When a neuron axon is at rest, there is a negative charge in relation to the outside. The membrane of the axon is semi-permeable to certain ions only. At rest, the inside of the neuron axon is 70mV less than the outside. When a depolarization current begins, (Click photo to see this process) the resting potential becomes 0mV. It will steadily move down towards -55mV until an action potential occurs. If the threshold is not reached, nothing will happen. A stimulation of the membrane will change the permeability of the membrane so that the ions can now go in and out, thus changing the charge and sending the impulse. As the action potential moves down, sodium ions are entering while potassium ions are exiting. Then, the sodium ions move down again to get ready to exit, while the potassium ions enter gain. As the potassium ions leave, the negative charge inside the neuron is restored. ATP drives this process.