What describes the discharge of electrical energy that causes contraction in heart muscle?

• A. Interval
• B. Electrodes
• C. Depolarization
• D. Electrocardiogram

Answer: (C)

Depolarization is the electrical discharge that triggers heart muscle contraction. When cardiac cells depolarize, their membrane potential shifts from negative to positive, opening channels that release calcium inside the cells. This calcium enables the myofilaments to slide past each other, causing the muscle fibers to contract. The impulse travels through the heart’s conduction system so the contraction occurs in a coordinated way. The other terms describe parts of the recording or tools, not the energy that causes contraction: an interval is a time segment on the ECG, electrodes are the sensing pads, and an electrocardiogram is the tracing itself. So the depolarization that spreads through the myocardium best explains the contraction.