Which term describes the electrical event in the heart during each heartbeat where sodium enters and potassium exits to stimulate contraction?

• A. Depolarization
• B. Repolarization
• C. Ischemia
• D. Hyperkalemia

Answer: (A)

Depolarization is the electrical change that occurs at the start of each heartbeat. It happens when voltage-gated sodium channels open and sodium ions rush into cardiac cells, making the inside of the cell more positive. This rapid influx triggers the excitation–contraction sequence, leading to contraction of the heart muscle. Potassium exiting the cell is more closely associated with repolarization, which resets the cell’s resting voltage after the contraction. Ischemia refers to reduced blood flow causing tissue damage, and hyperkalemia means high potassium in the blood, both of which are not the normal initiating event for contraction. So the description—sodium entering to stimulate contraction—fits depolarization.