Reversal potential
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In a biological membrane, the reversal potential (or Nernst potential) of a particular ion is the membrane voltage at which there is no net flow of ions from one side of the membrane to the other.
For a single-ion system only, the term reversal potential is synonymous with equilibrium potential. Their numerical values are identical. The two terms simply refer to different aspects of that potential difference. Equilibrium refers to the fact that the net ion flux at the voltage is zero (that is, the outward and inward rates are the same; the flux is in equilibrium). Reversal refers to the fact that perturbation of the membrane potential on either side of the equilibrium potential reverses the net direction of ion flux.
The reversal potential is also often called the "Nernst potential", as it can be calculated from the Nernst equation. Ion channels conduct most of the flow of simple ions in and out of cells. 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 to the reversal potential for that ion (i.e. assuming the outside of the cell is at 0 volts). For example, the resting potential of most cells is close to the potassium reversal potential because at rest, potassium conductance dominates. During a stereotypical action potential, the small resting conductance mediated by potassium channels is overwhelmed by the opening of a large number of sodium channels, which brings the membrane potential close to the reversal potential of sodium.
The identity between the terms reversal potential and equilibrium potential only holds true for single-ion systems. In multi-ion systems, there are places where the summed currents of those multiple ions will sum to zero. While this will be a reversal potential in the sense that membrane current reverses direction at this point, it is not an equilibrium potential, in that not all (or 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.
Another term that is related to equilibrium potential and is useful in understanding the flow of current in biological membranes is driving force. Driving force refers to the difference between an ion's equilibrium potential and the actual membrane potential. It is found in the equation
- Iion= gion(Vm-Eion)
Or in words, the ionic current (Iion) is equal to that ion's conductance (gion) times the driving force, represented by the difference between the membrane potential (Vm) and the ion's equilibrium potential (Eion). Note that the ionic current will be zero if the membrane is impermeable to the ion in question (gion = 0), regardless of the size of the driving force.
