What is the effect of M3 receptors on the bladder?

The M3 receptors play a crucial role in bladder function, specifically in the regulation of detrusor muscle activity. Activation of M3 receptors promotes the contraction of the detrusor muscle, which is the smooth muscle layer of the bladder wall responsible for pushing urine out during micturition. Additionally, this activation can lead to the relaxation of the internal urethral sphincter, which allows urine to flow from the bladder into the urethra.

This dual action—contracting the detrusor to promote bladder emptying while simultaneously relaxing the internal urethral sphincter—is essential for normal urination. By encouraging efficient bladder contraction and reducing resistance at the level of the sphincter, M3 receptors facilitate the overall process of micturition.

The other choices describe oppositional actions that do not accurately reflect the role of M3 receptors in bladder physiology. For instance, inhibiting detrusor contraction or facilitating storage by relaxing the detrusor would not align with the action mediated through M3 receptor activation, which aims to facilitate urination rather than hinder it.