What effect does parasympathetic innervation have on heart rate?

What effect does parasympathetic innervation have on heart rate?

• A. Increases heart rate
• B. Decreases heart rate
• C. No effect on heart rate
• D. Variable effect on heart rate

Answer: (B)

Parasympathetic innervation slows the heart. Nerves of the vagus release acetylcholine onto the pacemaker cells in the sinoatrial (and to some extent the AV) node. Acetylcholine binds to muscarinic M2 receptors, activating pathways that increase potassium conductance. This hyperpolarizes the pacemaker cells and slows the rate of spontaneous depolarization (the slope of phase 4), reducing the firing rate of the SA node. It also slows conduction through the AV node. The net effect is a slower heart rate, opposed by sympathetic input which increases rate.

Parasympathetic innervation slows the heart. Nerves of the vagus release acetylcholine onto the pacemaker cells in the sinoatrial (and to some extent the AV) node. Acetylcholine binds to muscarinic M2 receptors, activating pathways that increase potassium conductance. This hyperpolarizes the pacemaker cells and slows the rate of spontaneous depolarization (the slope of phase 4), reducing the firing rate of the SA node. It also slows conduction through the AV node. The net effect is a slower heart rate, opposed by sympathetic input which increases rate.