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Anal sphincter - Definition


The anal sphincter is a muscle located at the very end of the digestive tract, which enables bowel movement. There are two sphincters in this area of the body: one internal sphincter that functions automatically and is composed of so-called smooth muscle fibers, and an external sphincter that is controlled voluntarily. A bowel movement is achieved in the following stages: when there is fecal matter in the rectal ampulla, the body feels the need to open its bowels, and the anal sphincter relaxes. The external sphincter then automatically, and immediately, contracts. The patient can then keep it contracted until wishing to defecate, at which point the sphincter can be voluntarily relaxed. When there is dysfunction in these sphincters, this process is no longer possible, and the result can be involuntary defecation, called fecal or anal incontinence.