The Two Brains
There is one brain that sits at the top of the spinal cord, but there is a second brain equally powerful and equally controlling. With 100 million neurons, 90 % of the serotonin, 50% of the dopamine, neuropeptites, endorphins and benzodiazapines, the enteric nervous system (ENS) located in the sheaths of tissue lining the esophagus, stomach, small intestine and colon is the BRAIN in the GUT. Just like the larger brain in the head, the ENS sends and receives impulses, records experiences and respond to emotions. The gut can upset the brain just as the brain can upset the gut.
Following in-depth articles shed more light on this interplay and are very useful reading.
Adam Hadhazy, Scientific American
Harriet Brown, New York Times