"The membrane of an axon has K⁺ leak channels and very few Na⁺ leak channels similar to other cell membranes.

Like other cells we will find that there is 140 mM of K⁺ and only 5mM of Na⁺ inside the cell.

Outside this is reversed with 142 mM of Na⁺ and 4 mM of K⁺."

courtesy of Robert H. Parsons, Ph.D., Rensselaer Polytechnic Institute, used with permission