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The word comes from [[New Latin]] and is derived from the [[Greek language|Greek]] ''peristellein'', "to wrap around," from ''peri''-, "around" + ''stellein'', "draw in, bring together; set in order".<ref>{{Cite web|url=http://etymonline.com/index.php?allowed_in_frame=0&search=peristaltic|title=Online Etymology Dictionary|website=etymonline.com|access-date=2016-06-30}}</ref>
==Human physiology==
Peristalsis is generally directed caudal, that is, towards the [[anus]]. This sense of direction might be attributable to the polarisation of the [[myenteric plexus]]. Because of the reliance of the peristaltic reflex on the myenteric plexus, it is also referred to as the myenteric reflex.<ref>{{cite book|last1=Hall|first2=John E.|last2=Hall|first1=Michael E.|title=Guyton and Hall textbook of medical physiology|year=2021|publisher=Saunders/Elsevier|location=Philadelphia, Pa.|isbn=978-0-323-59712-8|edition=14th}}</ref>
===Mechanism of the peristaltic reflex===
The food bolus causes a stretch of the gut [[smooth muscle]] that causes [[serotonin]] to be secreted to sensory neurons, which then get activated. These sensory neurons, in turn, activate neurons of the [[myenteric plexus]], which then proceed to split into two [[cholinergic]] pathways: a retrograde and an anterograde. Activated neurons of the retrograde pathway release [[substance P]] and [[acetylcholine]] to contract the smooth muscle behind the bolus. The activated neurons of the anterograde pathway instead release [[nitric oxide]] and [[vasoactive 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.<ref>{{cite book|title=Ganong's Review of Medical Physiology|year=2019|isbn=978-1-26-012240-4|last1=Yuan|first1=Jason|last2=Brooks|first2=Heddwen L.|last3=Barman|first3=Susan M.|last4=Barrett|first4=Kim E.}}</ref>
===Esophagus===
After food is chewed into a bolus, it is [[swallowing|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 work to rapidly force the food into the stomach. The [[migrating 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).<ref name="ReferenceA"/>
[[Image:Peristalsis.gif|thumb|right|A simplified image showing peristalsis]]
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 continues 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), then 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 continue indefinitely until the bolus enters the stomach. The process of peristalsis is controlled by the medulla oblongata. Esophageal peristalsis is typically assessed by performing an [[esophageal motility study]].
* A third type of peristalsis, tertiary peristalsis, is dysfunctional and involves irregular, diffuse, simultaneous contractions. These contractions are suspect in esophageal dysmotility and present on a barium swallow as a "[[corkscrew esophagus]]".<ref>{{Cite book |url=https://www.ncbi.nlm.nih.gov/books/NBK54271/|title=Motor Patterns of the Esophagus – Aboral and Oral Transport|last=Mittal|first=Ravinder K.|date=2011|publisher=Morgan & Claypool Life Sciences|language=en}}</ref>
During [[vomiting]], the propulsion of food up the esophagus and out the mouth comes from the contraction of the [[abdominal muscles]]; peristalsis does not reverse in the esophagus.{{cn|date=July 2022}}
===Stomach===
When a peristaltic wave reaches at the end of the [[esophagus]], the [[cardiac 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 acidic [[gastric juice]], producing a mixture called the [[chyme]]. The [[Muscular layer|muscularis layer]] of the stomach is thickest and maximum peristalsis occurs here. After short intervals, the [[pyloric sphincter]] keeps on opening and closing so the chyme is fed into the intestine in installments.
===Small intestine===
Once processed and digested by the stomach, the semifluid chyme is passed through the [[pyloric sphincter]] into the [[small 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 the [[large intestine]].<ref name="ReferenceA"/>
In contrast to peristalsis, [[segmentation contractions]] result in that churning and mixing without pushing materials further down the digestive tract.
===Large intestine===
Although the [[large 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 the [[rectum]] part of the large intestine as a result of the thinnest [[Muscular layer|muscularis layer]].
===Sperm===
During [[ejaculation]], the smooth muscle in the walls of the [[vas deferens]] contracts reflexively in peristalsis, propelling [[sperm]] from the [[testicle]]s to the [[urethra]].<ref name="Reece2013">{{cite book|author=William O. Reece|title=Functional Anatomy and Physiology of Domestic Animals|url=https://books.google.com/books?id=C_q3Q-IpkNMC&pg=PA451|date=21 March 2013|publisher=John Wiley & Sons|isbn=978-1-118-68589-1|pages=451–}}</ref>
==Earthworms==
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