The exchange of ions across the cell membrane is most closely associated with which event in excitable cells?

• A. Hyperpolarization
• B. Repolarization
• C. Membrane polarization
• D. Depolarization

Answer: (D)

During depolarization, the exchange of ions across the membrane directly changes the membrane potential. Voltage-gated sodium channels open and sodium rushes into the cell, making the interior less negative and often reversing toward positive. This rapid inward current is the key ion movement that initiates the action potential.

Repolarization follows as potassium exits to restore the negative interior, and hyperpolarization occurs when potassium channels remain open a bit longer, dipping membrane potential below resting. Membrane polarization refers to the resting state. So the ion flow most closely tied to the change that begins the action potential is depolarization.