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Peristalsis (/ˌpɛrɪˈstælsɪs/PERR-ih-STAL-siss,US also/-ˈstɔːl-/-STAWL-)[1] is a type ofintestinal motility, characterized byradially symmetrical contraction and relaxation ofmuscles that propagate in awave down a tube, in ananterograde direction. Peristalsis is progression of coordinated contraction of involuntary circular muscles, which is preceded by a simultaneous contraction of the longitudinal muscle and relaxation of the circular muscle in the lining of the gut.[2]
In much of adigestive tract, such as the human gastrointestinal tract,smooth muscle tissue contracts in sequence to produce a peristaltic wave, which propels a ball offood (called abolus before being transformed intochyme in the stomach) along the tract. The peristaltic movement comprises relaxation of circular smooth muscles, then their contraction behind the chewed material to keep it from moving backward, then longitudinal contraction to push it forward.
Earthworms use a similar mechanism to drive their locomotion,[3][self-published source?] and some modern machinery imitate this design.[4]
The word comes fromNeo-Latin and is derived from theGreekperistellein, "to wrap around," fromperi-, "around" +stellein, "draw in, bring together; set in order".[5]
Peristalsis is generally directed caudal, that is, towards theanus. This sense of direction might be attributable to the polarisation of themyenteric plexus. Because of the reliance of the peristaltic reflex on the myenteric plexus, it is also referred to as the myenteric reflex.[6]
The food bolus causes a stretch of the gutsmooth muscle that causesserotonin to be secreted to sensory neurons, which then get activated. These sensory neurons, in turn, activate neurons of themyenteric plexus, which then proceed to split into twocholinergic pathways: a retrograde and an anterograde. Activated neurons of the retrograde pathway releasesubstance molecules alsoP andacetylcholine to contract the smooth muscle behind the bolus. The activated neurons of the anterograde pathway instead releasenitric oxide andvasoactive intestinal polypeptide to relax the smooth muscle caudal to the bolus. This allows the food bolus to effectively be pushed forward along the digestive tract.[7]
After food is chewed into a bolus, it isswallowed 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 work to rapidly force the food into the stomach. Themigrating motor complex (MMC) helps trigger peristaltic waves. This process works in one direction only, and its sole esophageal function is to move food from the mouth into the stomach (the MMC also functions to clear out remaining food in the stomach to the small bowel and remaining particles in the small bowel into the colon).[8]
In the esophagus, two types of peristalsis occur:
Duringvomiting, the propulsion of food up the esophagus and out the mouth comes from the contraction of theabdominal muscles; peristalsis does not reverse in the esophagus.[citation needed]
When a peristaltic wave reaches at the end of theesophagus, thecardiac sphincter (gastroesophageal sphincter) opens, allowing the passage of bolus into the stomach. The gastroesophageal sphincter normally remains closed and does not allow the stomach's food contents to move back. The churning movements of the stomach's thick muscular wall blend the food thoroughly with the acidicgastric juice, producing a mixture called thechyme. Themuscularis layer of the stomach is thickest and maximum peristalsis occurs here. After short intervals, thepyloric sphincter keeps on opening and closing so the chyme is fed into the intestine in installments.
Once processed and digested by the stomach, the semifluid chyme is passed through thepyloric sphincter into thesmall intestine. Once past the stomach, a typical peristaltic wave lasts only 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 thelarge intestine.[8]
In contrast to peristalsis,segmentation contractions result in that churning and mixing without pushing materials further down the digestive tract.
Although thelarge intestine has peristalsis of the type that the small intestine uses, it is not the primary propulsion. Instead, general contractions called mass action contractions occur one to three times per day in the large intestine, propelling the chyme (now feces) toward the rectum. Mass movements often tend to be triggered by meals, as the presence of chyme in the stomach and duodenum prompts them (gastrocolic reflex). Minimum peristalsis is found in therectum part of the large intestine as a result of the thinnestmuscularis layer.
The humanlymphatic system has no central pump. Instead,lymphcirculates through peristalsis in the lymph capillaries as well as valves in the capillaries, compression during contraction of adjacent skeletal muscle, and arterial pulsation.
Duringejaculation, the smooth muscle in the walls of thevasa deferentia contract reflexively in peristalsis, propellingsperm from thetesticles to theurethra.[10]
Theearthworm is a limblessannelid worm with ahydrostatic skeleton that moves by peristalsis. Its hydrostatic skeleton consists of a fluid-filled body cavity surrounded by an extensible body wall. The worm moves by radially constricting the anterior portion of its body, increasing 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-likesetae preventing backward slipping.[11] Various other invertebrates, such ascaterpillars andmillipedes, also move by peristalsis.
Aperistaltic pump is a positive-displacement pump in which a motor pinches advancing portions of a flexible tube to propel a fluid within the tube. The pump isolates the fluid from the machinery, which is important if the fluid is abrasive or must remain sterile.
Robots have been designed that use peristalsis to achieve locomotion, as the earthworm uses it.[12][13]
