Atropine is a drug that is used to treat symptoms of bradycardia (low heart rate). It affects the parasympathetic nervous system input to the heart. Knowing this, which of the following is atropine?
a) Agonist of parasympathetic receptors
b) Antagonist of parasympathetic receptors
c) Agonist of sympathetic receptors
d) Antagonist of sympathetic receptors