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. A diagram of two panels with an arrow between them showing Na+ ions outside a cell and inside a cell, separated by a plasma membrane with two Na+ channels labeled (a) and (b). In the first panel, the (a) channel is open and ions flow from outside to inside the cell. In the second panel, both channels are open.
Which statement correctly describes what causes the second voltage-gated Na+ channel to open?
- As Na+ ions enter the cell through the first channel, they spread out from the channel. When these Na+ ions reach the second channel, it opens.
- As Na+ ions enter the cell through the first channel, Na+ ions outside the cell move toward the open Na+ channel. When the concentration of Na+ ions near the second channel becomes low enough, the second channel opens.
- After the first channel opens, the movement of Na+ ions (both inside and outside the cell) alters the Na+ ion distribution across the membrane near the second channel, causing it 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.
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