Autonomic nervous system

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Autonomic nervous system
Latin divisio autonomica systematis nervosi peripherici
Anatomical terminology

The autonomic nervous system (ANS or efferent branch of the visceral nervous system or involuntary nervous system) is the part of the peripheral nervous system that acts as a control system that functions largely below the level of consciousness to control visceral functions,[1] including heart rate, digestion, respiratory rate, salivation, perspiration, pupillary dilation, micturition (urination), sexual arousal, breathing and swallowing. A common misconception about the ANS is that it is synonymous with the visceral nervous system. This is incorrect, as the ANS is only the motor side of the visceral nervous system.[2] Most autonomous functions are involuntary but they can often work in conjunction with the somatic nervous system which provides voluntary control.

Within the brain, the ANS is located in the medulla oblongata in the lower brainstem. The medulla's major ANS functions include respiration (the respiratory control center, or "rcc"), cardiac regulation (the cardiac control center, or "ccc"), vasomotor activity (the vasomotor center or "vmc"), and certain reflex actions (such as coughing, sneezing, swallowing and vomiting). Those are then subdivided into other areas and are also linked to ANS subsystems and nervous systems external to the brain. The hypothalamus, just above the brain stem, acts as an integrator for autonomic functions, receiving ANS regulatory input from the limbic system to do so.[3]

The ANS has three main branches: one branch is part of the parasympathetic nervous system (PSNS), the second branch is part of the sympathetic nervous system (SNS),[1][4] and the third branch is part of the enteric nervous system (ENS).[3] Depending on the circumstances, these sub-systems may operate independently of each other or interact co-operatively.

SNS is often considered the "fight or flight" system, while the PSNS is often considered the "rest and digest" or "feed and breed" system. In many cases, PSNS and SNS have "opposite" actions where one system activates a physiological response and the other inhibits it. An older simplification of the sympathetic and parasympathetic nervous systems as "excitory" and "inhibitory" was overturned due to the many exceptions found. A more modern characterization is that the sympathetic nervous system is a "quick response mobilizing system" and the parasympathetic is a "more slowly activated dampening system", but even this has exceptions, such as in sexual arousal and orgasm, wherein both play a role.[3]

In general, ANS functions can be divided into sensory (afferent) and motor (efferent) subsystems. Within both, there are inhibitory and excitatory synapses between neurons. Relatively recently, a third subsystem of neurons that have been named 'non-adrenergic and non-cholinergic' neurons (because they use nitric oxide as a neurotransmitter) have been described and found to be integral in autonomic function, in particular in the gut and the lungs.[5]


ANS innervation is divided into sympathetic nervous system and parasympathetic nervous system divisions. The sympathetic division has thoracolumbar “outflow”, meaning that the neurons begin at the thoracic and lumbar (T1-L2/3) portions of the spinal cord. The parasympathetic division has craniosacral “outflow”, meaning that the neurons begin at the cranial nerves (Cranial nerves III, VII, IX, and X) and sacral (S2-S4) spinal cord.

The ANS is unique in that it requires a sequential two-neuron efferent pathway; the preganglionic neuron must first synapse onto a postganglionic neuron before innervating the target organ. The preganglionic, or first, neuron will begin at the “outflow” and will synapse at the postganglionic, or second, neuron’s cell body. The postganglionic neuron will then synapse at the target organ.

Sympathetic division[edit]

The sympathetic division (thoracolumbar outflow) consists of cell bodies in the lateral horn of the spinal cord (intermediolateral cell columns) from T1 to L2/3. These cell bodies are GVE (general visceral efferent) neurons and are the preganglionic neurons. There are several locations upon which preganglionic neurons can synapse for their postganglionic neurons:

  1. cervical ganglia (3)
  2. thoracic ganglia (12) and rostral lumbar ganglia (2 or 3)
  3. caudal lumbar ganglia and sacral ganglia
  • Prevertebral ganglia (celiac ganglion, aorticorenal ganglion, superior mesenteric ganglion, inferior mesenteric ganglion)
  • Chromaffin cells of the adrenal medulla (this is the one exception to the two-neuron pathway rule: the synapse is directly efferent onto the target cell bodies)

These ganglia provide the postganglionic neurons from which innervation of target organs follows. Examples of splanchnic (visceral) nerves are:

  • Cervical cardiac nerves & thoracic visceral nerves, which synapse in the sympathetic chain
  • Thoracic splanchnic nerves (greater, lesser, least), which synapse in the prevertebral ganglia
  • Lumbar splanchnic nerves, which synapse in the prevertebral ganglia
  • Sacral splanchnic nerves, which synapse in the inferior hypogastric plexus

These all contain afferent (sensory) nerves as well, known as GVA (general visceral afferent) neurons.

Parasympathetic division[edit]

The parasympathetic division (craniosacral outflow) consists of cell bodies from one of two locations: the brainstem (Cranial Nerves III, VII, IX, X) or the sacral spinal cord (S2, S3, S4). These are the preganglionic neurons, which synapse with postganglionic neurons in these locations:

These ganglia provide the postganglionic neurons from which innervations of target organs follows. Examples are:

  • The postganglionic parasympathetic splanchnic (visceral) nerves
  • The vagus nerve, which passes through the thorax and abdominal regions innervating, among other organs, the heart, lungs, liver and stomach

Sensory neurons[edit]

Main article: Sensory neuron

The sensory arm is composed 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 and smell, which, unlike most functions of the ANS, is 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 or 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[edit]

Main article: Motor neuron

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 salivary 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. Preganglionic 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 the salivatory nuclei), and in the sacral spinal cord. Enteric neurons are also modulated by input from the CNS and from preganglionic neurons located, like parasympathetic ones, in the medulla oblongata (in the dmnX).


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. The sympathetic system is often considered the "fight or flight" system, while the parasympathetic system is often considered the "rest and digest" or "feed and breed" system.

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. In general, 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[edit]

Promotes a "fight or flight" response, corresponds with arousal and energy generation, and inhibits digestion

  • Diverts blood flow away from the gastro-intestinal (GI) tract and skin via vasoconstriction
  • Blood flow to skeletal muscles and the lungs is 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 enhanced blood flow to skeletal muscles
  • Dilates pupils and relaxes the ciliary muscle to the lens, allowing more light to enter the eye and far vision
  • Provides vasodilation for the coronary vessels of the heart
  • Constricts all the intestinal sphincters and the urinary sphincter
  • Inhibits peristalsis
  • Stimulates orgasm

Parasympathetic nervous system[edit]

Promotes a "rest and digest" response, promotes calming of the nerves return to regular function, and enhances digestion

  • The parasympathetic nerves dilate 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
  • Dedicated cardiac branches of the vagus and thoracic spinal accessory nerves impart parasympathetic control of the heart (myocardium)
  • During accommodation, the parasympathetic nervous system causes constriction of the pupil and contraction of the ciliary muscle to the lens, allowing for closer vision
  • The parasympathetic nervous system stimulates salivary gland secretion, and accelerates peristalsis, mediating digestion of food and, indirectly, the absorption of nutrients
  • The PNS is also involved in the erection of genital tissues via the pelvic splanchnic nerves 2–4.
  • The PNS is responsible for stimulating sexual arousal

Neurotransmitters and pharmacology[edit]

At the effector organs, sympathetic ganglionic neurons release noradrenaline (norepinephrine), along with other cotransmitters such as 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 medulla, 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 acts 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[edit]


Target Sympathetic (adrenergic) Parasympathetic (muscarinic)
cardiac output β1, (β2): increases M2: decreases
SA node: heart rate (chronotropic) β1, (β2):[6] increases M2: decreases
Atrial cardiac muscle: contractility (inotropic) β1, (β2):[6] increases M2: decreases
at AV node β1:
increases conduction
increases cardiac muscle automaticity[6]
decreases conduction
Atrioventricular block[6]
Ventricular cardiac muscle β1, (β2):
increases contractility (inotropic)
increases cardiac muscle automaticity[6]

Blood vessels[edit]

Target Sympathetic (adrenergic) Parasympathetic (muscarinic)
vascular smooth muscle in general α1:[2] contracts; β2:[2] relaxes M3: relaxes[6]
renal artery α1:[7] constricts ---
larger coronary arteries α1 and α2:[8] constricts[6] ---
smaller coronary arteries β2: dilates[9] ---
arteries to viscera α: constricts ---
arteries to skin α: constricts ---
arteries to brain α1:[10] constricts[6] ---
arteries to erectile tissue α1:[11] constricts M3: dilates
arteries to salivary glands α: constricts M3: dilates
hepatic artery β2: dilates ---
arteries to skeletal muscle β2: dilates ---
Veins α1 and α2:[12] constricts
β2: dilates


Target Sympathetic (adrenergic) Parasympathetic (muscarinic)
platelets α2: aggregates ---
mast cells - histamine β2: inhibits ---

Respiratory system[edit]

Target Sympathetic (adrenergic) Parasympathetic (muscarinic)
smooth muscles of bronchioles β2:[2] relaxes (major contribution)
α1: contracts (minor contribution)
M3:[2] contracts

The bronchioles have no sympathetic innervation, but are instead affected by circulating adrenaline[6]

Visual system[edit]

Target Sympathetic (adrenergic) Parasympathetic (muscarinic)
Pupil dilator muscle α1: Dilates
(causes mydriasis)
Iris sphincter muscle - M3: contracts
(causes miosis)
Ciliary muscle β2: relaxes
(causes long-range focus)
M3: contracts
(causes short-range focus)

Digestive system[edit]

Target Sympathetic (adrenergic) Parasympathetic (muscarinic)
salivary glands: secretions β: stimulates viscous, amylase secretions
α1: stimulates potassium secretions
M3: stimulates watery secretions
lacrimal glands (tears) β: stimulates protein secretion[13] secretion of tears by stimulating muscarinic receptors (M3)
juxtaglomerular apparatus of kidney β1:[2] renin secretion ---
parietal cells --- M1: Gastric acid secretion
liver α1, β2: glycogenolysis, gluconeogenesis ---
adipose cells β1,[2] β3: stimulates lipolysis ---
GI tract (smooth muscle) motility α1, α2,[14] β2: decreases M3, (M1):[6] increases
sphincters of GI tract α1,[2] α2,[6] β2: contracts M3:[2] relaxes
glands of GI tract no effect[6] M3: secretes

Endocrine system[edit]

Target Sympathetic (adrenergic) Parasympathetic (muscarinic)
pancreas (islets) α2: decreases insulin secretion from beta cells, increases glucagon secretion from alpha cells M3:[15][16] increases secretion of both insulin and glucagon.[15][16]
adrenal medulla N (nicotinic ACh receptor): secretes epinephrine and norepinephrine ---

Urinary system[edit]

Target Sympathetic (adrenergic) Parasympathetic (muscarinic)
Detrusor urinae muscle of bladder wall β2,[2] β3:[17] relaxes M3:[2] contracts
internal urethral sphincter α1:[2] contracts M3:[2] relaxes

Reproductive system[edit]

Target Sympathetic (adrenergic) Parasympathetic (muscarinic)
uterus α1: contracts (pregnant[6])
β2: relaxes (non-pregnant[6])
genitalia α1: contracts (ejaculation) M3: erection

Integumentary system[edit]

Target Sympathetic (muscarinic and adrenergic) Parasympathetic
sweat gland secretions α1: stimulates (minor contribution) M:[2] stimulates (major contribution)
arrector pili α1: stimulates ---

See also[edit]


Autonomic nervous system, showing splanchnic nerves in middle, and the vagus nerve as "X" in blue. The heart and organs below in list to right are regarded as viscera.

The viscera are mainly innervated parasympathetically by the vagus nerve and sympathetically by the splanchnic nerves. The sensory part of the latter reaches the spinal column at certain spinal segments. Pain in any viscera is perceived as referred pain, more specifically pain from the dermatome corresponding to the spinal segment.[18]

Viscus Nerves[19] Origin in spinal column[19]
stomach T6, T7, T8, T9 and, sometimes, T10
duodenum T5, T6, T7, T8, T9 and, sometimes, T10
pancreatic head T8, T9
jejunum and ileum T5, T6, T7, T8, T9
spleen T6, T7, T8
vermiform appendix T10
gallbladder and liver T6, T7, T8, T9
kidneys and ureters T11, T12


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  10. ^ Circulation & Lung Physiology I M.A.S.T.E.R. Learning Program, UC Davis School of Medicine
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  12. ^ Elliott, J. (1997). "Alpha-adrenoceptors in equine digital veins: Evidence for the presence of both alpha1 and alpha2-receptors mediating vasoconstriction". Journal of Veterinary Pharmacology and Therapeutics 20 (4): 308–17. doi:10.1046/j.1365-2885.1997.00078.x. PMID 9280371. 
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  17. ^ Kullmann, F. A.; Limberg, B. J.; Artim, D. E.; Shah, M.; Downs, T. R.; Contract, D.; Wos, J.; Rosenbaum, J. S.; De Groat, W. C. (2009). "Effects of 3-Adrenergic Receptor Activation on Rat Urinary Bladder Hyperactivity Induced by Ovariectomy". Journal of Pharmacology and Experimental Therapeutics 330 (3): 704–17. doi:10.1124/jpet.109.155010. PMC 2729793. PMID 19515967. 
  18. ^ Essential Clinical Anatomy. K.L. Moore & A.M. Agur. Lippincott, 2 ed. 2002. Page 199
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