Which physiological condition causes decreased heart rate?

Vagal nerve stimulation leads to a decreased heart rate as it is primarily associated with the activities of the parasympathetic nervous system, which is responsible for promoting a "rest and digest" state in the body. When the vagus nerve is stimulated, it releases acetylcholine that binds to receptors in the heart, resulting in a slower heart rate (bradycardia). This effect is particularly prominent in situations where the body is not under stress, allowing for relaxation and decreased cardiac demand.

In contrast, increased blood pressure typically triggers mechanisms that might elevate heart rate to maintain adequate cardiac output. Increased physical activity generally activates the sympathetic nervous system, which provides a fight-or-flight response, resulting in an increased heart rate to supply more oxygen and nutrients to the muscles. Similarly, sympathetic nervous system activation directly causes tachycardia, as it releases catecholamines that enhance heart rate and contractility. Thus, vagal nerve stimulation stands out as the physiological condition that specifically results in a decreased heart rate.