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The diffusion of ( sodium, potassium ) makes ( no, all the ) difference to the charge across the membrane. The diffusion of ( potassium, sodium ) out of the axon is the cause of a potential difference of ( -70, +40 )mV across the membrane. This charge is ( stable, unstable ) because the ( electrochemical, potassium concentration ) gradient goes from the outside to the inside of the axon whereas the ( potassium concentration, electrochemical ) gradient goes from the inside of the axon to the outside. Sodium cannot interfere due to the ( impermeability, permeability, dark forces of nature ).