What is the basic autonomic effect of the sympathetic and parasympathetic divisions on heart rate?

• A. Sympathetic increases heart rate and contractility; parasympathetic reduces heart rate.
• B. Sympathetic decreases HR; parasympathetic increases.
• C. Both increase heart rate.
• D. Both decrease heart rate.

Answer: (A)

Autonomic control of heart rate is antagonistic: the sympathetic division speeds up the heart, while the parasympathetic slows it down. The best statement reflects both the chronotropic and inotropic effects of sympathetic activation and the pure negative chronotropic effect of parasympathetic activity. When sympathetic nerves release norepinephrine onto beta-1 receptors in the SA node, heart rate rises and the force of contraction (contractility) also increases, through increased cAMP and enhanced calcium availability. In contrast, parasympathetic input via acetylcholine on M2 receptors in the SA and AV nodes slows the rate of depolarization, lengthens repolarization, and slows conduction, yielding a lower heart rate. Therefore, the described combination—sympathetic increasing heart rate and contractility, and parasympathetic reducing heart rate—most accurately captures the basic autonomic effects on heart rate. The other ideas implying that both divisions do the same thing or that sympathetic decreases heart rate contradict this established opposite control.