Enteric Nervous System: parts and functions

The enteric nervous system is the part of the autonomic nervous system responsible for regulating vital gastrointestinal functions. Among these functions is the regulation of the esophagus, stomach and colorectal functions; which in turn implies the absorption and digestion of nutrients, as well as the maintenance of the protective mucous membranes. The functioning of this system is the most complex of the set of elements that make up the autonomic nervous system.

Next we will see in more detail what the enteric nervous system is and what are some of its main functions and characteristics.

• Related article: "Parts of the Nervous System: anatomical structures and functions"

What is the Enteric Nervous System?

The enteric nervous system is the cellular structure responsible for controlling our gastrointestinal functions. The above includes the mobility, secretion, local immunity and inflammation of the organs that make up the digestive system.

In other words, the enteric nervous system is responsible for regulating important functions for the intake, absorption, metabolism and digestion of food. It is also responsible for preventing diseases related to these activities.

The enteric nervous system originates in the cells of the neural crest (a structure generated during embryonic development), which, in turn, divides into two large branches of nerve cells intertwined. These branches are called "Meissner's submucosal" and "Auerbach's myenteric," and they make up the two main components of the enteric nervous system.

This system is recognized as the most complex part of the peripheral nervous system and it is composed of a high concentration of neurons and glial cells. In fact, it contains the longest collection of neurons found outside of the brain.

• You may be interested: "Autonomic nervous system: structures and functions"

Origins and development of this system

The enteric nervous system is formed from embryonic development, from two main processes: cell proliferation and its differentiation with the great variety of glial cells and neuronal variants that make up the organism.

From the fourth week of gestation, a part of the neural crest cells, which give rise to most of the enteric nervous system, migrate through the entire digestive tract.

The other part of the same cells, which contributes to a lesser extent to the formation of the ENS, migrate from the cranial region to the caudal region (that is, from the head to the opposite end). The latter gradually spread through the gastrointestinal tract of the embryo in all its components:

• Fore bowel, which subsequently forms the esophagus, stomach and duodenum (connecting structure of the stomach with the one in charge of regulating the absorption of substances: the jejunum)
• Middle intestine, which will give rise to the small intestine, including its first proposition called "blind"; the ascending colon, the appendix, and a part of the transverse colon, called the "proximal segment."
• Posterior intestine, which makes up a part of the transverse colon called the “distal portion”, as well as its descending part, the sigmoids (parts of the colon that have an “S” shape) and the rectum.

Components of the SNE

As we saw before, the enteric nervous system is divided into two main segments that arise from the neural crest. Each one of them contains a great variety of glial and nerve cells, and together they are responsible for regulating the intake, absorption and metabolism of everything we eat. These segments, according to Oswaldo, et al. (2012), are the following:

Meissner submucosal plexus

It develops in the small intestine and colon mainly, and is responsible for regulating digestion and absorption in music and blood vessels.

Auerbach's myenteric plexus

It is found throughout the digestive tract, and is responsible for coordinate the activity of the muscular layers of said organ.

4 types of neurons that make it up

The large number of enteric neurons in the healthy adult small intestine remains constant for most of adult life, which seems to be the result of a continuous renewal process of neurons in the intestine (Kulkarni, S. et al, 2017).

The neurons that are part of the enteric nervous system, and therefore are in charge of regulating our gastrointestinal activity, are the following (Oswaldo, et al, 2012):

1. Primary intrinsic afferent neurons

Being afferents, they are neurons that carry nerve impulses from the organs to the central nervous system. However, being primary neurons, they do not conduct sensory information directly, but rather to through other cells located in the enteric epithelium (the cellular tissue that lines the nervous system enteric). Namely, its activity is mainly that of sensory transducers and in this way regulate physiological functions of the digestive tract.

2. Motor neurons

As its name says, it is responsible for activating the muscular layers that make up both the digestive tract and the blood vessels and some glands. They in turn divide into excitatory motor neurons (eg, acetylcholine), or inhibitory motor neurons (such as nitric oxide or GABA). The latter, the inhibitory neurons, are responsible for regulating water secretion, blood flow and the release of electrolytes.

3. Interneurons

These are the nerve cells responsible for connecting primary intrinsic afferent neurons with motor neurons. They can be ascending or descending, depending on whether they act from the head to the opposite extreme, or in the opposite direction.

5. Gut-leaking neurons

Its processes are located outside the digestive tract and connect with the nerve ganglia to form a new ganglion called the “prevertebral”. Its main function is to warn about changes in the activity of the intestine, so It's about mechanoreceptors (secondary neurons that fire action potentials under mechanical stimuli).

Main functions of the SNE and associated pathologies

According to Furness, 2012, the main functions performed by the enteric nervous system as a whole are the following:

• Determine the movement patterns of the gastrointestinal tract.
• Control gastric acid secretion.
• Regulate movement and fluids that cross the epithelium.
• Modify blood flow locally.
• Modify and regulate the absorption of nutrients.
• * Interact with the intestinal endocrine system and also with the immune system.
• Maintain the integrity of the epithelial barrier that divide the cells of the intestine.

Improper operation of this system affects the functions described above. For the most part, the inadequate functioning of the SNE neuropathies that make it difficult to control muscle activity and the movement of mucous fluid are associated. This is reflected in different ailments of the colon and digestive tract.

Furthermore, the malfunctioning of the ENS can be congenital in origin or acquired during postnatal development. Generally, the latter occurs due to a secondary medical condition that ends up significantly damaging the functioning of the SNE, although it can also occur due to an iatrogenic effect of some drug, or due to a neuropathology induced by consumption of drugs