Optical Recording of Impulses in Individual Neurones of an Invertebrate Central Nervous System

Abstract

FOR many purposes it would be a great advantage if one could measure the activity of individual neurones without the use of an electrode. Since the discovery of changes in the optical properties of axons that occur during the action potential^1,2, it has been possible, in principle, to accomplish this using optical techniques. Recently, this was achieved in a giant axon using a merocyanine dye; in a stained axon a single action potential gave rise to a fluorescence increase which was detectable with a signal-to-noise ratio greater than 10 : 1 (ref. 3). Because the membrane area observed in the giant axon experiments was about 10³ times larger than that of a 50 µm cell body, it was by no means evident that this fluorescent probe would be sensitive enough to monitor electrical activity in smaller cells. Happily, it is; with the merocyanine dye, we are able to detect action potentials in individual sensory neurones of leech segmental ganglia. We hope that this technique can be developed into a powerful tool for studying the functional organisation of populations of neurones.