Corpus: Internal anal sphincter

The internal anal sphincter is a smooth muscle that forms a cylindrical structure around the anus, playing a key role in preventing the involuntary discharge of feces.

The sphincter ani internus muscle arises directly from the inner circular muscle layer of the muscular layer of the rectum and surrounds approximately two-thirds of the anal canal in the form of lamellae. In its upper part, it lies adjacent to the corpus cavernosum recti. From the dentate line onward, the muscle is firmly fused with the anal mucosa. Its lower edge can be palpated as a ring at the anocutaneous line. The internal sphincter is separated from the external anal sphincter by the outer longitudinal muscle layer of the muscular layer, known as the musculus corrugator ani.

The sphincter ani internus muscle is part of the involuntary intestinal musculature and is innervated by the inferior hypogastric plexus.

The sphincter ani internus muscle works in conjunction with the sphincter ani externus muscle to close the anus, forming a functional unit. At rest, the muscle remains contracted, creating a barrier that prevents the passage of feces and intestinal gases. When the rectum is distended before defecation, the muscle's tonic contraction is inhibited, allowing it to briefly relax.