What happens during the depolarization phase of a nerve cell?

During the depolarization phase of a nerve cell, sodium ions enter the nerve membrane. This process begins when a stimulus causes the nerve's membrane potential to become less negative, eventually reaching a threshold that opens voltage-gated sodium channels. As these channels open, sodium ions, which are at a higher concentration outside the cell compared to the inside, rush into the cell. This influx of positively charged sodium ions causes the interior of the cell to become more positive, effectively depolarizing the membrane. This change in voltage is essential for the propagation of an action potential along the nerve fiber, leading to communication across the nervous system.

In this context, the importance of sodium's role during depolarization highlights the fundamental processes of excitability in nerve cells, contrasting with what occurs during repolarization, where potassium ions typically exit the cell to restore the resting membrane potential.