Membrane Potential
All cells manifest an electrical potential difference across their
cell membrane. This membrane potential (often given the symbol
Em is small in magnitude compared to voltages we encounter
in electrical power sources, but is extremely important to the cell.
Physiologists have developed a precise definition of membrane potential,
and the potentials can be measured reasonably accurately and reproducibly.
Definition
The membrane potential is transmembrane electrical potential difference
expressed inside with respect to outside. Using the outside as the
reference potential makes sense as all cells share that outside.
This choice of direction (or sign convention) affects other measures of
electrical properties, notably transmembrane current, which are all expressed
inside with respect to outside. Thus our current convention is that
current crossing the membrane from inside to outside is positive.
Typical values
Expressed with the above convention, all cells have membrane potentials which
are inside negative. The values observed vary a little from cell to
cell and across different species, but for nerve and muscle cells that are
very well studied, a typical value is -80mV (ranging from -90mV to -60mV).
Basis
The membrane potential is a manifestation of a small separation of charge
across the lipid bilayer of the membrane. The lipid is the dielectric
of a parallel plate capacitor in which the salty (conductive) solutions
separated by the membrane are the plates of the capacitor. The charges
involved are all ions, and the charge separation comes about through ion
movements resulting directly or indirectly from the action of ion pumps.
(see Why do cells have membrane potentials?
for a discussion of why this may be so.)
Impact
Although the membrane potentials observed in cells are less than 1/10th of
a volt, the electrical field is very large. Changes in this
field, brought about through changes in membrane potential can have
marked affects on membrane proteins that themselves carry charges.
Such influence on membrane proteins can markedly change cell activity.
Changes in Membrane Potential
Any process that changes the separation of charge across the membrane will
change membrane potential. These processes can be natural or experimental,
and an experimental one serves to illustrate the phenomenon well.
It is known as current injection, and schematically can be
represented as follows:
The microelectrode is a very fine glass tube which can penetrate the
membrane, The solution inside the electrode (usually concentrated KCl)
will then be connected to the inside of the cell and can be used to
carry current into the cell, hence the name current injection.
For this to happen the circuit must be completed: the switch must be
closed, and the current must go to the reference electrode
outside the cell by passing outwards through the membrane.
This is shown in the following diagram.
The positive charges reaching the inside of the membrane will tend to
reduce the inside negativity, and the positive charge carried away
from the outer surface will also do so. There are several important
points to note:
-
Although termed current injection the current is outward
from the membrane perspective. There must be a circuit: in through the
electrode, out through the cell.
-
The outward current will tend to make the membrane potential become
less negative. In other words the positive (outward)
transmembrane current will tend to make the membrane potential move
in a positive direction.
-
If the current could be adjusted in magnitude it could be made
large enough to eliminate the original charge separation altogether.
This particular case is very important in many attempts to understand
membrane phenomena because the membrane potential is temporarily
eliminated from influencing ion movements, merely because it is zero.
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©D.F. Davey,
Department of Physiology,
University of Sydney
Last updated 8 March 1998
