Answer:
Once the action potential has passed there are alot of K ions outside the cell and a alot of Na ions inside the cells. This would not allow another AP potential to pass as there is no concentration gradients for the ions to move down. To return the membrane to its resting potential an enzyme called sodium potassium ATPase used energy from ATP to pump 3 sodium out and 2 potassium in at the same time. This returns the concentration gradient and thanks to the difference in the number of each ion moved it also restores the electrical gradient of "membrane potnetial"