In the central nervous system, the activities of the autonomic nervous system are coordinated in the brainstem (especially in the nucleus of the tractus solitarius) and in the hypothalamus. In the peripheral nervous system, the autonomic nervous system comprises the visceral motor axons and (to most neuroscientists) the visceral sensory axons and the enteric nervous system (a neural net within the walls of the gastrointestinal tract). Compared to peripheral somatic axons, the peripheral autonomic axons tend to be small (less than 3 um in diameter), slowly conducting, and sparsely myelinated.
In the peripheral nervous system, autonomic motor pathways differ from somatic motor pathways. Somatic motor pathways -- which send signals to voluntary, skeletal muscles -- are only one axon long: the axons of somatic motor neurons of the spinal cord and brainstem synapse directly on the effector cells (muscle cells). In contrast, autonomic motor pathways are two axons long. The axons ( 'preganglionic axons') of visceral motor neurons of the spinal cord and brainstem synapse on interneurons in peripheral (autonomic) ganglia. In the autonomic motor pathways, it is the axons ('postganglionic axons') of these ganglion neurons that synapse on the effector cells (smooth muscles or secretory cells).
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The visceral motor circuitry of the autonomic nervous system is divided into two parallel subsystems, the sympathetic and the parasympathetic. SEE parasympathetic nervous system,. sympathetic nervous system. These subsystems differ in two major ways: (1) The central (preganglionic) neurons of the sympathetic system are located in the thoracic and lumbar segments of the spinal cord, while the central neurons of the parasympathetic system are located in the brainstem and in a short segment of the caudal end of the spinal cord. (2) The characteristic neurotransmitter of postganglionic sympathetic axons is norepinephrine, while the characteristic neurotransmitter of postganglionic parasympathetic axons is acetylcholine. (In both the sympathetic and parasympathetic systems, the characteristic neurotransmitter of the preganglionic axons is acetylcholine.) Besides their characteristic neurotransmitters, autonomic nerves influence surrounding tissues through the release other active chemicals; these additional neuroactive molecules include ATP, nitric oxide, and range of peptides (e.g., substance P and vasoactive intestinal peptide).
As a result of their different final transmitters, the effects of the two subsystems differ. Sympathetic stimulation readies an animal for interaction with the outside world and prepares the animal for "fight or flight"; for example, activation of sympathetic axons increases heart rate and decreases gastrointestinal peristalsis. On the other hand, parasympathetic stimulation relaxes and quiets an animal; for example, activation of parasympathetic axons decreases heart rate and increases gastrointestinal peristalsis. The accompanying table compares the effects of sympathetic and parasympathetic stimulation on specific tissues. SEE table SEE illus.
The autonomic nervous system is distributed throughout the body, and autonomic dysfunction can produce a wide range of symptoms, such as bladder malfunction, blood pressure abnormalities, breathing difficulty, gastrointestinal motility problems, heart arrhythmias, impotence, nasal congestion, sweating disorders, syncope, and visual symptoms. Drugs that act on or mimic autonomic neurotransmitters are commonly used to these symptoms as well as glaucoma, heart failure, shock, and thyroid storm. To assess the overall functioning of the autonomic nervous system, physicians often begin with simple measurements of the reflexive responses of the cardiovascular system, specifically, measuring the changes of blood pressure and heart rate to standing (from sitting) and to exercise.
Effects of the Autonomic Stimulation of Specific Tissues
Tissue Parasympathetic Stimulation Sympathetic Stimulation
adipose tissue metabolism lipolysis ---
adrenal cortex secretion (corticoids) --- increase
adrenal medulla secretion (adrenaline) --- increase
arterial wall muscle
abdominal organ arteries --- constriction
coronary arteries constriction dilation
peripheral arteries --- constriction
wall (detrusor) muscle contraction relaxation
sphincter muscle relaxation contraction
wall muscle contraction relaxation
duct muscle constriction dilation
AV node conduction velocity decrease increase
SA node rate decrease increase
ventricular contractility --- increase
wall muscle increase tone and motility decrease tone and motility
secretion increase decrease
rectal sphincter muscle relaxation contraction
kidney secretion (renin) --- increase
lacrimal gland secretion (tears) increase ---
metabolism glycogen synthesis glycogenolysis, gluconeogenesis
bile secretion increase decrease
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bronchial muscle constriction dilation
secretion (airway glands) increase ---
nasopharynx secretion (mucosal glands) increase ---
enzymes and insulin increase decrease
glucagon --- increase
pineal gland melatonin synthesis --- stimulation
pupil constriction dilation
salivary gland secretion increase decrease
reproductive tract muscular contractions, vasocontraction vasodilation, erection
skin arteries --- constriction
pilomotor muscle --- contraction
secretion (sweat) --- increase
wall muscle tone and motility increase decrease
secretion increase decrease
uterus muscle contraction
pregnant --- stimulate
nonpregnant --- inhibit
sympathetic nervous system The thoracolumbar division of the autonomic nervous system. Preganglionic fibers originate in the thoracic and lumbar segments of the spinal cord and synapse with postganglionic neurons in the sympathetic ganglia. Most of these ganglia are in two chains lateral to the backbone, and others are within the trunk; postganglionic fibers extend to the organs innervated. Some effects of sympathetic stimulation are increased heart rate, dilation of the bronchioles, dilation of the pupils, vasoconstriction in the skin and viscera, vasodilation in the skeletal muscles, slowing of peristalsis, conversion of glycogen to glucose by the liver, and secretion of epinephrine and norepinephrine by the adrenal medulla. Sympathetic effects are general rather than specific and prepare the body to cope with stressful situations. SEE: autonomic nervous system for illus. and table; parasympathetic nervous system
Sympathetic impulses have the following effects: vasodilation in skeletal muscle and vasoconstriction in the skin and viscera occur; heart rate and force are increased; the bronchioles dilate; the liver changes glycogen to glucose; sweat glands become more active; peristalsis and gastrointestinal secretions decrease; the pupils dilate; the salivary glands secrete small amounts of thick saliva; and the hair stands on end (gooseflesh). The sympathetic division dominates during stressful situations such as anger or fright, and the body responses contribute to fight or flight, with unimportant activities such as digestion markedly slowed. Most sympathetic neurons release the neurotransmitter norepinephrine at the visceral effector.
The sympathetic (thoracolumbar) division of the autonomic nervous system. Preganglionic fibers extend from the intermediolateral nucleus of the spinal cord to the peripheral autonomic ganglia, and postganglionic fibers extend from the peripheral ganglia to the effector organs, according to the scheme inÂ Fig. 26-1
TheÂ preganglionic neurons of the sympathetic division originateÂ in the intermediolateral cell column of the spinal gray matter, from the eighth cervical to the second lumbar segments.Â Low and Dyck (1977)Â have estimated that each segment of the cord contains approximately 5,000 lateral horn cells and that there is an attrition of 5 to 7 percent per decade in late adult life. Axons of the nerve fibers originating in the intermediolateral column are of small caliber and are myelinated; when grouped, they form theÂ white communicating rami. These preganglionic fibers synapse with the cell bodies of the postganglionic neurons, which are collected into two large ganglionated chains or cords, one on each side of the vertebral column (paravertebral ganglia), and several single prevertebral ganglia.
Axons of the sympathetic ganglion cells are also of small caliber but are unmyelinated. Most of the postganglionic fibers pass viaÂ gray communicating ramiÂ to spinal nerves of T5 to L2; they supply blood vessels, sweat glands, and hair follicles, and also form plexuses that supply the heart, bronchi, kidneys, intestines, pancreas, bladder, and sex organs. The postganglionic fibers of the prevertebral ganglia (located in the posterior abdomen rather than paravertebrally) form the hypogastric, splanchnic, and mesenteric plexuses, which innervate the glands, smooth muscle, and blood vessels of the abdominal and pelvic viscera (seeÂ Fig. 26-1).
The sympathetic innervation of the adrenal medulla is unique in that its secretory cells receive preganglionic fibers directly, via the splanchnic nerves. This is an exception to the rule that organs innervated by theÂ autonomic nervous systemÂ receive only postganglionic fibers. This special arrangement can be explained by the fact that cells of the adrenal medulla are the morphologic homologues of the postganglionic sympathetic neurons and secreteÂ epinephrineÂ andÂ norepinephrine(the postganglionic transmitters) directly into the bloodstream. In this way, the sympathetic nervous system and the adrenal medulla act in unison to produce diffuse effects, as one would expect from their role in emergency reactions. By contrast, the parasympathetic effects, as in the pupil and urinary bladder, are more discrete.
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There are 3 cervical (superior, middle, and inferior, or stellate), 11 thoracic, and 4 to 6 lumbar sympathetic ganglia. The head receives its sympathetic innervation from the eighth cervical and first two thoracic cord segments, the fibers of which pass through the inferior to the middle and superior cervical ganglia. Postganglionic fibers from cells of the superior cervical ganglion follow the internal and external carotid arteries and innervate the blood vessels and smooth muscle, as well as the sweat, lacrimal, and salivary glands of the head. Included among these postganglionic fibers, issuing mainly from T1, are the pupillodilator fibers and those innervating the Müller muscle of the upper eyelid (it connects the upper tarsus to the undersurface of the levator); there is a separate small tarsus muscle that is also sympathetically innervated. The arm receives its postganglionic innervation from the inferior cervical ganglion and uppermost thoracic ganglia (the two are fused to form the stellate ganglion). The cardiac plexus and other thoracic sympathetic nerves are derived from the stellate ganglion and the abdominal visceral plexuses, from the fifth to the ninth or tenth thoracic ganglia. The lowermost thoracic ganglia have no abdominal visceral connections; the upper lumbar ganglia supply the descending colon, pelvic organs, and legs.
The terminals of autonomic nerves and their junctions with smooth muscle and glands have been more difficult to visualize and study than the motor endplates of striated muscle. As the postganglionic axons enter an organ, usually via the vasculature, they ramify into many smaller branches and disperse, without a Schwann cell covering, to innervate the smooth muscle fibers, the glands, and, in largest number, the small arteries, arterioles, and precapillary sphincters (see Burnstock). Some of these terminals penetrate the smooth muscle of the arterioles; others remain in the adventitia. At the ends of the postganglionic fibers and in part along their course there are swellings that lie in close proximity to the sarcolemma or gland cell membrane; often the muscle fiber is grooved to accommodate these swellings. The axonal swellings contain synaptic vesicles, some clear and others with a dense granular core. The clear vesicles containÂ acetylcholineÂ and those with a dense core contain catecholamines, particularlyÂ norepinephrineÂ (Falck). This is well illustrated in the iris, where nerves to the dilator muscle (sympathetic) contain dense-core vesicles and those to the constrictor (parasympathetic) contain clear vesicles. A single nerve fiber innervates multiple smooth muscle and gland cells.
parasympathetic nervous system The craniosacral division of the autonomic nervous system .Preganglionic fibers originate from nuclei in the midbrain, medulla, and sacral portion of the spinal cord. They pass through the third, seventh, ninth, and tenth cranial nerves and the second, third, and fourth sacral nerves, and synapse with postganglionic neurons located in autonomic (terminal) ganglia that lie in the walls of or near the organ innervated. SEE: autonomic nervous system for table
Some effects of parasympathetic stimulation are constriction of the pupil, contraction of the smooth muscle of the alimentary canal, constriction of the bronchioles, slowing of the heart rate, and increased secretion of the digestive glands.
The parasympathetic division dominates during nonstressful situations, with the following effects: the heart slows to normal, the bronchioles constrict to normal, peristalsis and gastrointestinal secretion increase for normal digestion, the pupils constrict to normal, secretion of thin saliva increases, and the urinary bladder constricts normally. If parasympathetic supply to the bladder is impaired, there will be incomplete emptying and urinary retention. All parasympathetic neurons release the transmitter acetylcholine at the visceral effector.
The parasympathetic (craniosacral) division of the autonomic nervous system. Preganglionic fibers extend from nuclei of the brainstem and sacral segments of the spinal cord to peripheral ganglia. Short postganglionic fibers extend from the ganglia to the effector organs. The lateral-posterior hypothalamus is part of the supranuclear mechanism for the regulation of parasympathetic activities.Â
here are two divisions of the parasympathetic nervous system: cranial and sacral. TheÂ cranial divisionÂ originates in the visceral nuclei of the midbrain, pons, and medulla. These nuclei lie in close proximity to the somatic afferent nuclei and include the Edinger-Westphal pupillary nucleus, superior and inferior salivatory nuclei, dorsal motor nucleus of the vagus, and adjacent reticular nuclei.
Axons (preganglionic fibers) of the visceral cranial nuclei course through the oculomotor, facial, glossopharyngeal, and vagus nerves. The preganglionic fibers from the Edinger-Westphal nucleus traverse the oculomotor nerve and synapse in the ciliary ganglion in the orbit; axons of the ciliary ganglion cells innervate the ciliary muscle and pupillary sphincter (seeÂ Fig. 14-8). The preganglionic fibers of the superior salivatory nucleus enter the facial nerve and, at a point near the geniculate ganglion, form the greater superficial petrosal nerve, through which they reach the sphenopalatine ganglion; postganglionic fibers from the cells of this ganglion innervate the lacrimal gland (see also Fig. 47-3). Other fibers of the facial nerve traverse the tympanic cavity as the chorda tympani and eventually join the submandibular ganglion; cells of this ganglion innervate the submandibular and sublingual glands. Axons of the inferior salivatory nerve cells enter the glossopharyngeal nerve and reach the otic ganglion through the tympanic plexus and lesser superficial petrosal nerve; cells of the otic ganglion send fibers to the parotid gland. Preganglionic fibers, derived from the dorsal motor nucleus of the vagus and adjacent visceral nuclei in the lateral reticular formation (mainly the nucleus ambiguus), enter the vagus nerve and terminate in ganglia situated in the walls of many thoracic and abdominal viscera; the ganglionic cells give rise to short postganglionic fibers that activate smooth muscle and glands of the pharynx, esophagus, and gastrointestinal tract (the vagal innervation of the colon is somewhat uncertain but considered to extend up to the descending colon) and of the heart, pancreas, liver, gallbladder, kidney, and ureter.
TheÂ sacral part of the parasympathetic systemÂ originates in the lateral horn cells of the second, third, and fourth sacral segments. Axons of these sacral neurons, constituting the preganglionic fibers, traverse the sacral nerves and synapse in ganglia that lie within the walls of the distal colon, bladder, and other pelvic organs. Thus, the sacral autonomic neurons, like the cranial ones, have long preganglionic and short postganglionic fibers, a feature that permits a circumscribed influence upon the target organ.
Probably the neurons that activate striated muscle differ from those that innervate glands and smooth muscle. In the sacral segments, for example, the neurons that activate the external sphincters (voluntary muscle) differ from those that supply the smooth muscle of bladder and rectocolon. In 1900, Onufrowicz (calling himself Onuf) described a discrete, compact group of relatively small cells in the anterior horns of sacral segments 2 to 4. These neurons were originally thought to be autonomic in function, mainly because of their histologic features. There is now more compelling evidence that they are somatomotor, innervating the skeletal muscle of the external urethral and anal sphincters (Holstege and Tan). Neurons in the intermediolateral cell column of sacral cord segments innervate the detrusor of the bladder wall. In passing, it is worth noting that in motor system disease, in which bladder and bowel functions are usually preserved until late in the disease, the neurons in the Onuf nucleus, in contrast to other somatomotor neurons in the sacral cord, tend not to be involved in the degenerative process (Mannen et al).
enteric nervous system ABBR: ENS. A division of the autonomic nervous system (ANS) arising from its own line of neural crest cells and composed of the tens of millions of neurons and their supporting cells inside the walls of the gastrointestinal tract, pancreas, and gallbladder. Although the enteric nervous system is innervated (and modulated) by sympathetic and parasympathetic axons from the other divisions of the ANS, the enteric nervous system also acts independently. Reflex activities (e.g., maintaining gut wall tension and producing peristalsis) are initiated and coordinated via networks entirely inside the gut walls and organized via complex intrinsic ganglionated neural networks of two kinds: Auerbach's plexus and the submucous (Meissner's and Henle's) plexus lying between the circular and muscularis mucosae muscle layers.