How does the parasympathetic nervous system affect heart rate?

The parasympathetic nervous system primarily exerts its influence on heart rate through the release of the neurotransmitter acetylcholine, which acts on muscarinic receptors in the heart. When activated, the parasympathetic system generally works to slow down the heart rate. This occurs because acetylcholine decreases the pacemaker potential of the sinoatrial (SA) node, which is responsible for initiating the heartbeat. By reducing the rate at which the pacemaker cells reach the threshold for depolarization, the heart beats less frequently, effectively lowering the heart rate.

This action counterbalances the excitatory effects of the sympathetic nervous system, which typically increases heart rate during times of stress or physical activity. The parasympathetic nervous system’s role in slowing the heart rate is crucial for maintaining homeostasis, especially during rest and relaxation periods. Understanding this mechanism is vital for appreciating how autonomic regulation helps to manage cardiac function.