<prompt> Intramuscular injection of atropine causes initial bradycardia. The reason for this effect being seen is:Stimulation of medullary vagal centreStimulation of vagal gangliaBlockade of M2 receptors of SA nodal cellsBlockade of muscarinic autoreceptor on vagal nerve endings  <response>: 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. 