What occurs during depolarization?

What occurs during depolarization?

• A. Sodium enters the axoplasm; reverse polarity (sodium in, potassium out)
• B. Potassium enters the axoplasm
• C. Chloride exits the axoplasm
• D. Calcium enters the axoplasm

Answer: (A)

Depolarization is the phase of an action potential where the membrane potential rises toward zero and becomes positive because sodium ions rush into the axoplasm through voltage-gated Na+ channels after threshold is reached. This inward Na+ current reverses the usual charge difference, so the inside of the neuron becomes briefly positive relative to the outside. The description that sodium enters and the interior becomes more positive directly captures this depolarization event. Potassium moving in would not drive the upstroke of the action potential; potassium tends to move out later to restore the resting potential. Chloride movement or calcium entry can occur in other contexts, but they do not define the primary depolarization mechanism in most neurons.

Depolarization is the phase of an action potential where the membrane potential rises toward zero and becomes positive because sodium ions rush into the axoplasm through voltage-gated Na+ channels after threshold is reached. This inward Na+ current reverses the usual charge difference, so the inside of the neuron becomes briefly positive relative to the outside. The description that sodium enters and the interior becomes more positive directly captures this depolarization event. Potassium moving in would not drive the upstroke of the action potential; potassium tends to move out later to restore the resting potential. Chloride movement or calcium entry can occur in other contexts, but they do not define the primary depolarization mechanism in most neurons.