Which ion rushes into the cell during depolarization?

• A. Potassium
• B. Calcium
• C. Sodium
• D. Chloride

Answer: (C)

Depolarization is the rapid change in membrane potential that makes the inside of the cell less negative. This shift is driven mainly by the influx of sodium ions through voltage-gated sodium channels that open when the membrane reaches threshold. The inward Na+ current pushes the potential from around -70 mV toward +30 mV, producing the sharp upstroke of the action potential. Potassium channels open a bit later to let K+ exit and repolarize the cell. Calcium can contribute to depolarization in certain tissues (like the heart), but in the typical quick depolarization of neurons and skeletal muscle, sodium entry is the primary driver. Chloride movement generally stabilizes or hyperpolarizes the membrane rather than causes depolarization.