Question:medium
All are actions of muscarinic antagonists except?
All are actions of muscarinic antagonists except?
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How does atropine dilate the pupil: by contracting the radial muscle or relaxing the sphincter?
Updated On: Jun 24, 2026
- Decreases gastric secretion
- Prolongs A-V conduction
- Decreases tracheobronchial secretions
- Causes contraction of radial muscles of the iris
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The Correct Option is D
Solution and Explanation
Atropine-type muscarinic antagonists work by removing parasympathetic cholinergic tone, and the trick in this item lies in the mechanism of pupil dilation. Three of the listed effects are genuine antimuscarinic actions: blocking muscarinic receptors on parietal cells lowers gastric acid output, blocking them in the airway reduces tracheobronchial secretions (a property exploited in pre-anaesthetic medication), and blocking vagal influence at the heart alters AV nodal conduction. The false statement concerns the iris musculature. Antimuscarinics produce mydriasis, but they achieve it by PARALYSING the muscarinically driven circular sphincter pupillae, allowing the pupil to widen passively; they do not act on the radial dilator muscle, which is under sympathetic (alpha-adrenergic) control. Saying that a muscarinic antagonist contracts the radial muscle of the iris confuses an adrenergic action with a cholinergic-block action, so this is the incorrect option and the answer.\[\boxed{\text{Causes contraction of radial muscles of the iris}}\]
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