The voltage across a cell membrane is known as the cell membrane potential. Cells that generally have cell membrane potentials are nerve cells that are electrically active.
The cell membranes of nerve cells maintain a cell membrane potential in their resting state. This cell membrane potential is always negative in comparison to the outside of the cell. Normally, the cell membrane potential in its resting state is approximately -70 millivolts. Nerve cells have cell membrane potentials because of the differences in potassium and sodium ions in and outside of their cells. These ions are electrolytes, which can pass through the selectively permeable membrane of nerve cells to create a voltage-driven gradient.