Muscarinic receptors are responsible for what in the bladder?

• A. Bladder contraction
• B. Bladder relaxation
• C. Detrusor relaxation
• D. Urethral constriction

Answer: (A)

Muscarinic receptor activation in the bladder drives the muscle that controls voiding. When acetylcholine from parasympathetic nerves binds to these receptors, mainly the M3 subtype, it triggers a Gq-coupled cascade (IP3 and Ca2+ release) that causes detrusor smooth muscle contraction. This contraction pushes urine out of the bladder during micturition, so the action is bladder contraction rather than relaxation. Bladder relaxation occurs mainly through other pathways, such as β3-adrenergic signaling during filling, and urethral constriction is governed more by adrenergic and somatic controls of the sphincters, not by muscarinic actions on the detrusor. Therefore, muscarinic receptors are responsible for bladder contraction.