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In much of a [[digestive tract]], such as the human gastrointestinal tract, [[smooth muscle tissue]] contracts in sequence to produce a peristaltic wave, which propels a ball of [[food]] (called a [[bolus (digestion)|bolus]] before being transformed into [[chyme]] 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.
[[Earthworm]]s use a similar mechanism to drive their locomotion,<ref>{{Cite web|url=https://www.angelfire.com/de2/atoy/mus.htm|title=Earthworm - Muscular System|website=www.angelfire.com}}</ref>{{self-published inline|date=September 2014}} and some modern machinery imitate this design.<ref>{{Cite
The word comes from [[Neo-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>
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===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.|publisher=McGraw-Hill Education }}</ref>
===Esophagus===
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