Where exactly is the serotonin in the GI tract?

Serotonin is a transmitter in both the brain and elsewhere. In fact, although most non-scientists think only of brain serotonin, about 80 percent of the human body's total serotonin is actually located outside the brain in specialized cells (enterochromaffin cells) that are located in the membrane surrounding the intestines. Nerve endings around the intestines that release serotonin help to regulate intestinal movements. Medications that act on to stimulate a particular class of serotonin receptors (5-HT(4) receptors) are used in the treatment of irritable bowel syndrome and constipation. btw - if "bruce" is an MD, I hope he is not practicing in gastroenterology.