SPREAD OF DEPOLARIZATION DOWN AXON
When membrane resistance to Na+ decreases, Na+ rushes in shifting the resting potential from -70 to +30 millivolts inside of the neuron. This is immediately followed by K+ rushing out of the neuron to promote recovery.
The botton figure demonstrates the absolute recovery period (period when Na+ is rushing in) and the relative recovery period (period when K+ is rushing out).
This process (the action potential) moves down the axon until it reaches the terminal endings.