Autonomic nervous system
166189
226077326
2008-07-16T18:54:07Z
128.100.240.238
/* Sympathetic nervous system */
{{expert-portal|Neuroscience}}
[[Image:Gray839.png|thumb|300px|The autonomic nervous system<br/>Blue = parasympathetic<BR>Red = sympathetic]]
The '''autonomic nervous system''' ('''ANS''') (or '''visceral nervous system''') is the part of the [[peripheral nervous system]] that acts as a [[control system]], maintaining [[homeostasis]] in the body. These activities are generally performed without conscious control or sensation. The ANS affects heart rate, digestion, respiration rate, salivation, perspiration, diameter of the pupils, micturition (urination), and sexual arousal. Whereas most of its actions are involuntary, some, such as breathing, work in tandem with the conscious mind. Its main components are its sensory system, motor system (comprised of the [[parasympathetic nervous system]] and [[sympathetic nervous system]]), and the [[enteric nervous system]].
The ANS is a classical term, 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]]. If the central nervous system (CNS) is damaged above that level, life is still possible, because cardiovascular, digestive, and respiratory functions will continue to be adequately regulated.
==Anatomy==
The reflex arcs of the ANS comprise a sensory (afferent) arm, and a motor (efferent or effector) arm. Only the latter is shown in the illustration.
===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), that integrates all visceral information. The nTS also receives input from a nearby chemosensory center, the [[area postrema]], that detects toxins in the blood and the [[cerebrospinal fluid]] and is essential for chemically induced vomiting and conditional taste aversion (the memory that ensures that an animal which has been poisoned by a food never touches it again).
All these visceral sensory informations constantly and unconsciously modulate 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 (see below “Function”): 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 salivatory glands, paracardiac ganglia close to the heart etc…
Enteric ganglia, which as their 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 antagonistic fashion, to achieve homeostasis. Some typical actions of the sympathetic and parasympathetic systems are listed below:
===[[Sympathetic nervous system]]===
Promotes a "[[fight or flight]]" response, corresponds with arousal and energy generation, inhibits digestion:
* Diverts blood flow away from the gastro-intestinal (GI) tract and [[skin]] via vasoconstriction.
* Blood flow to [[skeletal muscle]]s, the [[lung]] is not only maintained, but enhanced (by as much as 1200%, 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]]===
Promotes a "rest and digest" response; promotes calming of the nerves return to regular function, and enhances digestion.
* 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), along with other [[cotransmittors]] such as [[adenosine triphosphate|ATP]], to act on [[adrenergic receptors]], with the exception of the sweat glands and the adrenal medulla:
* Acetylcholine is the preganglionic neurotransmitter for both divisions of the ANS, as well as the postganglionic neurotransmitter of parasympathetic neurons. Nerves that release acetylcholine are said to be cholinergic. In the parasympathetic system, ganglionic neurons use acetylcholine as a neurotransmitter, to stimulate 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.
The following table reviews the actions of these neurotransmitters as a function of their receptors.
===[[Circulatory system]]===
====[[Heart]]====
{| class="wikitable" width=100% width=100%
|-
| '''Target''' || '''[[Sympathetic nervous system|Sympathetic]]''' ([[adrenergic]]) || '''[[Parasympathetic]]''' ([[muscarinic]])
|-
| [[cardiac output]] || [[β1 receptor|β1]], ([[β2 receptor|β2]]): increases || [[M2 receptor|M2]]: decreases
|-
|}
:{| class="wikitable" width=100%
|-
| [[SA node]]: heart rate ([[chronotropic]]) || [[β1 receptor|β1]], ([[β2 receptor|β2]]) <ref name=Rangnot> "ref name=Rang" mentions only the one without brackets</ref>: increases || M2: decreases
|-
| [[heart atrium|Atrial]] [[cardiac muscle]]: contractility ([[inotropic]]) || β1, (β2)<ref name=Rangnot/>: increases || M2: decreases
|-
| [[heart ventricle|Ventricular]] [[cardiac muscle]] || β1, (β2): <BR> increases contractility ([[inotropic]]) <BR> increases [[cardiac muscle automaticity]] <ref name=Rang> Pharmacology, (Rang, Dale, Ritter & Moore, ISBN 0443071454, 5:th ed., Churchill Livingstone 2003). Page 127 </ref> || ---
|-
| at [[AV node]] || β1: <BR> increases conduction <BR> increases [[cardiac muscle automaticity]] <ref name=Rang/> || M2: <BR> decreases conduction <BR> [[Atrioventricular block]] <ref name=Rang/>
|-
|}
====[[Blood vessels]]====
{| class="wikitable" width=100%
|-
| '''Target''' || '''[[Sympathetic nervous system|Sympathetic]]''' ([[adrenergic]]) || '''[[Parasympathetic]]''' ([[muscarinic]])
|-
| [[vascular smooth muscle]] || α: contracts; β2: relaxes || [[M3 receptor|M3]]: relaxes <ref name=Rang/>
|-
|}
:{| class="wikitable" width=100%
|-
| [[renal artery]] || α1<ref>[http://jpet.aspetjournals.org/cgi/content/abstract/219/2/400 Renal alpha-1 and alpha-2 adrenergic receptors: biochemical and pharmacological correlations] JM Schmitz, RM Graham, A Sagalowsky and WA Pettinger </ref>: constricts|| ---
|-
| larger [[coronary arteries]] || α1 and α2<ref>[http://ajpheart.physiology.org/cgi/content/abstract/253/2/H388 Coronary vasoconstriction mediated by alpha 1- and alpha 2-adrenoceptors in conscious dogs] O. L. Woodman and S. F. Vatner </ref>: constricts <ref name=Rang/> || ---
|-
| smaller [[coronary arteries]] || β2:dilates <ref name=Rang270>{{cite book |author=Rang, H. P. |title=Pharmacology |publisher=Churchill Livingstone |location=Edinburgh |year=2003 |pages= |isbn=0-443-07145-4 |oclc= |doi=}} Page 270</ref> || ---
|-
| [[arteries]] to [[viscera]] || α: constricts|| ---
|-
| [[arteries]] to [[skin]] || α: constricts|| ---
|-
| [[arteries]] to [[brain]] || α1<ref>[http://cim.ucdavis.edu/masters/sessions2002/session4_ica.doc Circulation & Lung Physiology I] M.A.S.T.E.R. Learning Program, UC Davis School of Medicine</ref>: constricts <ref name=Rang/> || ---
|-
| [[arteries]] to [[erectile tissue]] || α1<ref>[http://www.nature.com/bjp/journal/v150/n1/full/0706956a.html 1A-Adrenoceptors mediate contractions to phenylephrine in rabbit penile arteries] J S Morton1, C J Daly1, V M Jackson2 and J C McGrath1: "1A-ARs could be utilized to aid the erectile response in male erectile dysfunction sufferers by reducing vasoconstriction of the penile arteries"</ref>: constricts|| M3: dilates
|-
| [[arteries]] to [[salivary gland]]s || α: constricts|| M3: dilates
|-
| [[hepatic artery]] || β2: dilates|| ---
|-
| [[arteries]] to [[skeletal muscle]] || β2: dilates|| ---
|-
|}
:{| class="wikitable" width=100%
|-
| [[Vein]]s || α1 and α2 <ref>[http://www.ingentaconnect.com/content/bsc/jvpt/1997/00000020/00000004/art00009;jsessionid=8lm5thggpj1x.alice?format=print IngentaConnect Alpha-adrenoceptors in equine digital veins ...</ref>: constricts <BR> β2: dilates || ---
|-
|}
====Other====
{| class="wikitable" width=100%
|-
| '''Target''' || '''[[Sympathetic nervous system|Sympathetic]]''' ([[adrenergic]]) || '''[[Parasympathetic]]''' ([[muscarinic]])
|-
| [[platelets]] || [[α2 receptor|α2]]: aggregates || ---
|-
| [[mast cells]] - [[histamine]] || β2: inhibits || ---
|-
|}
===[[Respiratory system]]===
{| class="wikitable" width=100%
|-
| '''Target''' || '''[[Sympathetic nervous system|Sympathetic]]''' ([[adrenergic]]) || '''[[Parasympathetic]]''' ([[muscarinic]])
|-
| [[smooth muscles]] of [[bronchioles]] || β2: relaxes (major contribution) <BR> [[alpha-1 adrenergic receptor|α1]]: contracts (minor contribution) || M3: contracts
|-
|}
The bronchioles have no sympathetic innervation, but are instead affected by circulating adrenaline <ref name=Rang/>
===[[Nervous system]]===
{| class="wikitable" width=100%
|-
| '''Target''' || '''[[Sympathetic nervous system|Sympathetic]]''' ([[adrenergic]]) || '''[[Parasympathetic]]''' ([[muscarinic]])
|-
| [[pupil]] of [[eye]] || [[α1 receptor|α1]]: relaxes || M3: contracts
|-
| [[ciliary muscle]] || β2: relaxes || M3: contracts
|-
|}
===[[Digestive system]]===
{| class="wikitable" width=100%
|-
| '''Target''' || '''[[Sympathetic nervous system|Sympathetic]]''' ([[adrenergic]]) || '''[[Parasympathetic]]''' ([[muscarinic]])
|-
| [[salivary glands]]: secretions || β: stimulates viscous, [[amylase]] secretions <BR> α1: stimulates [[potassium]] cation || M3: stimulates watery secretions
|-
| [[lacrimal glands]] (tears) || β2: Protein secretion <ref> [http://ajpcell.physiology.org/cgi/content/abstract/246/1/C37 Protein secretion induced by isoproterenol or pentoxifylline in lacrimal gland] P. Mauduit, G. Herman and B. Rossignol </ref> || M3: increases
|-
| [[kidney]] ([[renin]]) || β2: secretes || ---
|-
| [[parietal cells]] || --- || M1: [[Gastric acid]] secretion
|-
| [[liver]] || α1, β2: [[glycogenolysis]], [[gluconeogenesis]] || ---
|-
| [[adipose]] cells || β3: stimulates [[lipolysis]] || ---
|-
| [[GI tract]] ([[smooth muscle]]) motility || α1, α2<ref>[http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1433140 Involvement of alpha-1 and alpha-2 adrenoceptors in the postlaparotomy intestinal motor disturbances in the rat.] A Sagrada, M J Fargeas, and L Bueno</ref>, β2: decreases || M3, (M1) <ref name=Rangnot/>: increases
|-
| [[sphincters]] of [[GI tract]] || α2 <ref name=Rang/>, β2: contracts || M3: relaxes
|-
| [[glands]] of [[GI tract]] || no effect <ref name=Rang/> || M3: secretes
|-
|}
===[[Endocrine system]]===
{| class="wikitable" width=100%
|-
| '''Target''' || '''[[Sympathetic nervous system|Sympathetic]]''' ([[adrenergic]]) || '''[[Parasympathetic]]''' ([[muscarinic]])
|-
| [[pancreas]] ([[Islets of Langerhans|islets]]) || α2: decreases secretion from [[beta cell]]s, increases secretion from [[alpha cell]]s || M3<ref>{{cite journal |author=Verspohl EJ, Tacke R, Mutschler E, Lambrecht G |title=Muscarinic receptor subtypes in rat pancreatic islets: binding and functional studies |journal=Eur. J. Pharmacol. |volume=178 |issue=3 |pages=303–11 |year=1990 |pmid=2187704|doi=10.1016/0014-2999(90)90109-J}}</ref> increases stimulation from [[alpha cell]]s and [[beta cell]]s
|-
| [[adrenal medulla]] || [[Nicotinic acetylcholine receptor|N]]: secretes [[Acetylcholine]] || ---
|-
|}
===[[Urinary system]]===
{| class="wikitable" width=100%
|-
| '''Target''' || '''[[Sympathetic nervous system|Sympathetic]]''' ([[adrenergic]]) || '''[[Parasympathetic]]''' ([[muscarinic]])
|-
| [[Detrusor urinae muscle]] of [[Urinary bladder|bladder]] wall || β2: relaxes || contracts
|-
| [[ureter]] || α1: contracts || relaxes
|-
| [[sphincter]] || α1: contracts; β2 relaxes || relaxes
|-
|}
===[[Reproductive system]]===
{| class="wikitable" width=100%
|-
| '''Target''' || '''[[Sympathetic nervous system|Sympathetic]]''' ([[adrenergic]]) || '''[[Parasympathetic]]''' ([[muscarinic]])
|-
| [[uterus]] || α1: contracts (pregnant<ref name=Rang/>) <BR> β2: relaxes (non-pregnant<ref name=Rang/>) || ---
|-
| [[genitalia]] || α: contracts ([[ejaculation]]) || M3: erection
|-
|}
===[[Integument]]===
{| class="wikitable" width=100%
|-
| '''Target''' || '''[[Sympathetic nervous system|Sympathetic]]''' ('''[[muscarinic]]''' and [[adrenergic]]) || '''[[Parasympathetic]]''' ([[muscarinic]])
|-
| [[sweat gland]] secretions || M: stimulates (major contribution); α1: stimulates (minor contribution) || ---
|-
| [[erector pili]] || α1: stimulates || ---
|}
== See also ==
{{commonscat|Nervous system}}
* [[Reflex arc]]
* [[Central pattern generator]]
==References==
<references/>
==External links==
*[http://microvet.arizona.edu/Courses/VSC401/autonomicNervous.html Overview at arizona.edu]
*[http://www.rahulgladwin.com/blog/2006/07/autonomic-nervous-system-regulator-of.html ANS Medical Notes on rahulgladwin.com]
{{nervous_system}}
{{Autonomic}}
[[Category:Autonomic nervous system|*]]
[[Category:Nervous system]]
[[ar:جهاز عصبي ذاتي]]
[[bs:Autonomni nervni sistem]]
[[ca:Sistema nerviós autònom]]
[[da:Autonome nervesystem]]
[[de:Vegetatives Nervensystem]]
[[dv:އުޓޮނޮމިކް ނާރވަސް ސިސްޓަމް]]
[[es:Sistema nervioso autónomo]]
[[fa:سامانه عصبی خودگردان]]
[[fr:Système nerveux autonome]]
[[is:Dultaugakerfið]]
[[it:Sistema nervoso autonomo]]
[[he:מערכת העצבים האוטונומית]]
[[lt:Autonominė nervų sistema]]
[[mk:Вегетативен нервен систем]]
[[nl:Autonoom zenuwstelsel]]
[[ja:自律神経系]]
[[no:Det autonome nervesystem]]
[[pl:Autonomiczny układ nerwowy]]
[[pt:Sistema nervoso autônomo]]
[[ro:Sistem nervos vegetativ]]
[[ru:Вегетативная нервная система]]
[[simple:Autonomic nervous system]]
[[sl:Avtonomno živčevje]]
[[sr:Аутономни нервни систем]]
[[fi:Autonominen hermosto]]
[[uk:Автономна нервова система]]
[[zh:植物神经系统]]