Peristalsis



Peristalsis is a radially symmetrical contraction and relaxation of muscles which propagates in a wave down the muscular tube, in an anterograde fashion. In humans, peristalsis is found in the contraction of smooth muscles to propel contents through the digestive tract. Earthworms use a similar mechanism to drive their locomotion. The word is derived from New Latin and comes from the Greek peristallein, "to wrap around," from peri-, "around" + stallein, "to place".

In much of the gastrointestinal tract, smooth muscles contract in sequence to produce a peristaltic wave which forces a ball of food (called a bolus while in the esophagus and gastrointestinal tract and chyme in the stomach) along the gastrointestinal tract. 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. Catastalsis is a related intestinal muscle process.

Esophagus
After food is chewed into a bolus, it is swallowed and moved through the esophagus. Smooth muscles contract behind the bolus to prevent it from being squeezed back into 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 which occurs when 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.
 * 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 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.

Esophageal peristalsis is typically assessed by performing an esophageal motility study.

Small intestine
Once processed and digested by the stomach, the milky chyme is squeezed through the pyloric sphincter into the small intestine. Once past the stomach a typical peristaltic wave will only last for a few seconds, travelling 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.

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.

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 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.

Earthworms
The earthworm is a limbless annelid worm with a hydrostatic skeleton that moves by means of peristalsis. This hydrostatic skeleton consists of an extensible body wall surrounded by a fluid-filled body cavity. The worm moves by radially constricting the anterior portion of its body, resulting in an increase in length via hydrostatic pressure. This constricted region propagates posteriorly along the worm's body. As a result, each segment is extended forward, then relaxes and re-contacts the substrate, with hair-like setae preventing backwards slipping. Earthworms increase four orders of magnitude during their lifetime and during this period the dimensions increase according to geometric similarity, or 'isometry'. Unlike rigid skeletons which cannot exhibit both geometric and stress similarity, the hydrostatic skeleton can maintain both forms which may be due to decoupling of weight and skeletal function.