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MBBS QUESTION #10238
Question 1
The plateau (phase 2) of the ventricular myocardial action potential, mediated by a balance of inward calcium current and outward potassium current, is physiologically important because it:
  • Produces a prolonged refractory period that prevents tetanic contraction of cardiac muscle✔️
  • Is responsible for the rapid depolarization (phase 0) seen on the action potential upstroke
  • Causes the resting membrane potential to become unstable
  • Has no relationship to the QT interval on the surface ECG
Correct Answer Explanation
The prolonged plateau phase keeps the myocardium refractory to new stimuli for almost the entire duration of contraction, preventing summation and tetany — essential for the heart's rhythmic pump function. This phase corresponds to the ST segment/QT interval on the surface ECG, and its calcium influx also couples excitation to contraction.