As discussed previously, irritable bowel syndrome is believed to be due to the abnormal function (dysfunction) of the muscles of the organs of the gastrointestinal tract or the nerves controlling the organs. The nervous control of the gastrointestinal tract, however, is complex. A system of nerves runs the entire length of the gastrointestinal tract from the esophagus to the anus in the muscular walls of the organs. These nerves communicate with other nerves that travel to and from the spinal cord. Nerves within the spinal cord, in turn, travel to and from the brain. (The gastrointestinal tract is exceeded in the numbers of nerves it contains only by the spinal cord and brain.) Thus, the abnormal function of the nervous system in IBS may occur in a gastrointestinal muscular organ, the spinal cord, or the brain.