My watch list  

Membrane potential

Membrane potential (or transmembrane potential or transmembrane potential difference or transmembrane potential gradient), is the electrical potential difference (voltage) across a cell's plasma membrane. The plasma membrane bounds the cell to provide a stable environment for biological processes. Membrane potential arises from the action of ion transporters embedded in the membrane which maintain viable ion concentrations inside the cell. The term "membrane potential" is sometimes used interchangeably with cell potential but is applicable to any lipid bilayer or membrane.

The membrane potential of most cells is kept relatively stable. Unlike most cells, neurons are specialized to use changes in membrane potential for fast communication, primarily with other neurons. When a neuron fires, the action potential travels down the axon to the synapses: the magnitude of the axonal membrane potential varies dynamically along its length. On reaching a (chemical) synapse, a neurotransmitter is released causing a localized change in potential in the membrane of the target neuron by opening ion channels in its membrane.

Additional recommended knowledge


Graded membrane potential


The Ionic Basis of the resting potential

At its core, the origin of every resting membrane potential is some sort of ion pump exchanger or transporter. Any voltage — membrane potentials included — is a separation of electric charges across a resistive barrier. The typical membrane potential of a cell arises from the separation of sodium ions from intracellular immobile anions across the membrane of the cell. In order for this separation to occur, a concentration gradient of potassium ions must first be set up. This work is done by the ion pumps/transporters and/or exchangers and generally is powered by ATP.





In most quantitative treatments of membrane potential, such as the derivation of Goldman equation, electroneutrality is assumed; that is, that there is no measurable charge excess in any side of the membrane. So, although there is an electric potential across the membrane due to charge separation, there is no actual measurable difference in the global concentration of positive and negative ions across the membrane (as it is estimated below), that is, there is no actual measurable charge excess in either side. That occurs because the effect of charge on electrochemical potential is hugely greater than the effect of concentration so an undetectable change in concentration creates a great change on electric potential.

Generation of the resting potential




The number of ions involved in generating the upstroke of the action potential





  • dV: 100 mV
  • dt: 2 ms
  • A: cell membrane surface area
  • Length L: 100 μm
  • radius r: 5 μm
    • Thus, since the cell is a cylinder,
    • Area, A: 3.3×10-9 m2
    • Volume, V: 7.85×10-12 liters







Equilibrium potentials

An equilibrium potential (also called reversal potential or Nernst Potential) is the membrane voltage at which the distribution across the membrane of a particular ion is in equilibrium. This means that the transmembrane voltage exactly matches (resists) the diffusive tendency of the ion (or vice versa), such that the net current of the ion across the membrane is zero and unchanging. The equilibrium potential of a particular ion is designated by the notation Eion. In the previous section, the equilibrium potential specifically for potassium ions was described, but any ion present will have an equilibrium potential that is determined by the concentration difference for that ion across the membrane and temperature. For potassium, EK is typically around −80 mV. For sodium, ENa is typically around +60 mV. Note that the sign of ENa and EK are opposite. This is because the concentration gradient for potassium is directed out of the cell, while the concentration gradient for sodium is directed into the cell.

Resting potential revisited

The resting potential of a cell can be most thoroughly understood by thinking of it in terms of equilibrium potentials. In the example diagram above, the model cell was given only one permeant ion (potassium). In this case, the resting potential of this cell would be the same as the equilibrium potential for potassium.

However, a real cell is more complicated, having permeabilities to many ions, each of which contributes to the resting potential. To understand better, consider a cell with only two permeant ions, potassium and sodium. Consider a case where these two ions have equal concentration gradients directed in opposite directions, and that the membrane permeabilities to both ions are equal. K+ leaving the cell will tend to drag the membrane potential toward EK. Na+ entering the cell will tend to drag the membrane potential toward ENa. Since the permeabilities to both ions were set to be equal, the membrane potential will, at the end of the Na/K tug-of-war, end up halfway between ENa and EK. As ENa and EK were equal but of opposite signs, halfway in between is zero, meaning that the membrane will rest at 0 mV. Note that even though the membrane potential at 0 mV is stable, it is not an equilibrium condition because neither of the contributing ions are in equilibrium. Ions diffuse down their electrochemical gradients through ion channels, but the membrane potential is upheld by continual K influx and Na efflux via ion pumps.

Closer to the actual case for a real resting potential is that neither the equilibrium potentials nor the permeabilities of Na and K are equal. Na permeability is rather about 5% of the K permeability, whereas EK is about -80 mV and ENa about + 60 mV. Thus the membrane potential will not be right at EK, but rather depolarized from EK by an amount of approximately 5% of the 140 mV difference between EK and ENa. Thus, the cell's resting potential will be about −73 mV.

In a more formal notation, the membrane potential is the weighted average of each contributing ion's equilibrium potential. The size of each weight is the relative permeability of each ion. In the normal case, where three ions contribute to the membrane potential:

E_{m} = \frac{P_{K^+}} {P_{tot}} E_{K^+} + \frac{P_{Na^+}} {P_{tot}} E_{Na^+} + \frac{P_{Cl^-}} {P_{tot}} E_{Cl^-},


  • Em is the membrane potential, measured in volts
  • EX is the equilibrium potential for ion X, also in volts
  • PX is the relative permeability of ion X in arbitrary units (e.g. siemens for electrical conductance)
  • Ptot is the total permeability of all permeant ions, in this case PK+ + PNa+ + PCl-

All other values of membrane potential

From the viewpoint of biophysics, there is nothing particularly special about the resting membrane potential. It is merely the membrane potential that results from the membrane permeabilities that predominate when the cell is resting. The above equation of weighted averages always applies, but the following approach may be easier to visualize. At any given moment, there are two factors for an ion that determine how much influence that ion will have over the membrane potential of a cell.

  1. That ion's driving force and,
  2. That ion's permeability


  • Permeability: is simply a measure of how easily an ion can cross the membrane. It is normally measured as the (electrical) conductance and the unit, siemens, corresponds to 1 C·s-1·V-1, that is one charge per second per volt of potential.

So in a resting membrane, while the driving force for potassium is low, its permeability is very high. Sodium has a huge driving force, but almost no resting permeability. In this case, the math tells us that potassium carries about 20 times more current than sodium, and thus has 20 times more influence over Em than does sodium.



Effects and implications




See also

  • Action potential
  • Electrochemical potential
  • Goldman Equation
  • Membrane biophysics
  • Signal (biology)


This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Membrane_potential". A list of authors is available in Wikipedia.