The small intestine is divided into duodenum, jejunum, and ileum, with the duodenum and jejunum being the major site of digestion and absorption. Chyme takes 2 to 4 hours to move through the 5 m of the small intestine. Segmentation, which mixes intestinal contents and enhances contact of the chyme with the intestinal microvilli, is the most frequent type of intestinal contraction. Segmentation can be rhythmic, with adjacent sites alternating contraction and relaxation. Eating increases segmentation.
Peristalsis moves the chyme aborally an average of 10 cm per contraction. Eating actually slows the aboral movement of chyme. An intestinal pacemaker and the enteric nervous system control the frequency of segmentation and short peristalsis. Contractions occur at an intrinsic rate of 11 to 13 per minute in the duodenum, which declines to 8 to 9 per minute by the terminal ileum. This rate is modified by extrinsic neural and hormonal inputs.
Reflex activity helps coordinate intestinal motility with events elsewhere in the GI system. Distention of one segment of intestine relaxes the remainder of the intestine (intestinointestinal reflex). Distention of the ileum decreases gastric motility (ileogastric reflex). Distention of the stomach increases movement of material out of the ileum (gastroileal reflex) and colon (gastrocolic reflex), a reflex especially evident in the newborn.
Migrating Myoelectric Complex
During fasting, periods of intense electrical activity are followed by long quiescent periods. The electrical activity, called the migrating myoelectric complex, is initiated in the stomach and traverses the entire small intestine. There is evidence for both a vagal and a hormonal (motilin) role in its initiation. The frequency of the migrating myoelectric complex is altered by substance P, somatostatin, and neurotensin. This contraction sweeps the intestines clean and inhibits the retrograde movement of bacteria from the colon into the small intestine. The electrical and contractile activity require the enteric nervous system for propagation.