Friday, January 9, 2009

Peristalsis is the rhythmic contraction of smooth muscles to push contents through the digestive tract. The word derives Greek peristaltikos, which means “to wrap around.” In much of the gastrointestinal tract, smooth muscles contract in sequence to produce a peristaltic wave which forces a ball of food along the gastrointestinal tract. This ball of food is called a bolus while in the esophagus and gastrointestinal tract, and chyme in the stomach. Peristaltic movement is initiated by circular smooth muscles contracting behind the chewed material to prevent it from moving back into the mouth, followed by a contraction of longitudinal smooth muscles which pushes the digested food forward.

Peristalsis begins just after food has been chewed into a bolus and swallowed down into the esophagus. Smooth muscles will contract behind the bolus to prevent it from being squeezed back onto the mouth, then rhythmic, unidirectional waves of contractions will work to rapidly force the food into the stomach. This process works in one direction only and its sole purpose is to move food from the mouth into the stomach.

In the esophagus, two types of peristalsis occur. First, there is a primary peristaltic wave. Once the bolus enters the esophagus during swallowing. The primary peristaltic wave forces the bolus down the esophagus and into the stomach in a wave lasting about 8-9 seconds. The wave travels 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 esophagus.

If the bolus gets stuck or moves slower than the primary peristaltic wave (as can happen when it is poorly lubricated), stretch receptors in the esophageal lining are stimulated and a local reflex response causes a secondary peristaltic wave around the bolus, forcing it further down the esophagus, and these secondary waves will continue indefinitely until the bolus enters the stomach. During vomiting the propulsion of food up the esophagus and out the mouth comes from contraction of the abdominal muscles. Peristalsis does not reverse in the esophagus.

When the chyme has been processed and digested by the stomach, it is squeezed through the pyloric valve down into the small intestine. Once past the stomach a typical peristaltic wave will only last for a few seconds, traveling at only a few centimeters per second. Its primary purpose is to mix the chyme in the intestine rather than to move it forward in the intestine. Through this process of mixing and continued digestion and absorption of nutrients, the chyme gradually works its way through the small intestine to the large intestine.

As opposed to the more continuous peristalsis of the small intestines, fecal contents are propelled into the large intestine by periodic mass movements. These mass movements occur one to three times per day in the large intestines and colon, and help propel the contents from the large intestine through the colon to the rectum.