An action potential moves along an axon due to the sequential opening of voltage-gated Na+ channels. The diagram below shows voltage-gated Na+ channels separated by a short distance in the plasma membrane of an axon. Initially (left panel), only channel (a) is open. Within a very short time (right panel), channel (b) also opens.
What causes the second voltage-gated Na+ channel to open?
After the first channel opens, the movement of many types of ions (both inside and outside the cell) alters the distribution of charges near the second channel, causing it to open.
When Na+ ions enter the cell through the first channel, the charge distribution across the membrane changes. Inside the cell, the increase in Na+ ions near the first channel makes that region more positive; as a result, negative ions are attracted to the region, while positive ions are repelled. Conversely, outside of the cell, the loss of Na+ ions makes the region near the first channel more negative; as a result, positive ions are attracted to that region, while negative ions are repelled. Together, all of these ion movements alter the charge (and thus the membrane potential) at the neighboring channel, allowing it to reach threshold.