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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. |
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 |
However, there is concern about the demise of some good bacteria which allow us to absorb nutrients and develop immunity. The make-up of human microbiomes is shifting, with diversity declining and key-stone species disappearing. The problems are related to over-use of antibiotic, germ-killing hand sanitizers, Caesarean sections that rob new-borns of the bacterial kick-start when passing through the birth canal, and feeding small doses of antibiotics to meat animals as growth promoters. In "Missing Microbes" by M. Blaser, he stresses that our ancient microbiome is akin to an essential organ; we unknownly excised it, and only now are waking up to the implications. | |
Figure 08c Bacteria in Gut [view large image] |