Anal sphincter

The anal sphincter is a system of muscles surrounding the anal canal that controls its closure and opening, helping maintain continence and permit defecation.

Internal anal sphincter: The involuntary smooth-muscle ring surrounding the upper anal canal. It supplies most resting closure through sustained involuntary contraction.
Anal canal: The short terminal passage of the digestive tract between the rectum and anus. The sphincter muscles surround this passage and regulate its opening.
Anal fissure: A tear in the lining of the anal canal, often associated with pain during defecation. Internal sphincter spasm can contribute to pain and hinder healing.
Dyssynergic defecation: A defecation disorder in which pelvic-floor muscles fail to relax or contract appropriately during evacuation. It involves faulty coordination rather than simply weak sphincter closure.
External anal sphincter: The striated skeletal-muscle ring surrounding the anal canal, under voluntary control. It provides voluntary tightening and helps delay defecation.
Rectum: The final section of the large intestine, where feces are held before defecation. Rectal filling triggers sensations and reflexes that interact with sphincter control.
Obstetric anal sphincter injury: A tear involving the anal sphincter during childbirth. Such injuries can impair continence and may require repair.
Rectal prolapse: A condition in which the rectum descends and protrudes through the anus. Prolapse is a structural displacement, though it can coexist with sphincter weakness.
Rectoanal inhibitory reflex: A reflex relaxation of the internal anal sphincter when the rectum distends. This reflex briefly opens the canal as rectal contents arrive.
Anus: The external opening at the end of the digestive tract. The sphincter system controls passage through this opening.

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