Which statement describes the effect of parasympathetic activity on heart rate?

Which statement describes the effect of parasympathetic activity on heart rate?

• A. Increases heart rate by raising SA node firing.
• B. Decreases heart rate by signaling SA and AV nodes.
• C. Has no effect on heart rate.
• D. Only affects the contractions of the ventricular muscle.

Answer: (B)

Parasympathetic input to the heart primarily slows the rate by acting on the pacing and conducting pathways—the SA and AV nodes. The neurotransmitter acetylcholine binds to M2 receptors on these nodal cells, which increases potassium efflux and makes the cells more negative. This hyperpolarizes the pacemaker cells and reduces the slope of their spontaneous depolarization, so the SA node fires more slowly. At the AV node, acetylcholine slows conduction, increasing the delay between atrial and ventricular activation. Together, these actions lower heart rate. Parasympathetic influence on the ventricles is minimal, so the primary effect is not on ventricular contraction but on rate and nodal conduction. That’s why the statement that parasympathetic activity decreases heart rate by signaling SA and AV nodes is the best description.

Parasympathetic input to the heart primarily slows the rate by acting on the pacing and conducting pathways—the SA and AV nodes. The neurotransmitter acetylcholine binds to M2 receptors on these nodal cells, which increases potassium efflux and makes the cells more negative. This hyperpolarizes the pacemaker cells and reduces the slope of their spontaneous depolarization, so the SA node fires more slowly. At the AV node, acetylcholine slows conduction, increasing the delay between atrial and ventricular activation. Together, these actions lower heart rate. Parasympathetic influence on the ventricles is minimal, so the primary effect is not on ventricular contraction but on rate and nodal conduction. That’s why the statement that parasympathetic activity decreases heart rate by signaling SA and AV nodes is the best description.