sentences:  If atropine is given by intramuscular or subcutaneous injection, it causes initial bradycardia.This is because when administered intramuscularly or subcutaneously atropine acts on presynaptic M1 receptors (autoreceptors).Intake of acetylcholine in axoplasm is prevented and the presynaptic nerve releases more acetylcholine into the synapse that initially causes bradycardia.