What is the effect of the vagus nerve on heart rate?

What is the effect of the vagus nerve on heart rate?

• A. Slows the heart rate
• B. Increases the heart rate
• C. No effect on heart rate
• D. Causes tachycardia

Answer: (A)

Parasympathetic control of the heart via the vagus nerve slows the heart rate. The vagus releases acetylcholine onto muscarinic M2 receptors in the SA and AV nodes, which increases potassium conductance and decreases the slope of phase 4 depolarization in pacemaker cells. This makes it take longer for the SA node to reach threshold and slows conduction through the AV node, producing a lower heart rate and slower conduction. This negative chronotropic effect contrasts with sympathetic stimulation, which would increase heart rate. Vagal activity helps explain why certain maneuvers or early rest can reduce heart rate during tachyarrhythmias.

Parasympathetic control of the heart via the vagus nerve slows the heart rate. The vagus releases acetylcholine onto muscarinic M2 receptors in the SA and AV nodes, which increases potassium conductance and decreases the slope of phase 4 depolarization in pacemaker cells. This makes it take longer for the SA node to reach threshold and slows conduction through the AV node, producing a lower heart rate and slower conduction. This negative chronotropic effect contrasts with sympathetic stimulation, which would increase heart rate. Vagal activity helps explain why certain maneuvers or early rest can reduce heart rate during tachyarrhythmias.