Which statement accurately describes how the autonomic divisions regulate heart rate and contractility?

• A. Sympathetic decreases heart rate; parasympathetic increases contractility.
• B. Sympathetic stimulation via β-adrenergic receptors increases heart rate and contractility; parasympathetic stimulation (vagus) decreases heart rate and has modest effects on contractility.
• C. Parasympathetic via β-adrenergic receptors increases heart rate; sympathetic decreases.
• D. Both divisions increase heart rate and contractility.

Answer: (B)

The autonomic nervous system modulates heart rate and the force of contraction in opposite ways: sympathetic activation increases both how fast the heart beats and how forcefully it contracts, while parasympathetic activation slows the heart rate and has only a small effect on contraction.

Sympathetic stimulation works mainly through β1 receptors in the SA node, AV node, and myocardium. This raises cAMP, which enhances funny channels and calcium entry, speeding up depolarization and AV conduction and boosting calcium availability for stronger contraction. So heart rate and contractility rise.

Parasympathetic stimulation, via the vagus nerve, acts mainly on M2 receptors in the SA and AV nodes. This activates Gi to lower cAMP, opens potassium channels that hyperpolarize cells, and slows conduction and firing rate, reducing heart rate. Its effect on ventricular contractility is minimal because parasympathetic input to the ventricles is limited, so the strength of contraction doesn’t change much.

That’s why the statement describing sympathetic increases in heart rate and contractility and parasympathetic decreases heart rate with only modest effects on contractility best fits how the divisions regulate the heart.