Each sodium channel has an activation gate and an inactivation gate , while potassium channels have only one gate. At the resting state the sodium activation gates are closed, sodium inactivation gates are open, and potassium gates are closed. Resting membrane potential is at around -70 mv inside the cell. The sodium inactivation gates close and potassium gates open. Until the membrane is repolarized it cannot be stimulated, called the absolute refractory period .
Excess potassium leaves the cell causing a brief hyperpolarization .
Sodium activation gates close and potassium gates begin closing. During hyperpolarization the membrane can be stimulated but only with a greater than normal depolarization, the relative refractory period . Action potentials are self-propagated , and once started the action potential progresses along the axon membrane. You either have one or you don't.