part of part of innervated by innervated_by urethra superior hypogastric nerve plexus urethral sphincter internal urethral sphincter http://upload.wikimedia.org/wikipedia/commons/b/ba/Gray407.png internal urethral sphincter muscle preprostatic sphincter FMA:45769 musculus sphincter urethrae internus musculus sphincter urethrae internus sphincter urethrae internus musculus sphincter supracollicularis 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]. supracollicular sphincter Wikipedia:Internal_sphincter_muscle_of_urethra SCTID:277856008 UBERON:0004918 internal sphincter of urethra internal sphincter muscle of urethra hypogastric nerve