During an action potential, which movement of ions depolarizes the cell?

• A. Potassium exits the cell
• B. Chloride enters the cell
• C. Sodium enters the cell, providing a positive charge
• D. Calcium leaves the cell

Answer: (C)

Depolarization during an action potential happens when positively charged ions rush into the cell, making the inside less negative. In neurons, this is driven by voltage-gated sodium channels that open quickly and allow Na+ to enter, bringing a surge of positive charge inside and driving the membrane potential toward zero and above. Potassium leaving is what helps reset the membrane later (repolarization), chloride entering would make the inside more negative (inhibitory effect), and calcium leaving would reduce positive charge rather than depolarize. So the movement that depolarizes the cell is sodium entering, providing a positive charge.