internal anal sphinctermedical dictionary

<anatomy> A smooth muscle ring, formed by an increase of the circular fibres of the rectum, situated at the upper end of the anal canal, internal to the outer voluntary external anal sphincter.

This sphincter is maximally-contracted when the rectal ampulla is at rest -empty or relaxed to accommodate a distending faecal mass. It is inhibited with filling of the ampulla, increased distension and peristalsis.

Synonyms: musculus sphincter ani internus, internal sphincter muscle of anus.

(05 Mar 2000)