Which statement about depolarization and contraction is correct?

• A. Depolarization is associated with contraction; repolarization is the return to resting state.
• B. Depolarization is relaxation; repolarization is contraction.
• C. Depolarization and repolarization are unrelated to contraction.
• D. Depolarization stops the heart.

Answer: (A)

Depolarization triggers the heart muscle to contract, and repolarization lets it relax. When cardiac cells depolarize, ions flow through voltage-gated channels, calcium enters, and the muscle fibers form cross-bridges that produce contraction (systole). After the peak of the action potential, repolarization restores the resting membrane potential by moving ions out, which allows calcium to be removed from the cytoplasm and the muscle to relax (diastole). In ECG terms, ventricular depolarization (the QRS complex) leads to contraction, and ventricular repolarization (the T wave) leads to relaxation. Therefore, saying depolarization is associated with contraction and repolarization is the return to resting state accurately describes how electrical activity governs the heart’s mechanical function.