| Enteric nervous system | |
|---|---|
 The enteric nervous system is embedded in the lining of thegastrointestinal system. | |
| Synonyms | intrinsic nervous system |
| Identifiers | |
| Acronym(s) | ENS |
| MeSH | D017615 |
| FMA | 66070 |
| Anatomical terminology | |
Theenteric nervous system (ENS) is one of the three divisions of theautonomic nervous system (ANS), the others being thesympathetic nervous system (SNS) andparasympathetic nervous system (PSNS). It consists of a mesh-like system ofneurons that governs the function of thegastrointestinal tract.[1] The ENS is nicknamed the "second brain".[2][3] It is derived fromneural crest cells.[4][5]
The enteric nervous system is capable of operating independently of the brain and spinal cord,[6] but is thought to rely on innervation from thevagus nerve andprevertebral ganglia in healthy subjects. However, studies have shown that the system is operable with a severed vagus nerve.[7] The neurons of the enteric nervous system control the motor functions of the system, in addition to the secretion of gastrointestinal enzymes. These neurons communicate through manyneurotransmitters similar to the CNS, includingacetylcholine,dopamine, andserotonin. The large presence of serotonin and dopamine in theintestines are key areas of research forneurogastroenterology.[8][9][10]
The enteric nervous system in humans consists of some 500 millionneurons[11] (including the various types ofDogiel cells),[1][12] 0.5% of the number of neurons in thebrain, five times as many as the one hundred million neurons in the human spinal cord,[13] and about2⁄3 as many as in thewhole nervous system of a cat. The enteric nervous system is embedded in the lining of thegastrointestinal system, beginning in theesophagus and extending down to the anus.[13]
The neurons of the ENS are collected into two types ofganglia:myenteric (Auerbach's) andsubmucosal (Meissner's) plexuses.[14] Myenteric plexuses are located between the inner and outer layers of themuscularis externa, while submucosal plexuses are located in thesubmucosa.
Auerbach's plexus, also known as the myenteric plexus, is a collection of fibers and postganglionic autonomic cell bodies that lie between the circular and longitudinal layers of the muscularis externa in the gastrointestinal tract.[citation needed] It was discovered and named by German neuropathologistLeopold Auerbach. These neurons provide motor inputs to both layers of the muscularis externa and provide both parasympathetic and sympathetic input. The anatomy of the plexus is similar to the anatomy of thecentral nervous system. The plexus includes sensory receptors, such aschemoreceptors andmechanoreceptors, that are used to provide sensory input to the interneurons in the enteric nervous system. The plexus is the parasympathetic nucleus of origin for the vagus nerve and communicates with themedulla oblongata through both the anterior and posterior vagal nerves.
The submucosal plexus (also known as Meissner's plexus) is found in the submucosal layer of thegastrointestinal tract.[15] It was discovered and named by German physiologistGeorg Meissner. It functions as a pathway for the innervation in the mucosa layer of the gastrointestinal wall.
The ENS is capable of autonomous functions[16] like the coordination ofreflexes; although it receives considerable innervation from the autonomic nervous system, it can and does operate independently of the brain and the spinal cord.[17] Its study is the focus ofneurogastroenterology.
The enteric nervous system is a complex part of the nervous system. The enteric nervous system can operate autonomously. It normally communicates with thecentral nervous system (CNS) through theparasympathetic (e.g., via thevagus nerve) andsympathetic (e.g., via theprevertebral ganglia) nervous systems. However,vertebrate studies show that when thevagus nerve is severed, the enteric nervous system continues to function.[7]
In vertebrates, the enteric nervous system includesefferent neurons,afferent neurons, andinterneurons, all of which make the enteric nervous system capable of carrying reflexes and acting as anintegrating center in the absence of CNS input. The sensory neurons report on mechanical and chemical conditions. Through intestinal muscles, the motor neurons controlperistalsis and churning of intestinal contents. Other neurons control the secretion ofenzymes. The enteric nervous system also makes use of more than 30 neurotransmitters, most of which are identical to the ones found in CNS, such asacetylcholine,dopamine, andserotonin. More than 90% of the body's serotonin lies in the gut, as well as about 50% of the body's dopamine, which is currently being studied to further our understanding of its utility in the brain.[18][19][20]
The enteric nervous system has the capacity to alter its response depending on such factors as bulk and nutrient composition.[21] In addition, the ENS contains support cells which are similar toastroglia of the brain and a diffusion barrier around the capillaries surrounding ganglia which is similar to theblood–brain barrier ofcerebral blood vessels.[22]
Peristalsis is a series of radially symmetrical contractions and relaxations of muscles which propagate down a muscular tube. In humans and other mammals, peristalsis is found in the smooth muscles of the digestive tract to propel contents through the digestive system. The word is derived from New Latin and comes from the Greek peristallein, "to wrap around," from peri-, "around" + stellein, "draw in, bring together; set in order". Peristalsis was discovered in 1899 by the work of physiologistsWilliam Bayliss andErnest Starling. Working on thesmall intestines of dogs, they found that the response of increasing the pressure in the intestine caused the contraction of the muscle wall above the point of stimulation and the relaxation of the muscle wall below the point of stimulation.[23][6]
Segmentation contractions are the contractions in intestines carried out by the smooth muscle walls. Unlike peristalsis, which involves the contraction and relaxation of muscles in one direction, segmentation occurs simultaneously in both directions as the circular muscles alternatively contract. This allows for thorough mixing of intestinal contents, known aschyme, to allow greater absorption.
The secretion ofgastrointestinal hormones, such asgastrin andsecretin, is regulated through cholinergic neurons residing in the walls of the digestive tract. Hormone secretion is controlled by thevagovagal reflex, where the neurons in the digestive tract communicate through bothafferent andefferent pathways with thevagus nerve.[24]
Neurogastroenterology encompasses the study of the brain, the gut, and their interactions with relevance to the understanding and management of gastrointestinalmotility and functional gastrointestinal disorders. Specifically, neurogastroenterology focuses on the functions, malfunctions, and the malformations of thesympathetic,parasympathetic, and enteric divisions of the digestive tract.[25] The term also describes a medical sub-specialism of gastroenterology dedicated to the treatment of motility and functional gastrointestinal disorders.
Functional gastrointestinal (GI) disorders are a class of gastrointestinal disorders where there is a malfunction in the normal activities of the gastrointestinal tract, but there are no structural abnormalities that can explain the cause. There are rarely any tests that can detect the presence of these disorders. Clinical research in neurogastroenterology focuses mainly on the study of common functional gastrointestinal disorders such asirritable bowel syndrome, the most common functional GI disorder.[26]
Motility disorders are the second classification of gastrointestinal disorder studied by neurogastroenterologists. Motility disorders are divided by what they affect, with four regions: The esophagus, the stomach, the small intestines, and the large intestines. Clinical research in neurogastroenterology focuses mainly on the study of common motility disorders such asgastroesophageal reflux disease, the damage of the mucosa of the esophagus caused by rising stomach acid through the lower esophageal sphincter.[27]
ENS function can be damaged byischemia.[28] Transplantation, previously described as a theoretical possibility,[29] has been a clinical reality in the United States since 2011 and is regularly performed at some hospitals.[citation needed]
