What physiological response occurs when atropine is administered?

When atropine is administered, it acts as an anticholinergic agent, which means it blocks the effects of the neurotransmitter acetylcholine at muscarinic receptors in the heart. Normally, acetylcholine, released by the vagus nerve, slows down the heart rate by exerting a parasympathetic influence. By inhibiting this action, atropine effectively reduces the vagal tone, leading to an increase in heart rate. This increase occurs as the heart becomes less influenced by the calming effects of parasympathetic activity, allowing the intrinsic firing rate of the sinoatrial (SA) node to prevail.

In the case of the other choices, the administration of atropine does not lead to a decreased heart rate, stable vascular resistance, or no change in heart rhythm, as these outcomes would be indicative of either continued parasympathetic activity or neutral effects, which is contrary to the mechanism of action of atropine. Thus, the physiological response to atropine is marked by an increased heart rate due to the decrease in parasympathetic influence on the heart.