Comprehensive Osteopathic Medical Licensing Examination (COMLEX USA) Practice

What is the main response of the parasympathetic nervous system on heart rate?

1. Increases heart rate

2. Decreases heart rate

3. No change

4. Irregular heart rhythm

The correct answer is: Decreases heart rate

The main response of the parasympathetic nervous system on heart rate is to decrease it. This is primarily mediated through the vagus nerve, which releases acetylcholine at the cardiac pacemaker cells. Acetylcholine binds to muscarinic receptors, leading to a reduction in the heart rate by slowing the depolarization of the sinoatrial node, which is responsible for initiating the heartbeat. In a resting state or during activities that require a calming effect, such as digestion, the parasympathetic system predominates, resulting in bradycardia or a generally slower heart rate. This contrasts with the sympathetic nervous system, which would typically increase the heart rate in response to stress or excitement. The impact of the parasympathetic nervous system is significant in maintaining homeostasis and ensuring that the body conserves energy during less active periods.