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. The larger the diameter of the axon, the lower the resistance for ions moving inside the cytoplasm of the axon, and the faster the action potential frequency will be. Since mammals have too many neurons to have extremely large axons, many mammalian neurons have myelin sheaths. Thus a chemical is released by one cell and diffuses across the gap to the next cell to initiate its electrical activity. The chemical released at the synapse is known as a neurotransmitter .