Which muscle functions involuntarily in urination?

The detrusor muscle is responsible for the involuntary contraction of the bladder during urination. It is a smooth muscle layer lining the bladder wall, and its activity is autonomic, meaning it operates without conscious control. When the bladder fills with urine and reaches a certain volume, stretch receptors send signals to the brain, which in turn stimulates the detrusor muscle to contract and expel urine through the urethra.

Other muscles involved in urination, such as the external and internal urethral sphincters, primarily function under voluntary and involuntary control, respectively. The external urethral sphincter is composed of skeletal muscle and is under conscious control, allowing an individual to hold urine until they consciously decide to release it. The internal urethral sphincter, while it works involuntarily to help retain urine, does not actively facilitate the act of urination in the same manner as the detrusor muscle. Skeletal muscles, in general, are also under voluntary control, which differs from the involuntary action of the detrusor during urination. Therefore, the detrusor muscle's role as the primary involuntary muscle in the urination process makes it the correct answer.