Membrane potential

The electrical charge across a cell membrane resulting from different concentrations of ions on the inside of the cell vs. the outside of the cell.
Key: Blue pentagons – sodium ions; Purple squares – potassium ions; Yellow circles – chloride ions; Orange rectangles – membrane-impermeable anions (these arise from a variety of sources including proteins).
The large purple structure with an arrow represents a transmembrane potassium channel and the direction of net potassium movement.

Membrane potential (also transmembrane potential or membrane voltage) is the difference in electric potential between the interior and the exterior of a biological cell.

By convention it is written as Vm=Vinside−Voutside, thus a negative membrane potential means the cell interior is negative relative to the outside.It equals the interior potential minus the exterior potential. This is the energy per charge which is required to move a very small positive charge at constant velocity across the cell membrane from the exterior to the interior. Note, though, that if the charge is allowed to change velocity, the change of kinetic energy and production of radiation must be taken into account.

Typical values of membrane potential is normally given in units of millivolts and denoted as mV. In many animal cells, Vm is typically in the order of tens of millivolts. It commonly falls in the range of about −20 mV to −200 mV depending on the cell type and state. For such typical negative membrane potentials, positive work is required to move a positive charge from the interior to the exterior. However, thermal kinetic energy allows ions to overcome the potential difference. For a selectively permeable membrane, this permits a net flow against the gradient.

Although the entire cell is considered negative inside, the fluids in the cell are electrically neutral. The presence of the voltage is due to the thin layer of charge that accumulates at the surfaces of the cell membrane. The cell membrane is only a few nanometers thick, and as such, even a voltage of 100 mV across this thickness generates an electric field that is extremely strong on the cell membrane.

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