Peristalsis
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In much of the digestive tract, the circular and longitudinal muscle of the muscularis externa (which lies beneath the submucosa) contract in sequence to produce a peristaltic wave which forces the bolus (in the oesophagus) or chyme (below the oesophagus) along the alimentary canal. During a peristaltic movement, the circular muscles contract behind the digested material ; then a contraction of the longitudinal muscle follows which pushes the digested food further along the oesophagus. In the small intestine, a typical peristaltic wave will only last for a few seconds and travel quite slowly, at only a few cm a second - its purpose is to keep the chyme moving through the intestine rather than to move it from A to B, as is the function of the oesophagus.
In the oesophagus two types of peristalsis occurs. Primarily there is a primary peristaltic wave; this is initiated by the entrance of food to the oesophagus in the oesophageal phase of swallowing, and is controlled remotely by the swallowing centre of the brain. The primary peristaltic wave forces the bolus of food down the oesophagus and into the stomach in a wave lasting about 8-9 seconds; this wave will travel down to the stomach even if the bolus of food descends at a greater rate than the wave itself, and will continue even if for some reason the bolus gets stuck further up the oesophagus.
In the event that the bolus gets stuck or moves slower than the primary peristaltic wave (as can happen when it is poorly lubricated), stretch receptors in the oesophageal lining are stimulated and a local reflex response causes a secondary peristaltic wave around the bolus, forcing it further down the oesophagus, and these secondary waves will continue indefinitely until the bolus enters the stomach.
The word is derived from New Latin and comes from the Greek peristaltikos, peristaltic, from peristellein, "to wrap around," and stellein, "to place."
As opposed to the more continuous peristalsis of the small intestines, faecal contents are propelled into the large intestine by periodic Mass Movements. These mass movements occur anywhere from 1-3 times per day, occur in the large intestines and colon, and help propel the contents from the large intestine, through the colon, to the rectum.
- See also: Borborygmus
