Corpus: External anal sphincter

The external anal sphincter muscle is a striated muscle arranged in a ring around the anus that closes the anus. Its fibres run in a loop shape.

The sphincter ani externus muscle is a muscle fibre cylinder that is about 3 to 4 cm long. There are three parts of the muscle:

• Pars subcutanea: Also known as the sphincter ani externus subcutaneus muscle. The muscle fibres of this part insert into the subcutis and are divided into septa by connective tissue. The pars subcutanea runs from the skin behind the anus to the skin in front of the anus.
• Pars superficialis: This part runs from the anococcygeal ligament to the tendineal centre of the perineum.
• Pars profunda: It runs in a loop from the centrum tendineum perinei around the anus back to the centrum tendineum perinei.

The musculus sphincter ani externus protrudes over the musculus sphincter ani internus aborally. This creates a noticeable depression at the level of the anocutaneous line. From the outside, the puborectalis muscle wraps around the external sphincter muscle.

The external sphincter muscle is voluntarily innervated by the pudendal nerve, which contains fibres from segments S2-S4. The controlling motor neurons are localised in the Onuf nucleus.

The sphincter ani externus muscle ensures the closure of the anus through continuous tone. Defecation is initiated by the voluntary relaxation of the muscle. In addition, the muscle ensures congestion and protrusion of the rectal venous plexus and thus a fine seal that prevents the escape of intestinal gases.

• Image source for podcast: © Jacob Rice / Unsplash