How atropine speeds the heart rate?

• To understand how atropine affects the heart, first consider how the heart's electrical conduction system functions.

Without the drug
When the neurotransmitter acetylcholine is released, the vagus nerve stimulates the sinoatrial (SA) node (the heart's pacemaker) and atrioventricular (AV) node, which controls conduction between the atria and ventricles of the heart. This inhibits electrical conduction and causes the heart rate to slow down.

With the drug
When the patient receives atropine, a cholinergic blocking drug, it competes with acethylcoline for for binding with the cholinergic receptors on the SA and AV nodes. By blocking acetylcholine, atropine speeds up the heart rate.

(Clinical Pharmacology: Incredibly Easy pg. 31)