Cell Membrane Potentials
Cell membranes in general, and membranes of nerve cells in particular, maintain a small voltage or "potential" across the membrane in its normal or resting state. In the rest state, the inside of the nerve cell membrane is negative with respect to the outside (typically about -70 millivolts). The voltage arises from differences in concentration of the electrolyte ions K+ and Na+.
If the cell membranes were simply permeable to these ions, they would approach an equilibrium with equal concentrations on each side of the membrane, and hence no voltage difference. This makes it clear that the processes which produce the membrane potential are not simply diffusion and osmosis.
In the selectively permeable cell membranes are ion channels which allow K+ ions to pass to the interior of the cell, but block Na+ ions. Negatively charged proteins on the interior of the cell are also denied passage across the membrane. In addition, there are active transport mechanisms at work. There is a process called the sodium-potassium pump which utilizes ATP to pump out three Na+ ions and pump in two K+ ions. The collective action of these mechanisms leaves the interior of the membrane about -70 mV with respect to the outside.
For the nerve cell, this equilibrium is disturbed by the arrival of a suitable stimulus. The dynamic changes in the membrane potential in response to the stimulus is called an action potential. After the action potential the mechanisms described above bring the cell membrane back to its resting state.