An increase in parasympathetic activity in the heart will cause what?

• A. Tachycardia
• B. Heart block
• C. Bradycardia
• D. Increased conduction velocity

Answer: (C)

An increase in parasympathetic activity slows the heart. Parasympathetic nerves release acetylcholine onto the SA and AV nodes, binding to M2 receptors. This opens potassium channels and reduces calcium currents, hyperpolarizing nodal cells and decreasing the rate of spontaneous firing as well as slowing conduction through the AV node. The net result is a slower heart rate with sometimes a longer PR interval—bradycardia. Tachycardia comes from sympathetic stimulation, and increased conduction velocity would be the opposite of what parasympathetic activation does. In normal physiology, the dominant effect of increased parasympathetic tone is bradycardia.