Enteric nervous system

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Enteric nervous system
GI Organization.svg
The enteric nervous system is embedded in the lining of the gastrointestinal system.
Identifiers
Acronym(s)ENS
MeSH D017615
FMA 66070
Anatomical terminology

The 'enteric nervous system' (ENS) or intrinsic nervous system is one of the main divisions of the peripheral nervous system (PNS) and consists of a mesh-like system of neurons that governs the function of the gastrointestinal tract. [1] It is capable of acting independently of the sympathetic and parasympathetic nervous systems, although it may be influenced by them. The ENS is nicknamed the "second brain". [2] [3] It is derived from neural crest cells. [4] [5]

Contents

The enteric nervous system is capable of operating independently of the brain and spinal cord, [6] but does rely on innervation from the vagus nerve and prevertebral 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 many neurotransmitters similar to the CNS, including acetylcholine, dopamine, and serotonin. The large presence of serotonin and dopamine in the gut are key areas of research for neurogastroenterologists. [8] [9] [10]

Structure

Layers of the Alimentary Canal. The wall of the alimentary canal has four basic tissue layers: the mucosa, submucosa, muscularis, and serosa. 2402 Layers of the Gastrointestinal Tract.jpg
Layers of the Alimentary Canal. The wall of the alimentary canal has four basic tissue layers: the mucosa, submucosa, muscularis, and serosa.

The enteric nervous system in humans consists of some 500 million neurons [11] (including the various types of Dogiel cells), [1] [12] 0.5% of the number of neurons in the brain, five times as many as the one hundred million neurons in the human spinal cord, [13] and about 2⁄3 as many as in the whole nervous system of a cat. The enteric nervous system is embedded in the lining of the gastrointestinal system, beginning in the esophagus and extending down to the anus. [13]

The neurons of the ENS are collected into two types of ganglia: myenteric (Auerbach's) and submucosal (Meissner's) plexuses. [14] Myenteric plexuses are located between the inner and outer layers of the muscularis externa, while submucosal plexuses are located in the submucosa.

Auerbach's plexus

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 neuropathologist Leopold 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 the central nervous system. The plexus includes sensory receptors, such as chemoreceptors and mechanoreceptors, 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 the medulla oblongata through both the anterior and posterior vagal nerves.

Submucosal plexus

The submucosal plexus (also known as Meissner's plexus) is found in the submucosal layer of the gastrointestinal tract. [15] It was discovered and named by German physiologist Georg Meissner. It functions as a pathway for the innervation in the mucosa layer of the gastrointestinal wall.

Function

The ENS is capable of autonomous functions [16] like the coordination of reflexes; 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 of neurogastroenterology.

Complexity

The enteric nervous system has been described as a "second brain" for several reasons. The enteric nervous system can operate autonomously. It normally communicates with the central nervous system (CNS) through the parasympathetic (e.g., via the vagus nerve) and sympathetic (e.g., via the prevertebral ganglia) nervous systems. However, vertebrate studies show that when the vagus nerve is severed, the enteric nervous system continues to function. [7]

In vertebrates, the enteric nervous system includes efferent neurons, afferent neurons, and interneurons, all of which make the enteric nervous system capable of carrying reflexes and acting as an integrating center in the absence of CNS input. The sensory neurons report on mechanical and chemical conditions. Through intestinal muscles, the motor neurons control peristalsis and churning of intestinal contents. Other neurons control the secretion of enzymes. The enteric nervous system also makes use of more than 30  neurotransmitters, most of which are identical to the ones found in CNS, such as acetylcholine, dopamine, and serotonin. 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 to astroglia of the brain and a diffusion barrier around the capillaries surrounding ganglia which is similar to the blood–brain barrier of cerebral blood vessels. [22]

Peristalsis

A simplified image showing peristalsis Peristalsis.gif
A simplified image showing peristalsis

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" + stallein, "to place". Peristalsis was discovered in 1899 by the work of physiologists William Bayliss and Ernest Starling. Working on the small 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

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 as chyme, to allow greater absorption.

Secretion

The secretion of gastrointestinal hormones, such as gastrin and secretin, is regulated through cholinergic neurons residing in the walls of the digestive tract. Hormone secretion is controlled by the vagovagal reflex, where the neurons in the digestive tract communicate through both afferent and efferent pathways with the vagus nerve. [24]

Clinical significance

Neurogastroenterology encompasses the study of the brain, the gut, and their interactions with relevance to the understanding and management of gastrointestinal motility and functional gastrointestinal disorders. Specifically, neurogastroenterology focuses on the functions, malfunctions, and the malformations of the sympathetic, 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 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 as irritable bowel syndrome, the most common functional GI disorder. [26]

Motility disorders

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 as gastroesophageal reflux disease, the damage of the mucosa of the esophagus caused by rising stomach acid through the lower esophageal sphincter. [27]

Gut ischaemia

ENS function can be damaged by ischemia. [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 ]

Additional images

Neurogastroenterology societies

See also

Related Research Articles

<span class="mw-page-title-main">Peripheral nervous system</span> Part of the nervous system excluding the brain and spinal cord

The peripheral nervous system (PNS) is one of two components that make up the nervous system of bilateral animals, with the other part being the central nervous system (CNS). The PNS consists of nerves and ganglia, which lie outside the brain and the spinal cord. The main function of the PNS is to connect the CNS to the limbs and organs, essentially serving as a relay between the brain and spinal cord and the rest of the body. Unlike the CNS, the PNS is not protected by the vertebral column and skull, or by the blood–brain barrier, which leaves it exposed to toxins.

<span class="mw-page-title-main">Vagus nerve</span> Cranial nerve X, for visceral innervation

The vagus nerve, also known as the tenth cranial nerve, cranial nerve X, or simply CN X, is a cranial nerve that carries sensory fibers that create a pathway that interfaces with the parasympathetic control of the heart, lungs, and digestive tract. It comprises two nerves—the left and right vagus nerves—but they are typically referred to collectively as a single subsystem. The vagus is the longest nerve of the autonomic nervous system in the human body and comprises both sensory and motor fibers. The sensory fibers originate from neurons of the nodose ganglion, whereas the motor fibers come from neurons of the dorsal motor nucleus of the vagus and the nucleus ambiguus. The vagus was also historically called the pneumogastric nerve.

<span class="mw-page-title-main">Stomach</span> Digestive organ

The stomach is a muscular, hollow organ in the gastrointestinal tract of humans and many other animals, including several invertebrates. The stomach has a dilated structure and functions as a vital organ in the digestive system. The stomach is involved in the gastric phase of digestion, following chewing. It performs a chemical breakdown by means of enzymes and hydrochloric acid.

<span class="mw-page-title-main">Gastrointestinal tract</span> Organ system within humans and other animals

The gastrointestinal tract is the tract or passageway of the digestive system that leads from the mouth to the anus. The GI tract contains all the major organs of the digestive system, in humans and other animals, including the esophagus, stomach, and intestines. Food taken in through the mouth is digested to extract nutrients and absorb energy, and the waste expelled at the anus as faeces. Gastrointestinal is an adjective meaning of or pertaining to the stomach and intestines.

<span class="mw-page-title-main">Peristalsis</span> Radially symmetrical contraction and relaxation of muscles

Peristalsis is a type of intestinal motility, characterized by radially symmetrical contraction and relaxation of muscles that propagate in a wave down a tube, in an anterograde direction. Peristalsis is progression of coordinated contraction of involuntary circular muscles, which is preceded by a simultaneous contraction of the longitudinal muscle and relaxation of the circular muscle in the lining of the gut.

<span class="mw-page-title-main">Autonomic nervous system</span> Division of the nervous system supplying internal organs, smooth muscle and glands

The autonomic nervous system (ANS), formerly referred to as the vegetative nervous system, is a division of the nervous system that operates internal organs, smooth muscle and glands. The autonomic nervous system is a control system that acts largely unconsciously and regulates bodily functions, such as the heart rate, its force of contraction, digestion, respiratory rate, pupillary response, urination, and sexual arousal. This system is the primary mechanism in control of the fight-or-flight response.

<span class="mw-page-title-main">Esophagus</span> Vertebrate organ through which food passes to the stomach

The esophagus or oesophagus, colloquially known also as the food pipe, food tube, or gullet, is an organ in vertebrates through which food passes, aided by peristaltic contractions, from the pharynx to the stomach. The esophagus is a fibromuscular tube, about 25 cm (10 in) long in adults, that travels behind the trachea and heart, passes through the diaphragm, and empties into the uppermost region of the stomach. During swallowing, the epiglottis tilts backwards to prevent food from going down the larynx and lungs. The word oesophagus is from Ancient Greek οἰσοφάγος (oisophágos), from οἴσω (oísō), future form of φέρω + ἔφαγον.

<span class="mw-page-title-main">Parasympathetic nervous system</span> Division of the autonomic nervous system

The parasympathetic nervous system (PSNS) is one of the three divisions of the autonomic nervous system, the others being the sympathetic nervous system and the enteric nervous system. The enteric nervous system is sometimes considered part of the autonomic nervous system, and sometimes considered an independent system.

<span class="mw-page-title-main">Solitary nucleus</span> Sensory nuclei in medulla oblongata

The solitary nucleus is a series of sensory nuclei forming a vertical column of grey matter in the medulla oblongata of the brainstem. It receives general visceral and/or special visceral inputs from the facial nerve, glossopharyngeal nerve and vagus nerve ; it receives and relays stimuli related to taste and visceral sensation. It sends outputs to various parts of the brain. Neuron cell bodies of the SN are roughly somatotopically arranged along its length according to function.

<span class="mw-page-title-main">Myenteric plexus</span> Part of the enteric nervous system

The myenteric plexus provides motor innervation to both layers of the muscular layer of the gut, having both parasympathetic and sympathetic input, whereas the submucous plexus provides secretomotor innervation to the mucosa nearest the lumen of the gut.

<span class="mw-page-title-main">Enterochromaffin cell</span> Cell type

Enterochromaffin (EC) cells are a type of enteroendocrine cell, and neuroendocrine cell. They reside alongside the epithelium lining the lumen of the digestive tract and play a crucial role in gastrointestinal regulation, particularly intestinal motility and secretion. They were discovered by Nikolai Kulchitsky.

<span class="mw-page-title-main">Nerve plexus</span> Network of nerve fibres

A nerve plexus is a plexus of intersecting nerves. A nerve plexus is composed of afferent and efferent fibers that arise from the merging of the anterior rami of spinal nerves and blood vessels. There are five spinal nerve plexuses, except in the thoracic region, as well as other forms of autonomic plexuses, many of which are a part of the enteric nervous system. The nerves that arise from the plexuses have both sensory and motor functions. These functions include muscle contraction, the maintenance of body coordination and control, and the reaction to sensations such as heat, cold, pain, and pressure. There are several plexuses in the body, including:

<span class="mw-page-title-main">Submucosal plexus</span>

The submucosal plexus lies in the submucosa of the intestinal wall. The nerves of this plexus are derived from the myenteric plexus which itself is derived from the plexuses of parasympathetic nerves around the superior mesenteric artery. Branches from the myenteric plexus perforate the circular muscle fibers to form the submucosal plexus. Ganglia from the plexus extend into the muscularis mucosae and also extend into the mucous membrane.

Gastrointestinal physiology is the branch of human physiology that addresses the physical function of the gastrointestinal (GI) tract. The function of the GI tract is to process ingested food by mechanical and chemical means, extract nutrients and excrete waste products. The GI tract is composed of the alimentary canal, that runs from the mouth to the anus, as well as the associated glands, chemicals, hormones, and enzymes that assist in digestion. The major processes that occur in the GI tract are: motility, secretion, regulation, digestion and circulation. The proper function and coordination of these processes are vital for maintaining good health by providing for the effective digestion and uptake of nutrients.

A slow-wave potential is a rhythmic electrophysiological event in the gastrointestinal tract. The normal conduction of slow waves is one of the key regulators of gastrointestinal motility. Slow waves are generated and propagated by a class of pacemaker cells called the interstitial cells of Cajal, which also act as intermediates between nerves and smooth muscle cells. Slow waves generated in interstitial cells of Cajal spread to the surrounding smooth muscle cells and control motility.

The basal or basic electrical rhythm (BER) or electrical control activity (ECA) is the spontaneous depolarization and repolarization of pacemaker cells known as interstitial cells of Cajal (ICCs) in the smooth muscle of the stomach, small intestine, and large intestine. This electrical rhythm is spread through gap junctions in the smooth muscle of the GI tract. These pacemaker cells, also called the ICCs, control the frequency of contractions in the gastrointestinal tract. The cells can be located in either the circular or longitudinal layer of the smooth muscle in the GI tract; circular for the small and large intestine, longitudinal for the stomach. The frequency of contraction differs at each location in the GI tract beginning with 3 per minute in the stomach, then 12 per minute in the duodenum, 9 per minute in the ileum, and a normally low one contraction per 30 minutes in the large intestines that increases 3 to 4 times a day due to a phenomenon called mass movement. The basal electrical rhythm controls the frequency of contraction but additional neuronal and hormonal controls regulate the strength of each contraction.

The nervous system, and endocrine system collaborate in the digestive system to control gastric secretions, and motility associated with the movement of food throughout the gastrointestinal tract, including peristalsis, and segmentation contractions.

<span class="mw-page-title-main">Gut–brain axis</span> Biochemical signaling between the gastrointestinal tract and the central nervous system

The gut–brain axis is the two-way biochemical signaling that takes place between the gastrointestinal tract and the central nervous system (CNS). The "microbiota–gut–brain axis" includes the role of gut microbiota in the biochemical signaling events that take place between the GI tract and the CNS. Broadly defined, the gut–brain axis includes the central nervous system, neuroendocrine system, neuroimmune systems, the hypothalamic–pituitary–adrenal axis, sympathetic and parasympathetic arms of the autonomic nervous system, the enteric nervous system, vagus nerve, and the gut microbiota.

<span class="mw-page-title-main">Gastrointestinal wall</span> Digestive system structure

The gastrointestinal wall of the gastrointestinal tract is made up of four layers of specialised tissue. From the inner cavity of the gut outwards, these are:

  1. Mucosa
  2. Submucosa
  3. Muscular layer
  4. Serosa or adventitia
<span class="mw-page-title-main">Human digestive system</span> Digestive system in humans

The human digestive system consists of the gastrointestinal tract plus the accessory organs of digestion. Digestion involves the breakdown of food into smaller and smaller components, until they can be absorbed and assimilated into the body. The process of digestion has three stages: the cephalic phase, the gastric phase, and the intestinal phase.

References

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  12. ↑ p. 921
  13. 1 2 Hall JE (2011). "General Principles of Gastrointestinal Function". Guyton and Hal Textbook of Medical Physiology (12th ed.). Saunders Elsevier. p. 755. ISBN   978-1416045748.
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  19. ↑ Martinucci I, Blandizzi C, de Bortoli N, Bellini M, Antonioli L, Tuccori M, et al. (2015). "Genetics and pharmacogenetics of aminergic transmitter pathways in functional gastrointestinal disorders". Pharmacogenomics. 16 (5): 523–539. doi:10.2217/pgs.15.12. hdl: 11577/3166305 . PMID   25916523.
  20. ↑ Smitka K, Papezova H, Vondra K, Hill M, Hainer V, Nedvidkova J (2013). "The role of "mixed" orexigenic and anorexigenic signals and autoantibodies reacting with appetite-regulating neuropeptides and peptides of the adipose tissue-gut-brain axis: relevance to food intake and nutritional status in patients with anorexia nervosa and bulimia nervosa". International Journal of Endocrinology. 2013: 483145. doi: 10.1155/2013/483145 . PMC   3782835 . PMID   24106499.
  21. ↑ Neunlist M, Schemann M (July 2014). "Nutrient-induced changes in the phenotype and function of the enteric nervous system". The Journal of Physiology. Wiley. 592 (14): 2959–2965. doi: 10.1113/jphysiol.2014.272948 . PMC   4214652 . PMID   24907307. S2CID   37969390.
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  25. ↑ Wood JD, Alpers DH, Andrews PL (September 1999). "Fundamentals of neurogastroenterology". Gut. 45 (Suppl 2): II6–II16. doi:10.1136/gut.45.2008.ii6. PMC   1766686 . PMID   10457039.
  26. ↑ Kumar A, Rinwa P, Sharma N (2012). "Irritable Bowel Syndrome: A Review". J Phys Pharm Adv. 2 (2): 97–108.
  27. ↑ DeVault KR, Castell DO (June 1999). "Updated guidelines for the diagnosis and treatment of gastroesophageal reflux disease. The Practice Parameters Committee of the American College of Gastroenterology". The American Journal of Gastroenterology. 94 (6): 1434–1442. doi: 10.1111/j.1572-0241.1999.1123_a.x . PMID   10364004. S2CID   32509777.
  28. ↑ Linhares GK, Martins JL, Fontanezzi F, Patrício F, Montero EF (2007). "Do lesions of the enteric nervous system occur following intestinal ischemia/reperfusion?". Acta Cirurgica Brasileira. 22 (2): 120–124. doi: 10.1590/S0102-86502007000200008 . PMID   17375218.
  29. ↑ Gershon MD (April 2007). "Transplanting the enteric nervous system: a step closer to treatment for aganglionosis". Gut. 56 (4): 459–461. doi:10.1136/gut.2006.107748. PMC   1856867 . PMID   17369379.
  30. ↑ ANMS - American Neurogastroenterology and Motility Society
  31. ↑ ESNM - European Society for Neurogastroenterology & Motility

Further references

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