describe in detail a neuronal axon at resting potential, including the charges and ions present inside and outside the membrane. outline the processes of depolarization, repolarization, hyperpolarization, and return to resting potential and give a step-wise description of the events, including initiation of action potential at the trigger zone, the roles of the various ion-gated channels and the sodium/potassium pump, and the changes in voltage across the membrane. indicate where absolute and relative refractory periods are located and explain what each type of refractory period represents and how another action potential can be sent during this time period, if possible.