Describe the ion movement across the cell membrane during the following stages: resting potential, threshold potential, depolarization, repolarization.

Resting potential: the inside of the neuron's axon is -70mV versus the outside of the membrane 

Threshold potential: If the impulse that is received by this neuron's dendrtes meets or exceeds its threshold level enough Na+ activation gates (which are protein ion channels in the axon membrane) will open and Na+ will start to flow into the axon following a charge and concentration gradient.

Depolarization: If the action potential has been initiated (and Na+ has started to move into the axon) then the internal negative charged areas of the membrane will open their Na+ gates as well. 

Repolarization: Once K+ have started to move outside of the axon, the original charges along the axon membrane start to become reestablished