include the descending colon, sigmoid colon, rectum, urinary bladder, and sexual organs.

Upon reaching their target organs by traveling with the blood vessels that supply them, sympathetic fibers terminate as a series of varicosities close to the end organ. Because of this anatomical arrangement, autonomic transmission takes place across a junction rather than a synapse. “Presynaptic” sites can be identified because they contain aggregations of synaptic vesicles and membrane thickenings; postjunctional membranes, on the other hand, rarely possess morphological specializations, but they do contain specific receptors for various neurotransmitters. The distance between pre- and postsynaptic elements can be quite large as compared to typical synapses. For instance, the gap between cell membranes of a typical chemical synapse is 30–50 nanometres, while in blood vessels the distance is often greater than 100 nanometres and, in some cases, 1–2 micrometres (1,000–2,000 nanometres). Owing to these relatively large gaps between autonomic nerve terminals and their effector cells, transmitters tend to act slowly; they become inactivated rather slowly as well. To compensate for this apparent inefficiency, many effector cells, such as those in smooth and cardiac muscle, are connected by low-resistance pathways that allow for electrotonic coupling of the cells. In this way, if only one cell is activated, multiple cells will respond and work as a group.

At a first approximation, chemical transmission in the sympathetic system appears simple: preganglionic neurons use acetylcholine as a neurotransmitter, whereas most postganglionic neurons utilize norepinephrine (noradrenaline)—with the major exception that postganglionic neurons innervating sweat glands use acetylcholine. On closer inspection, however, neurotransmission is seen to be more complex, because multiple chemicals are released, and each functions as a specific chemical code affecting different receptors on the target cell. In addition, these chemical codes are self-regulatory, in that they act on presynaptic receptors located on their own axon terminals.

The chemical codes are specific to certain tissues. For example, most sympathetic neurons that innervate blood vessels secrete both norepinephrine and neuropeptide Y, sympathetic neurons that innervate the submucosal neural plexus of the gut contain both norepinephrine and somatostatin, and sympathetic neurons that innervate sweat glands contain calcitonin gene-related peptide, vasoactive intestinal polypeptide, and acetylcholine. In addition, other chemicals besides the neuropeptides mentioned above are released from autonomic neurons along with the so-called classical neurotransmitters, norepinephrine and acetylcholine. For instance, some neurons synthesize a gas, nitric oxide, that functions as a novel type of neuronal messenger molecule. Thus, neural transmission in the autonomic nervous systems involves the release of combinations of different neuroactive agents that affect both pre- and postsynaptic receptors.

Neurotransmitters released from nerve terminals bind to specific receptors, which are specialized macromolecules embedded in the cell membrane. The binding action initiates a series of specific biochemical reactions in the target cell that produce a physiological response. These effects can be modified by various drugs that act as agonists or antagonists. In the sympathetic nervous system, for example, there are five types of adrenergic receptors (receptors binding epinephrine): a 1, a 2, b 1, b 2, and b 3. These are found in different combinations in various cells throughout the body. Activation of a 1 receptors in arterioles causes blood-vessel constriction, whereas stimulation of a 2 autoreceptors (receptors located in sympathetic presynaptic nerve endings) function to inhibit the release of norepinephrine. Other types of tissue have unique adrenergic receptors. Heart rate and myocardial contractility, for example, is controlled by b 1 receptors, bronchial smooth muscle relaxation is mediated by b 2 receptors, and lipolysis is controlled by b 3 receptors.

Cholinergic receptors (receptors binding acetylcholine) also are found in the sympathetic system (as well as the parasympathetic system). Nicotinic cholinergic receptors cause sympathetic postganglionic neurons, adrenal chromaffin cells, and parasympathetic postganglionic neurons to fire and release their chemicals. Muscarinic receptors are associated mainly with parasympathetic functions and are located in peripheral tissues (e.g., glands, smooth muscle). Peptidergic receptors exist in target cells as well.

The length of time that each type of chemical acts on its target cell is variable. As a rule, peptides cause slowly developing, long-lasting effects (one or more minutes), whereas the classical transmitters produce short-term effects (about 25 milliseconds).

The sympathetic nervous system normally functions to produce localized adjustments (such as sweating) and reflex adjustments of the cardiovascular system. Under conditions of stress, however, the entire sympathetic nervous system is activated, producing an immediate, widespread response that has been called the “fight or flight” response. This is characterized by the release of large quantities of epinephrine from the adrenal gland, an increase in heart rate, an increase in cardiac output, skeletal muscle vasodilation, cutaneous and gastrointestinal vasoconstriction, pupillary dilation, bronchial dilation, and piloerection. The overall effect is to prepare the individual for imminent danger.

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The parasympathetic nervous system is organized in a manner similar to the sympathetic nervous system. Its motor component consists of a two-neuron system. The preganglionic neurons lie in specific cell groups (also called nuclei) in the brain stem or in the lateral horns of the spinal cord at sacral levels (segments S2–S4). Because parasympathetic fibers exit from these two sites, the system is sometimes referred to as the craniosacral outflow. Preganglionic axons emerging from the brain stem project to parasympathetic ganglia that are located in the head (ciliary, pterygopalatine [also called sphenopalatine], and otic ganglia) or near the heart (cardiac ganglia), embedded in the end organ itself (e.g., the trachea, bronchi, and gastrointestinal tract), or situated a short distance from the urinary bladder (pelvic ganglion). Both pre- and postganglionic neurons secrete acetylcholine as a neurotransmitter, but, like sympathetic ganglion cells, they also contain other neuroactive chemical agents that function as cotransmitters. To view a figure depicting the difference in function between the sympathetic and parasympathetic nervous system click here.

The parasympathetic nervous system modulates mainly visceral organs such as glands. These are never activated en masse as in the “fight or flight” sympathetic response. While providing important control of many tissues, the parasympathetic system, unlike the sympathetic system, is not crucial for the maintenance of life.

The third cranial nerve ( oculomotor nerve) contains parasympathetic nerve fibers that regulate the iris and lens of the eye. From their origin in the Edinger-Westphal nucleus of the midbrain, preganglionic axons travel to the orbit and synapse on the ciliary ganglion. The ciliary ganglion contains two types of postganglionic neurons: one innervates smooth muscle of the iris and is responsible for pupillary constriction, and the other innervates ciliary muscle and controls the curvature of the lens.

Various secretory glands located in the head are under parasympathetic control. These include the lacrimal gland, which supplies tears to the cornea of the eye; salivary glands (sublingual, submandibular, and parotid glands), which produce saliva; and nasal mucous glands, which secrete mucus throughout the nasal air passages. The parasympathetic preganglionic neurons that regulate these originate in the reticular formation of the medulla oblongata. One group belongs to the superior salivatory nucleus and lies in the rostral part of the medullary reticular formation. These neurons send axons out of the medulla in a separate part of the seventh cranial nerve (facial nerve) called the intermediate nerve. Some of the axons innervate the pterygopalatine ganglion, and others project to the submandibular ganglion. Pterygopalatine ganglion cells innervate the vasculature of the brain and eye as well as the lacrimal gland, nasal glands, and palatine glands, while neurons of the submandibular ganglion innervate the submandibular and sublingual salivary glands. A second group of parasympathetic preganglionic neurons belongs to the inferior salivatory nucleus, a group lying in the caudal part of the medullary reticular formation. Its neurons send axons out of the medulla in the ninth cranial ( glossopharyngeal) nerve and to the otic ganglion. From this site, postganglionic fibers travel to and innervate the parotid salivary gland.

Preganglionic parasympathetic fibers of the tenth cranial nerve (vagus) arise from two different sites in the medulla. Neurons that slow heart rate arise from a part of the ventral medulla called the nucleus ambiguus, while those that control the gastrointestinal tract arise from the dorsal vagal nucleus. After exiting the medulla in the vagus nerve and traveling to their respective organs, the fibers synapse on ganglion cells embedded in the organs themselves. The vagus nerve also contains visceral afferent fibers that carry sensory information from organs of the neck (larynx, pharynx, and trachea), chest (heart and lungs), and gastrointestinal tract into a visceral sensory nucleus located in the medulla and called the solitary tract nucleus.

The enteric nervous system is made up of two plexuses, or networks of neurons, embedded in the wall of the gastrointestinal tract. The outermost collection, lying between the inner circular and outer longitudinal smooth-muscle layers of the gut, is called the myenteric (or Auerbach's) plexus. Neurons of this plexus regulate the peristaltic waves, consisting of polarized muscular activity, that move digestive products from oral to anal openings. In addition, myenteric neurons control local muscular contractions that are responsible for stationary mixing and churning. The innermost group of neurons is called the submucosal (or Meissner's) plexus. This group regulates the configuration of the luminal surface, controls glandular secretions, alters electrolyte and water transport, and regulates local blood flow.

Three functional classes of intrinsic enteric neurons are recognized: sensory neurons, interneurons, and motor neurons. Sensory neurons, activated by either mechanical or chemical stimulation of the innermost surface of the gut, transmit information to interneurons located within the myenteric and submucosal plexi, and the interneurons relay the information to motor neurons. Motor neurons in turn modulate the activity of a variety of target cells, including mucous glands, smooth muscle cells, endocrine cells, epithelial cells, and blood vessels.

Extrinsic neural pathways also are involved in the control of gastrointestinal functions. Three types exist: intestinofugal, sensory, and motor. Intestinofugal neurons reside in the gut wall; they send their axons to the preaortic sympathetic ganglia and control reflex arcs that involve large portions of the gastrointestinal tract. Sensory neurons relay information regarding distention (pain) and acidity into the central nervous system. There are two types of sensory neurons: sympathetic neurons, which originate from dorsal-root ganglia found at the thoracic and lumbar levels; and parasympathetic neurons, which originate in the nodose ganglion of the tenth cranial nerve (vagus) or in dorsal-root ganglia at sacral levels S2–S4. The former innervate the entire gastrointestinal tract from the pharynx to the left colic flexure, and the latter innervate the distal colon and rectum. Each portion of the gastrointestinal tract receives a dual sensory innervation: pain sensations travel via sympathetic afferents, and sensations that signal information regarding the chemical milieu of the gut travel by way of parasympathetic fibers and are not consciously perceived.

The third extrinsic pathway, exercising motor control over the gut, arises from parasympathetic preganglionic neurons found in the dorsal vagal nucleus of the medulla and from sympathetic preganglionic neurons in the lateral horns of the spinal cord. These outflows provide modulatory commands to the intrinsic enteric motor system and are nonessential in that basic functions can be maintained in their absence.

Through the pathways described above, the parasympathetic system activates digestive processes while the sympathetic system inhibits them. The sympathetic system inhibits digestive processes by two mechanisms: (1) contraction of circular smooth muscle sphincters located in the distal portion of the stomach (pyloric sphincter), small intestine (ileo-cecal sphincter), and rectum (internal anal sphincter), which act as valves to prevent the oral-to-anal passage (as well as reverse passage) of digestive products; and (2) inhibition of motor neurons throughout the length of the gut. In contrast, the parasympathetic system provides messages only to myenteric motor neurons.

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