A Large Change in Axon Fluorescence that Provides a Promising Method for Measuring Membrane Potential

Abstract

THE changes in birefringence¹, light scattering¹, and extrinsic fluorescence² of axons that occur during the action potential have been studied in some detail^3–5 in an attempt to understand the molecular and structural basis of action potential propagation. These optical changes could also be useful in another context. Because many of the changes occur with the same time course as the action potential, it is theoretically possible to use optical techniques to measure membrane potentials. The previously described optical changes are relatively small and to measure a change in a single axon the results of many trials had to be averaged. We had already tried over a hundred fluorescent dyes, the best giving a signal-to-noise ratio of 2:1 during an action potential in a giant axon, when our attention was called to merocyanine dyes. One of these dyes gives a rather large signal, easily visible in a single sweep. This fluorescence change could provide a powerful tool for measuring membrane potential in systems where the use of electrodes is inconvenient or impossible.