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Autonomic nervous system

Last updated: April 21, 2021

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The autonomic nervous system (ANS) is part of the peripheral nervous system and regulates involuntary, visceral body functions in different organ systems (e.g., the cardiovascular, gastrointestinal, genitourinary systems). It is divided into the sympathetic and parasympathetic nervous systems. The sympathetic nervous system has a thoracolumbar outflow and is activated during fight or flight response, while the parasympathetic nervous system has a craniosacral outflow and is activated during digestion and rest. The sympathetic and parasympathetic nervous systems consist of preganglionic and postganglionic neurons. The preganglionic fibers of both ANS divisions and the postganglionic fibers of the parasympathetic division are cholinergic fibers (release acetylcholine) that act on cholinergic receptors (nicotinic or muscarinic). All postganglionic fibers of the sympathetic division are adrenergic fibers (release norepinephrine) that act on adrenergic alpha or beta receptors for neurotransmission, with the exception of the fibers innervating the sweat glands, which are cholinergic. The adrenal medulla does not have a postsynaptic neuron. The sympathetic preganglionic fibers stimulate the chromaffin cells of the adrenal medulla directly via acetylcholine on nicotinic receptors, which results in the release of norepinephrine and epinephrine mediating the fight or flight response. The sympathetic and parasympathetic nervous systems have antagonistic effects in some organ systems. The effects of the sympathetic nervous system on target organs include mydriasis, increased heart rate, contractility, and conduction velocity, bronchodilation, sweat secretion, decreased intestinal motility, and increased renin release. The effect of the parasympathetic nervous system on target organs include miosis, decreased heart rate, contractility, and conduction velocity; increased intestinal motility; and bronchoconstriction. In addition to the sympathetic and parasympathetic nervous systems, there is the enteric nervous system, which consists of enteric ganglia, the myenteric (Auerbach) and the submucosal (Meissner) plexuses, and the interstitial cells of Cajal. The enteric nervous system controls gastrointestinal motility and secretion. Autonomic function can be affected by medications (e.g., selective serotonin reuptake inhibitors, anticholinergics) as well as diseases (e.g., diabetes mellitus, Parkinson disease, multiple system atrophy).

  • Function: controls unconscious, involuntary, and visceral body functions, i.e., the cardiovascular, thermoregulatory, gastrointestinal, genitourinary, and pupillary systems
  • Components
Overview of the sympathetic and parasympathetic nervous system
Sympathetic nervous system Parasympathetic nervous system
Effect
Location
  • Thoracolumbar outflow
  • Craniosacral outflow
Characteristics
Neurotransmitter
Most common cotransmitters
Receptors

Types of receptors

Types of receptors
Class Structure Most important location Effect of stimulation
Metabotropic receptors: G-protein-coupled receptors acting through second messengers (see signal transduction) Adrenergic receptors Alpha-adrenergic receptors Alpha-1 receptor
Alpha-2 receptor
Beta-adrenergic receptors Beta-1 receptor
Beta-2 receptor
Beta-3 receptor
Muscarinic acetylcholine receptors M1
M2
M3
M4
M5
Ionotropic receptors Nicotinic acetylcholine receptors N subtype
  • ↑ Pre- and postsynaptic excitation
M subtype

All beta receptors are coupled with Gs proteins. Odd alpha receptors and muscarinic acetylcholine receptors (alpha-1, M1, M3, and M5) are coupled with Gq proteins. Even alpha receptors and muscarinic acetylcholine receptors (alpha-2, M2, and M4) are coupled with Gi proteins.

Types of nerve fibers

  • Small myelinated fibers: transmit preganglionic autonomic efferents and somatic afferents (fast)
  • Unmyelinated fibers (e.g., which innervate sweat glands): transmit postganglionic autonomic efferents and somatic/autonomic afferents (slow)

Neurotransmitters

Transmitters of the sympathetic nervous system are acetylcholine (preganglionic postganglionic neurons) and norepinephrine (postganglionic neurons → effector organ).

ANS innervation of organs

Overview of ANS innervation of organs
Organs Sympathetic Parasympathetic
Head and Neck T1–T4 CN III, XII, IX, X
Heart T1–T5 Vagus nerve (occiput, C1, C2)
Lung T2–T7
Esophagus T2–T8
Stomach T5–T9
Liver
Gallbladder
Spleen
Pancreas
Proximal duodenum
Distal duodenum T10–L1
Jejunum
Ileum
Ascending colon
Proximaltransverse colon
Kidney
Proximal ureter
Adrenal glands T8–T10
Ovaries/Testes T10–T11
Uterus T10–L2 S2–S4
Cervix
Prostate
Distal ureter T11–L2
Bladder
Distaltransverse colon L1–L2
Descending colon
Sigmoid colon
Rectum
  • The corresponding segmental innervations may vary between sources.

For a rapid overview see table “Overview of the sympathetic and parasympathetic nervous system.” [1]

Sympathetic nervous system

Sympathetic ganglia

Paravertebral ganglia

Prevertebral ganglia (or preaortic ganglia)

Injury to the cervical sympathetic trunk (esp. the superior cervical ganglion, which supplies the visceral structures of the head and neck) may result in Horner syndrome (partial ptosis, miosis, anhidrosis).

For a rapid overview see table “Overview of the sympathetic and parasympathetic nervous system.”

General information

Cranial outflow

The cranial outflow supplies visceral structures of the head, neck and face via CN III, CN VII, CN IX and of the thorax and upper abdomen via CN X.

Sacral outflow

The sympathetic and parasympathetic nervous systems mediate numerous, sometimes antagonistic effects in the organs they innervate. In general, the sympathetic nervous system stimulates the body’s fight-or-flight response, while the parasympathetic nervous system controls homeostasis and the body at rest.

Target organ Sympathetic nervous system

Parasympathetic nervous system

Brain
Eye
Salivary glands
Blood vessels
Heart
Lungs
  • β2: bronchodilation
  • M3: bronchoconstriction
Digestive system

Reproductive organs

Bladder
  • α1: contraction of vesical sphincter muscle
  • β2 and β3: relaxation of detrusor muscle
Adrenal medulla
  • None
Kidneys
  • None
Skin
  • None
Blood
  • None
Adipose tissue
  • None

The sympathetic nervous system is for fight or flight. The parasympathetic nervous system is for rest and digest.

The enteric nervous system has complex networks of afferent and efferent nerve fibers. It can operate independently of the brain and the spinal cord but its activity is usually modulated by the sympathetic and parasympathetic nervous systems.

Plexuses

Whereas the sympathetic system has an inhibitory effect on the gastrointestinal tract, the parasympathetic system promotes secretion and motility. However, removal of vagal or sympathetic connections with the gastrointestinal tract only has a minor effect on GI function because of the autonomy of the enteric nervous system.

  1. Brenner GM, Stevens C. Pharmacology. Elsevier Health Sciences ; 2009
  2. Love JA, Yi E, Smith TG. Autonomic pathways regulating pancreatic exocrine secretion. Auton Neurosci. 2007; 133 (1): p.19-34. doi: 10.1016/j.autneu.2006.10.001 . | Open in Read by QxMD
  3. Steers WD. Pharmacologic treatment of erectile dysfunction.. Rev Urol. 2002; 4 Suppl 3 : p.S17-25.
  4. The Autonomic Nervous System. https://www.britannica.com/science/human-nervous-system/The-autonomic-nervous-system#ref605928. Updated: June 5, 2018. Accessed: June 7, 2018.