Actions of muscarinic antagonists – Pharmacology MCQ

Which of the following is not an action of muscarinic antagonists?

A. Reduce respiratory secretions
B. Reduce gastric secretions
C. Contraction of radial muscles of iris
D. Increase rate of conduction in the heart

Correct answer : C. Contraction of radial muscles of iris

Contraction of radial muscles of iris is brought about by the sympathetic nervous system (produces mydriasis).
They have no parasympathetic innervation.
Muscarinic antagonists produce mydriasis by inducing relaxation of circular muscles of iris.

Reduction of secretions in the gastrointestinal tract and respiratory system (used as pre anaesthetic medication).
Increases the heart rate and rate of conduction.
Some anticholinergics (like hyoscine) have antiemetic action and are used for treatment of motion sickness.
Smooth muscle relaxation
- Bronchodilation – used for treatment of asthma.
- Relaxation of circular muscle of iris (mydriasis) – used in ophthalmological diagnosis.
- Antispasmodic action for treatment of GI hypermotility and overactive bladder.