Phototransduction in Rods and Cones

Abstract

Light is detected by visual pigment, and this detection signal is converted to an electrical signal in the photoreceptor cell. In vertebrates, there are two classes of photoreceptors, rods and cones, and they respond to light by membrane hyperpolarization. The mechanism that produces a hyperpolarizing light response, the phototransduction cascade consisting of a series of enzymatic reactions, is now well understood in rods. Rods and cones function under different light conditions. Rods are highly light sensitive, so that they are functional in very dim light. Cones are less light sensitive and they are functional in daylight. Thanks to these two classes of cells, our vision covers a light intensity range more than 108 fold from the darkness under starlight to dazzling brightness at high noon on a tropical island. In addition to the difference in light sensitivity, the time required for recovery of a hyperpolarizing response to a flash of light is much shorter in cones than in rods. This higher time resolution in cones makes it possible to detect an object moving quickly in daylight. In cones, the phototransduction cascade is similar to that in rods. The differences in light sensitivity and time resolution between rods and cones could, therefore, stem from differences in the efficiencies of reactions or the lifetime of an active species in the cascade. This chapter describes the similarities and the differences in the phototransduction cascade between rods and cones studied biochemically in carp (Cyprinus carpio).