ENTERIC NERVOUS SYSTEM: CHARACTERISTICS, PARTS, FUNCTIONS - BIOLOGY - 2025

The enteric nervous system is an intrinsic network of neurons of the autonomic system that is distributed in the walls of the intestine and that has semi-independent functions. The number of neurons in the wall of the intestine (100 million) is almost as numerous as those in the spinal cord.

The enteric nervous system is often considered the third division of the autonomic system and is therefore called the "enteric division of the autonomic system." Other authors consider it as a displacement of the central nervous system (CNS) for the regulation of the gastrointestinal system.

This enteric division works relatively independently, but connects to the central nervous system through the sympathetic and parasympathetic systems. Its function is to control intestinal motility, secretion and absorption of nutrients.

It includes sensory neurons that detect chemical changes, pain, and distention of the digestive tract; motor neurons that coordinate the activity of intestinal smooth muscle and interneurons that integrate intrinsic activity and that receive signals from the sympathetic and parasympathetic divisions.

Although the function of the enteric nervous system is autonomous, it is regulated and controlled by the extrinsic innervation of the digestive system, constituted by the sympathetic and parasympathetic division of the autonomic nervous system.

The effects of these extrinsic innervation systems on the function of the digestive system are antagonistic, that is, they are opposite.

The enteric nervous system is organized into two different but interconnected neuronal plexuses: the myenteric or Auerbach plexus and the submucosal or Meissner plexus.

The Auerbach plexus lies between and innervates the inner longitudinal and inner circular smooth muscle layers of the gastrointestinal tract. This system is responsible for coordinating the peristaltic movements of the intestine and is linked to the Meissner submucosal plexus.

Meissner's plexus is located along the digestive tract in the submucosal layer of the wall. It innervates the glandular epithelium, the intestinal endocrine cells, and the blood vessels of the submucosa. Among its functions is to regulate the transport of ions and water through the wall of the intestine.

The main neurotransmitters in this enteric system are acetylcholine, norepinephrine, serotonin, GABA, ATP, nitric oxide, carbon monoxide and many peptides and polypeptides such as VIP (vasoactive peptide) and YY peptide, among others..

Diseases such as achalasia, paralytic or adynamic ileus, megacolon and chronic diarrhea are some examples of diseases caused by alterations of the enteric nervous system.

characteristics

The digestive system has a double innervation, an intrinsic and an extrinsic. The enteric nervous system is the intrinsic innervation system of the digestive system, while extrinsic innervation is represented by the autonomic system with its sympathetic and parasympathetic division.

The enteric nervous system acts quite independently, but is regulated by the autonomic nervous system, which is the extrinsic innervation system of the digestive tract.

An example of this double innervation is the innervation of the blood vessels that nourish the digestive system. These are innervated by the intrinsic or enteric nervous system and by the extrinsic system through sympathetic division.

It is not yet known whether there is any cholinergic parasympathetic innervation (by acetylcholine) of the enteric vascular system.

The enteric nervous system innervates these blood vessels and through the neurotransmitters nitric oxide (NO) and vasoactive peptide (VIP) causes hyperemia or increased blood flow due to vasodilation, which accompanies digestion.

On the other hand, these enteric vessels are innervated by the sympathetic nervous system through sympathetic postganglionic fibers that release noradrenaline (noradrenergic). When this system is stimulated, vasoconstriction occurs and blood flow in the area decreases.

The sympathetic and parasympathetic effects on the function of the digestive system are antagonistic. Sympathetic stimulation decreases motility, secretion, absorption, and digestive blood flow.

Parasympathetic increases motility, absorption, and secretion. Sympathetic stimulation increases the tone of the sphincters of the gastrointestinal system, while parasympathetic stimulation decreases it.

Parts

The enteric nervous system is organized into two large extended groups of interconnected neurons and nerve fibers called plexuses.

These plexuses are distributed among the different layers that make up the wall of the digestive tract and are known as the Auerbach and Meissner plexus.

Description of the digestive tract

Histological diagram of the digestive tract (Source: Posible2006 via Wikimedia Commons)

The layers of the wall of the digestive tract are similar throughout the entire tube, but show particular characteristics in each segment.

These are four concentric layers that, from the inside out, are: the mucosa, the submucosa, the external muscularis and the serosa or adventitia. All four are found throughout the digestive tract.

- The mucosa is composed of an epithelium, a lamina propria and the muscularis mucosae with two smooth muscular layers. It also contains glands, lymphatic vessels, and lymphoid nodes.

- The submucosa is a layer of loose tissue that only has glands in the esophagus and duodenum.

- The external muscle layer is composed of two layers of smooth muscle, one arranged longitudinally on the outside and the other arranged circularly on the inside.

- The serosa or adventitia is a thin layer of connective tissue and is the outermost layer of the tube wall.

Distribution of enteric plexuses

In the external muscular layer of the digestive tract, between the circular and longitudinal layers, is the Auerbach plexus, also called the Myenteric plexus. This plexus innervates both layers of smooth muscle and is responsible for peristalsis.

The fibers of the sympathetic and parasympathetic neurons are also distributed around the Auerbach plexus.

In the submucosal layer, the Meissner plexus or submucosal plexus of the enteric nervous system is distributed throughout the digestive tract. Fibers of the parasympathetic nervous system also exist in this area.

The Meissner submucosal plexus innervates the glandular epithelium, the intestinal endocrine cells, and the blood vessels of the submucosa. This plexus regulates secretory function, mucosal movements, and local blood flow.

Distributed in the wall of the digestive tract are many sensory fibers that carry information directly about the luminal content and the local secretory and muscular state to the nearby and distant plexuses.

This sensory information is also transmitted to the central nervous system through the autonomic system.

Anatomical organization of the innervation of the digestive system

The general organization of the enteric nervous system and the autonomic system that innervates the digestive tract is complex and interconnected.

In general, most of the parasympathetic fibers connect with the ganglion cells of the enteric plexuses and not directly on the smooth muscle cells or the glands.

Parasympathetic fibers reach the digestive tract through the vagus and pelvic nerves, and parasympathetic stimulation increases the motility and secretory activity of the intestine.

The celiac, superior and inferior mesenteric plexuses, and the hypogastric plexus provide the sympathetic innervation of the intestine. Most of these fibers end in the Auerbach and Meissner plexuses.

Sympathetic activation decreases motor activity, reduces secretions, and produces local vasoconstriction. Some fibers end directly in the outer muscle layers, in the muscularis mucosae, and in some sphincters.

Graphic summary of the enteric nervous system (Source: Mewtow via Wikimedia Commons; modified by Raquel Parada)

In the outer muscle layers, the sympathetic decreases motor activity by acting through the Myenteric plexus, which is in contact with the outer muscle cells. In the muscularis mucosae and in the sphincters, sympathetic activity causes their contraction.

The contraction of the muscularis mucosae generates the folds and crypts of the mucosa.

There are afferent fibers that are part of local and central reflexes. For central reflexes, the afferent fibers are those that are directed and connected with neurons located in the central nervous system.

These afferent fibers send information detected by chemoreceptors, mechanoreceptors, and other sensory receptors.

The local reflexes are established by direct connections of the sensory fibers with the nerve cells of the Myenteric and submucosal plexuses, which send a response that can be directed to the activity of the outer muscular layer, the glands, the endocrine cells, the blood vessels or the muscularis mucosae.

Features

The two plexuses of the enteric nervous system serve different functions. The Auerbach plexus is associated with peristalsis, with contractions aimed at mixing the chyme, and with the tone of the smooth muscle.

The Meissner plexus is associated with local secretory functions, with some hormonal secretions, and with local regulation of blood flow.

Peristalsis and external musculature activity

Peristalsis can be defined as a reflex response that is initiated by the distention that occurs in the wall of the digestive tract when the food bolus enters. This reaction occurs throughout the entire digestive tract, from the esophagus to the rectum.

Initially, the distention or elongation of the tube generates a circular contraction of the anterior zone, that is, the one that is located behind the stimulus (food bolus or luminal content) and a frontal relaxation zone or in front of the stimulus.

The stretching that occurs in the wall of the digestive tract when the food bolus enters activates the sensory neurons which, in turn, activate the neurons of the Myenteric plexus. Cholinergic neurons in the area are distributed in opposite directions.

Some neurons emit cholinergic fibers in an antegrade direction and others do so in a retrograde direction. That is, some are directed caudally (towards the rectum) and others orally (towards the mouth).

Those that are directed upwards generate contraction of the smooth muscle and those that are directed downwards generate relaxation of the smooth muscle.

This zone of contraction and relaxation around the food bolus generates a contraction wave that propels the luminal content and directs it caudally into the tube.

Basal electrical activity

In addition to this peristaltic activity, the digestive tract exhibits a basic electrical activity that allows regulating the motility of the system. This electrical activity originates in specialized cells called stellate cells of Cajal or pacemaker cells.

The stellate cells of Cajal are found in the inner circular muscle layer of smooth muscle, near the Myenteric plexus. The esophagus and the upper part of the stomach do not have these types of cells.

Rhythmic electrical activity is initiated in Cajal cells that trigger a spontaneous depolarization of the membrane potential, called basic electrical rhythm (REB), which generally does not produce muscle jerks alone, but waves of depolarization.

The function of the REB is to coordinate and regulate peristalsis and other motor activities of the system, they also regulate the tone of the smooth muscle of the walls of the digestive tract.

Neurotransmitters

The neurotransmitters of the digestive system are many. In the first instance there are the neurotransmitters of the sympathetic and parasympathetic postganglionic fibers such as norepinephrine and acetylcholine, respectively.

Noradrenaline

For the enteric nervous system there is a long list of neurotransmitters and neuromodulators with a great variety of receptors that determine the function of the local activation of said system.

Acetylcholine molecular structure

Among these, the most important are acetylcholine, norepinephrine, serotonin, dopamine, glycine, GABA (γ-aminobutyric acid), NO, CO, ATP (adenosine triphosphate), CCK (cholecystokinin), VIP and YY peptide, etc.

Many of the descriptions of each of the intracellular pathways, connections and mechanisms are under study and have not yet been fully elucidated.

Diseases

There are multiple pathologies related to alterations of the enteric nervous system, examples of which are:

Achalasia

It is a disease that affects the motility of the esophagus and that prevents efficient emptying of the esophagus, as a consequence food accumulates and the esophagus dilates. It is due to an increase in the tone of the lower esophageal sphincter, so it does not relax completely after swallowing.

In this pathology there is an alteration of the Myenteric plexus in the lower esophageal sphincter with an alteration in the release of VIP and NO.

Gastroesophageal reflux

It is an esophageal dysfunction that occurs when the lower esophageal sphincter becomes incompetent, that is, it does not close well and this causes gastroesophageal reflux.

In other words, part of the stomach contents back up into the esophagus, causing irritation of the mucosa, heartburn and esophageal ulcers.

Paralytic ileus

Another dysfunction of the motility of the intestine is the so-called "paralytic or adynamic ileus". In this pathology, due to direct trauma to the intestines or abdominal surgical interventions, there is a diffuse inhibition of peristalsis, especially in the small intestine.

The reduction of peristalsis in the area prevents intestinal emptying in the colon, so the small intestine becomes distended, filled with fluid and gases. The peristaltic activity of the small intestine returns in about 6 to 8 hours and that of the colon after about 2 to 3 days.

Aganglionic megacolon and chronic diarrhea

The congenital absence of ganglion cells from the Myenteric and submucosal plexuses in the distal portions of the colon generates what is called “aganglionic megacolon” ​​or Hirschsprung's disease. It is accompanied by severe constipation and abdominal and colon distention.

Chronic diarrhea lasting more than two weeks is associated with irritable bowel syndrome, a disease that affects the function of the colon.

It can occur due to increased muscular contractions of the colon wall due to alterations in the functional coordination between the central nervous system and the enteric nervous system.

References

1. Berne, R., & Levy, M. (1990). Physiology. Mosby; International Ed edition.
2. Dudek, RW (1950). High-Yield Histology (2nd ed.). Philadelphia, Pennsylvania: Lippincott Williams & Wilkins.
3. Guyton, A., & Hall, J. (2006). Textbook of Medical Physiology (11th ed.). Elsevier Inc.
4. Johnson, K. (1991). Histology and Cell Biology (2nd ed.). Baltimore, Maryland: The National medical series for independent study.
5. Kuehnel, W. (2003). Color Atlas of Cytology, Histology, and Microscopic Anatomy (4th ed.). New York: Thieme.
6. Ross, M., & Pawlina, W. (2006). Histology. A Text and Atlas with correlated cell and molecular biology (5th ed.). Lippincott Williams & Wilkins.
7. William, FG, & Ganong, MD (2005). Review of medical physiology. Printed in United States of America, Seventeenth Edition, Pp-781.