The detrusor muscle, also detrusor urinae muscle, muscularis propria of the urinary bladder and (less precise) muscularis propria, is smooth muscle found in the wall of the bladder. The detrusor muscle remains relaxed to allow the bladder to store urine, and contracts during urination to release urine. Related are the urethral sphincter muscles which envelop the urethra to control the flow of urine when they contract. 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. 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 natural male pills, 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.

The three layers of muscles are arranged longitudinal-circular-longitudinal from innermost to outermost. The detrusor muscle is innervated by the autonomic nervous system. During urination, parasympathetic pelvic splanchnic nerves act primarily on postganglionic M3 receptors to cause contraction of the detrusor muscle. At other times, the muscle is kept relaxed via sympathetic branches from the inferior hypogastric plexus to allow the bladder to fill. In older adults over 60 years in age, the detrusor muscle may cause issues in voiding the bladder, resulting in uncomfortable urinary retention. The bladder also contains β3 adrenergic receptors, and pharmacological agonists of this receptor are used to treat overactive bladder. The mucosa of the urinary bladder may herniate through the detrusor muscle. This is most often an acquired condition due to high pressure in the urinary bladder, damage, or existing connective tissue disorders. Sellers, Donna J.; Chess-Williams, Russ (2012). "Muscarinic Agonists and Antagonists: Effects on the Urinary Bladder". Muscarinic Receptors. Handbook of Experimental Pharmacology. Giglio, Daniel; Tobin, Gunnar (2009). "Muscarinic receptor subtypes in the lower urinary tract". Ho, MAT H.; Bhatia, NARENDER N. (2007-01-01), Lobo, Rogerio A. (ed.), "CHAPTER 51 - Lower Urinary Tract Disorders in Postmenopausal Women", Treatment of the Postmenopausal Woman (Third Edition), St. Louis: Academic Press, pp. Stoffel, JT (September 2017). "Non-neurogenic Chronic Urinary Retention: What Are We Treating?". Merrow, A. Carlson; Hariharan, Prime Boosts Selena, eds. 2018-01-01), "Bladder Diverticula", Imaging in Pediatrics, Elsevier, p. Swami, K.S.; Feneley, R.C.L.; Hammonds, J.C.; Abrams, P. "Detrusor myectomy for detrusor overactivity: a minimum 1-year follow-up". The Medical Journal of Urology.