No one knows for sure what causes IBS. This much is known: IBS is neither a psychiatric disorder, nor is it caused by a specific biological or anatomical abnormality. IBS is considered a functional disorder. This means that physical examinations and laboratory testing like x-rays, blood tests, or endoscopies show nothing wrong with the physical structure of the bowel. Instead, the problem is how the bowel functions. Sometimes muscles of the gastrointestinal tract occur too forcefully or weakly, and sometimes they contract too slowly or rapidly. Experts now believe that one clue to IBS lies in "faulty wiring" between the network of nerves connecting the brain and the gastrointestinal system - the part of the body responsible for digesting food and removing waste material. In people with IBS, nerves that help control pain and muscle contractions in the bowel are more sensitive to certain foods, hormonal changes, and stress. This sensitivity can trigger a painful chain reaction that speeds (resulting in diarrhea) or slows (resulting in constipation) the passage of waste material through the colon. Some people experience both diarrhea and constipation at different times.