The enteric nervous system (ENS) is a quasi-autonomous part of the nervous system. It includes a number of neural circuits that control motor functions, local blood flow, mucosal transport and secretion, and modulate immune and endocrine functions.
Structure
The human enteric nervous system is made up of about 500 million neurons (including various types of Dogel cells). It is embedded in the lining of the gastrointestinal (GI) tract, from the esophagus to the anus.
The neurons of the enteric system are assembled into two types of ganglia: myenteric and submucosal plexuses. The first are located between the inner and outer layers of the muscles, and the second - in the submucosa.
The enteric nervous system also includes:
- primary afferent neurons;
- excitatory motive muscles of motor neurons;
- long muscles of motor neurons;
- ascending and descending internal neurons.
Organization and relationships
Physiology of the enteric nervous systemoriginates from neural crest cells that colonize the intestines during fetal life. It becomes functional in the last third of pregnancy and continues to develop after birth.
ENS receives input from the parasympathetic and sympathetic nervous systems, and the GI tract has an abundant supply of afferent nerve fibers through vagus nerves and spinal afferent pathways. Thus, there is a rich interaction in both directions between the enteric nervous system, the sympathetic prevertebral ganglia and the central nervous system.
Types of intestinal neurons
Approximately 20 types of intestinal neurons can be identified by their functions. Three groups stand out among them:
- Own primary afferent. They determine the physical state of organs (for example, tension in the intestinal wall) and the chemical characteristics of the contents of the lumen.
- Motor. Includes muscle, secretomotor, and vasodilator neurons.
- Interneurons. Connect with the above.
Motor control
The gastrointestinal tract has an outer muscular layer. Its purpose is to mix food so that it is exposed to the digestive enzymes and absorbent membrane and to move the contents of the digestive tube. The gut reflex circuits regulate movement by controlling the activity of both excitatory and inhibitory neurons that innervate the muscle. They have co-transmitters for excitatory neurons, acetylcholine and tachykinins. Enter althe nervous system organizes the mixing and movement of food. In this case, the digestion and absorption of nutrients occurs.
Internal ENS reflexes are essential for generating small and large intestinal motility patterns. Basic muscle movements in the small intestine:
- mixing activities;
- motor reflexes;
- migrating myoelectric complex;
- perist altic impulses;
- retropulsion associated with vomiting.
The enteric nervous system is programmed to produce these different results.
Regulation of fluid exchange and local blood flow
ENS regulates the movement of water and electrolytes between the intestinal lumen and tissue fluid. This is done by directing the activity of secretomotor neurons that innervate the mucosa in the small and large intestines and control its permeability to ions.
Local mucosal blood flow is regulated by enteric vasodilator neurons. The mucosal circulation is suitable for balancing the nutritional needs of the mucosa and for accommodating fluid exchange between the vasculature, interstitial fluid, and intestinal lumen. General blood flow in the intestine is coordinated by the central nervous system through sympathetic vasoconstrictor neurons.
Regulation of gastric and pancreatic secretions
The secretion of gastric acid is regulated by both neurons andhormones of the enteric system. Regulation is carried out through cholinergic neurons with cell bodies in the wall of the stomach. They receive excitatory signals from both intestinal sources and vagus nerves.
The secretion of bicarbonate from the pancreas to neutralize the contents of the duodenum is controlled by the hormone secretin 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 their primary neurotransmitter. Endocrine cells sample the luminal environment and release metabolic molecules into the mucosal tissue where nerve endings are found. This is a necessary relationship because the nerve endings are separated from the lumen by mucosal epithelium.
Defensive reactions
Intestinal neurons are involved in a number of gut defenses. They include:
- diarrhea to dissolve and remove toxins;
- exaggerated propulsive activity of the colon, which occurs when there are pathogenic microorganisms in the intestine;
- vomit.
Fluid secretion is triggered by noxious stimuli, specifically the intraluminal presence of certain viruses, bacteria, and bacterial toxins. It is conditionedstimulation of intestinal secretomotor reflexes. The physiological goal is to rid the body of pathogens and their products.
Enteric nervous system and bacteria
The gut is 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 he alth and preventing chronic inflammation. The enteric nervous system is the main regulator of physiological processes in the intestine. It profoundly affects the composition of the gut microbiota.
ENS-CNS interactions
The digestive system is in two-way communication with the CNS (central nervous system). Afferent neurons transmit information about its state. It consists of:
- pain and discomfort from the intestines;
- conscious feeling of hunger and satiety;
- other signals (blood glucose, for example).
Afferent signals regarding nutritional load in the small intestine or stomach acidity usually do not reach consciousness. The CNS provides signals to control the intestines, which are transmitted through the ENS. For example, the sight and smell of food triggers preparations in the gastrointestinal tract, including salivation and gastric acid secretion. Other central influences come through sympathetic pathways.
