In order for an action potential to be initiated, enough of an ion change to depolarize the cell to threshold is required at the axon hillock. Each action potential is identical to every other action potential and action potential strength is the same for the first one at the axon hillock and for the last one at the axon terminal and all the ones in between. The neuron is at resting membrane potential . Voltage-gated sodium channels open and sodium ions rush in to the cell. The rapid entry of sodium ions into the cell depolarizes the cell even further.
Potassium ions move out of the cell through the open potassium channels.
The voltage-gated potassium channels begin to close and some potassium ions begin to enter the cell through other kinds of potassium channels. The entry of the positive potassium ions makes the membrane potential return to its resting level and then the closing of ion channels returns the cell to resting levels of ion permeability. It occurs because the voltage-gated sodium channels have not had a chance to reset. Refractory periods limit the rate at which signals can be transmitted down the neuron. The stronger the stimulus, the more frequent the action potentials that result.