The esophagus, stomach, small intestine, and large intestine are the main regions of the gastrointestinal (GI) tract. Gut motility is the term given to the stretching and contractions of the muscles in the GI tract. The synchronized contraction of these muscles is called peristalsis. These movements enable food to progress along the digestive tract while, at the same time, ensuring the absorption of the important nutrients.
Techniques of measuring these movements of the gut enable us to recognize the normal patterns of contraction in each of the regions. The types of contraction in the gut differ depending on the region and the type of food which has been eaten. Some contractions cause onward movement of the food, others cause mixing and grinding.
The four regions of the GI tract are separated from each other by special muscles, called sphincters, which regulate the movement of ingested material from one part to another. Each part of the GI tract has a unique function to perform in digestion, and each has a distinct type of motility and sensation.
The esophagus propels food from the mouth to the stomach. The stomach is large enough to temporarily store the food eaten at each meal. Solid food is gradually broken down by powerful muscle contractions in the lower end of the stomach. This muscular activity produces small food particles suitable to enter the small bowel, where processes of nutrient absorption begin. Different types of food empty from the stomach at different rates; for example, fatty foods take longer to leave the stomach than other foods. Beverages are handled differently by the stomach, emptying more quickly into the small bowel and not requiring break-down into smaller particles. Normally, most of an average-sized meal has left the stomach after about 2 hours.
In the small intestine, the muscular contractions occur irregularly, varying in strength and type. Here also, the different nutrients in food affect the type of contractions generated. After an average sized meal, the contractions continue for several hours, mixing the food and moving it along the intestine. These types of contractions last until most of the meal residues enter the large intestine. Different foods travel at different rates along the small intestine; for example, foods high in fat travel more slowly than fiber-rich foods. After most of the food has left the small intestine, a different pattern of contractions appears. Bursts of powerful contraction, occurring about every 90 minutes during fasting and particularly at night, progress slowly down the intestine. These bursts clear residual food and secretions from the upper intestine, and thus act as a "housekeeper" in the intestine.
In the large intestine (colon), water and salts are absorbed from the food residues and further mixing of the residues occurs. The patterns of contraction in the colon are not as well understood as those in the small intestine. It is known, however, that eating a meal stimulates contractions in the colon – the larger the meal the greater is the response. Stretching of the rectum by stool produces relaxation of the muscles of the anus and surrounding structures. The rectal contents can then be discharged voluntarily.