Parasympathetic neurons control which aspect of heart function?

• A. Resting heart rate
• B. Heart rate variability
• C. Contractility
• D. Venous return

Answer: (A)

Parasympathetic input to the heart mainly slows the heart rate at rest. The vagus nerve releases acetylcholine onto the SA and AV nodes, increasing potassium conductance and reducing the rate of spontaneous depolarization. This lowers the firing rate of the SA node and slows AV conduction, setting the baseline, resting heart rate. When vagal influence withdraws, heart rate rises as sympathetic effects become more prominent.

Heart rate variability is shaped by autonomic balance and can be influenced by parasympathetic activity, but it isn’t the primary function parasympathetic nerves regulate. Contractility is mainly increased by sympathetic activation, not parasympathetic. Venous return depends more on blood volume and venous tone, with little direct parasympathetic control.