The enteric nervous system plays an important role in physical and mental well being. Michael Gershon, professor of cell biology at Columbia Prebisterian Medical Center in NY, says that the enteric nervous system, the autonomic nervous system, is full of neurons and functions as a second brain. When a pathogen cross the gut lining immune cells present in the gut secrete inflammatory substances including histamines, which are detected by neurons in the enteric nervous system. The gut brain then triggers defence mechanisms such as diarrhea. About 90 percent of the signals passing along the vagus nerve comes from the enteric nervous system. The neurons in the gut, continues Michael Gershon, are thought to generate as much dopamine as those in the head. 95 percent of the serotonin present in the body is located in the enteric nervous system.