What Is the Enteric Nervous System?

The enteric nervous system is composed of a large number of neurons buried in the gastrointestinal wall. These cells vary in size and shape, and form many ganglions of the gastrointestinal tract. They are also called plexus, mainly including myenteric plexus and submucosal plexus. These ganglia are different from the ganglia of the peripheral nervous system. There are no blood vessels and connective tissues between the neurons of the enteric nervous system. Part of the membrane surface of the neuron is in direct contact with the interstitial space and absorbs nutrients. The protrusions of the glial cells in the ganglion wrap the nerve components, forming an external basement membrane, limiting the capillaries outside the basement membrane. And these capillary endothelial cells that provide nutrition to the ganglia are continuous and have no window holes, forming a "blood-gut barrier" similar to the "blood-brain barrier". ^[1]

Enteric nervous system

The enteric nervous system consists of ganglion cells of the submucosal plexus (Meissner's plexus) and intermuscular plexus (Orbach's plexus) of the gastrointestinal tract, intermediate connecting fibers, and the supply from the plexus to the gastrointestinal tract. Nerve fibers of smooth muscle, glands and blood vessels. There are more than 100 million ganglion cells in the wall of the human intestine, which is about the same as the total number of neurons contained in the spinal cord. The post-sympathetic ganglion fibers and pre-sympathetic ganglion fibers that enter the intestinal wall can only form synaptic connections with some intestinal ganglion cells, transmit information of the central nervous system, affect the release of excitatory or inhibitory neurotransmitters, and thus regulate Gastrointestinal function. There are also a large number of enteric ganglion cells that do not directly receive impulses from the central nervous system.

The neurons in the enteric nervous system are diverse. There are pseudounipolar and bipolar sensory neurons in the submucosal plexus, which can sense pressure and stretch stimuli on the mucosal surface. The enteric nervous system also has intermediate neurons that process input information and generate outgoing impulses, as well as excitatory and inhibitory motor neurons. The structure of the enteric nervous system is similar to that of the central nervous system. For example: the nerve fibers of the intermuscular plexus are not surrounded by connective tissue such as nerve bundle membrane and endoneurium, and are supported by glial cells; distributed in the intermuscular plexus The neural network composed of the neurons and their cell processes is intertwined with the glial cells, leaving a small extracellular space; There are almost no blood vessels in the intramuscular plexus, and the capillaries supplying nerve components are distributed in Outside the glial sheath; Due to the thicker capillary walls and denser connections between vascular endothelial cells, proteins and other macromolecules cannot pass through, forming the blood-gut muscle plexus barrier, its structure and central nervous system The blood-brain barrier of the system is similar; Similar to the central nervous system, there are many types of neurons in the enteric nervous system. Not only are there significant differences in morphology and structure, but also neurotransmitters are diverse. In addition to acetylcholine and norepinephrine, serotonin (5-HT), adenosine triphosphate (ATP), and various neuropeptides are possible neurotransmitters that have been discovered in the enteric nervous system, including:

The enteric nerve can be divided into the following categories based on the different transmitters and their functions released by various nerves:

Cholinergic nerve

The cell body of cholinergic nerves is scattered in the submucosa and the intermuscular plexus, and the nerve endings supply the gastrointestinal longitudinal and ring muscles, or form synaptic connections with other neurons in the plexus. The released neurotransmitter acetylcholine excites M choline receptors on smooth muscle or N choline receptors on ganglion cells, causes gastrointestinal muscle excitatory effects, and participates in intestinal peristaltic reflexes.

Non-adrenaline inhibits nerves

There are few fibers after entering the gastrointestinal circular muscles or longitudinal muscle layers that dominate the smooth muscles of the sympathetic ganglia. Gastrointestinal smooth muscle relaxation is mainly regulated by a class of non-adrenergic inhibitory neurons present in the intestinal wall. Pharmacological evidence shows that the effects of such nerves are not affected by factors such as alpha or beta adrenergic blockers, chemical resection of sympathetic nerves, exhaustion of catecholamines in tissues, or inhibition of norepinephrine release. Non-adrenergic inhibitory neurotransmitters have not been identified. Some people have suggested that adenosine triphosphate or related purine compounds may be its transmitters, and said this type of nerve is purinergic nerve. There is also evidence that VIP may be its neurotransmitter. Immunocytochemical methods have shown that 42% of guinea pig ileum's submucosal plexus and 2.4% of neurons in the myenteric plexus have VIP-like immune activity. Non-adrenaline can inhibit the function of nerves and is related to the intestinal downward reflex, which is conducive to chyme through the digestive tract.

Intermediate neuron

A large number of neurons in the enteric nervous system are intermediate neurons, and little is known about their properties and functions. More research has been done on serotonergic and peptidergic nerves: serotonergic nerve. The neurotransmitter serotonin released by this type of nerve mainly acts on other neurons in the intestinal wall. It can excite cholinergic nerves and promote the release of acetylcholine, which causes gastrointestinal smooth muscle contraction. It can also stimulate non-adrenaline to inhibit nerves. When excitatory neuromuscular transmission is blocked, it can cause intestinal muscle relaxation. Peptidergic nerve. Immunocytochemical methods have revealed the presence of multiple peptidergic nerves in the enteric nervous system. Most of the neurons of the submucosal plexus and more than half of the intermuscular plexus neurons contain neuropeptides. In terms of quantity, peptidergic nerves are mainly concentrated in the myenteric plexus, but neurons containing certain neuropeptides (for example, somatostatin and VIP) can dominate the submucosal plexus. There may also be two or more neuropeptides coexisting in certain enteric neurons, or the neuropeptides coexist with classic neurotransmitters (eg, acetylcholine). The function of peptidergic nerves is unclear. Somatostatin can inhibit cholinergic nerve excitement and reduce the release of acetylcholine; it can also activate non-adrenaline to inhibit nerves, which may participate in the downward reflex of gastrointestinal motility. Enkephalin acts on opioid receptors on cholinergic neuronal processes and blocks the release of acetylcholine. In addition to having a direct excitatory effect on the intestinal muscle, substance P can also stimulate cholinergic nerves to release acetylcholine, causing intestinal muscle contraction. VIP may not only be an inhibitory transmitter that directly affects the intestinal muscles, but VIP energy nerves may also be an intermediate neuron that can form synaptic connections with other neurons in the intermuscular plexus.

Gastrointestinal motor function (eg, segmental movement of the small intestine and peristalsis) is mainly regulated by the local enteric nervous system, but is relatively independent of the central nervous system. Intestinal peristaltic reflexes can be performed in vitro. Cutting off the vagus or sympathetic nerves also has little effect on gastrointestinal movement. Deficiency or dysfunction of the enteric nervous system can cause gastrointestinal dysfunction. Intestinal infarction is caused by the endogenous inhibitory nerves that innervate the ring muscles. The intestinal spasm and esophageal flaccidity are the opposite. It is caused by the endogenous inhibitory nerves. The cause of infantile megacolon (Hirscnsprung's disease) is that due to the congenital loss of ganglion cells in the wall of the intestinal segment of the distal colon, rectum or anal canal, this intestinal segment cannot produce normal bowel movements, making it more than The intestinal cavity at the site is dilated, with severe cases involving the entire colon.

Common diseases of the enteric nervous system are poisoning, nerve damage, vascular disease, inflammation, degeneration and neuromas. Bacterial, viral and parasitic infections are also very common.