How does parasympathetic innervation affect the heart?

How does parasympathetic innervation affect the heart?

• A. It decreases heart rate.
• B. It increases heart rate.
• C. It increases force of contraction.
• D. It has no effect.

Answer: (A)

Parasympathetic input to the heart slows the rate by acting on the SA and AV nodes. Acetylcholine released from the vagus nerve binds M2 muscarinic receptors in these nodal tissues, activating Gi proteins, reducing cAMP, and opening certain potassium channels. This makes the nodal cells fire more slowly and slows conduction through the AV node. The result is a slower heart rate. Ventricular contraction is not enhanced by parasympathetic activity because the ventricles receive little parasympathetic innervation; sympathetic input mainly increases contractility. So the primary and best-described effect is a decrease in heart rate.

Parasympathetic input to the heart slows the rate by acting on the SA and AV nodes. Acetylcholine released from the vagus nerve binds M2 muscarinic receptors in these nodal tissues, activating Gi proteins, reducing cAMP, and opening certain potassium channels. This makes the nodal cells fire more slowly and slows conduction through the AV node. The result is a slower heart rate. Ventricular contraction is not enhanced by parasympathetic activity because the ventricles receive little parasympathetic innervation; sympathetic input mainly increases contractility. So the primary and best-described effect is a decrease in heart rate.