why a neuron's voltage returns to about -70 mV following a graded potential that does not cause voltage to reach threshold.

a.      why a neuron's voltage returns to about -70 mV following a graded potential that does not cause voltage to reach threshold. That is to say, what changes in ion flow occur before RMP before RMP is reached? Why is there no net movement of ions (except via the pump and continuous inflow of sodium) once RMP is reached? 

b. if the RMP would increase, decrease, or stay the same if the sodium-potassium ATPases stop working for several hours. What is the rationale for your prediction? How would the persistent failure of the pumps influence the ability of the neuron to generate action potentials? (Assume that the neuron will not burst because of a possible increase in the concentration of ions in the neuron.) reached? Why is there no net movement of ions (except via the pump and continuous inflow of sodium) once RMP is reached?