Difference between revisions of "Autonomic nervous system" - New World Encyclopedia

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[[Image:Gray839.png|thumb|360px|The autonomic nervous system<br/>Blue = parasympathetic<BR>Red = sympathetic]]
  
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The '''autonomic nervous system''' (ANS) is that part of the [[peripheral nervous system]] that largely acts independent of conscious control (involuntarily) and consists of nerves in cardiac [[muscle]], smooth muscle, and exocrine and [[endocrine system|endocrine]] glands. It is responsible for maintenance functions ([[metabolism]], cardiovascular activity, temperature regulation, digestion) that have a reputation for being outside of conscious control. The other main subdivision of the peripheral nervous system, the '''somatic nervous system,''' consists of cranial and spinal nerves that innervate skeletal muscle tissue and are more under voluntary control (Anissimov 2006; Towle 1989).
  
The '''autonomic nervous system''' (ANS) is the part of the [[nervous system]] of the higher life forms that is not consciously controlled. It is commonly divided into two usually antagonistic subsystems: the [[sympathetic nervous system|sympathetic]] and [[parasympathetic nervous system]], and involves the [[homeostasis]] of organs and physiological functions.
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The autonomic nervous system is typically divided into two main subsystems, the '''[[sympathetic nervous system]]''' and the '''[[parasympathetic nervous system]].''' These tend to balance each other, offering opposite and yet complementary effects reflective of the philosophy of [[Yin and Yang]]. The sympathetic nervous system deals with the response to [[stress (medicine)|stress]] and danger, releasing epinephrines ([[adrenaline]]), and in general increasing activity and [[metabolism|metabolic rate]]. The parasympathetic nervous system counters this, and is central during rest, sleeping, and digesting food and, in general, lowers metabolic rate, slows activity, and restores blood pressure and resting heartbeat, and so forth (Chamberlain and Narins 2005). Just as Yin and Yang are opposing, yet complementary and interdependent forces, the sympathetic and parasympathetic systems complement each other and are both necessary to create overall harmony and balance in the living organism.
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{{toc}}
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A third subsystem, the '''enteric nervous system,''' is classified as a division of the autonomic nervous system as well. This subsystem has nerves around the intestines, [[pancreas]], and [[gall bladder]].  
  
A third and less commonly considered part of the autonomic nervous system is the [[enteric nervous system]], which controls the digestive organs, and is, for the most part, independent of [[central nervous system]] (CNS) input.
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==Overview==
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The [[vertebrate]] [[nervous system]] is divided into the [[central nervous system]] (CNS), comprising the [[brain]] and [[spinal cord]], and the [[peripheral nervous system]] (PNS), consisting of all the nerves and [[neuron]]s that reside or extend outside the central nervous system, such as to serve the limbs and organs.  
  
In general, the [[parasympathetic nervous system]] is involved with digestion and energy conservation, while the [[sympathetic nervous system]] is involved with energy expenditure and the 'fight or flight' response.
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The peripheral nervous system, in turn, is commonly divided into two subsystems, the ''somatic nervous system'' and the ''autonomic nervous system.''
  
== Function ==
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The '''somatic nervous system''' or '''sensory-somatic nervous system''' involves nerves just under the [[skin]] and serves as the sensory connection between the outside environment and the CNS. These nerves are under conscious control, but most have an automatic component, as is seen in the fact that they function even in the case of a coma (Anissimov 2007). In humans, the somatic nervous system consists of 12 pairs of cranial nerves and 31 pairs of spinal nerves (Chamberlin and Narins 2005).  
The autonomic nervous system regulates bodily functions and the activity of specific organs. As examples, the ANS plays a role in the diameter of [[blood vessels]], [[heart rate]], force of contraction of the heart, diameter of the [[pupils]], [[salivation]], perspiration, [[bronchiole]] diameter, [[peristaltic]] movements in the [[intestine]], spinctor diameter, [[erection]], [[ejaculation]], and [[parturition]].
 
  
Although the bodily functions that the ANS regulates are typically portrayed as being involuntary, they are not completely outside our awareness, and some schools of thought believe that one's state of mind impacts the functioning of the ANS. It remains open to debate whether the term 'involuntary nervous system' is a precise description of the ANS. Many autonomic functions are beyond conscious control, but others are impacted voluntarily including the sphincters in [[urination]] (micturition).  
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The '''autonomic nervous system''' is typically presented as that portion of the peripheral nervous system that is independent of conscious control, acting involuntarily and subconsciously (reflexively), and innervating heart muscle, [[endocrine system|endocrine glands]], exocrine glands, and smooth muscle (Chamberlin and Narins 2005). In contrast, the somatic nervous system innervates skeletal muscle tissue, rather than smooth, cardiac, or glandular tissue.
  
The autonomic nervous system is divided into subsystems: the '''[[sympathetic nervous system|sympathetic]]''' (SNS) and the '''[[parasympathetic nervous system|parasympathetic]]''' (PNS) nervous systems. The SNS and PNS often have opposing effects in the same organs or physiological systems, and the ANS is a major factor in [[homeostasis]].
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The autonomic nervous system is subdivided into the [[sympathetic nervous system]], the [[parasympathetic nervous system]], and the enteric nervous system. In general, the sympathetic nervous system increases activity and [[metabolism|metabolic]] rate (the "fight or flight response"), while the parasympathetic slows activity and metabolic rate, returning the body to normal levels of function (the "rest and digest state") after heightened activity from sympathetic stimulation (Chamberlin and Narins 2005). The enteric nervous system innervates areas around the intestines, pancreas, and gall bladder, dealing with digestion, and so forth.
  
The SNS is frequently referred to as the "[[fight or flight]]" system, as it has a stimulating effect on organs and physiological systems. For example, the SNS constricts blood vessels feeding blood to the GI tract and [[skin]], while dilating [[skeletal muscle]] and [[lung]] blood vessels. Bronchioles also dilate allowing more oxygen to be exchanged at the lungs. At the same time, the SNS increases [[heart rate]] and [[contractility]] of the heart. This vastly increases blood flow to the skeletal muscles and diverts blood away from organs such as the GI tract which are not important during the "[[fight or flight]]" response. Sympathetic nerves also dilate the pupils and relax the lenses, allowing more light to enter the eyes and enabling one to see further.  
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Unlike the somatic nervous system, which always excites [[muscle]] tissue, the autonomic nervous system can either excite or inhibit innervated tissue (Chamberlin and Narins 2005). Most associated tissues and organs have nerves of both the sympathetic and the parasympathetic nervous systems. The two systems can stimulate the target organs and tissues in opposite ways, such as sympathetic stimulation to increase heart rate and parasympathetic to decrease heart rate, or the sympathetic stimulation resulting in pupil dilation, and the parasympathetic in pupil constriction or narrowing (Chamberlin and Narins 2005). Or, they can both stimulate activity in concert, but in different ways, such as both increasing saliva production by salivary glands, but with sympathetic stimulation yielding viscous or thick saliva and parasympathetic yielding watery saliva.  
  
The parasympathetic nervous system has sometimes been called the "rest and digest" response. The PNS slows and relaxes many functions of organs and body systems. For example, the PNS will dilate blood vessels to the GI tract, while slowing the heart beat and decreasing the force of the heart's contractions. These effects help to lower the metabolic strain on the body, resulting in energy conservation. The PNS can divert blood back to the skin and the [[gastrointestinal tract]]. Increased blood flow to the GI tract aids digestion. The PNS also constricts the bronchioles when the need for oxygen has diminished. During [[Accommodation_(eye)|accommodation]], the PNS causes the constriction of the pupils and lenses. The PNS stimulates [[salivary gland]] secretion, and accelerates [[peristalsis]], so although the PNS generally has a calming effect on the body, it does stimulate activity too.
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In general, the autonomic nervous system controls [[homeostasis]], that is the constancy of the content of tissues in gases, ions, and nutrients. It does so mostly by controlling cardiovascular, digestive, and respiratory functions, but also salivation, perspiration, diameter of the pupils, micturition (the discharge of urine), and erection. While many of the activities of the ANS are involuntary, [[breathing]], for example, can be in part consciously controlled. Indeed, although breathing is a purely homeostatic function in aquatic [[vertebrate]]s, in land vertebrates it accomplishes much more than oxygenating the blood: It is essential to sniff prey or a [[flower]], to blow out a candle, to talk or sing. This example, among others, illustrates that the so-called “autonomic nervous system” is not truly autonomous. It is anatomically and functionally linked to the rest of the nervous system and a strict delineation is impossible.
  
The cell bodies of [[preganglionic]] autonomic nerve cells are situated in the [[central nervous system]]. Those of the sympathetic nervous system arise in the thoracic and lumbar segments of the spinal cord. The preganglionic parasympathetic cell bodies are situated in the [[brain stem]] (cranial parasympathetic) and in the sacral spinal cord (sacral parasympathetic).  
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The ANS is nevertheless a classical term, still widely used throughout the scientific and medical community. Its most useful definition could be: The sensory and motor neurons that innervate the [[viscera]]. These neurons form [[reflex arcs]] that pass through the lower [[brainstem]] or [[medulla oblongata]]. This explains that when the central nervous system (CNS) is damaged experimentally or by accident above that level, a vegetative life is still possible, whereby cardiovascular, digestive, and respiratory functions are adequately regulated.
  
In order to reach the target organs and glands, the axons of neurons in the SNS and PNS often must travel long distances in the body. In the SNS and PNS, neurons from the CNS synapse at ganglions; a site where a group of neurons of similar function (called presynaptic neurons) connect to another group of neurons (called postsynaptic neurons), by means of a synapse. [[Ganglion|Ganglions]] allow for the modulation of the presynaptic input before it is sent along the postsynaptic neurons to their effector sites.
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==Sensory and motor neurons==
  
The main neurotransmitter that is located at the ganglion is [[acetylcholine]]. Acetylcholine is released from the presynaptic neuron and acts on postsynaptic [[nicotinic receptors]] in both the SNS and PNS. Postsynaptic cells pass signals to the effector organs. At the effector organs, SNS postsynaptic neurons release [[noradrenaline]] (norepinephrine) to act on [[adrenoceptors]], with the exception of the sweat glands and the adrenal medulla. At sweat glands, the neurotransmitter is acetylcholine, which acts on [[muscarinic receptors]]. At the adrenal cortex, there is no postsynapic neuron. Instead the presynaptic neuron releases acetylcholine to act on [[nicotinic receptors]]. Stimulation of the adrenal medulla releases [[adrenaline]] (epinephrine) into the bloodstream which will act on adrenoceptors, producing a widespread increase in sympathetic activity. In the PNS, all postsynaptic cells use acetylcholine as a neurotransmitter, to stimulate [[muscarinic receptors]].
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Neurons active in the autonomic nervous system (and the PNS in general) can be divided into sensory neurons and motor neuron (Chamberlin and Narins 2005). Sensory neurons act as a conduit between sensory receptors, which sense stimuli such as cold, heat, and [[pain]], and the CNS. Motor neurons act as a conduit between the CNS and various [[muscle]]s and glands (effectors). Or, looked at another way, receptors are cells or groups of cells that receive information from stimuli (external or internal), and effectors are cells or groups of cells that received information from the nervous system.
  
The sympathetic axons build a chain of 22 ganglia, the so-called [[paravertebral ganglia]], on each side of the [[spinal column]]. From these the [[splanchnic nerves]] run to the prevertebral ganglia, which lie in front of the [[aorta]], at the level where its unpaired visceral arteries branch off. The left and right trunks of the sympathetic nerve fuse to form an unpaired ganglion in the pelvic area. Organs innervated by sympathetic fibres include the [[heart]], lungs, [[esophagus]], [[stomach]], small and large intestine, [[liver]], [[gallbladder]] and [[genital organs]].
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Although commonly the ANS is looked at, and even defined, as if limited to motor fibers and excluding sensory fibers, a more comprehensive definition is that the reflex arcs of the ANS comprises both a sensory (or afferent) arm, and a motor (or efferent, or effector) arm.  
  
These organs are also innervated by the part side of the parasympathetic nervous system. The digestive system distal to the lower part of the [[colon (anatomy)|colon]] is regulated by the [[sacral parasympathetic fibres]] via the pelvic ganglia. The more proximal digestive tract is controlled by the [[vagus nerve]], the largest element of the cranial parasympathetic system. Like those of the vagus, other cranial parasympathetic fibers arise in the brain stem before exiting the skull with various [[cranial nerves]], en route to the cranial parasympathetic ganglia and the innervation of the eye muscles and salivary glands.
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===Sensory neurons===
  
==Individual components==
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The sensory arm is made of “primary visceral sensory neurons” found in the peripheral nervous system (PNS) in “cranial sensory ganglia:" the [[geniculate]], [[petrosal]], and [[nodose]] ganglia, appended respectively to [[cranial nerves]] VII, IX, and X. These sensory neurons monitor the levels of [[carbon dioxide]], [[oxygen]], and [[sugar]] in the blood; arterial pressure; and the chemical composition of the stomach and gut content. They also convey the sense of taste, a conscious perception.
<center>[[image:Gray838.png]]<br>
 
'''Figure 1:''' The right sympathetic chain and its connections with the thoracic, abdominal, and pelvic plexuses. (After Schwalbe.)</center>
 
  
The peripheral portion of the sympathetic nervous system is characterized by the presence of numerous [[ganglion|ganglia]] and complicated plexuses. These ganglia are connected with the central nervous system by three groups of sympathetic efferent or preganglionic fibers, ''i. e.,'' the cranial, the thoracolumbar, and the sacral. These outflows of sympathetic fibers are separated by intervals where no connections exist. The cranial and sacral sympathetics are often grouped together owing to the resemblance between the reactions produced by stimulating them and by the effects of certain drugs. Acetylcholine, for example, when injected intravenously in very small doses, produces the same effect as the stimulation of the cranial or sacral sympathetics, while the introduction of epinephrine produces the same effect as the stimulation of the thoracolumbar sympathetics. Much of our present knowledge of the sympathetic nervous system has been acquired through the application of various drugs, especially [[nicotine]] which paralyzes the connections or [[synapse]]s between the preganglionic and postganglionic fibers of the sympathetic nerves. When it is injected into the general circulation all such synapses are jerkylike paralyzed; when it is applied locally on a ganglion only the synapses occurring in that particular ganglion are paralyzed.
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Blood oxygen and carbon dioxide are, in fact, directly sensed by the [[carotid body]], a small collection of chemosensors at the bifurcation of the carotid artery, innervated by the [[petrosal]] (IXth) ganglion.
Langley, 138 who has contributed greatly to our knowledge, adopted a terminology somewhat different from that used here and still different from that used by the pharmacologists. This has led to considerable confusion, as shown by the arrangement of the terms in the following columns. Gaskell has used the term involuntary nervous systems.
 
  
<table align=center border=1>
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Primary sensory neurons project (synapse) onto “second order” or relay visceral sensory neurons located in the medulla oblongata, forming the nucleus of the solitary tract (nTS), which integrates all visceral information. The nTS also receives input from a nearby chemosensory center, the [[area postrema]], which detects toxins in the blood, and the [[cerebrospinal fluid]]. It is essential for chemically induced vomiting and conditional taste aversion (the memory that ensures that an animal that has been poisoned by a food never touches it again).
<tr><td>Gray</td><td>Langley</td><td>Meyer and Gottlieb</td></tr>
 
<td>Sympathetic nervous system</td><td>Autonomic nervous system</td><td>Vegetative nervous system</td></tr>
 
<td>Cranio-sacral sympathetics</td><td>Parasympathetics</td><td>Autonomic</td></tr>
 
<td>Oculomotor sympathetics</td><td>Tectal autonomics</td><td>Cranial autonomics</td></tr>
 
<td>Facial sympathetics</td><td>Bulbar autonomics</td><td></td></tr>
 
<td colspan=3>Glossopharyngeal sympathetics</td></tr>
 
<td colspan=3>Vagal sympathetics</td></tr>
 
<td>Sacral sympathetics</td><td colspan=2>Sacral autonomics</td></tr>
 
<td>Thoracolumbar sympathetics</td><td colspan=2>Sympathetic.</td></tr>
 
<td colspan=3>Thoracic autonomic</td></tr>
 
<td colspan=3>Enteric</td></tr>
 
</table>
 
  
== The cranial autonomics==
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All this visceral sensory information constantly, and unconsciously, modulates the activity of the motor neurons of the ANS.
{{disputed}}
 
The cranial parasympathetics include parasympathetic efferent fibers in the oculomotor, facial, glossopharyngeal and vagus nerves, as well as parasympathetic afferent in the last three spinal nerves i.e.S1 to S4.
 
  
The parasympathetic efferent fibers of the oculomotor nerve probably arise from cells in the anterior part of the oculomotor nucleus which is located in the tegmentum of the mid-brain. These preganglionic fibers run with the third nerve into the orbit and pass to the ciliary ganglion where they terminate by forming synapses with parasympathetic motor neurons whose axons, postganglionic fibers, proceed as the short ciliary nerves to the eyeball. Here they supply motor fibers to the ciliaris muscle and the sphincter pupillæ muscle. So far as known there are no parasympathetic afferent fibers connected with the nerve.
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===Motor neurons===
  
The parasympathetic efferent fibers of the facial nerve are supposed to arise from the small cells of the facial nucleus. According to some authors the fibers to the salivary glands arise from a special nucleus, the superior salivatory nucleus, consisting of cells scattered in the reticular formation, dorsomedial to the facial nucleus. These preganglionic fibers are distributed partly through the chorda tympani and lingual nerves to the submaxillary ganglion where they terminate about the cell bodies of neurons whose axons as postganglionic fibers conduct secretory and vasodilator impulses to the submaxillary and sublingual glands. Other preganglionic fibers of the facial nerve pass via the great superficial petrosal nerve to the sphenopalatine ganglion where they form synapses with neurons whose postganglionic fibers are distributed with the superior maxillary nerve as vasodilator and secretory fibers to the mucous membrane of the nose, soft palate, tonsils, uvula, roof of the mouth, upper lips and gums, parotid and orbital glands.
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Motor neurons of the ANS are also located in ganglia of the PNS, called “autonomic ganglia.” They belong to three categories with different effects on their target organs: Sympathetic, parasympathetic, and enteric.
  
<center>[[image:Gray839.png]]<br>
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Sympathetic ganglia are located in two sympathetic chains close to the spinal cord: The prevertebral and pre-aortic chains.
'''Figure 2:''' Diagram of efferent autonomic nervous system. Blue, cranial and sacral parasympathetic outflow. Red, thoracohumeral sympathetic outflow. - -, Postganglionic fibers to spinal and cranial nerves to supply vasomotors to head, trunk and limbs, motor fibers to smooth muscles of skin and fibers to sweat glands. (Modified after Meyer and Gottlieb.)</center>
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Parasympathetic ganglia, in contrast, are located in close proximity to the target organ: The [[submandibular ganglion]] close to salivary glands, paracardiac ganglia close to the heart, and so forth. Enteric ganglia, which, as the name implies, innervate the digestive tube, are located inside its walls and collectively contain as many [[neuron]]s as the entire spinal cord, including local sensory neurons, motor neurons, and interneurons. It is the only truly autonomous part of the ANS and the digestive tube can function surprisingly well even in isolation. For that reason, the [[enteric nervous system]] has been called “the second brain.
  
There are supposed to be a few parasympathetic afferent fibers connected with the facial nerve, whose cell bodies lie in the [[lateral geniculate body|geniculate ganglion]], but very little is known about them.
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The activity of autonomic ganglionic neurons is modulated by “preganglionic neurons” (also called, improperly but classically, "visceral motoneurons") located in the central nervous system. Preganglionc sympathetic neurons are in the spinal cord, at thoraco-lumbar levels. Preganglionic, parasympathetic neurons are in the [[medulla oblongata]] (forming visceral motor nuclei: The [[dorsal motor nucleus of the vagus nerve]] (dmnX), the [[nucleus ambiguus]], and salivatory nuclei) and in the sacral spinal cord. Enteric neurons are also modulated by input from the CNS, from preganglionic neurons located, like parasympathetic ones, in the medulla oblongata (in the dmnX).
  
<center>[[image:Gray840.png]]<br>
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The feedback from the sensory to the motor arm of visceral reflex pathways is provided by direct or indirect connections between the nucleus of the solitary tract and visceral motoneurons.
'''Figure 3:''' Parasympathetic (blue, note mislabeling as sympathetic) and sympathetic (red) connections of the ciliary and superior cervical ganglia.</center>
 
  
The parasympathetic afferent fibers of the glossopharyngeal nerve are supposed to arise either in the dorsal nucleus (nucleus ala cinerea) or in a distinct nucleus, the inferior salivatory nucleus, situated near the dorsal nucleus. These preganglionic fibers pass into the tympanic branch of the glossopharyngeal and then with the small superficial petrosal nerve to the otic ganglion. Postganglionic fibers, vasodilator and secretory fibers, are distributed to the parotid gland, to the mucous membrane and its glands on the tongue, the floor of the mouth, and the lower gums.
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== Function ==
 
 
parasympathetic afferent fibers, whose cells of origin lie in the superior or inferior ganglion of the trunk, are supposed to terminate in the dorsal nucleus. Very little is known of the peripheral distribution of these fibers.
 
The parasympathetic efferent fibers of the vagus nerve are supposed to arise in the dorsal nucleus (nucleus ala cinerea). These preganglionic fibers are all supposed to end in parasympathetic ganglia situated in or near the organs supplied by the vagus parasympathetics. The inhibitory fibers to the heart probably terminate in the small ganglia of the heart wall especially the atrium, from which inhibitory postganglionic fibers are distributed to the musculature. The preganglionic motor fibers to the esophagus, the stomach, the small intestine, and the greater part of the large intestine are supposed to terminate in the plexuses of Auerbach, from which postganglionic fibers are distributed to the smooth muscles of these organs. Other fibers pass to the smooth muscles of the bronchial tree and to the gall-bladder and its ducts. In addition the vagus is believed to contain secretory fibers to the stomach and pancreas. It probably contains many other efferent fibers than those enumerated above.
 
 
 
<center>[[image:Gray841.png]]<br>
 
'''Figure 4 :''' parasympathetic (blue) and sympathetic (red) connections of the sphenopalatine and superior cervical ganglia.</center>
 
  
parasympathetic afferent fibers of the vagus, whose cells of origin lie in the jugular ganglion or the ganglion nodosum, probably terminate in the dorsal nucleus of the medulla oblongata or according to some authors in the nucleus of the tractus solitarius. Peripherally the fibers are supposed to be distributed to the various organs supplied by the parasympathetic efferent fibers.
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Sympathetic and parasympathetic divisions typically function in opposition to each other. But this opposition is better termed complementary in nature rather than antagonistic. For an analogy, one may think of the sympathetic division as the accelerator and the parasympathetic division as the brake. The sympathetic division typically functions in actions requiring quick responses. The parasympathetic division functions with actions that do not require immediate reaction. Consider sympathetic as "fight or flight" and parasympathetic as "rest and digest."
  
The sacral parasympathetic efferent fibers leave the spinal cord with the anterior roots of the second, third and fourth sacral nerves. These small medullated preganglionic fibers are collected together in the pelvis into the nervus erigentes or pelvic nerve which proceeds to the hypogastric or pelvic plexuses from which postganglionic fibers are distributed to the pelvic viscera. Motor fibers pass to the smooth muscle of the descending colon, rectum, anus and bladder. Vasodilators are distributed to these organs and to the external genitalia, while inhibitory fibers probably pass to the smooth muscles of the external genitalia. Afferent parasympathetic fibers conduct impulses from the pelvic viscera to the second, third and fourth sacral nerves. Their cells of origin lie in the spinal ganglia.
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However, many instances of sympathetic and parasympathetic activity cannot be ascribed to "fight" or "rest" situations. For example, standing up from a reclining or sitting position would entail an unsustainable drop in blood pressure if not for a compensatory increase in the arterial sympathetic tonus. Another example is the constant, second-to-second modulation of heart rate by sympathetic and parasympathetic influences, as a function of the respiratory cycles. More generally, these two systems should be seen as permanently modulating vital functions, in usually opposing fashion, to achieve homeostasis. Some typical actions of the sympathetic and parasympathetic systems are listed below:
  
<center>[[image:Gray842.png]]<br>
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===[[Sympathetic nervous system]]===
'''Figure 5 :''' parasympathetic (blue) connections of the submaxillary and sympathetic (red) connections superior cervical ganglia.</center>
 
  
The thoracolumbar parasympathetic fibers arise from the dorsolateral region of the anterior column of the gray matter of the spinal cord and pass with the anterior roots of all the thoracic and the upper two or three lumbar spinal nerves. These preganglionic fibers enter the white rami communicantes and proceed to the sympathetic trunk where many of them end in its ganglia; others pass to the prevertebral plexuses and terminate in its collateral ganglia. The postganglionic fibers have a wide distribution. The vasoconstrictor fibers to the blood vessels of the skin of the trunk and limbs, for example, leave the spinal cord as preganglionic fibers in all the thoracic and the upper two or three lumbar spinal nerves and terminate in the ganglia of the parasympathetic trunk, either in the ganglion directly connected with its ramus or in neighboring ganglia. Postganglionic fibers arise in these ganglia, pass through gray rami communicantes to all the spinal nerves, and are distributed with their cutaneous branches, ultimately leaving these branches to join the small arteries. The postganglionic fibers do not necessarily return to the same spinal nerves which contain the corresponding preganglionic fibers. The vasoconstrictor fibers to the head come from the upper thoracic nerves, the preganglionic fibers end in the superior cervical ganglion. The postganglionic fibers pass through the internal carotid nerve and branch from it to join the sensory branches of the various cranial nerves, especially the trigeminal nerve; other fibers to the deep structures and the salivary glands probably accompany the arteries.
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* Diverts blood flow away from the gastro-intestinal (GI) tract and [[skin]] via vasoconstriction.
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* Blood flow to [[skeletal muscle]]s and the [[lung]] is not only maintained, but enhanced (by as much as 1200 percent, in the case of skeletal muscles).  
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* Dilates bronchioles of the lung, which allows for greater alveolar oxygen exchange.  
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* Increases [[heart rate]] and the contractility of cardiac cells ([[myocytes]]), thereby providing a mechanism for the enhanced blood flow to skeletal muscles.  
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* Dilates pupils and relaxes the lens, allowing more light to enter the [[eye]].
  
<center>[[image:Gray843.png]]<br>
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===[[Parasympathetic nervous system]]===
'''Figure 6 :''' parasympathetic (blue) connections of the otic and sympathetic (red) connections of the superior cervical ganglia.</center>ãñÆÓ
 
  
The postganglionic vasoconstrictor fibers to the blood vessels of the abdominal viscera arise in the prevertebral or collateral ganglia in which terminate many preganglionic fibers. Vasoconstrictor fibers to the pelvic viscera arise from the inferior mesenteric ganglia.
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* Dilates blood vessels leading to the GI tract, increasing blood flow. This is important following the consumption of food, due to the greater [[metabolism|metabolic]] demands placed on the body by the gut.
The pilomotor fibers to the hairs and the motor fibers to the sweat glands apparently have a distribution similar to that of the vasoconstrictors of the skin.
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*The parasympathetic nervous system can also constrict the bronchiolar diameter when the need for oxygen has diminished.  
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* During accommodation, the parasympathetic nervous system causes constriction of the pupil and lens.  
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* The parasympathetic nervous system stimulates salivary gland secretion, and accelerates [[peristalsis]], so, in keeping with the rest and digest functions, appropriate PNS activity mediates digestion of food and indirectly, the absorption of nutrients.
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* Is also involved in erection of genitals, via the [[pelvic splanchnic nerves]] 2–4.
  
A vasoconstrictor center has been located by the physiologists in the neighborhood of the facial nucleus. Axons from its cells are supposed to descend in the spinal cord to terminate about cell bodies of the preganglionic fibers located in the dorsolateral portion of the anterior column of the thoracic and upper lumbar region.
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==Neurotransmitters and pharmacology==
  
The motor supply to the dilator pupillæ muscle of the eye comes from preganglionic parasympathetic fibers which leave the spinal cord with the anterior roots of the upper thoracic nerves. These fibers pass to the parasympathetic trunk through the white rami communicantes and terminate in the superior cervical ganglion. Postganglionic fibers from the superior cervical ganglion pass through the internal carotid nerve and the ophthalmic division of the trigeminal nerve to the orbit where the long ciliary nerves conduct the impulses to the eyeball and the dilator pupillæ muscle. The cell bodies of these preganglionic fibers are connected with fibers which descend from the mid-brain.
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At the effector organs, sympathetic ganglionic neurons release [[noradrenaline]] (norepinephrine) to act on adrenergic receptors, with the exception of the sweat glands and the adrenal medulla:
  
Other postganglionic fibers from the superior cervical ganglion are distributed as secretory fibers to the salivary glands, the lacrimal glands and to the small glands of the mucous membrane of the nose, mouth and pharynx.
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* At [[sweat glands]], the neurotransmitter is [[acetylcholine]], which acts on [[muscarinic receptors]].  
The thoracic parasympathetics supply accelerator nerves to the heart. They are supposed to emerge from the spinal cord in the anterior roots of the upper four or five thoracic nerves and pass with the white rami to the first thoracic ganglion, here some terminate, and others pass in the ansa subclavia to the inferior cervical ganglion. The postganglionic fibers pass from these ganglia partly through the ansa subclavia to the heart, on their way they intermingle with parasympathetic fibers from the vagus to form the cardiac plexus.
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* At the [[adrenal gland|adrenal cortex]], there is no postsynaptic neuron. Instead, the presynaptic neuron releases acetylcholine to act on nicotinic receptors.  
Inhibitory fibers to the smooth musculature of the stomach, the small intestine and most of the large intestine are supposed to emerge in the anterior roots of the lower thoracic and upper lumbar nerves. These fibers pass through the white rami and parasympathetic trunk and are conveyed by the splanchnic nerves to the prevertebral plexus where they terminate in the collateral ganglia. From the celiac and superior mesenteric ganglia postganglionic fibers (inhibitory) are distributed to the stomach, the small intestine and most of the large intestine. Inhibitory fibers to the descending colon, the rectum and internal sphincter ani are probably postganglionic fibers from the inferior mesenteric ganglion.
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* Stimulation of the [[adrenal gland|adrenal medulla]] releases [[adrenaline]] (epinephrine) into the bloodstream, which will act on adrenoceptors, producing a widespread increase in sympathetic activity.  
  
The thoracolumbar parasympathetics are characterized by the presence of numerous ganglia which may be divided into two groups, central and collateral.
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In the parasympathetic system, ganglionic neurons use acetylcholine as a neurotransmitter, to stimulate muscarinic receptors.
  
The central ganglia are arranged in two vertical rows, one on either side of the middle line, situated partly in front and partly at the sides of the vertebral column. Each ganglion is joined by intervening nervous cords to adjacent ganglia so that two chains, the parasympathetic trunks, are formed. The collateral ganglia are found in connection with three great prevertebral plexuses, placed within the thorax, abdomen, and pelvis respectively.
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The following table reviews the actions of these neurotransmitters as a function of their receptors.
  
The parasympathetic trunks (''truncus parasympathicus; gangliated cord'') extend from the base of the skull to the coccyx. The cephalic end of each is continued upward through the carotid canal into the skull, and forms a plexus on the internal carotid artery; the caudal ends of the trunks converge and end in a single ganglion, the ganglion impar, placed in front of the coccyx. The ganglia of each trunk are distinguished as cervical, thoracic, lumbar, and sacral and, except in the neck, they closely correspond in number to the vertebræ. They are arranged thus:
+
{| class="wikitable"
* Cervical portion3 ganglia
+
|-
* Thoracic portion12 ganglia
+
|  || [[Sympathetic nervous system|Sympathetic]] ([[adrenergic]], with exceptions)  || [[Parasympathetic]] (muscarinic)
* Lumbar portion4 ganglia
+
|-
* Sacral portion4 or 5 ganglia
+
| '''[[circulatory system]]''' ||  || 
In the neck the ganglia lie in front of the transverse processes of the vertebræ; in the thoracic region in front of the heads of the ribs; in the lumbar region on the sides of the vertebral bodies; and in the sacral region in front of the sacrum.
+
|-
 +
| cardiac output || increases  || M2: decreases
 +
|-
 +
| SA node: heart rate (chronotropic) || β1, β2: increases  || M2: decreases
 +
|-
 +
| cardiac muscle: contractility (inotropic) || β1, β2: increases  || M2: decreases ([[atria]] only)
 +
|-
 +
| conduction at AV node || β1: increases  || M2: decreases
 +
|-
 +
| vascular smooth muscle || M3: contracts; α: contracts; β2: relaxes  || ---
 +
|-
 +
| [[platelets]] || α2: aggregates  || ---
 +
|-
 +
| [[renal artery]] || constricts|| ---
 +
|-
 +
| [[hepatic artery]] || dilates|| ---
 +
|-
 +
| [[mast cells]] - [[histamine]] || β2: inhibits  || ---
 +
|-
 +
| '''[[respiratory system]]''' ||  || 
 +
|-
 +
| [[smooth muscles]] of [[bronchioles]] || β2: relaxes (major contribution); α1: contracts (minor contribution)  || M3: contracts
 +
|-
 +
| '''[[nervous system]]''' ||  || 
 +
|-
 +
| [[pupil]] of [[eye]] || α1: relaxes  || M3: contracts
 +
|-
 +
| ciliary muscle || β2: relaxes  || M3: contracts
 +
|-
 +
| '''[[digestive system]]''' ||  || 
 +
|-
 +
| [[salivary gland]]s: secretions || β: stimulates viscous, [[amylase]] secretions; α1 = stimulates [[potassium]] cation  || stimulates watery secretions
 +
|-
 +
| [[lacrimal glands]] (tears) || decreases  || M3: increases
 +
|-
 +
| [[kidney]] ([[renin]]) || secretes  || ---
 +
|-
 +
| [[parietal cells]] || ---  || M1: secretion
 +
|-
 +
| [[liver]] || α1, β2: [[glycogenolysis]], [[gluconeogenesis]]  || ---
 +
|-
 +
| [[adipose]] cells || β3: stimulates [[lipolysis]]  || ---
 +
|-
 +
| [[GI tract]] motility || decreases  || M1, M3: increases
 +
|-
 +
| [[smooth muscles]] of [[GI tract]] || α, β2: relaxes  || M3: contracts
 +
|-
 +
| [[sphincters]] of [[GI tract]] || α1: contracts  || M3: relaxes
 +
|-
 +
| [[glands]] of [[GI tract]] || inhibits  || M3: secretes
 +
|-
 +
| '''[[endocrine system]]''' ||  || 
 +
|-
 +
| [[pancreas]] ([[Islets of Langerhans|islets]]) || α2: decreases secretion from [[beta cell]]s, increases secretion from [[alpha cell]]s  || increases stimulation from alpha cells and beta cells
 +
|-
 +
| [[adrenal medulla]] || [[Nicotinic acetylcholine receptor|N]]: secretes [[epinephrine]]  || ---
 +
|-
 +
| '''[[urinary system]]''' ||  || 
 +
|-
 +
| [[bladder]] wall || β2: relaxes  || contracts
 +
|-
 +
| [[ureter]] || α1: contracts  || relaxes
 +
|-
 +
| [[sphincter]] || α1: contracts; β2 relaxes  || relaxes
 +
|-
 +
| '''[[reproductive system]]''' ||  || 
 +
|-
 +
| [[uterus]] || α1: contracts; β2: relaxes  || ---
 +
|-
 +
| [[genitalia]] || α: contracts  || M3: erection
 +
|-
 +
| '''[[integument]]''' ||  || 
 +
|-
 +
| [[sweat gland]] secretions || M: stimulates (major contribution); α1: stimulates (minor contribution)  || ---
 +
|-
 +
| [[arrector pili]]  || α1: stimulates  || ---
  
== Connections with the spinal nerves ==
+
|}
Communications are established between the sympathetic and spinal nerves through what are known as the gray and white rami communicantes; the gray rami convey sympathetic fibers into the spinal nerves and the white rami transmit spinal fibers into the sympathetic. Each spinal nerve receives a gray ramus communicans from the sympathetic trunk, but white rami are not supplied by all the spinal nerves. White rami are derived from the first thoracic to the first lumbar nerves inclusive, while the visceral branches which run from the second, third, and fourth sacral nerves directly to the pelvic plexuses of the sympathetic belong to this category. The fibers which reach the sympathetic through the white rami communicantes are medullated; those which spring from the cells of the sympathetic ganglia are almost entirely non-medullated. The sympathetic nerves consist of efferent and afferent fibers. The three great gangliated plexuses (''collateral ganglia'') are situated in front of the vertebral column in the thoracic, abdominal, and pelvic regions, and are named, respectively, the cardiac, the solar or epigastric, and the hypogastric plexuses. They consist of collections of nerves and ganglia; the nerves being derived from the sympathetic trunks and from the cerebrospinal nerves. They distribute branches to the viscera.
 
  
== Development ==
+
==References==
The ganglion cells of the sympathetic system are derived from the cells of the neural crests. As these crests move forward along the sides of the neural tube and become segmented off to form the spinal ganglia, certain cells detach themselves from the ventral margins of the crests and migrate toward the sides of the aorta, where some of them are grouped to form the ganglia of the sympathetic trunks, while others undergo a further migration and form the ganglia of the prevertebral and visceral plexuses. The ciliary, sphenopalatine, otic, and submaxillary ganglia which are found on the branches of the trigeminal nerve are formed by groups of cells which have migrated from the part of the neural crest which gives rise to the semilunar ganglion. Some of the cells of the ciliary ganglion are said to migrate from the neural tube along the oculomotor nerve.
 
  
''This article is based on an entry from the 1918 edition of [[Gray's Anatomy]], which is in the [[public domain]]. As such, some of the information contained herein may be outdated. Please edit the article if this is the case, and feel free to remove this notice when it is no longer relevant.''
+
* Anissimov, M. 2007. [http://www.wisegeek.com/how-does-the-nervous-system-work.htm How does the nervous system work?] ''Conjecture Corporation: Wise Geek.'' Retrieved May 13, 2007.
 +
* Chamberlin, S. L., and B. Narins. 2005. ''The Gale Encyclopedia of Neurological Disorders.'' Detroit: Thomson Gale. ISBN 078769150X
 +
* Gershon, M. D. 1998. ''The Second Brain: The Scientific Basis of Gut Instinct and a Groundbreaking New Understanding of Nervous Disorders of the Stomach and Intestine.'' New York, NY: HarperCollins Publishers. ISBN 0060182520
 +
* Towle, A. 1989. ''Modern Biology.'' Austin, TX: Holt, Rinehart and Winston. ISBN 0030139198
  
==External links==
 
{{commons|Category:Nervous system|Autonomic nervous system}}
 
*[http://www.dyansys.com Monitoring the Autonomic Nervous System]
 
*[http://www.fmpartnership.org/Lavin%20article.htm A Novel Holistic Explanation for the Fibromyalgia Enigma - ANS Dysfunction]
 
*[http://www.dinet.org Dysautonomia Information Network]
 
Autonomic Nervous System Dysfunction
 
  
 
{{nervous_system}}
 
{{nervous_system}}
  
  
{{credit|59714154}}
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{{credit|128225878}}
  
 
[[Category:Life sciences]]
 
[[Category:Life sciences]]
 +
[[Category:Anatomy and physiology]]

Latest revision as of 22:28, 5 July 2013

The autonomic nervous system
Blue = parasympathetic
Red = sympathetic

The autonomic nervous system (ANS) is that part of the peripheral nervous system that largely acts independent of conscious control (involuntarily) and consists of nerves in cardiac muscle, smooth muscle, and exocrine and endocrine glands. It is responsible for maintenance functions (metabolism, cardiovascular activity, temperature regulation, digestion) that have a reputation for being outside of conscious control. The other main subdivision of the peripheral nervous system, the somatic nervous system, consists of cranial and spinal nerves that innervate skeletal muscle tissue and are more under voluntary control (Anissimov 2006; Towle 1989).

The autonomic nervous system is typically divided into two main subsystems, the sympathetic nervous system and the parasympathetic nervous system. These tend to balance each other, offering opposite and yet complementary effects reflective of the philosophy of Yin and Yang. The sympathetic nervous system deals with the response to stress and danger, releasing epinephrines (adrenaline), and in general increasing activity and metabolic rate. The parasympathetic nervous system counters this, and is central during rest, sleeping, and digesting food and, in general, lowers metabolic rate, slows activity, and restores blood pressure and resting heartbeat, and so forth (Chamberlain and Narins 2005). Just as Yin and Yang are opposing, yet complementary and interdependent forces, the sympathetic and parasympathetic systems complement each other and are both necessary to create overall harmony and balance in the living organism.

A third subsystem, the enteric nervous system, is classified as a division of the autonomic nervous system as well. This subsystem has nerves around the intestines, pancreas, and gall bladder.

Overview

The vertebrate nervous system is divided into the central nervous system (CNS), comprising the brain and spinal cord, and the peripheral nervous system (PNS), consisting of all the nerves and neurons that reside or extend outside the central nervous system, such as to serve the limbs and organs.

The peripheral nervous system, in turn, is commonly divided into two subsystems, the somatic nervous system and the autonomic nervous system.

The somatic nervous system or sensory-somatic nervous system involves nerves just under the skin and serves as the sensory connection between the outside environment and the CNS. These nerves are under conscious control, but most have an automatic component, as is seen in the fact that they function even in the case of a coma (Anissimov 2007). In humans, the somatic nervous system consists of 12 pairs of cranial nerves and 31 pairs of spinal nerves (Chamberlin and Narins 2005).

The autonomic nervous system is typically presented as that portion of the peripheral nervous system that is independent of conscious control, acting involuntarily and subconsciously (reflexively), and innervating heart muscle, endocrine glands, exocrine glands, and smooth muscle (Chamberlin and Narins 2005). In contrast, the somatic nervous system innervates skeletal muscle tissue, rather than smooth, cardiac, or glandular tissue.

The autonomic nervous system is subdivided into the sympathetic nervous system, the parasympathetic nervous system, and the enteric nervous system. In general, the sympathetic nervous system increases activity and metabolic rate (the "fight or flight response"), while the parasympathetic slows activity and metabolic rate, returning the body to normal levels of function (the "rest and digest state") after heightened activity from sympathetic stimulation (Chamberlin and Narins 2005). The enteric nervous system innervates areas around the intestines, pancreas, and gall bladder, dealing with digestion, and so forth.

Unlike the somatic nervous system, which always excites muscle tissue, the autonomic nervous system can either excite or inhibit innervated tissue (Chamberlin and Narins 2005). Most associated tissues and organs have nerves of both the sympathetic and the parasympathetic nervous systems. The two systems can stimulate the target organs and tissues in opposite ways, such as sympathetic stimulation to increase heart rate and parasympathetic to decrease heart rate, or the sympathetic stimulation resulting in pupil dilation, and the parasympathetic in pupil constriction or narrowing (Chamberlin and Narins 2005). Or, they can both stimulate activity in concert, but in different ways, such as both increasing saliva production by salivary glands, but with sympathetic stimulation yielding viscous or thick saliva and parasympathetic yielding watery saliva.

In general, the autonomic nervous system controls homeostasis, that is the constancy of the content of tissues in gases, ions, and nutrients. It does so mostly by controlling cardiovascular, digestive, and respiratory functions, but also salivation, perspiration, diameter of the pupils, micturition (the discharge of urine), and erection. While many of the activities of the ANS are involuntary, breathing, for example, can be in part consciously controlled. Indeed, although breathing is a purely homeostatic function in aquatic vertebrates, in land vertebrates it accomplishes much more than oxygenating the blood: It is essential to sniff prey or a flower, to blow out a candle, to talk or sing. This example, among others, illustrates that the so-called “autonomic nervous system” is not truly autonomous. It is anatomically and functionally linked to the rest of the nervous system and a strict delineation is impossible.

The ANS is nevertheless a classical term, still widely used throughout the scientific and medical community. Its most useful definition could be: The sensory and motor neurons that innervate the viscera. These neurons form reflex arcs that pass through the lower brainstem or medulla oblongata. This explains that when the central nervous system (CNS) is damaged experimentally or by accident above that level, a vegetative life is still possible, whereby cardiovascular, digestive, and respiratory functions are adequately regulated.

Sensory and motor neurons

Neurons active in the autonomic nervous system (and the PNS in general) can be divided into sensory neurons and motor neuron (Chamberlin and Narins 2005). Sensory neurons act as a conduit between sensory receptors, which sense stimuli such as cold, heat, and pain, and the CNS. Motor neurons act as a conduit between the CNS and various muscles and glands (effectors). Or, looked at another way, receptors are cells or groups of cells that receive information from stimuli (external or internal), and effectors are cells or groups of cells that received information from the nervous system.

Although commonly the ANS is looked at, and even defined, as if limited to motor fibers and excluding sensory fibers, a more comprehensive definition is that the reflex arcs of the ANS comprises both a sensory (or afferent) arm, and a motor (or efferent, or effector) arm.

Sensory neurons

The sensory arm is made of “primary visceral sensory neurons” found in the peripheral nervous system (PNS) in “cranial sensory ganglia:" the geniculate, petrosal, and nodose ganglia, appended respectively to cranial nerves VII, IX, and X. These sensory neurons monitor the levels of carbon dioxide, oxygen, and sugar in the blood; arterial pressure; and the chemical composition of the stomach and gut content. They also convey the sense of taste, a conscious perception.

Blood oxygen and carbon dioxide are, in fact, directly sensed by the carotid body, a small collection of chemosensors at the bifurcation of the carotid artery, innervated by the petrosal (IXth) ganglion.

Primary sensory neurons project (synapse) onto “second order” or relay visceral sensory neurons located in the medulla oblongata, forming the nucleus of the solitary tract (nTS), which integrates all visceral information. The nTS also receives input from a nearby chemosensory center, the area postrema, which detects toxins in the blood, and the cerebrospinal fluid. It is essential for chemically induced vomiting and conditional taste aversion (the memory that ensures that an animal that has been poisoned by a food never touches it again).

All this visceral sensory information constantly, and unconsciously, modulates the activity of the motor neurons of the ANS.

Motor neurons

Motor neurons of the ANS are also located in ganglia of the PNS, called “autonomic ganglia.” They belong to three categories with different effects on their target organs: Sympathetic, parasympathetic, and enteric.

Sympathetic ganglia are located in two sympathetic chains close to the spinal cord: The prevertebral and pre-aortic chains. Parasympathetic ganglia, in contrast, are located in close proximity to the target organ: The submandibular ganglion close to salivary glands, paracardiac ganglia close to the heart, and so forth. Enteric ganglia, which, as the name implies, innervate the digestive tube, are located inside its walls and collectively contain as many neurons as the entire spinal cord, including local sensory neurons, motor neurons, and interneurons. It is the only truly autonomous part of the ANS and the digestive tube can function surprisingly well even in isolation. For that reason, the enteric nervous system has been called “the second brain.”

The activity of autonomic ganglionic neurons is modulated by “preganglionic neurons” (also called, improperly but classically, "visceral motoneurons") located in the central nervous system. Preganglionc sympathetic neurons are in the spinal cord, at thoraco-lumbar levels. Preganglionic, parasympathetic neurons are in the medulla oblongata (forming visceral motor nuclei: The dorsal motor nucleus of the vagus nerve (dmnX), the nucleus ambiguus, and salivatory nuclei) and in the sacral spinal cord. Enteric neurons are also modulated by input from the CNS, from preganglionic neurons located, like parasympathetic ones, in the medulla oblongata (in the dmnX).

The feedback from the sensory to the motor arm of visceral reflex pathways is provided by direct or indirect connections between the nucleus of the solitary tract and visceral motoneurons.

Function

Sympathetic and parasympathetic divisions typically function in opposition to each other. But this opposition is better termed complementary in nature rather than antagonistic. For an analogy, one may think of the sympathetic division as the accelerator and the parasympathetic division as the brake. The sympathetic division typically functions in actions requiring quick responses. The parasympathetic division functions with actions that do not require immediate reaction. Consider sympathetic as "fight or flight" and parasympathetic as "rest and digest."

However, many instances of sympathetic and parasympathetic activity cannot be ascribed to "fight" or "rest" situations. For example, standing up from a reclining or sitting position would entail an unsustainable drop in blood pressure if not for a compensatory increase in the arterial sympathetic tonus. Another example is the constant, second-to-second modulation of heart rate by sympathetic and parasympathetic influences, as a function of the respiratory cycles. More generally, these two systems should be seen as permanently modulating vital functions, in usually opposing fashion, to achieve homeostasis. Some typical actions of the sympathetic and parasympathetic systems are listed below:

Sympathetic nervous system

  • Diverts blood flow away from the gastro-intestinal (GI) tract and skin via vasoconstriction.
  • Blood flow to skeletal muscles and the lung is not only maintained, but enhanced (by as much as 1200 percent, in the case of skeletal muscles).
  • Dilates bronchioles of the lung, which allows for greater alveolar oxygen exchange.
  • Increases heart rate and the contractility of cardiac cells (myocytes), thereby providing a mechanism for the enhanced blood flow to skeletal muscles.
  • Dilates pupils and relaxes the lens, allowing more light to enter the eye.

Parasympathetic nervous system

  • Dilates blood vessels leading to the GI tract, increasing blood flow. This is important following the consumption of food, due to the greater metabolic demands placed on the body by the gut.
  • The parasympathetic nervous system can also constrict the bronchiolar diameter when the need for oxygen has diminished.
  • During accommodation, the parasympathetic nervous system causes constriction of the pupil and lens.
  • The parasympathetic nervous system stimulates salivary gland secretion, and accelerates peristalsis, so, in keeping with the rest and digest functions, appropriate PNS activity mediates digestion of food and indirectly, the absorption of nutrients.
  • Is also involved in erection of genitals, via the pelvic splanchnic nerves 2–4.

Neurotransmitters and pharmacology

At the effector organs, sympathetic ganglionic neurons release noradrenaline (norepinephrine) to act on adrenergic receptors, with the exception of the sweat glands and the adrenal medulla:

  • At sweat glands, the neurotransmitter is acetylcholine, which acts on muscarinic receptors.
  • At the adrenal cortex, there is no postsynaptic neuron. Instead, the presynaptic neuron releases acetylcholine to act on nicotinic receptors.
  • Stimulation of the adrenal medulla releases adrenaline (epinephrine) into the bloodstream, which will act on adrenoceptors, producing a widespread increase in sympathetic activity.

In the parasympathetic system, ganglionic neurons use acetylcholine as a neurotransmitter, to stimulate muscarinic receptors.

The following table reviews the actions of these neurotransmitters as a function of their receptors.

Sympathetic (adrenergic, with exceptions) Parasympathetic (muscarinic)
circulatory system
cardiac output increases M2: decreases
SA node: heart rate (chronotropic) β1, β2: increases M2: decreases
cardiac muscle: contractility (inotropic) β1, β2: increases M2: decreases (atria only)
conduction at AV node β1: increases M2: decreases
vascular smooth muscle M3: contracts; α: contracts; β2: relaxes ---
platelets α2: aggregates ---
renal artery constricts ---
hepatic artery dilates ---
mast cells - histamine β2: inhibits ---
respiratory system
smooth muscles of bronchioles β2: relaxes (major contribution); α1: contracts (minor contribution) M3: contracts
nervous system
pupil of eye α1: relaxes M3: contracts
ciliary muscle β2: relaxes M3: contracts
digestive system
salivary glands: secretions β: stimulates viscous, amylase secretions; α1 = stimulates potassium cation stimulates watery secretions
lacrimal glands (tears) decreases M3: increases
kidney (renin) secretes ---
parietal cells --- M1: secretion
liver α1, β2: glycogenolysis, gluconeogenesis ---
adipose cells β3: stimulates lipolysis ---
GI tract motility decreases M1, M3: increases
smooth muscles of GI tract α, β2: relaxes M3: contracts
sphincters of GI tract α1: contracts M3: relaxes
glands of GI tract inhibits M3: secretes
endocrine system
pancreas (islets) α2: decreases secretion from beta cells, increases secretion from alpha cells increases stimulation from alpha cells and beta cells
adrenal medulla N: secretes epinephrine ---
urinary system
bladder wall β2: relaxes contracts
ureter α1: contracts relaxes
sphincter α1: contracts; β2 relaxes relaxes
reproductive system
uterus α1: contracts; β2: relaxes ---
genitalia α: contracts M3: erection
integument
sweat gland secretions M: stimulates (major contribution); α1: stimulates (minor contribution) ---
arrector pili α1: stimulates ---

References
ISBN links support NWE through referral fees

  • Anissimov, M. 2007. How does the nervous system work? Conjecture Corporation: Wise Geek. Retrieved May 13, 2007.
  • Chamberlin, S. L., and B. Narins. 2005. The Gale Encyclopedia of Neurological Disorders. Detroit: Thomson Gale. ISBN 078769150X
  • Gershon, M. D. 1998. The Second Brain: The Scientific Basis of Gut Instinct and a Groundbreaking New Understanding of Nervous Disorders of the Stomach and Intestine. New York, NY: HarperCollins Publishers. ISBN 0060182520
  • Towle, A. 1989. Modern Biology. Austin, TX: Holt, Rinehart and Winston. ISBN 0030139198


Nervous system
v·d·e
Brain | Spinal cord | Nerve cord | Central nervous system | Peripheral nervous system | Somatic nervous system | Autonomic nervous system | Sympathetic nervous system | Parasympathetic nervous system | Neuron | Axon | Soma (biology) | Dendrite | Hindbrain


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