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At rest, most of the Na⁺ channels are closed most of the time, and the membrane potential is mainly due to the open K⁺ channels. During the action potential both types of channel are more likely to be open, but the Na⁺ conductance increases sooner and far more than the K⁺ conductance.

The ion conductance channels are the gateways through which the ions must pass in moving between inside and outside of the cell. The gate in each channel bears an electric charge and is held at a particular position depending on the membrane potential. If the negativity of the inside end of the channel is diminished (that is, the membrane becomes depolarized), the gate will become open, the "permeability" of the channel to its specific ion is increased, and the contribution of that ion to the membrane potential rises. Most of the Na⁺ conductance channels are closed during "rest." A decrease in membrane polarization ("depolarization," lessened internal negativity) persuades the channels to open. The Na⁺ concentration gradient can then express itself more effectively, and in fact, Na⁺ ions flood inward, driven by that gradient.

Thus the depolarized zone becomes even more depolarized (more + inside), and moves toward the theoretical Na⁺ potential, +50 mV inside. As the inside is more positive, the outside is less; i.e. at that zone, the outside of the membrane becomes less positive than at rest.

 
Figure 9. Channels and conductance changes.

 
 
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