This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Other topics and Adverse effects, Subject name: Anatomy
Intramuscular injection of atropine causes initial bradicardia. The reason for this effective being seen is 
Select and explain. 
A. Stimulation of medullary vagal centre, B. Stimulation of vagal ganglia, C. Blockade of ME receptor and SA nodal cells, D. Blockade of muscarinic auto receptor on vagal nerve ending
D: Blockade of muscarinic auto receptor on vagal nerve ending
Answer: D: Blockade of muscarinic auto receptor on vagal nerve ending 
Explanation: Ref-KDT 6/e p107 Atropine is a non-selective antagonist of M1, M2, and M3 muscarinic receptors. M2, cholinergic receptors are responsible for bradycardia and blockade of these receptors can result in tachycardia. Atropine initially acts on presynaptic M1, receptors (normally decrease the release of ACh) and result in greater release of ACh which is responsible for bradycardia. Later on, blockade of M2 receptors will lead to tachycardia.