It is no surprise that there is a direct relationship between emotional stress and physical distress. "Clinicians are finally acknowledging that a lot of dysfunction in GI disorders involves changes in the central nervous system," said Gary M. Mawe, a professor of anatomy and neurobiology at the University of Vermont.  The big question is which comes first, physiology or psychology?  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.  When Dr. Gershon, whose work has been supported by Novartis, studied mice without SERT, he found that they developed a condition very much like I.B.S. in humans.  Several new serotonin-based drugs - intestinal antidepressants, in a way - have brought hope for those with chronic gut disorders.  Another mechanism that lends credence to physiology as the source of intestinal dysfunctions is the system of mast cells in the gut that have an important role in immune response.  "During stress, trauma or 'fight or flight' reactions, the barrier between the lumen, the interior of the gut where food is digested, and the rest of the bowel could be broken, and bad stuff could get across," Dr. Wood said. "So the big brain calls in more immune surveillance at the gut wall by activating mast cells."  These mast cells release histamines and other inflammatory agents, mobilizing the enteric nervous system to expel the perceived intruders, and causing diarrhea.  Inflammation induced by mast cells may turn out to be crucial in understanding and treating GI disorders. Inflamed tissue becomes tender. A gut under stress, with chronic mast cell production and consequent inflammation, may become tender, as well.  In animals, Dr. Mawe said, inflammation makes the sensory neurons in the gut fire more often, causing a kind of sensory hyperactivity. "I have a theory that some chronic disorders may be caused by something like attention deficit disorder in the gut," he said.  Dr. Gershon, too, theorizes that physiology is the original culprit in brain-gut dysfunctions. "We have identified molecular defects in the gut of everyone who has irritable bowel syndrome," he said. "If you were chained by bloody diarrhea to a toilet seat, you, too, might be depressed."  Still, psychology clearly plays a role. Recent studies suggest that stress, especially early in life, can cause chronic GI diseases, at least in animals. "If you put a rat on top of a little platform surrounded by water, which is very stressful for a rat, it develops the equivalent of diarrhea," Dr. Mayer said.  Another experiment showed that when young rats were separated from their mothers, the layer of cells that line the gut, the same barrier that is strengthened by mast cells during stress, weakened and became more permeable, allowing bacteria from the intestine to pass through the bowel walls and stimulate immune cells.  "In rats, it's an adaptive response," Dr. Mayer said. "If they're born into a stressful, hostile environment, nature programs them to be more vigilant and stress responsive in their future life."  He said up to 70 percent of the patients he treats for chronic gut disorders had experienced early childhood traumas like parents' divorces, chronic illnesses or parents' deaths. "I think that what happens in early life, along with an individual's genetic background, programs how a person will respond to stress for the rest of his or her life," he said.  Either way, what is good for one brain is often good for the other, too. A team of researchers from Penn State University recently discovered a possible new direction in treating intestinal disorders, biofeedback for the brain in the gut.  In an experiment published in a recent issue of Neurogastroenterology and Motility, Robert M. Stern, a professor of psychology at Penn State, found that biofeedback helped people consciously increase and enhance their gastrointestinal activity. They used the brains in their heads, in other words, to help the brains in their guts, proving that at least some of the time two brains really are better than one.  Correction: Aug. 28, 2005, Sunday: An article in Science Times last Tuesday about the enteric nervous system, the body's gastrointestinal "second brain," misstated the context of a remark by Dr. Michael D. Gershon, chairman of the department of anatomy and cell biology at Columbia, about an intestinal disease's potential to bring on depression. He was referring to ulcerative colitis, not irritable bowel syndrome.  A version of this article appears in print on , on page D8 of the National edition with the headline: The Other Brain, the One With Butterflies, Also Deals With Many Woes. Order Reprints| Today's Paper|Subscribe
The connection between the brain in the head and the hidden but powerful one in the gut lies at the heart of many woes, physical and psychiatric.