Which statement best describes depolarization and repolarization in cardiac cells?

• A. Depolarization causes heart contraction; Repolarization returns the heart to resting state.
• B. Depolarization causes relaxation; Repolarization causes contraction.
• C. Depolarization and repolarization are unrelated to contraction.
• D. Depolarization stops the heart.

Answer: (C)

Depolarization of cardiac cells triggers contraction through excitation-contraction coupling. When the cell depolarizes, calcium influx and release within the sarcomeres initiate the interaction of actin and myosin that pulls the muscle and produces a heartbeat (systole). Repolarization then restores the resting membrane potential as potassium exits the cell, allowing the muscle to relax (diastole) and be ready for the next beat. So, contraction follows depolarization, and relaxation follows repolarization. The statements that depolarization is linked to relaxation, or that depolarization and repolarization are unrelated to contraction, don’t fit how the heart actually works, and a description implying stopping the heart isn’t accurate.