During an action potential, the rapid decrease in sodium ion permeability and the simultaneous and rapid increase in potassium ion permeability is responsible for

The answer is repolarization. In the first phases of an action potential,
Sodium ions flow into the cell, through the voltage-gated ion channels, causing depolarization from the resting potential of -70 mV. In the later stages of the action potential, the potassium ion selective filter channels open and potassium ions move from inside of the cell to the outside. This repolarizes the cell before sodium ions begin to be pumped out of the cell.