How does the axonal membrane potential become restored following action potential depolarization?
The depolarizing event not only opens voltage-gated Na+ channels. It also opens voltage-gated K+ channels. These potassium channels open more slowly than the sodium channels. As a result, they open about the same time as the sodium channels are closing. These two events together, result in a repolarization of the membrane potential. As the sodium channels are inactivitated, Na+ movement into the cell slows down. Concurrently, as K+ channels open, potassium ions move out of the cell. These two actions together change the membrane potential from about +30 mV to -70 mV.
Interestingly, the outflow of K+ can be so great and so rapid, that it actually leads to a hyperpolarizing effect. In other words, for a brief amount of time, the membrane potential can go all the way down to -90 mV. As these K+ channels ...