The Enteral Nervous System (ENS) is a quasi-autonomous part of the nervous system. It includes a number of nerve chains that control motor function, local blood flow, transport and secretion of the mucous membrane and modulate immune and endocrine functions.
Structure
The human enteric nervous system consists of about 500 million neurons (including various types of Dogel cells). It is embedded in the mucous membrane of the gastrointestinal tract (GIT), starting from the esophagus and ending with the anus.
The neurons of the enteral system are assembled into two types of ganglia: mental and submucosal plexuses. The former are located between the inner and outer layers of the muscles, and the latter are located in the submucosa.
Also include the enteric nervous system:
- primary afferent neurons;
- exciting motor muscles of motor neurons;
- long muscles of motor neurons;
- ascending and descending internal neurons.
Organization and Relationships
The physiology of the enteric nervous system originates from neural crest cells that colonize the intestine during fetal life. It becomes functional in the last third of pregnancy and continues to develop after birth.
The ENS receives data from the parasympathetic and sympathetic parts of the nervous system, and the gastrointestinal tract has abundant reserves of afferent nerve fibers due to the vagus nerves and spinal afferent pathways. Thus, between the enteric nervous system, the sympathetic prevertebral ganglia and the central nervous system, there is a rich interaction in both directions.
Types of intestinal neurons
About 20 types of intestinal neurons can be identified by their functions. Among them, three groups stand out:
- Own primary afferents. The physical state of organs (for example, tension in the intestinal wall) and the chemical characteristics of the contents of the lumen are determined.
- Motorized. They include muscle, secretory motor and vasodilator neurons.
- Interneurons. Connect with the above.
Motor control
The gastrointestinal tract has an external muscle membrane. Its purpose is to mix food so that it is exposed to digestive enzymes and an absorbent membrane and promote the contents of the digestive tube. The contours of the intestinal reflex regulate movement, controlling the activity of both excitatory and inhibitory neurons that innervate the muscle. They have co-transmitters for exciting neurons, acetylcholine and tachykinins. The enteric nervous system organizes the mixing and movement of food. In this case, the digestion and assimilation of nutrients occurs.
Internal ENS reflexes are important for generating patterns of motility of the small and large intestines. The main movements of the muscles in the small intestine:
- mixing activities;
- motor reflexes;
- migratory myoelectric complex;
- peristaltic gusts;
- retropulsion associated with vomiting.
The enteric nervous system is programmed to produce these various results.
Regulation of fluid metabolism and local blood flow
The ENS regulates the movement of water and electrolytes between the intestinal lumen and tissue fluid. This is done by directing the activity of secretory motor neurons that innervate the mucous membrane in the small and large intestines and control its permeability to ions.
Local blood flow into the mucous membrane is regulated by the use of enteric vasodilating neurons. Mucous blood flow is suitable for balancing the nutritional needs of the mucous membrane and for adjusting fluid exchange between the vascular system, interstitial fluid and the intestinal lumen. Total blood flow in the intestines is coordinated by the central nervous system through sympathetic vasoconstrictor neurons.
Regulation of gastric and pancreatic secretion
The secretion of gastric acid is regulated by both neurons and hormones of the enteric system. Regulation is carried out through cholinergic neurons with cell bodies in the wall of the stomach. They receive stimulating signals from both intestinal sources and vagus nerves.
Pancreatic secretion by bicarbonate to neutralize the contents of the duodenum is controlled by the secretion hormone in combination with the activity of cholinergic and non-cholinergic intestinal neurons.
Regulation of gastrointestinal endocrine cells
Nerve fibers pass close to the endocrine cells of the gastrointestinal mucosa. Some of them are under nervous control. For example, gastrin cells in the antrum of the stomach are innervated by excitatory neurons that use the releasing peptide as the primary neurotransmitter. Endocrine cells try the lumen and release metabolism molecules into the tissue of the mucous membrane, where nerve endings are detected. This is a necessary relationship, because the nerve endings are separated from the lumen by the epithelium of the mucous membrane.
Defense reactions
Intestinal neurons are involved in a number of bowel defense reactions. They include:
- diarrhea for dissolving and eliminating toxins;
- exaggerated propulsive activity of the colon that occurs in the presence of pathogenic microorganisms in the intestine;
- vomiting.
Liquid secretion is caused by harmful stimuli, in particular, by the intraluminal presence of certain viruses, bacteria and bacterial toxins. It is due to the stimulation of intestinal secretory motor reflexes. The physiological goal is to rid the body of pathogens and their products.
Enteral nervous system and bacteria
The intestines are colonized by trillions of bacteria that regulate the body's production of several signaling molecules, including serotonin, hormones, and neurotransmitters. Maintaining a balanced microbial community is critical to maintaining health and preventing chronic inflammation. The enteric nervous system is the main regulator of physiological processes in the intestine. It deeply affects the composition of the intestinal microbiota.
ENS-CNS interaction
The digestive system is in two-way communication with the central nervous system (central nervous system). Afferent neurons transmit information about his condition. It consists in:
- pain and discomfort from the intestines;
- conscious feelings of hunger and satiety;
- other signals (blood glucose, for example).
Afferent signals regarding nutrient loading in the small intestine or stomach acidity usually do not reach consciousness. The central nervous system provides signals for intestinal control, which are transmitted through the ENS. For example, the appearance and smell of food causes preparatory measures in the gastrointestinal tract, including salivation and secretion of gastric acid. Other central influences pass through sympathetic paths.