Spread of activation and desensitisation in rod outer segments

Visual transduction in vertebrate photoreceptors is thought to involve a diffusible internal transmitter which links photon absorption in disk membranes to the conductance decrease in the plasma membrane of the outer segment ^1-5. The desensitisation of the photoresponse which occurs during and after illumination may also involve a diffusible substance, because a photon absorbed in one disk can desensitise the cell's response to a subsequent photon absorbed in a different disk⁶. Although both activation and desensitisation spread longitudinally from the site of photon absorption, neither spreads over the entire length of the outer segment^7,8. We report here experiments which indicate that, with dim illumination, both effects decline to be half-maximal in less than 8 m from the site of light absorption.