An action potential is caused by positive ions moving in and then out of the neuron at a certain spot on the neuron membrane. An action potential is initiated by a stimulus above a certain intensity or threshold. Not all stimuli initiate an action potential. The stimulus could be a pin prick, light, heat, sound or an electrical disturbance in another part of the neuron. Depolarization A stimulus causes a gate in the Na+ Channel to open. Since there is a high concentration of Na+ outside, Na+ diffuses into the neuron. The electrical potential changes to ~ +40 mV. Repolarization Depolarization causes the K+ Channel gate to immediately open. K+ diffuses out of the neuron. This reestablishes the initial electrical potential of ~-60 mV. Refractory Period During this time (~ 1 msec), the Na+ and K+ Channels cannot be opened by a stimulus. The Na+/K+ Pump actively pumps Na+ out of the neuron and K+ into the neuron. This reestablishes the initial ion distribution of the resting neuron.