What response occurs when the vagus nerve is stimulated?

What response occurs when the vagus nerve is stimulated?

• A. Enhanced myocardial contractility
• B. Slowed conduction through the AV node
• C. Accelerated automaticity
• D. Tachycardia

Answer: (B)

When the vagus nerve is stimulated, parasympathetic input to the heart increases. The transmitter acetylcholine acts on M2 receptors in the SA and AV nodes, causing hyperpolarization and a slower rate of spontaneous firing. In the AV node, this effect slows impulse conduction and lengthens the AV nodal refractory period. The net result is slower conduction through the AV node, which is why this option best describes the response. Think of it as a brake on the heart's electrical system: vagal stimulation lowers heart rate and particularly reduces how quickly impulses pass from the atria to the ventricles. Enhanced contractility, accelerated automaticity, and tachycardia would be effects you'd expect from sympathetic stimulation or reduced vagal tone, not from vagal activation.

When the vagus nerve is stimulated, parasympathetic input to the heart increases. The transmitter acetylcholine acts on M2 receptors in the SA and AV nodes, causing hyperpolarization and a slower rate of spontaneous firing. In the AV node, this effect slows impulse conduction and lengthens the AV nodal refractory period. The net result is slower conduction through the AV node, which is why this option best describes the response.

Think of it as a brake on the heart's electrical system: vagal stimulation lowers heart rate and particularly reduces how quickly impulses pass from the atria to the ventricles. Enhanced contractility, accelerated automaticity, and tachycardia would be effects you'd expect from sympathetic stimulation or reduced vagal tone, not from vagal activation.