Action Potential in Nerve cell: Moving Depolarization of axon
Resting Potential = Inside of cell is negative (-) charged; outside of cell is positive (+) charged. Na+ is outside, K+ is inside.
Polarization = Resting Potential; inside of cell is (-) charged.
Depolarization (Action potential) = Inside of cell is negative (-). K channel is closed, Na channel is open; Na+ flood into cell causing cell to become (+) charged on inside.
Repolarization = Return to resting potential. Na channel is closed and K channel is open; K+ flood out of cell and and restore (+) charge outside cell. Inside of cell becomes polar ( or negative charged). NA+/K+ pump fully restore the concentration gradient by transporting Na+ out of cell and K+ into cell to resting membrane potential.
Useful Links for Animations:
Very useful animation from Harvard: http://outreach.mcb.harvard.edu/animations/actionpotential_short.swf
Animation of AP: http://bcs.whfreeman.com/thelifewire/content/chp44/4402s.swf
Animation of AP: http://bcs.whfreeman.com/thelifewire/content/chp44/4402002.html