What is the primary effect of parasympathetic stimulation on the heart?

What is the primary effect of parasympathetic stimulation on the heart?

• A. Increase heart rate.
• B. Increase conduction velocity.
• C. Nerves that decrease heart rate.
• D. Decrease heart rate.

Answer: (D)

Parasympathetic stimulation slows the heart. The vagus nerve releases acetylcholine that acts on M2 receptors in the SA and AV nodes, increasing potassium leakage and reducing calcium entry. This hyperpolarizes pacemaker cells and lowers the slope of spontaneous depolarization, producing a slower heart rate (negative chronotropy) and slower AV conduction. The dominant effect is a decrease in heart rate. Increasing heart rate is a sympathetic effect, and increasing conduction velocity is not how parasympathetic input works; the description about nerves that decrease heart rate refers to the nerves themselves rather than the direct outcome on the heart.

Parasympathetic stimulation slows the heart. The vagus nerve releases acetylcholine that acts on M2 receptors in the SA and AV nodes, increasing potassium leakage and reducing calcium entry. This hyperpolarizes pacemaker cells and lowers the slope of spontaneous depolarization, producing a slower heart rate (negative chronotropy) and slower AV conduction. The dominant effect is a decrease in heart rate. Increasing heart rate is a sympathetic effect, and increasing conduction velocity is not how parasympathetic input works; the description about nerves that decrease heart rate refers to the nerves themselves rather than the direct outcome on the heart.