Final answer:
Depolarization reduces charge separation by allowing positively charged sodium ions to enter the neuron, reversing the membrane potential. This phase is followed by repolarization when potassium ions leave the cell to return it to the resting state.
Step-by-step explanation:
Depolarization is a reduction of charge separation within a neuron. During an action potential, the membrane potential reverses from typically around -70 mV (millivolts) to about +30 mV due to the influx of sodium ions (Na+). This influx is caused by the opening of voltage-gated Na+ channels, which allows the positively charged sodium ions to rush in, making the inside of the cell membrane less negative relative to the outside, hence reducing the charge separation.
Following this, repolarization begins as potassium ions (K+) exit the cell through newly opened voltage-gated K+ channels, returning the membrane potential towards its resting state. This repolarization moves the membrane potential back down, passing through the resting state into a briefly more negative state known as hyperpolarization. After the cell returns to its resting membrane potential, it is ready to undergo another action potential if stimulated.
It's important to note that the action potential moves down the axon in one direction due to the inactivation of the sodium channels immediately after depolarization, which prevents the action potential from traveling backward. This movement of the membrane potential during depolarization and repolarization constitutes a primary mechanism of nerve impulse transmission.