What effect does the parasympathetic nervous system have on heart rate?

The parasympathetic nervous system plays a crucial role in heart rate regulation, primarily functioning through the release of acetylcholine at the sinoatrial (SA) node, which is the heart's natural pacemaker. When acetylcholine binds to its receptors on the cells of the SA node, it leads to a decrease in heart rate by slowing the depolarization of heart muscle cells. This action is a key aspect of the "rest and digest" response, as it promotes a state of calm and relaxation in the body, reducing the heart's workload and energy expenditure during non-stressful situations.

In contrast, the sympathetic nervous system would mediate an increase in heart rate through the action of adrenaline, but this is separate from the parasympathetic influence. The parasympathetic system's effect stands out for its role in decreasing heart rate, particularly during times of rest, as opposed to during exercise when heart rate typically increases due to sympathetic stimulation. Hence, the correct answer effectively captures the primary mechanism of the parasympathetic nervous system related to heart rate control.