Depolarization is characterized by the movement of which ion into the cell to initiate an action potential?

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

Answer: (D)

The key idea is that depolarization of a neuron’s membrane is driven by a rapid inward flow of sodium ions. When a stimulus reaches threshold, voltage-gated sodium channels open, allowing Na+ to rush into the cell. This inward movement makes the inside of the cell less negative (more positive), pushing the membrane potential toward and past the threshold to trigger the action potential. Potassium, on the other hand, mainly moves out during repolarization to restore the negative resting potential. Calcium can contribute to other processes in muscle and certain neurons, but not the primary initiation of a typical neuronal action potential. Chloride movement generally stabilizes or hyperpolarizes the membrane rather than initiating depolarization. So, the ion responsible for starting depolarization is sodium.