Editing Autonomic nervous system (section)

==Structure==
[[File:Gray839.png|thumb|300px|Autonomic nervous system, showing [[splanchnic nerve]]s in middle, and the vagus nerve as "X" in blue. The heart and organs below in list to right are regarded as viscera.]]
The autonomic nervous system has been classically divided into the [[sympathetic nervous system]] and [[parasympathetic nervous system]] only (i.e., exclusively motor). The sympathetic division emerges from the [[spinal cord]] in the [[Thoracic vertebrae|thoracic]] and [[Lumbar vertebrae|lumbar]] areas, terminating around L2-3. The parasympathetic division has craniosacral "outflow", meaning that the neurons begin at the [[cranial nerve]]s (specifically the [[oculomotor nerve]], [[facial nerve]], [[glossopharyngeal nerve]] and [[vagus nerve]]) and [[Sacral vertebrae|sacral]] (S2-S4) spinal cord.{{cn|date=April 2024}}

The autonomic nervous system 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.{{cn|date=April 2024}}

===Sympathetic division===
{{Main|Sympathetic nervous system}}
The sympathetic nervous system consists of cells with bodies in the [[lateral grey column]] from T1 to L2/3. These cell bodies are [[General visceral efferent fibers|"GVE" (general visceral efferent) neurons]] and are the preganglionic neurons. There are several locations upon which preganglionic neurons can synapse for their postganglionic neurons:
* [[paravertebral ganglia]] (3) of the sympathetic chain (these run on either side of the vertebral bodies)
# [[cervical ganglia]] (3)
# [[thoracic ganglia]] (12) and rostral [[lumbar ganglia]] (2 or 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 nerves|splanchnic (visceral) nerves]] are:
* cervical cardiac nerves and 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 [[General visceral afferent fibers|GVA (general visceral afferent) neurons]].

===Parasympathetic division===
{{Main|Parasympathetic nervous system}}
The parasympathetic nervous system consists of cells with bodies in 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:
* [[parasympathetic ganglia]] of the head: ciliary ([[cranial nerve III]]), geniculate (cranial nerve VII),
* pterygopalatine ([[cranial nerve VII]] and IX), and submandibular ([[cranial nerve VII]] and IX),
* ottic in inner ear space ([[cranial nerve IX]])
* tympanic nerve of VII with C9, C10, C5 (cranial nerves [[Cranial nerve VII|VII]], XI, X, V) in promontory plexus in middle ear space
* trigeminal ganglion specially sensory (only mastication motor) is common with other ones
* in or near the wall of an organ innervated by the vagus ([[cranial nerve X]]) or [[sacral nerves]] plexus (S2, S3, S4)
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

===Enteric nervous system===
{{Main|Enteric nervous system}}
====Development of the enteric nervous system====
The intricate process of enteric nervous system (ENS) development begins with the migration of cells from the vagal section of the neural crest. These cells embark on a journey from the cranial region to populate the entire gastrointestinal tract. Concurrently, the sacral section of the neural crest provides an additional layer of complexity by contributing input to the hindgut ganglia. Throughout this developmental journey, numerous receptors exhibiting tyrosine kinase activity, such as Ret and Kit, play indispensable roles. Ret, for instance, plays a critical role in the formation of enteric ganglia derived from cells known as vagal neural crest. In mice, targeted disruption of the RET gene results in renal agenesis and the absence of enteric ganglia, while in humans, mutations in the RET gene are associated with megacolon. Similarly, Kit, another receptor with tyrosine kinase activity, is implicated in Cajal interstitial cell formation, influencing the spontaneous, rhythmic, electrical excitatory activity known as slow waves in the gastrointestinal tract. Understanding the molecular intricacies of these receptors provides crucial insights into the delicate orchestration of ENS development.<ref>{{Cite journal |last1=Goyal |first1=Raj K. |last2=Hirano |first2=Ikuo |date=1996-04-25 |title=The Enteric Nervous System |url=http://dx.doi.org/10.1056/nejm199604253341707 |journal=New England Journal of Medicine |volume=334 |issue=17 |pages=1106–1115 |doi=10.1056/nejm199604253341707 |pmid=8598871 |issn=0028-4793|url-access=subscription }}</ref>

====Structure of the enteric nervous system====
Originally perceived as postganglionic parasympathetic neurons, the ENS earned recognition for its autonomy in the early 1900s. Boasting approximately 100 million neurons, a quantity comparable to the spinal cord, the ENS is often described as a "brain of its own." This description is rooted in the ENS's ability to communicate independently with the central nervous system through parasympathetic and sympathetic neurons. At the core of this structure are the myenteric plexus (Auerbach's) and the submucous plexus (Meissner's), two main plexuses formed by the grouping of nerve-cell bodies into tiny ganglia connected by bundles of nerve processes. The myenteric plexus extends the full length of the gut, situated between the circular and longitudinal muscle layers. Beyond its primary motor and secretomotor functions, the myenteric plexus exhibits projections to submucosal ganglia and enteric ganglia in the pancreas and gallbladder. Additionally, the myenteric plexus plays a unique role in innervating motor end plates with the inhibitory neurotransmitter nitric oxide in the striated-muscle segment of the esophagus, a feature exclusive to this organ. Meanwhile, the submucous plexus, most developed in the small intestine, occupies a crucial position in secretory regulation. Positioned in the submucosa between the circular muscle layer and the muscularis mucosa, the submucous plexus's neurons innervate intestinal endocrine cells, submucosal blood arteries, and the muscularis mucosa. Furthermore, the ENS also has ganglionated plexuses in the pancreatic, cystic duct, common bile duct, and gallbladder, resembling submucous plexuses. Glial cells are also involved , outnumbering enteric neurons and covering the majority of the surface of enteric neuronal-cell bodies with laminar extensions. Resembling the astrocytes of the central nervous system, enteric glial cells respond to cytokines by expressing MHC class II antigens and generating interleukins. Thus, they have a pivotal role in modulating inflammatory responses in the intestine. The varied morphological shapes of enteric neurons further contribute to the structural diversity of the ENS, with neurons capable of exhibiting up to eight different morphologies. These neurons are primarily categorized into type I and type II, where type II neurons are multipolar with numerous long, smooth processes, and type I neurons feature numerous club-shaped processes along with a single long, slender process. <ref>{{Cite journal |last1=Goyal |first1=Raj K. |last2=Hirano |first2=Ikuo |date=1996-04-25 |title=The Enteric Nervous System |url=http://dx.doi.org/10.1056/nejm199604253341707 |journal=New England Journal of Medicine |volume=334 |issue=17 |pages=1106–1115 |doi=10.1056/nejm199604253341707 |pmid=8598871 |issn=0028-4793|url-access=subscription }}</ref>

===Sensory neurons===
{{Main|Sensory neuron}}
The visceral sensory system - technically not a part of the autonomic nervous system - is composed of primary neurons located in cranial sensory ganglia: the [[geniculate ganglion|geniculate]], [[petrosal ganglion|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" 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.

===Innervation===
Autonomic nerves travel to organs throughout the body. Most organs receive parasympathetic supply by the [[vagus nerve]] and sympathetic supply by [[splanchnic nerve]]s. The sensory part of the latter reaches the [[spinal column]] at certain [[spinal segment]]s. Pain in any internal organ is perceived as [[referred pain]], more specifically as pain from the [[dermatome (anatomy)|dermatome]] corresponding to the spinal segment.<ref name=Moore199>Essential Clinical Anatomy.  K. L. Moore and A. M. Agur. Lippincott, 2 edition (2002). Page 199</ref>

{{Table of autonomic innervation of organs}}

===Motor neurons===
{{Main|Motor neuron}}
Motor neurons of the autonomic nervous system are found in "autonomic ganglia". Those of the parasympathetic branch are located close to the target organ whilst the ganglia of the sympathetic branch are located close to the spinal cord.

The sympathetic ganglia here, are found in two chains: the pre-vertebral and pre-aortic chains. The activity of autonomic ganglionic neurons is modulated by "preganglionic neurons" located in the central nervous system. Preganglionic sympathetic neurons are located in the spinal cord, at the thorax and upper lumbar levels. Preganglionic parasympathetic neurons are found in the medulla oblongata where they form visceral motor nuclei; the dorsal motor nucleus of the vagus nerve; the nucleus ambiguus,  the [[salivatory nuclei]], and in the sacral region of the spinal cord.