musculus sphincter urethrae internus

[TA] internal sphincter muscle of urethra: a circular layer of smooth muscle fibers surrounding the internal urethral orifice in males, innervated by the vesical nerve, and acting to close the internal orifice of the urethra. No such structure exists in females. Called also internal sphincter of urethra; internal urethral sphincter; preprostatic, smooth, or supracollicular sphincter; m. sphincter supracollicularis [TA alternative]; and m. sphincter vesicae urinariae.