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Nervous System

Enteric Nervous System

Nerves to the Bowel Enteric Nervous System The vertebrate nervous system used to divide into the central nervous system (CNS) including the brain and the nerves running down the spinal cord, and the peripheral nervous system (PNS) encompassed the rest such as the sympathetic and para-sympathetic nerves (collectively called the ANS - Autonomic Nervous System). The brain is at the top, the effectors (muscles) and sensory receptors are at the bottom with the information travels back and forth via the PNS. A careful examination of the ANS reveals that there is an odd man standing out within. This the enteric (intestinal) nervous system (ENS). As the name implied, this nervous system is related to the intestine. It is different from the others in its autonomy from the tyranny of the brain especially in the middle of the digestive system (Figure 08a, ENS motor neurons and interneurons in blue,

Figure 08a ENS, Global View [view large image]

Figure 08b ENS, Sectional View [view large image]

sensory neurons in purple). The brain retains essential control at both ends. Such design of distributed processing is to free the brain from running the very complicated task of digestion.

The ENS was first introduced in 1921 and had been ignored until recently in the 21st century. It consists of ~ 100 million neurons, 1/1000 of the number of neurons in the brain, and about equal to the 100 million neurons in the spinal cord. The enteric nervous system is embedded in the lining of the gastrointestinal system, beginning in the esophagus and extending down to the anus. The neurons of the ENS are collected into two types of ganglia: myenteric (Auerbach's) and submucosal (Meissner's) plexuses (Figure 08b). Table 05a lists the functions of the nervous systems including the ENS for the various digestive organs. It is followed by further explanation to supplement the very brief description in the table.

Component Type of Nerve Function
Esophagus Glossopharyngeal nerve Swallowing
Esophagus ENS Generate peristaltic movement (e.g., to relieve jammed bolus in the throat)
Sphincters Vagus nerves Enforce one-way movement downward
Stomach Vagus nerves Production of HCl and B12/intrinsic-factor complex (cruical for life)
Stomach ENS Evoke secretion of digestive materials (HCl, pepsin, alkaline mucous gel)
Pancreas Vagus nerves Control secretion of the precursors (the inactive form) of the pancreatic enzyme
Pancreas ENS Override adverse command by the vagus nerves (with the serotonin neurotransmitter)
Gall Bladder ENS Control bile secretion
Duodenum ENS Neutralizing stomach acid and activating pancreatic enzymes
Small Intestine Sympathetic nerves Nutrient absorption
Small Intestine ENS Nutrient absorption
Colon Sacral nerves Control the reabsorption of water and propulsion of the fecal material
Colon ENS Turn on the immune response to deal with bad intestinal flora (e.g., in tourist diarrhea)
Control the reabsorption of water and propulsion of the fecal material
Rectum Sacral nerves Holding defecation in coordination with central command
Rectum ENS Provide automatic reflex to defecate

Table 05a Nerves in the Digestive System

See "The Second Brain" by M. D. Gershon for more details.

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