The voltage-gated sodium channel has several functional parts. One portion of the channel determines its ion selectivity. This particular channel is quite selective for sodium ions. Even the chemically similar potassium ions cannot pass through the channel.
Another portion of the channel serves as a gate that can open and close. The gate is controlled by a voltage sensor, which responds to the level of the membrane potential.
An inactivation gate: this limits the period of time the channel remains open, despite steady stimulation.
At a typical resting membrane potential (for example, -70 mV) the channel is closed. Then should any factor depolarize the membrane potential sufficiently (for example, to -50 mV), the voltage sensor moves outward and the gate opens.