Peristalsis – the rhythmic waves of contraction that move food through the gastrointestinal tract. Pressure against the inner walls of the intestines, such as occurs when food enters an intestinal segment, signals the muscles to contract and relax in a progressive pattern. The muscles ahead of the pressure relax, widening the intestinal passage, and the muscles behind the pressure contract, narrowing the passage.
This action “massages” the intestinal contents forward. Peristalsis is totally involuntary, under the control of the autonomic nervous system.
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