Diffusion Potentials

A diffusion potential describes the voltage generated by ions that diffuse across the membrane. Two conditions are necessary for a membrane potential to occur by diffusion: the membrane must be selectively permeable, allowing a single type of ion to diffuse through membrane pores, and the concentration of the diffusible ion must be greater on one side of the membrane than on the other. In the rest­ing or unexcited state, when the membrane is highly per­meable to potassium, the concentration of potassium ions inside the cell is approximately 35 times greater than out­side. Because of the large concentration gradient existing across the cell membrane, potassium ions tend to diffuse outward. As they do so, they carry their positive charges with them, and the inside becomes negative in relation to

the outside. This new potential difference repels further outward movement of the positively charged potassium ions. The same phenomenon occurs during an action po­tential, when the membrane is highly permeable to sodium. Sodium ions move inside the cell, creating a membrane potential of the opposite polarity. An equilibrium potential is one in which no net movement of ions occurs because the diffusion and electrical forces are exactly balanced (see accompanying box). For potassium, this represents the resting membrane potential.