because the sodium channels have a refractory period following activation, during which they cannot open again.
A slight depolarization of the membrane potential causes the sodium channels in the neuronal membrane to open. Therefore, the leading edge of additional nearby sodium channels is activated when an action potential depolarizes the membrane. This results in another depolarization spike, the leading edge of which opens up more nearby sodium channels, etc. So, from the point of start, a wave of depolarization propagates. If this were all there was to it, an action potential would move along an axon in every direction. Action potentials, however, go one way. This is made possible by the sodium channels' ability to close after activation, known as their refractory period. This guarantees that the action potential travels along the axon in a particular direction.
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