Which statement best describes how antimuscarinics work in overactive bladder?

• A. They reduce detrusor contractions by blocking muscarinic receptors.
• B. They work by beta-adrenergic receptor activation.
• C. They increase bladder capacity by relaxing urethral sphincter.
• D. They have no side effects.

Answer: (A)

Antimuscarinics work by blocking muscarinic receptors on the bladder detrusor muscle, especially the M3 receptors that acetylcholine normally activates to cause contraction. In overactive bladder, these involuntary detrusor contractions during the filling phase drive urgency and urge incontinence. By blocking these receptors, acetylcholine can’t stimulate the detrusor as effectively, so contractions decrease, bladder capacity increases, and urgency is reduced. This mechanism is distinct from beta-adrenergic receptor activation, which relaxes the detrusor via a different pathway (beta-3 agonists). They don’t primarily increase capacity by relaxing the urethral sphincter, and they do have side effects like dry mouth and constipation, not none.