1. When the membrane is at rest, the channel is blocked by a gating portion of the protein. The resting membrane maintains a positive charge on the outside and a negative charge on the inside. This charge disparity accrues as a consequence of two factors. A sodium-potassium pump (not shown here) forces sodium (red, positive charge) outside the cell membrane, leaving the anionic (blue, negative charge) proteins behind in the cytoplasm.
2. When the sodium channel protein senses a change in voltage resulting from the opening of adjacent pores (depolarization), a solenoid-like portion of the protein in the wall of the channel (red coil) begins to contract.
3. The coil opens the gate as it contracts, allowing sodium ions to rush through the channel into the cell, motivated by diffusion and the potential gradient. The timing of this process is critical. If the channel continues to admit sodium, the cell cannot respond to further stimuli and would quickly die.
4. Since channel cannot close while the membrane is depolarized, a special portion of the protein (green ball) acts like a plug to close the channel until the sodium-potassium pump can restore the resting membrane potential.
5. In a few milliseconds, the pump has removed the excess sodium, the membrane potential is restored, the gate is closed, and the plug relaxes, ready for the next stimulus.