The positive feedback of the rising phase slows and comes to a halt as the sodium ion channels become maximally open. At the peak of the action potential, the sodium permeability is maximized and the membrane voltage V is nearly equal to the sodium equilibrium voltage E. However, the same raised voltage that opened the sodium channels initially also slowly shuts them off, by closing their pores; the sodium channels become inactivated. This lowers the membranes permeability to sodium relative to potassium, driving the membrane voltage back towards the resting value. At the same time, the raised voltage opens voltage-sensitive potassium channels; the increase in the membranes potassium permeability drives V towards E. Combined, these changes in sodium and potassium permeability cause V to drop quickly, repolarizing the membrane and producing the "falling phase" of the action potential.