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 segment7,8. 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.
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