Molecular indicators of membrane potential may be used to obtain sub-millisecond time resolved images of transient changes in membrane voltage in a variety of biological systems. These probes are small amphipathic molecules having molecular weights of 400-500, and dimensions on the order of 10 Angstroms, which bind to, but do not cross cell membranes, and change either their absorbance or fluorescence in response to membrane voltage. These extrinsic optical signals depend linearly upon membrane potential, and the best of the dyes respond to a step change in voltage in less than 1.5 μsec at room temperature. The salient properties of fast potentiometric probes will be discussed, and the fidelity of optical recordings to transmembrane voltage changes will be considered.
Since voltage changes in excitable cells take place on a time scale that is determined by the kinetics of conformational changes in membrane proteins, and by membrane electrical time constants, these changes tend to be very rapid, and resolving them requires imaging systems that are frequently orders of magnitude faster than usual video rates.