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Reversal potential
 
 
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In a biological membrane
Biological membrane

A biological membrane or biomembrane is an enclosing or separating amphipathic layer that acts as a barrier within or around a cell . It is, almost invariably, a lipid bilayer, composed of a double layer of lipid-class molecules, specifically phospholipids and cholesterol, with occasional integral membrane protein intertwined, some o...
, the reversal potential (also known as the Nernst potential) of an ion
Ion

An ion is an atom or molecule which has lost or gained one or more electrons, giving it a positive or negative electrical charge. According to the Bohr_model this will be from or in the outer shield 'n'....
 is the membrane potential
Membrane potential

Membrane potential , is the voltage difference between the interior and exterior of a cell. Because the fluid inside and outside a cell is highly conductive, whereas a cell's plasma membrane is highly resistive, the voltage change in moving from a point outside to a point inside occurs largely within the narrow width of the membrane itself...
 at which there is no net (overall) flow of ions from one side of the membrane to the other. In the case of post-synaptic neuron
Neuron

Neurons are responsive cell in the nervous system that process and transmit information by electrochemical Signal . They are the core components of the brain, the vertebrate spinal cord, the invertebrate ventral nerve cord, and the peripheral nerves....
s, the reversal potential is the membrane potential at which a given neurotransmitter
Neurotransmitter

Neurotransmitters are chemistry which relay, amplify and modulate signals between a neuron and another cell . Neurotransmitters are packaged into vesicles that cluster beneath the membrane on the presynaptic side of a synapse, and are released into the synaptic cleft, where they bind to receptors in the membrane on the postsynaptic side of...
 causes no net current flow of ions.

In a single-ion system, reversal potential is synonymous with equilibrium potential; their numerical values are identical.






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In a biological membrane
Biological membrane

A biological membrane or biomembrane is an enclosing or separating amphipathic layer that acts as a barrier within or around a cell . It is, almost invariably, a lipid bilayer, composed of a double layer of lipid-class molecules, specifically phospholipids and cholesterol, with occasional integral membrane protein intertwined, some o...
, the reversal potential (also known as the Nernst potential) of an ion
Ion

An ion is an atom or molecule which has lost or gained one or more electrons, giving it a positive or negative electrical charge. According to the Bohr_model this will be from or in the outer shield 'n'....
 is the membrane potential
Membrane potential

Membrane potential , is the voltage difference between the interior and exterior of a cell. Because the fluid inside and outside a cell is highly conductive, whereas a cell's plasma membrane is highly resistive, the voltage change in moving from a point outside to a point inside occurs largely within the narrow width of the membrane itself...
 at which there is no net (overall) flow of ions from one side of the membrane to the other. In the case of post-synaptic neuron
Neuron

Neurons are responsive cell in the nervous system that process and transmit information by electrochemical Signal . They are the core components of the brain, the vertebrate spinal cord, the invertebrate ventral nerve cord, and the peripheral nerves....
s, the reversal potential is the membrane potential at which a given neurotransmitter
Neurotransmitter

Neurotransmitters are chemistry which relay, amplify and modulate signals between a neuron and another cell . Neurotransmitters are packaged into vesicles that cluster beneath the membrane on the presynaptic side of a synapse, and are released into the synaptic cleft, where they bind to receptors in the membrane on the postsynaptic side of...
 causes no net current flow of ions.

In a single-ion system, reversal potential is synonymous with equilibrium potential; their numerical values are identical. The two terms refer to different aspects of the difference in membrane potential. Equilibrium refers to the fact that the net ion flux at a particular voltage is zero. That is, the outward and inward rates of ion movement are the same; the ion flux is in equilibrium. Reversal refers to the fact that a change of membrane potential on either side of the equilibrium potential reverses the overall direction of ion flux.

The reversal potential is often called the "Nernst potential", as it can be calculated from the Nernst equation
Nernst equation

In electrochemistry, the Nernst equation is an equation which can be used to determine the equilibrium reduction potential of a half-cell in an electrochemical cell....
. Ion channel
Ion channel

Ion channels are pore-forming proteins that help establish and control the small voltage gradient across the plasma membrane of all living cell s by allowing the flow of ions down their electrochemical gradient....
s conduct most of the flow of simple ions in and out of cells
Cell (biology)

The cell is the structural and functional unit of all known Life organisms. It is the smallest unit of an organism that is classified as living, and is often called the building bricks of life....
. When a channel type that is selective to one species of ion dominates within the membrane of a cell (because other ion channels are closed, for example) then the voltage inside the cell will equilibrate (i.e. become equal) to the reversal potential for that ion (assuming the outside of the cell is at 0 volts). For example, the resting potential
Resting potential

Relatively static membrane potential of quiescent cells is called resting membrane potential , as opposed to the specific dynamic electrochemical phenomenona called action potential and graded membrane potential....
 of most cells is close to the K
Potassium

Potassium is a chemical element. It has the symbol K , atomic number 19, and atomic mass 39.0983. Potassium was first isolated from potash, hence the name....
+ (potassium ion) reversal potential. This is because at resting potential, potassium conductance dominates. During a typical action potential
Action potential

An action potential is a self-regenerating wave of electrochemical activity that allows nerve cells to carry a signal over a distance. It is the primary electrical signal generated by nerve cells, and arises from changes in the permeability of the nerve cell's axonal Cell membranes to specific ions....
, the small resting ion conductance mediated by potassium channels is overwhelmed by the opening of a large number of Na
Sodium

Sodium is an element which has the symbol Na , atomic number 11, atomic mass 23 amu , and a common oxidation number +1. Sodium is a soft, silvery white, highly reactive element and is a member of the alkali metals within "group 1" ....
+ (sodium ion) channels, which brings the membrane potential close to the reversal potential of sodium.

The relationship between the terms "reversal potential" and "equilibrium potential" only holds true for single-ion systems. In multi-ion systems, there are areas of the cell membrane where the summed currents of the multiple ions will equal zero. While this is a reversal potential in the sense that membrane current reverses direction, it is not an equilibrium potential because not all (and in some cases, none) of the ions are in equilibrium and thus have net fluxes across the membrane. When a cell has significant permeabilities to more than one ion, the cell potential can be calculated from the Goldman-Hodgkin-Katz equation rather than the Nernst equation.

Mathematical models

The term driving force is related to equilibrium potential, and is likewise useful in understanding the current in biological membranes. Driving force refers to the difference between an ion's equilibrium potential and the actual membrane potential. It is defined by the following equation:

In words, this equation says that: the ionic current (Iion) is equal to that ion's conductance (gion) multiplied by the driving force, which is represented by the difference between the membrane potential and the ion's equilibrium potential (i.e. Vm-Eion). Note that the ionic current will be zero if the membrane is impermeable (gion = 0) to the ion in question, regardless of the size of the driving force.

A related equation (which is derived from the more general equation above) determines the magnitude of an end plate current (EPC), at a given membrane potential, in the neuromuscular junction
Neuromuscular junction

A neuromuscular junction is the synapse or junction of the axon terminal of a motoneuron with the motor end plate, the highly-excitable region of muscle plasma membrane responsible for initiation of action potentials across the muscle's surface, ultimately causing the muscle to contract....
:

where EPC is the end plate current, GACh is the ionic conductance activated by acetylcholine
Acetylcholine

The chemical compound acetylcholine is a neurotransmitter in both the peripheral nervous system and central nervous system in many organisms including homo sapiens....
, Vm is the membrane potential, and Erev is the reversal potential. When the membrane potential is equal to the reversal potential, Vm-Erev is equal to 0 and there is no driving force on the ions involved.

Use in research

When Vm is at the reversal potential (Vm-Erev is equal to 0), the identity of the ions that flow during an EPC can be deduced by comparing the reversal potential of the EPC to the equilibrium potential for various ions. For instance, if acetylcholine opens an ion channel that is permeable only to K+, then the reversal potential of the EPC is at the equilibrium potential for K+, which is around -100mV in muscle cells. If, however, the acetylcholine-activated channels were permeable only to Na+, the reversal potential would then be around +70mV (which is the Na+ equilibrium potential of muscle cells). If the channels were only permeable to Cl-, then the reversal potential would be around -50mV. On the other hand, if the acetylcholine-activated channels were permeable to both Na+ and K+, then the reversal potential would be somewhere between +70mV and -100mV.

This line of reasoning led to the development of experiments (by Akira Takeuchi
Akira Takeuchi

Akira Takeuchi is an active Japanese fashion designer. Along with Tayuka Nakanishi, he is founder and main designer of the Theatre Products fashion brand, which is popular in the Harajuku and Shibuya, Tokyo areas of Tokyo, known as centers of youth fashion....
 and Noriko Takeuchi in 1960) that proved that acetylcholine-activated ion channels are approximately equally permeable to Na+ and K+ ions. The experiment was performed by lowering the external Na+ concentration, which lowers (more negative) the Na+ equilibrium potential and produces a negative shift in reversal potential. Conversely, increasing the external K+ concentration raises (more positive) the K+ equilibrium potential and produces a positive shift in reversal potential.

See also

  • Electrochemical potential
    Electrochemical potential

    In electrochemistry, the electrochemical potential, , sometimes confusingly abbreviated to ECP, is a thermodynamic measure that combines the concepts of energy stored in the form of chemical potential and electric charge....
  • Cell potential