" The actual potential difference is closer to the K⁺ equilibrium potential because in a resting cell the membrane has a large number of K⁺ channels; the K⁺ permeability is high. However while there are not a lot of Na⁺ channels there are a few.

This low Na⁺ permeability allows a few Na⁺ ions to enter the cell. The inward movement of Na⁺ ions represents a positive change that cancels out the effect of a few K⁺ ions moving out.

This causes a slight decrease in the potential difference across the membrane.

Thus the relative permeability of the cell membrane to K⁺ and Na⁺ determines the actual membrane potential.

• What do you think would happen if more Na⁺ channels were added?

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