We all know that one of the pharmacological action of Atropine that it leads to cardiac stimulation as atropine blocks M2 receptors so why sometimes there is initial bradycardia with atropine?
The initial bradycardia may be explained by: 1) Blockade of pre-synaptic M2 receptors on the vagal fibers to the heart with subsequent increase in Ach release. This Ach stimulates cardiac M2 receptors which are not blocked by atropine yet. Once these cardiac M2 receptors get blocked the bradycardia will be replaced by tachycardia 2) Central action of atropine on M2 receptors on the vagal nucleus (CIC). This blockade leads to activation of CIC with release of Ach from the vagal nerve endings. Ach will leads to bradycardia before blockade of cardiac M2 receptors by atropine.
