Which ion is primarily responsible for the depolarization phase of an action potential?

During the depolarization phase of an action potential, the membrane potential of a neuron becomes more positive, which is mainly driven by the influx of sodium ions (Na+). When a neuron is stimulated, voltage-gated sodium channels open, allowing Na+ to flow into the cell. This influx occurs because the concentration of sodium is much higher outside the cell compared to the inside, and the electrical gradient also favors the entry of positive sodium ions as the internal environment becomes less negative.

As sodium enters the neuron, it causes the membrane potential to rise rapidly from its resting state, creating a positive change that is characteristic of the depolarization phase. This rapid change in voltage is critical for the propagation of action potentials along the neuron, facilitating neural communication. The subsequent repolarization phase that follows is primarily characterized by the efflux of potassium ions (K+), but during depolarization, it is the sodium ions that play the dominant role in driving the membrane potential to positive values.