The rapid repolarization of a neuron during the second half of an action potential is due mostly to the

Passage of Sodium ion particles into the cell through gated sodium channels.  

Explanation:

Action Potential results when various particles cross the neuron film. An upgrade first causes sodium channels to open. Since there are a lot more sodium particles outwardly, and within the neuron is negative comparative with the outside, sodium particles race into the neuron.   Repolarization is a phase of an activity potential where the cell encounters a lessening of voltage because of the efflux of potassium ion particles along its electro-chemical angle. This stage happens after the cell arrives at its most noteworthy voltage from depolarization.