Reversal potential
From Freepedia
Reversal potential is the following:
- In electrochemistry, reversal potential is the potential difference across a reversible cell.
- In the context of biological membranes, the reversal potential of a particular ion species is the membrane voltage at which there is no net flow of ions from one side of the membrane to the other. The term reversal potential is more or less interchangeable with the term equilibrium potential'. They are, in fact, the same potential. The two terms simply refer to different aspects of that potential. Equilibrium refers to the fact that the net ion flux at The potential is zero, that is, the outward and inward rates are the same. In other words, the flux is in equilibrium. Reversal refers to the fact that perterbation of the membrane potential on either side of the equilibrium potential reverses the net direction of ion flux. The reversal/equilibrium potential is also often call 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. 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) or 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 described by the equation
Vion= gion(Em-Eion)
In words: The driving force acting on an ion (Vion) is equal to that ion's conductance (gion) times the difference between the membrane potential (Em) and the ion's equilibrium potential (Eion)
See also
