Library · Physiology
The defecation reflex
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When stool reaches the rectum, the last stretch of the large intestine, its wall stretches. That wall contains stretch receptors, sensors that signal there is now volume inside. That signal is what starts the defecation reflex, meaning the urge to go to the bathroom.
The signal travels along the myenteric plexus, the intestine's own nerve network, tucked between its muscle layers. What that network does is loosen the internal anal sphincter, a ring of smooth muscle that is not under your control: it opens on its own, like an inner door that gives way without you deciding.
Behind that door there is a second one, the external anal sphincter. This one is striated muscle, the same kind as in your arms, and it is under your control. That is why the bowel movement happens at the moment you decide to relax it.
The reflex on its own is weak; what reinforces it are impulses from the parasympathetic nervous system, the part of the nervous system that works at rest. And if you put it off, the rectal wall relaxes and the urge goes away.
In brief
The defecation reflex begins when stool distends the rectum and stimulates its stretch receptors.
The signal travels through the myenteric plexus, which relaxes the internal anal sphincter, made of involuntary smooth muscle.
The inner door opens on its own.
The external anal sphincter is voluntary striated muscle: defecation happens when you deliberately relax it.
The myenteric reflex alone is weak, and parasympathetic signals reinforce it; if you delay, the rectum relaxes and the urge fades.
Sources (1)
- National Center for Biotechnology Information, StatPearls PublishingPhysiology, Defecation, StatPearlsPublished
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