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 involves the absorption and digestion of nutrients, as well as the maintenance of protective mucous membranes. The functioning of this system is the most complex of all the elements that make up the autonomic nervous system.
Next we will see in more detail what the enteric nervous system is and what some of its main functions and characteristics are.
What is the Enteric Nervous System?
The enteric nervous system is the cellular structure responsible for controlling our gastrointestinal functions. This 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 in charge of 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 (structure generated during embryonic development), which, in turn, divides into two large branches of intertwined nerve cells. These branches are called “Meissner’s submucosus” 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 is composed of a high concentration of neurons and glial cells . In fact, it contains the largest collection of neurons that are found outside the brain.
Origins and development of this system
The enteric nervous system is formed from the embryonic development, from two main processes: the cellular proliferation and its differentiation with the great variety of glial cells and neuronal variants that compose the organism.
From the fourth week of pregnancy, part of the cells of the neural crest, which give rise to most of the enteric nervous system, migrate through the entire digestive tract .
The other part of the same cells, which contribute less to the formation of the NES, migrate from the cranial region to the caudal region (i.e. from the head to the opposite end). The latter gradually spread throughout the gastrointestinal tract of the embryo in all its components:
- Anterior intestine , which later forms the esophagus, stomach and duodenum (connecting structure of the stomach to 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 “cecum”; the ascending colon, the appendix and a part of the transverse colon, called the “proximal segment”.
- Posterior intestine , which forms a part of the transverse colon called the “distal portion”, as well as its descending part, the sigmoids (S-shaped parts of the colon) 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 contains a wide 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 as follows:
Meissner’s submucosal plexus
It develops in the small intestine and colon mainly, and is responsible for regulating digestion and absorption into the music and blood vessels .
Auerbach’s Myenteric Plexus
It is found throughout the digestive tract, and is responsible for coordinating the activity of the muscular layers of that organ .
4 types of neurons that form it
The large number of enteric neurons in the healthy adult small intestine remains constant throughout most of adulthood, which seems to be the result of a process of continuous renewal of the neurons in the intestine (Kulkarni, S. et al, 2017).
The neurons that form part of the enteric nervous system, and which therefore are responsible for regulating our gastrointestinal activity are the following (Oswaldo, et al, 2012):
1. Primary intrinsic afferent neurons
As afferent, they are neurons that transport nerve impulses from the organs to the central nervous system. However, as primary neurons, they do not conduct sensory information directly, but rather through other cells located in the enteric epithelium (the cellular tissue that covers the enteric nervous system). That is, their activity is mainly that of sensory transducers and in this way they regulate physiological functions of the digestive tract.
2. Motor neurons
As its name suggests, it is responsible for activating the muscle layers that make up the digestive tract, as well as the blood vessels and some glands. They are further divided into excitatory motor neurons (e.g. acetylcholine), or inhibitory motor neurons (e.g. nitric oxide or GABA). The latter, inhibitory neurons, are responsible for regulating water secretion, blood flow and electrolyte release.
These are the nerve cells responsible for connecting the primary intrinsic afferent neurons to the motor neurons. They can be ascending or descending , depending on whether they act from the head to the opposite end, or in the opposite direction.
5. Intestinal Neurons
Its extensions 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 is treated as mechanoreceptors (secondary neurons that trigger action potentials in the face of 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 as follows:
- Determine the movement patterns of the gastrointestinal tract.
- Controlling gastric acid secretion .
- Regulate movement and fluids that cross the epithelium.
- Modify blood flow at the local level.
- 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 divides the cells of the intestine.
Improper operation of this system affects the functions described above. Most of the malfunctioning of the NES is related to neuropathies that make it difficult to control muscle activity and mucosal fluid movement . This is reflected in different conditions of the colon and digestive tract.
In addition, inadequate functioning of the NHS may be congenital or acquired during postnatal development. Usually the latter occurs due to a secondary medical condition that ends up significantly damaging the functioning of the NES, although it can also occur due to an iatrogenic effect of some drug, or to a drug-induced neuropathology.
- Kulkarni, S., Micci, M-A., Leser, J., Shin, Ch., Tang, S-Ch., Fu, Y-Y., …, Pasricha, P. (2017). Adult enteric nervous system in health is maintained by a dynamic balance between neuronal apoptosis and neurogenesis. Proceedings of the National Academy of Science of the United States of America, 114(18): E3709-E3718.
- Furness, J. (2012). The enteric nervous system and neurogastroenterology. Nature Reviews Gastroenterology & Hepatology, 9: 286-294.
- Oswaldo, J., Frank-Márquez, N., Cervantes-Bustamante, R., Cadena-León, J., Montijo-Barrios, E., … Ramírez-Mayans, J. (2012). Enteric nervous system and gastrointestinal motility
- Grundy, D. and Schemann, M. (2007). Enteric nervous system. Current Opinion in Gastroenterology, 23(2): 121-126.