Open at depolarization (~−55 mV), allowing rapid Na+ influx. Have activation gate (fast) and inactivation gate (faster), producing transient inward current.
To understand voltage-gated sodium channels, start with what you know about the resting membrane potential: at rest, the inside of the neuron is about −70 mV relative to the outside, maintained largely by the selective permeability of potassium channels and the sodium-potassium pump. Sodium ions are abundant outside the cell, and there is a strong electrochemical drive pushing them inward — but at rest, voltage-gated sodium channels are closed, so they cannot enter.
When a depolarizing stimulus brings the membrane to threshold (~−55 mV), voltage-gated sodium channels sense the change in electric field and undergo a conformational shift: the activation gate (the m gate) swings open. Sodium ions rush inward down their electrochemical gradient, rapidly depolarizing the membrane further toward +40 mV. This is the rising phase of the action potential. The key feature is that the channel has a second gate — the inactivation gate (h gate) — that responds to depolarization more slowly. Within a millisecond of opening, the inactivation gate swings into the pore and plugs it, terminating Na+ influx even while the membrane is still depolarized. The current is transient precisely because of this dual-gate design.
The inactivated state is fundamentally different from the closed state. A closed channel can open if threshold is reached; an inactivated channel cannot, regardless of voltage. It can only recover — reset to the closed state — after the membrane repolarizes. This recovery takes time, which creates the absolute refractory period: a window after each action potential during which no stimulus, however strong, can trigger another. The refractory period ensures that action potentials travel in only one direction along an axon and limits the maximum firing rate of neurons.
Compare this to ligand-gated channels you've encountered: those open in response to neurotransmitter binding, not voltage, and they don't have the same fast inactivation mechanism. Voltage-gated channels are tuned for speed and precision — the activation and inactivation kinetics are carefully matched to produce the sharp, stereotyped spike that makes action potentials reliable digital signals over long distances.