When do voltage gated Na channels open?

Voltage-gated sodium channels open when the membrane potential reaches a certain threshold level, which is typically around -55 mV in neurons. This threshold is crucial because it is the point at which the influx of sodium ions (Na+) becomes more favorable due to the opening of these channels. When the channels open, sodium rushes into the cell, causing rapid depolarization – a key event in the generation of action potentials.

This process is fundamental for the rapid transmission of electrical signals along neurons and is an essential function of excitatory cells in both nervous and muscular tissues. The other choices describe different phases or states in the action potential cycle, but they do not pertain specifically to the critical moment when voltage-gated sodium channels become activated.