Which comes first in the cardiac cycle: electrical activity or contraction?

• A. Contraction precedes electrical activity.
• B. Electrical activity must precede contraction.
• C. They occur simultaneously.
• D. Electricity and contraction are unrelated.

Answer: (B)

Electrical activity must precede contraction. The heart’s pumping action is driven by electrical impulses that trigger the muscle fibers to contract through excitation-contraction coupling. In a normal cycle, an electrical impulse starts in the sinoatrial node and spreads across the atria, causing atrial contraction. The impulse then passes through a brief delay at the atrioventricular node, allowing filling, before traveling to the ventricles and producing ventricular contraction. Because the contraction relies on the preceding electrical depolarization, it cannot occur first. The other ideas—contraction occurring before electrical activity, happening simultaneously, or being unrelated—don’t fit how the heart coordinates its rhythm and force.