Detrusor urinae muscle

The detrusor urinae muscle, also detrusor muscle, muscularis propria of the urinary bladder and (less precise) muscularis propria, contracts when urinating to squeeze out urine. Otherwise, it remains relaxed to allow the bladder to fill. Related are the urethral sphincter muscles which envelop the urethra to control the flow of urine when they contract.

Structure
The fibers of the detrusor muscle arise from the posterior surface of the body of the pubis in both sexes (musculi pubovesicales), and in the male from the adjacent part of the prostate and its capsule. These fibers pass, in a more or less longitudinal manner, up the inferior surface of the bladder, over its apex, and then descend along its fundus to become attached to the prostate in the male, and to the front of the vagina in the female. At the sides of the bladder the fibers are arranged obliquely and intersect one another.