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Pharmacology

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Autonomic Nervous Systems

Question
24 out of 75
 

All of the following are actions of muscarinic antagonists except (AI 2011)



A Decreases gastric secretion

B Prolongs A-V conduction

C Decreases tracheobronchial secretions

D Causes contraction of radial muscle of iris

Ans. B Prolongs A-V conduction (Raf: Katzug, 11th /113-123)

d. Muscarinic antagonists will produce actions opposite to parasympathetic system, thus it will decrease gastric as well as respiratory secretions.

e. Action of ACh through muscrainic receptors is to increase the A V conduction, thus resulting in bradycardia.

f. Antagonist will thus result in opposite effect i.e. shorten the A V conduction.

g. ACh via muscarinic receptors cause contraction of sphictor pupillae muscle and result in miosis. Muscarinic

h. antagonists will have opposite effect i.e. mydriasis. However, this mydriasis is a passive effect due to relaxation of

i. constrictor pupillae. These agents themselves do not cause contraction of radial muscle of iris; however, they stop the counteracting effect of sphincter pupillae and allow the radial muscle to do its work.

j. Therefore, although both B. and D. can be the answers, if only one option need to be choosen, We will definitely go with answer as B..

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