Coordinated contractions of smooth muscle participate in several ways to facilitate digestion and absorption in the small intestine:
- foodstuffs are mixed with digestive enzymes from the pancreas and bile salts from the biliary system
- nutrient molecules in the lumen are constantly dispersed, allowing them to contact the epithelium where enzymatic digestion is completed and absorption occurs
- chyme is moved down the digestive tube, making way for the next load and also eliminating undigestable, perhaps toxic substances
In most animals, the small intestine cycles through two states, each of which is associated with distinctive patterns of motility:
- Following a meal, when the lumen of the small intestine contains chyme, two types of motility predominate: segmentation contractions chop, mix and roll the chyme and peristalsis slowly propels it toward the large intestine.
- The interdigestive period is seen between meals, when the lumen is largely devoid of contents. During such times, so-called housekeeping contractions propagate from the stomach through the entire small intestine, sweeping it clear of debris. This complex pattern of motility is also known as the migrating motor complex and is the cause of "growling".
Motility in the small intestine, as in all parts of the digestive tube, is controlled predominantly by excitatatory and inhibitory signals from the enteric nervous system. These local nervous signals are however modulated by inputs from the central nervous system, and a number of gastrointestinal hormones appear to affect intestinal motility to some degree.