Anal sphincter

The anal sphincter is a complex of muscles encircling the anal canal that is essential for maintaining fecal continence and regulating defecation. It consists of two main components: the internal anal sphincter, which is a thickened continuation of the circular smooth muscle layer of the rectum, and the external anal sphincter, which is composed of striated skeletal muscle fibers derived from the levator ani muscle group.

The internal anal sphincter provides involuntary tonic contraction, while the external anal sphincter allows voluntary control over anal closure. These muscles are anatomically distinct but functionally integrated, with the internal sphincter lying between the anal mucosa and the external sphincter, and the external sphincter encircling the distal anal canal.