I will explain some more, but first a little about how the colon works.The colon a muscle when distended mechanically say by foods, works by releasing serotonin from specialize cells in the gut called enterochromaffin cells which release serotonin which then intiates gut contractions."The enteric and central nervous systems use the same hardware, as it were, to run two very different programs. Serotonin, for instance, is crucial to feelings of well-being. Hence the success of the antidepressants known as S.S.R.I.'s that raise the level of serotonin available to the brain. But 95 percent of the body's serotonin is housed in the gut, where it acts as a neurotransmitter and a signaling mechanism. The digestive process begins when a specialized cell, an enterochromaffin, squirts serotonin into the wall of the gut, which has at least seven types of serotonin receptors. The receptors, in turn, communicate with nerve cells to start digestive enzymes flowing or to start things moving through the intestines. Serotonin also acts as a go-between, keeping the brain in the skull up to date with what is happening in the brain below. Such communication is mostly one way, with 90 percent traveling from the gut to the head.Many of those messages are unpleasant, and serotonin is involved in sending them. Chemotherapy drugs like doxorubicin, which is used to treat breast cancer, cause serotonin to be released in the gut, leading to nausea and vomiting. "The gut is not an organ from which you wish to receive frequent progress reports," Dr. Gershon said.Serotonin is also implicated in one of the most debilitating gut disorders, irritable bowel syndrome, or I.B.S., which causes abdominal pain and cramping, bloating and, in some patients, alternating diarrhea and constipation. "You can run any test you want on people with I.B.S., and their GI tracts look essentially normal," Dr. Mawe said. The default assumption has been that the syndrome is a psychosomatic disease. But it turns out that irritable bowel syndrome, like depression, is at least in part a function of changes in the serotonin system. In this case, it is too much serotonin rather than too little.In a healthy person, after serotonin is released into the gut and initiates an intestinal reflex, it is whisked out of the bowel by a molecule known as the serotonin transporter, or SERT, found in the cells that line the gut wall.People with irritable bowel syndrome do not have enough SERT, so they wind up with too much serotonin floating around, causing diarrhea. The excess serotonin then overwhelms the receptors in the gut, shutting them down and causing constipation."
http://ibsgroup.org/eve/forums/a/tpc/f/71210261/m/369100861I will tie this into foods more. But when you eat serotonin is released that starts contractions. That happens in all people. IN IBS something is wrong with that mechanism.