Across the cell membrane of a skeletal muscle cell in the resting state with a non-electrogenic pump
Only if the membrane potential (MP) is at the value of electrochemical equilibrium for sodium (around +60 mv) is there no net movement of sodium across the cell membrane; only at electrochemical equilibrium for potassium (around -100 mv) is there no net movement of potassium. For all MP values between +60 and -100 there is an influx of sodium and an efflux of potassium. When the pump is non-electrogenic (does not contribute to the MP), the resting MP occurs at that value at which the rate of sodium influx equals the rate of potassium efflux. If the membrane is equally permeable to both sodium and potassium, then the resting MP will be about midway between +60 and -100 mv. Because the rate of flux of an ion is a product of permeability to that ion and the distance from electrochemical equilibrium for that ion, then when potassium is more permeable than sodium, as it is in a typical cell at rest, the point at which the rate of sodium influx equals that of potassium efflux is much closer to the potassium electrochemical equilibrium.
Both sodium and potassium are in electrochemical equilibrium
The rate of sodium influx is equal to the rate of potassium efflux
The rate of sodium influx is much less than the rate of potassium efflux
The rate of sodium influx is much greater than the rate of potassium efflux
General physiology