Peristalsis and Movements of the Alimentary Canal

Digestion is not simply a matter of the right enzyme meeting the right substrate at the right time; food must also be actively and continuously moved along the alimentary canal, and mixed thoroughly with each successive digestive secretion, for digestion to proceed efficiently. Both of these mechanical tasks are carried out by the smooth muscle of the gut wall, principally through two related but functionally distinct types of movement: peristalsis, which propels food onward, and segmentation, which mixes and churns it in place.

Peristalsis is a wave of alternating contraction and relaxation that travels along the muscular wall of the alimentary canal, always in the direction from mouth to anus, and that has the effect of squeezing a bolus of food, or later a mass of chyme or faecal matter, steadily onward ahead of it. The muscularis externa of the gut wall consists of an inner layer of circular muscle fibres, which encircle the tube, and an outer layer of longitudinal muscle fibres, which run along its length; a peristaltic wave begins when the circular muscle just behind the food mass contracts, narrowing the lumen there and so pushing the mass forward, while the circular muscle immediately ahead of the mass simultaneously relaxes, widening the lumen there to receive it, and the longitudinal muscle of the segment just traversed shortens to draw the gut wall up over the advancing mass. This coordinated wave then repeats itself continuously along the length of the canal, so that food is driven onward even against gravity, which is why swallowing and normal digestion continue perfectly well whether a person is standing, lying down or even upside down. Peristaltic waves occur throughout the alimentary canal — carrying the bolus down the oesophagus after swallowing, mixing and slowly emptying the stomach, propelling chyme along the small intestine, and, as strong "mass movements," driving faecal matter through the colon toward the rectum a few times a day, typically after a meal (the gastrocolic reflex).

Segmentation is a different kind of movement, confined mainly to the small intestine, in which rings of circular muscle contract and relax at more or less fixed points along a short stretch of gut, dividing it into a series of adjacent segments and then, moments later, contracting at new points located at what were previously the mid-points of the earlier segments. Unlike peristalsis, segmentation produces no significant net forward movement of the food itself; instead, it repeatedly divides and re-divides the chyme back and forth, thoroughly mixing it with the surrounding digestive juices (pancreatic juice, bile and succus entericus) and repeatedly bringing fresh portions of the chyme into close contact with the absorptive brush-border surface of the intestinal villi, which is essential for efficient digestion and absorption. …