In which of the following phase sodium voltage gated channel is open but sodium–potassium pump is closed?

(a) Polarised

(b) Repolarised

(c) Depolarised

Sodium voltage gated channel is open but sodium–potassium pump is closed is depolarised phase.

In the depolarised phase of an action potential, sodium voltage-gated channels are indeed open, allowing sodium ions to flow into the cell. This influx of sodium ions is what causes the rapid depolarization of the cell membrane.

However, it's important to note that the sodium-potassium pump (Na⁺/K⁺ pump) is not directly involved in the depolarisation phase of the action potential. The sodium-potassium pump operates continuously to maintain the resting membrane potential and to restore the ion gradients after the action potential has occurred. It pumps sodium ions out of the cell and potassium ions into the cell, helping to restore the resting state of the neuron.

So, during the depolarisation phase, while sodium voltage-gated channels are open and sodium ions are entering the cell, the sodium-potassium pump is not directly responsible for these changes. It becomes more active during the repolarization and hyperpolarisation phases of the action potential to restore the ionic balance across the cell membrane.