During depolarization, which ions move in which direction across the neural membrane?

• A. Sodium enters and Potassium exits
• B. Sodium exits and Potassium enters
• C. Calcium enters and Chloride exits
• D. Chloride enters and Sodium exits

Answer: (A)

Depolarization is driven by a rapid inward current of sodium ions as voltage-gated sodium channels open, making the inside of the neuron more positive. The defining moment is sodium entering the cell, which short-circuits the resting negative charge. Potassium movement plays its major role later, as channels open to let K+ exit and help restore the resting potential during repolarization. While some potassium current can occur as depolarization begins, the key feature of depolarization is the influx of sodium. Calcium and chloride movements aren’t the rapid depolarization mechanism in most neurons, so they don’t describe the main event of this phase.