Spatial Origin of the Fast Photovoltage in Retinal Rods

Abstract

Fast photoelectric effects, of which the “early receptor potential” (1) is the first to be discovered, are found in many types of retinas (2,3,4; for reviews). There is now little doubt that the currents which cause fast photo voltages (FPV’s) originate from changes in the distribution of charges in the photopigment molecules themselves, but the electrical arrangements which convert these into externally observable voltages are not yet fully understood. This paper reports some experiments designed to explore the electrical circuitry underlying FPV’s of vertebrate photoreceptors. Our aim is to locate the sources of the charge displacements, to estimate their absolute magnitudes on a molecular basis and to determine the cable constants of the rods of isolated rat retinas. The experimental procedure is to compare the FPV’s recorded with extracellular microelectrodes from various depths in the receptor layer of the retinas with theoretical solutions of the cable equation for nonuniform cells with the anatomical dimensions of rat rods. The experimental techniques are described previously (5).