During depolarization, which ion movement occurs?

During depolarization, which ion movement occurs?

• A. Sodium enters the axoplasm; reverse polarity
• B. Potassium enters the axoplasm
• C. Sodium exits the axoplasm
• D. Chloride enters the axoplasm

Answer: (A)

Depolarization is driven by a rapid influx of sodium ions into the neuron. When the membrane reaches threshold, voltage-gated sodium channels open and Na+ moves from outside into the axoplasm, making the inside of the cell more positive. This inward Na+ current flips the membrane polarity from negative toward positive. Potassium leaving the cell is what mainly mediates repolarization, not depolarization, so potassium entering would not produce the depolarization phase. Sodium exiting the axoplasm would reduce positive charge inside and oppose depolarization, and chloride entering would typically hyperpolarize or stabilize the resting potential rather than cause the depolarization peak.

Depolarization is driven by a rapid influx of sodium ions into the neuron. When the membrane reaches threshold, voltage-gated sodium channels open and Na+ moves from outside into the axoplasm, making the inside of the cell more positive. This inward Na+ current flips the membrane polarity from negative toward positive. Potassium leaving the cell is what mainly mediates repolarization, not depolarization, so potassium entering would not produce the depolarization phase. Sodium exiting the axoplasm would reduce positive charge inside and oppose depolarization, and chloride entering would typically hyperpolarize or stabilize the resting potential rather than cause the depolarization peak.