A urethral sphincter muscle which constricts the internal urethral orifice. It is the junction of the urethra with the urinary bladder. The muscle is made of smooth muscle, so therefore it is under involuntary control. It is kept tonically contracted by the sympathetic nervous system and during micturition, is relaxed via the parasympathetic nervous system. This is the primary muscle for preventing the release of urine[WP]. [ http://en.wikipedia.org/wiki/Internal_sphincter_muscle_of_urethra ]