Abstract

In vertebrates, the cylindrically shaped rod photoreceptors are oriented with their long axes facing the incoming light. Therefore, the rhodopsin molecules housed in the photoreceptors will not bleach uniformly when the eye is exposed to light. Rhodopsins located at the base, closest to the inner segment, will receive the incoming light first and momentarily screen the rhodopsins located at the more sclerad distal tip. As the basal rhodopsins bleach, photons become available farther along the outer segment to bleach more distal rhodopsins. An effort was made to describe this bleaching gradient. A mathematical model was proposed for a case in which the following assumption was made: if photoexcitation caused a rhodopsin to bleach, the bleaching occurred immediately and permanently. The validity of the model was tested with the aid of a microspectrophotometer. Absorbance was measured as a function of location along the outer segment in individual photoreceptors. Finding good agreement, the model is currently being extended to the case of steady state bleaching, by taking into account the recent finding that bleached pigments regenerate nonrandomly in situ.¹ These spatiotemporal patterns of bleach are likely to have a functional significance with respect to light and dark adaptational processes.

© 1985 Optical Society of America