Increased vagal tone to the heart directly causes which of the following effects on the cardiac action potential?
- A Shortening of the action potential duration in atrial muscle ✓
- B Increased slope of phase 4 depolarization in the SA node
- C Increased conduction velocity through the AV node
- D Increased contractility of the ventricles
Correct answer: A. Shortening of the action potential duration in atrial muscle
Explanation
Vagal stimulation activates muscarinic receptors, increasing potassium conductance and decreasing cyclic AMP. This shortens the atrial action potential duration and decreases the slope of phase 4 in the SA node, slowing the heart rate. It also slows AV conduction. Vagal tone has minimal direct effect on ventricular contractility.
Reference: Ganong's Review of Medical Physiology, 26th ed.
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