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Differences in concentration of ions on opposite sides of a cellular membrane
lead to a voltage called the membrane potential. Typical values of
membrane potential are in the range –40 mV to –80 mV. Many ions have a
concentration gradient across the membrane, including potassium (K+
), which is at a high inside and a low concentration outside the membrane. Sodium (Na+
) and chloride (Cl–
) ions are at high concentrations in the extracellular region, and low concentrations in the intracellular
regions. These concentration gradients provide the potential energy to
drive the formation of the membrane potential. This voltage is
established when the membrane has permeability to one or more ions. In
the simplest case, illustrated here, if the membrane is selectively
permeable to potassium, these positively charged ions can diffuse down
the concentration gradient to the outside of the cell, leaving behind
uncompensated negative charges. This separation of charges is what
causes the membrane potential. Note that the bulk solutions of either
side of the membrane are electo-neutral. Likewise, the system as a whole
is electro-neutral. The "uncompensated" positive charges outside the
cell, and the uncompensated negative charges inside the cell, physically
line up on the membrane surface and attract each other across membrane.
Thus, the membrane potential is physically located only in the
immediate vicinity of the membrane. It is the separation of these
charges across the membrane that is the basis of the membrane voltage.
Note also that this diagram is only an approximation of the ionic
contributions to the membrane potential. Other ions including sodium,
chloride, calcium and others play a more minor role, even though they
have strong concentration gradients, because they have more limited
permeability than potassium. Key: Blue pentagons - sodium ions; Purple
squares - potassium ions; Yellow circles - Choloride ions; Orange
rectangles - 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
Membrane potential (also transmembrane potential or membrane voltage) is the difference in electrical potential between the interior and the exterior of a biological cell. Typical values of membrane potential range from –40 mV to –80 mV.
All animal cells are surrounded by a plasma membrane composed of a lipid bilayer
with a variety of types of proteins embedded in it. The membrane
potential arises primarily from the interaction between the membrane and
the actions of two types of transmembrane proteins embedded in the plasma membrane. The membrane serves as both an insulator and a diffusion barrier to the movement of ions. Ion transporter/pump proteins actively push ions across the membrane to establish concentration gradients across the membrane, and ion channels allow ions to move across the membrane down those concentration gradients, a process known as facilitated diffusion. In the most fundamental example of this, the ion transporter Na+/K+-ATPase pumps sodium cations
from the inside to the outside, and potassium cations from the outside
to the inside of the cell. This establishes two concentration gradients:
a gradient for sodium where its concentration is much higher outside
than inside the cell, and a gradient for potassium where its
concentration is much higher inside the cell than outside. Transmembrane
potassium-selective leak channels
allow potassium ions to diffuse across the membrane, down the
concentration gradient that was established by the ATPase, creating a
charge separation, and thus a voltage, across the membrane. In almost
all cases, the ion that determines the so-called "resting" membrane potential of a cell is K+,
although other ions do contribute in more minor ways. By convention,
the sign of the membrane potential is the voltage inside relative to ground outside the cell. In the case of K+, its diffusion
down its concentration gradient (toward the outside of the cell, in
this case) creates transmembrane voltage that is negative relative to
the outside of the cell, and typically –60 to –80 millivolts (mV) in
Virtually all eukaryotic
cells (including cells from animals, plants, and fungi) maintain a
nonzero transmembrane potential, usually with a negative voltage in the
cell interior as compared to the cell exterior. The membrane potential
has two basic functions. First, it allows a cell to function as a battery,
providing power to operate a variety of "molecular devices" embedded in
the membrane. Second, in electrically excitable cells such as neurons and muscle cells,
it is used for transmitting signals between different parts of a cell.
Signals are generated by opening or closing of ion channels at one point
in the membrane, producing a local change in the membrane potential.
This change in the electric field can quickly be detected by either
adjacent or more distant ion channels in the membrane. Those ion
channels can then depolarize, reproducing the signal.
In non-excitable cells, and in excitable cells in their baseline states, the membrane potential is held at a relatively stable value, called the resting potential.
For neurons, typical values of the resting potential range from –70 to
–80 millivolts; that is, the interior of a cell has a negative baseline
voltage of a bit less than one tenth of a volt. The opening and closing
of ion channels can induce a departure from the resting potential. This
is called a depolarization if the interior voltage becomes more positive (say from –70 mV to –60 mV), or a hyperpolarization
if the interior voltage becomes more negative (say from –70 mV to –80
mV). In excitable cells, a sufficiently large depolarization can evoke
an action potential,
in which the membrane potential changes rapidly and significantly for a
short time (on the order of 1 to 100 milliseconds), often reversing its
polarity. Action potentials are generated by the activation of certain voltage-gated ion channels.
the factors that influence the membrane potential are diverse. They
include numerous types of ion channels, some that are chemically gated
and some that are voltage-gated. Because voltage-gated ion channels are
controlled by the membrane potential, while the membrane potential
itself is influenced by these same ion channels, feedback loops that
allow for complex temporal dynamics arise, including oscillations and
regenerative events such as action potentials.
- 1 Physical basis
- 1.1 Voltage
- 1.2 Ions and the forces driving their motion
- 1.3 Plasma membranes
- 1.4 Facilitated diffusion and transport
- 1.5 Ion pumps
- 1.6 Ion channels
- 1.6.1 Leakage channels
- 1.6.2 Ligand-gated channels
- 1.6.3 Voltage-dependent channels
- 1.7 Reversal potential
- 1.8 Equivalent circuit
- 2 Resting potential
- 3 Graded potentials
- 4 All other values of membrane potential
- 5 Effects and implications
- 6 See also
- 7 Notes
- 8 References
- 9 Further reading
- 10 External links
The membrane potential in a cell derives ultimately from two factors:
electrical force and diffusion. Electrical force arises from the mutual
attraction between particles with opposite electrical charges (positive
and negative) and the mutual repulsion between particles with the same
type of charge (both positive or both negative). Diffusion arises from
the statistical tendency of particles to redistribute from regions where
they are highly concentrated to regions where the concentration is low
(due to thermal energy).
Main article: Voltage
Electric field (arrows) and contours of constant voltage created by a
pair of oppositely-charged objects. The electric field is at right
angles to the voltage contours, and the field is strongest where the
spacing between contours is the smallest.
Voltage, which is synonymous with difference in electrical potential, is the ability to drive an electric current across a resistance. Indeed the simplest definition of a voltage is given by Ohm's law:
V=IR, where V is voltage, I is current and R is resistance. If a
voltage source such as a battery is placed in an electrical circuit, the
higher the voltage of the source, the greater the amount of current
that it will drive across the available resistance. The functional
significance of voltage lies only in potential differences
between two points in a circuit. The idea of a voltage at a single point
is meaningless. It is conventional in electronics to assign a voltage
of zero to some arbitrarily chosen element of the circuit, and then
assign voltages for other elements measured relative to that zero point.
There is no significance in which element is chosen as the zero
point—the function of a circuit depends only on the differences, not on
voltages per se. However, in most cases and by convention, the
zero level is most often assigned to the portion of a circuit that is in
contact with ground.
The same principle applies to voltage in cell biology. In
electrically active tissue, the potential difference between any two
points can be measured by inserting an electrode at each point, for
example one inside and one outside the cell, and connecting both
electrodes to the leads of what is in essence a specialized voltmeter.
By convention, the zero potential value is assigned to the outside of
the cell and the sign of the potential difference between the outside
and the inside is determined by the potential of the inside relative to
the outside zero.
In mathematical terms, the definition of voltage begins with the concept of an electric field E, a vector field assigning a magnitude and direction to each point in space. In many situations, the electric field is a conservative field, which means that it can be expressed as the gradient of a scalar function V, that is, E = –∇V. This scalar field V
is referred to as the voltage distribution. Note that the definition
allows for an arbitrary constant of integration—this is why absolute
values of voltage are not meaningful. In general, electric fields can be
treated as conservative only if magnetic fields do not significantly
influence them, but this condition usually applies well to biological
Because the electric field is the gradient of the voltage
distribution, rapid changes in voltage within a small region imply a
strong electric field; on the converse, if the voltage remains
approximately the same over a large region, the electric fields in that
region must be weak. A strong electric field, equivalent to a strong
voltage gradient, implies that a strong force is exerted on any charged
particles that lie within the region.
Ions and the forces driving their motion
Main articles: Ion, Diffusion, Electrochemical gradient, and Electrophoretic mobility
Ions (pink circles) will flow across a membrane from the higher
concentration to the lower concentration (down a concentration
gradient), causing a current. However, this creates a voltage across the
membrane that opposes the ions' motion. When this voltage reaches the
equilibrium value, the two balance and the flow of ions stops.
Electrical signals within biological organisms are, in general, driven by ions. The most important cations for the action potential are sodium (Na+) and potassium (K+). Both of these are monovalent cations that carry a single positive charge. Action potentials can also involve calcium (Ca2+), which is a divalent cation that carries a double positive charge. The chloride anion (Cl−) plays a major role in the action potentials of some algae, but plays a negligible role in the action potentials of most animals.
Ions cross the cell membrane under two influences: diffusion and electric fields.
A simple example wherein two solutions—A and B—are separated by a
porous barrier illustrates that diffusion will ensure that they will
eventually mix into equal solutions. This mixing occurs because of the
difference in their concentrations. The region with high concentration
will diffuse out toward the region with low concentration. To extend the
example, let solution A have 30 sodium ions and 30 chloride ions. Also,
let solution B have only 20 sodium ions and 20 chloride ions. Assuming
the barrier allows both types of ions to travel through it, then a
steady state will be reached whereby both solutions have 25 sodium ions
and 25 chloride ions. If, however, the porous barrier is selective to
which ions are let through, then diffusion alone will not determine the
resulting solution. Returning to the previous example, let's now
construct a barrier that is permeable only to sodium ions. Since
solution B has a lower concentration of both sodium and chloride, the
barrier will attract both ions from solution A. However, only sodium
will travel through the barrier. This will result in an accumulation of
sodium in solution B. Since sodium has a positive charge, this
accumulation will make solution B more positive relative to solution A.
Positive sodium ions will be less likely to travel to the
now-more-positive B solution. This constitutes the second factor
controlling ion flow, namely electric fields. The point at which this
electric field completely counteracts the force due to diffusion is
called the equilibrium potential. At this point, the net flow of this
specific ion (in this case sodium) is zero.
The cell membrane, also called the plasma membrane or plasmalemma, is a semipermeable
lipid bilayer common to all living cells. It contains a variety of
biological molecules, primarily proteins and lipids, which are involved
in a vast array of cellular processes.
Every animal cell is enclosed in a plasma membrane, which has the structure of a lipid bilayer
with many types of large molecules embedded in it. Because it is made
of lipid molecules, the plasma membrane intrinsically has a high
electrical resistivity, in other words a low intrinsic permeability to
ions. However, some of the molecules embedded in the membrane are
capable either of actively transporting ions from one side of the
membrane to the other or of providing channels through which they can
In electrical terminology, the plasma membrane functions as a combined resistor and capacitor.
Resistance arises from the fact that the membrane impedes the movement
of charges across it. Capacitance arises from the fact that the lipid
bilayer is so thin that an accumulation of charged particles on one side
gives rise to an electrical force that pulls oppositely-charged
particles toward the other side. The capacitance of the membrane is
relatively unaffected by the molecules that are embedded in it, so it
has a more or less invariant value estimated at about 2 µF/cm2
(the total capacitance of a patch of membrane is proportional to its
area). The conductance of a pure lipid bilayer is so low, on the other
hand, that in biological situations it is always dominated by the
conductance of alternative pathways provided by embedded molecules.
Thus, the capacitance of the membrane is more or less fixed, but the
resistance is highly variable.
The thickness of a plasma membrane is estimated to be about 7-8
nanometers. Because the membrane is so thin, it does not take a very
large transmembrane voltage to create a strong electric field within it.
Typical membrane potentials in animal cells are on the order of 100
millivolts (that is, one tenth of a volt), but calculations show that
this generates an electric field close to the maximum that the membrane
can sustain—it has been calculated that a voltage difference much larger
than 200 millivolts could cause dielectric breakdown, that is, arcing across the membrane.
Facilitated diffusion and transport
Facilitated diffusion in cell membranes, showing ion channels and carrier proteins