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A time-space diagram of a peristaltic wave after a water swallow. High pressure values are red, zero pressure is blue-green. The ridge in the upper part of the picture is the high pressure of the upper esophageal sphincter which only opens for a short time to let water pass.
A time-space diagram of a peristaltic wave after a water swallow. High pressure values are red, zero pressure is blue-green. The ridge in the upper part of the picture is the high pressure of the upper esophageal sphincter which only opens for a short time to let water pass.

Peristalsis is a radially symmetrical contraction and relaxation of muscles that propagates in a wave down a tube, in an anterograde 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.[1]

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 before being transformed into chyme in the stomach) along the tract. 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,[2][self-published source?] and some modern machinery imitate this design.

The word comes from New Latin and is derived from the Greek peristellein, "to wrap around," from peri-, "around" + stellein, "draw in, bring together; set in order".[3]

Human physiology

Peristalsis is generally directed caudad, 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.[4]

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 caudad to the bolus. This allows the food bolus to effectively be pushed forward along the digestive tract.[5]

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 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).[6]

A simplified image showing peristalsis
A simplified image showing peristalsis

In the oesophagus, two types of peristalsis occur:

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.

Stomach

When a peristaltic wave reaches at the end of the oesophagus, the cardiac sphincter (gastroesophageal sphincter) opens allowing the passage of bolus into the stomach. Gastroesophageal sphincter normally remains closed and does not allow food contents of the stomach to move back. The churning movements of stomach's thick muscular wall mixes the food thoroughly with the acidic gastric juice and is called the chyme. The 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, 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.[6]

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 large intestine as a result of thinnest muscularis layer.

Lymph

The human lymphatic system has no central pump. Instead, lymph circulates through peristalsis in the lymph capillaries, as well as valves in the capillaries, compression during contraction of adjacent skeletal muscle, and arterial pulsation.

Sperm

During ejaculation, the smooth muscle in the walls of the vas deferens contracts reflexively in peristalsis, propelling sperm from the testicles to the urethra.[8]

Earthworms

A simplified image showing earthworm movement via peristalsis
A simplified image showing earthworm movement via peristalsis

The earthworm is a limbless annelid worm with a hydrostatic 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, 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 backward slipping.[9] Various other invertebrates, such as caterpillars and millipedes, also move by peristalsis.

Machinery

A peristaltic 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.[10][11]

Related terms

References

  1. ^ Mittal, Ravinder K. (2011). Peristalsis in the Circular and Longitudinal Muscles of the Esophagus. Morgan & Claypool Life Sciences.
  2. ^ "Earthworm - Muscular System".[self-published source]
  3. ^ "Online Etymology Dictionary". etymonline.com. Retrieved 2016-06-30.
  4. ^ Hall, Michael E.; Hall, John E. (2021). Guyton and Hall textbook of medical physiology (14th ed.). Philadelphia, Pa.: Saunders/Elsevier. ISBN 978-0-323-59712-8.
  5. ^ Yuan, Jason; Brooks, Heddwen L.; Barman, Susan M.; Barrett, Kim E. (2019). Ganong's Review of Medical Physiology. ISBN 978-1-26-012240-4.
  6. ^ a b c Marieb, Elaine N. & Hoehn, Katja "Human Anatomy & Physiology" 8th Ed., Benjamin Cummings/Pearson, 2010[page needed]
  7. ^ Mittal, Ravinder K. (2011). Motor Patterns of the Esophagus – Aboral and Oral Transport. Morgan & Claypool Life Sciences.
  8. ^ William O. Reece (21 March 2013). Functional Anatomy and Physiology of Domestic Animals. John Wiley & Sons. pp. 451–. ISBN 978-1-118-68589-1.
  9. ^ Quillin KJ (May 1998). "Ontogenetic scaling of hydrostatic skeletons: geometric, static stress and dynamic stress scaling of the earthworm lumbricus terrestris". The Journal of Experimental Biology. 201 (12): 1871–83. PMID 9600869.
  10. ^ Sangok Seok, C.D. Onal; et al. (2010-05-07). "Peristaltic locomotion with antagonistic actuators in soft robotics" (PDF). Massachusetts Institute of Technology. Retrieved 2014-11-20.
  11. ^ Alexander Boxerbaum (2010-05-10). "A New Form of Peristaltic Locomotion in a Robot". YouTube. Retrieved 2014-11-20.