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Biology Articles » Neurobiology » Enteric Nervous System

Enteric Nervous System

Intuitively we understand that there is a link between the mind and the gut and our digestion works best when we are in a harmonious mood..  Think of a fine dinner with a lakefront view, good company, your favorite music, courteous service and a relaxed time frame.  Then think of nervously chomping on French fries in your car in stop and go traffic on the way to an important  business meeting where you are late.  When we are stressed we tend to overeat or undereat. We speak of a "visceral sense".
 
Current research supports our intuition of a link between our brain and gut. The seminal work on the enteric nervous system (ENS was done independently by Byron Robinson and Johannis Langley I the early twentieth century.  Subsequently, it was neglected until Gershon picked up the trail in the 90s.
 
The enteic nervous system( ENS ) consists of two layers within the intestinal wall, the myenteric plexus and the sjubmucosal plexus. The functions are intertwined but the myenteric layer is more involved with muscle control and the submucosal with  managing secretion.  Accordingly,  that the myenteric plexus lies between the muscle layers and the submucosal layer is nearer to the inner layer. Although the  central nervous system( CNS) and ENS are connected by the vagus nerve, they continue to operate independently when the nerve is severed. 
 
The ENS develops from the same embryonic neural crest as the CNS. and throughout life the enteric nervous system continues to resemble the central nervous system.  Like the CNS, the ENS makes use of neurotransmitters such as seratonin, acetylcholine, nitric oxide. and dopamine. For this reason, pharmaceuticals that modulate our neurotransmitters may have untoward effects on our intestines. The mix of neurons in the ENS include motor neurons, sensory neurons, support neurons called glia and neurons that communicate with the CNS. The interstial cells of Cajal send out rhythmic pulses that are believed to collectively function as a pacemaker. The ENS even mirrors diseases of the CNS. In Alzheimer's, amyloid plaques manifest in both the brain and intestines. Lewy bodies appear in both structures in the case of Parkinson's.
 
The complexity of the ENS is not surprising when you consider its function.  Its task is to take everything you eat, whether it be a plate of enchiladas, a smoked salmon or a chocolate torte, and break it down into building blocks that your body can reuse. This alchemy requires complex chemical processing.  The ENS must be  sensitive to the chemical composition of stomach contents to secrete the appropriate mix of enzymes and regulate the blood supply. It must adjust its squeezing, churning action to mechanical conditions in the gut.
 
The enteric brain is ancient in evolutionary terms and probably antedates the CNS.  It has been discovered in a sea cucumber.  Sea cucumbers belong to the echinoderms, a sister phylum to our phylum, the chordates. It was believed that the nervous system of echinoderms consisted of only a ring of nervous tissue but recent research shows that echinoderms have a sophisticated enteric nervous system.  When folks at a party start  trading believe it or not factoids, someone may say "Did you know there is a fish that eats its brain?". This is a melodramatic exaggeration and distortion of a true scientific fact.  An ascidian ( a fish shaped chordate not really a fish. ) reabsorbs its central brain when it transitions from larval  to adult form.    At the same time it develops its visceral ganglion, the term for a primitive enteric brain.  The metamorphosis is a sensible adaptation to a change in lifestyle, analogous to trading in scuba equipment for skis after moving from Barbados to Colorado. The larval sea squirt is motile and needs a CNS to direct motion.  The primary activity of the adult sessile form is eating. Recent genomic work on the sea squirt, Ciona Intestinalis, may reveal more detail of the evolution of the enteric nervous system.
 
In vertebrates the ENS shuts down digestion in conditions of stress.  Stress also triggers an inflammatory response in the alimentary canal. All this is was adaptive in our prehistory when the "tiger in waiting" was very likely a literal one. Energy from the digestive process could be diverted to fight or flight. Should the gut sustain an injury the immune system would be primed to ward off pathogens and begin the repair process. However in modern times the stress may be a difficult boss or the Dow Jonest average. These stressors are persistent and they provoke a persistent response.  A chronic state of alert may help explain migraine, indigestion, irritable bowel syndrome. The links are under investigation.  A few overly enthusiastic proponents may have jumped the gun and started offering remedies before the data are in. Caveat emptor!

It is always a temptation to speculate about historical figures.  Was it coincidence that  Soren Kierkegaard, who produced titles such as "Dread" and "Fear and Trembling" complained of chronic abdominal pain?  Such questions are currently beyond the purview of science.
 
Developmental errors account for other than psychosomatic alimentary canal diseases.  It is an unfolding story with many universities and research centers using knockout genes, electron microscopes and microarrays to unravel the details. But, beyond medical insights, the enteric nervous system is fascinating because of the light it throws on our evolution.  It is also a mirror into our nervous system and therefore into our mind.
 
Further reading:
 
The Other Brain Also Deals With Many Woes NYTimes article
The Enteric Nervous System: The Brain in the Gut http://whyfiles.org/026fear/physio2.html
The Enteric nervous system (ENS) http://www.anaesthetist.com/anaes/patient/ans/ens.htm
The Enteric Nervous System (ENS) http://www.open.ac.uk/science/biosci/research/saffrey/saffrey.htm
The Enteric Nervous System http://content.nejm.org/cgi/content/short/334/17/1106?query=nextarrow
The Enteric Nervous System: The Brain in the Gut http://www.psyking.net/id36.htm


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