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Peristalsis
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Peristalsis is the movement that carries food along the digestive tract. It is involuntary: you do not tell it to happen. The wall of the tract has two layers of muscle, a circular one that wraps around it and a longitudinal one that runs along its length, and the two contract and relax in a precise order from the pharynx, which is the back of the throat, down to the anus.
The order is what matters. Behind the contents the muscle contracts and ahead of them it relaxes, so the bolus, the portion of food already swallowed, has only one path open: forward. It is the same motion as squeezing a tube of toothpaste from the bottom.
What coordinates all of that lives in the wall itself: the myenteric plexus, a network of nerves sitting right between the two muscle layers, where the interstitial cells of Cajal are also found.
And it does not always start the same way. When the wave arises from swallowing, it is called primary peristalsis. When it arises from distension of the esophagus, that is, from something stretching it from the inside, it is called secondary.
In brief
Peristalsis is the involuntary contraction and relaxation of the circular and longitudinal muscles of the digestive tract, and that movement pushes contents from the pharynx to the anus.
It is governed by the myenteric plexus, which sits between those two muscle layers, and by the interstitial cells of Cajal: together they alternate contraction behind and relaxation ahead.
Peristalsis triggered by swallowing is called primary; peristalsis triggered by distension of the esophagus is called secondary.
Sources (1)
- National Center for Biotechnology Information, StatPearls PublishingPhysiology, Peristalsis, StatPearlsPublished
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