What is the role of the detrusor muscle in the voiding phase?

The detrusor muscle is a smooth muscle that forms the wall of the bladder. During the voiding phase, its primary role is to contract, which facilitates bladder emptying. When the bladder fills with urine, stretch receptors in the bladder wall signal the brain to initiate the process of urination. The contraction of the detrusor muscle reduces the volume of the bladder, increasing the intravesical pressure. This pressure forces urine out of the bladder and through the urethra, enabling voiding.

The other options are incompatible with the physiological role of the detrusor muscle during urination. For instance, relaxation of the detrusor would actually obstruct the process of emptying, while remaining unchanged during urination would prevent effective voiding. The statement regarding contraction only occurring if the bladder is full simplifies the complex interaction of neural signals involved in the micturition reflex, as the muscle needs to contract in response to both the fullness and the decision to void, rather than solely based on bladder fullness.