Sodium and potassium are involved in action potential. Sodium ions flow into the cell through ion channels and cause depolarization, where the potential of the cell is higher than the cell's resting potential. Then the sodium ion channels close, and potassium is released from the cell, returning it to its resting state.
I don't know if you mean the equation simplified, but the first property to any equation or expression is "given." (without the quotation marks, of course.)
