When an action potential is transmitted along the axon, the membrane reaches its depolarized state by opening Na+ channels. Both K+ (potassium) and Na+ (sodium) are now on both sides with close to equal concentration-ratios. It is the next step that confuses me: the K+ rushing out of the cell, thus repolarizing the membrane.
Wouldn't the two substances balance themselves out according to the Electric-/Substance-Concentration-Gradient once the Na+/K+ channels are open? I don't understand where that energy comes from, the energy that not only mediates polarization, but also the subsequent re-polarization.