Processing of visual signals in vertebrate photoreceptors

Photoreceptors of the vertebrate retina hyperpolarize in response to illumination. The conductance changes in the plasma membrane associated with the electrical response are the final step of chain of events initiated by light absorption at the outer segment of the visual cell. The mechanism whereby the free energy of photons in converted into neural information is largely unknown. Present knowledge is consistent with the idea that an internal transmitter is modulated by light and modifies the ionic permeability of the plasma membrane. As to the identity of the internal messenger two candidates have been proposed: Ca2+ and cyclic GMP respectively. Increasing evidence suggests that both substances may be involved in the process of phototransduction. The electrical response of photoreceptors does not simply reflect the light absorbed by the cell: complex interactions occurring between adjacent photoreceptors and between photoreceptors and second order neurons cooperate with the initial process in determining the final shape of the receptor message. Recurrent interactions involve particularly cones: their membrane potential can be modified at least by three distinct mechanisms; i) by light absorption at their outer segment; ii) by light absorption at the outer segment of neighbouring cones, and iii) by potential changes occurring in horizontal cells.