Overall blood flow to the gut is regulated from the CNS, via sympathetic vasoconstrictor neurons. The sympathetic vasoconstrictor neurons act in concert with the autonomic control of other vascular beds, to distribute cardiac output in relation to the relative needs of all organs.
Thus in times of need, even during digestion, the sympathetic can divert blood flow away from the gastrointestinal tract. Gastric acid secretion is regulated both by neurons and by hormones. Neural regulation is through cholinergic neurons with cell bodies in the wall of the stomach.
These receive excitatory inputs both from enteric sources and from the vagus nerves. Gastric secretion of HCl and pepsinogen in the stomach, and secretion of pancreatic enzymes, is largely dependent on vago-vagal reflexes.
Enteric motor neurons are the final common pathway, but the roles of intrinsic reflexes are minor. Pancreatic secretion of bicarbonate, to neutralise the duodenal contents, is controlled secretin, a hormone released from the duodenum, in synergy with activity of cholinergic and non-cholinergic enteric neurons. Secretion into the gall-bladder and bicarbonate secretion in the distal stomach are also nerve controlled. Nerve fibres run close to endocrine cells of the mucosa of the gastro-intestinal tract, some of which are under neural control.
For example, gastrin cells in the antrum of the stomach are innervated by excitatory neurons that utilize gastrin releasing peptide as the primary neurotransmitter. Conversely, hormones released by gastrointestinal endocrine cells influence the endings of enteric neurons.
In a sense, the endocrine cells act like taste cells, that sample the luminal environment, and release messenger molecules into the tissue of the mucosa, where the nerve endings are found.
This is a necessary communication, because the nerve endings are separated from the lumen by the mucosal epithelium. An important communication is with serotonin 5-hydroxytryptamine, 5-HT containing endocrine cells which activate motility reflexes. Excessive release of serotonin can cause nausea and vomiting, and antagonists of the 5-HT3 receptor are anti-nauseants.
Enteric neurons are involved in a number of defense reactions of the gut. Defense reactions include diarrhea to dilute and eliminate toxins, exaggerated colonic propulsive activity that occurs when there are pathogens in the gut, and vomiting. Fluid secretion is provoked by noxious stimuli, particularly by the intraluminal presence of certain viruses, bacteria and bacterial toxins. This secretion is due in large part to the stimulation of enteric secretomotor reflexes.
The physiological purpose is undoubtedly to rid the body of pathogens and their products. Signals between gut regions are carried both by hormones such as cholecystokinin, gastrin and secretin and by nerve circuits. Entero-enteric reflexes regulate one region in relation to others. For example, when nutrients enter the small intestine, secretion of digestive enzymes from the pancreas occurs.
A series of nerve circuits that carry signals from one region of intestine, to sympathetic ganglia, and back to the gut wall provide a regulatory system that is unique to the gastrointestinal tract. Neurons with cell bodies in enteric ganglia and terminals in pre-vertebral sympathetic ganglia form the afferent limbs of these reflexes.
The gastrointestinal tract is in two way communication with the CNS. Afferent neurons convey information about the state of the gastrointestinal tract. Some of this reaches consciousness , including pain and discomfort from the gut and the conscious feelings of hunger and satiety, which are integrated perceptions derived from the gastrointestinal tract and other signals blood glucose, for example.
Other afferent signals, concerning, for example, the nutrient load in the small intestine, or the acidity of the stomach, do not normally reach consciousness. For example, the sight and smell of food elicits preparatory events in the gastro-intestinal tract, including salivation and gastric acid secretion. This is termed the cephalic phase of digestion. Swallowed food stimulates the pharynx and upper esophagus, eliciting afferent signals that are integrated in the brainstem, and subsequently provide efferent signals to enteric neurons in the stomach that cause acid secretion and increased gastric volume, in preparation for the arrival of the food.
At the other end of the gut, signals from the colon and rectum are relayed to defecation centres in the spinal cord, from which a programmed set of signals is conveyed to the colon, rectum and anal sphincter to cause defecation.
The defecation centres are under inhibitory control from higher CNS regions, and inhibition that can be released when it is chosen to defecate.
The other central influences are through sympathetic pathways, which have been discussed under the sections on control of motility and regulation of fluid exchange and local blood flow, above. There are a large number of pathologies associated with the neural regulation of digestion, many of these arising from abnormalities of the enteric nervous system De Giorgio and Camilleri ; Spiller and Grundy It is fatal if untreated. The irritable bowel syndrome IBS is sometimes considered to be an enteric neuropathy, although IBS covers a spectrum of conditions.
Two-way communication occurs between the enteric nervous system and the immune system of the gastrointestinal tract, that is, transmitters released by the terminals of enteric neurons in the mucosa influence immune-related cells, such as mast cells, and the cells of the mucosa release active substances, including cytokines and mast cell tryptase, that act on enteric neurons De Giorgio et al.
Gastroenterology The intestine as a sensory organ: neural, endocrine, and immune responses. Gershon MD Nerves, reflexes, and the enteric nervous system. Spiller R, Grundy D Pathophysiology of the enteric nervous system, a basis for understanding functional diseases. Blackwell, Oxford. Gut i6-i A division of the autonomic nervous system whose component neurons lie within the walls of the digestive organs esophagus, stomach, intestines, pancreas, gall bladder and pancreato-biliary ducts.
The enteric nervous system contains entire nerve circuits for digestive organ control, and can function autonomously. A neuron whose cell body is in a ganglion within the wall of the digestive tract, biliary system or pancreas. Conversely, parasympathetic stimuli typically stimulate these digestive activities.
Some of the prominent communiques enabled by nervous interconnections within the digestive tract have been named as reflexes and serve to illustrate a robust system of control. Examples include the gastrocolic reflex, where distention of the stomach stimulates evacuation of the colon, and the enterogastric reflex, in which distention and irritation of the small intestine results in suppression of secretion and motor activity in the stomach.
Congenital and acquired derangements in the structure or function of the enteric nervous system are well recognized as causes of digestive tract disease. Examples include small intestinal motility disorders, gastric outlet obstructions and megacolon. Control of Digestive Function. Enteric Endocrine System.
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Citation, DOI and article data. Morgan, M. Myenteric plexus. Reference article, Radiopaedia. Auerbach plexuses Auerbach's plexus Auerbach plexus. URL of Article.
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