How vision begins: an odyssey

Abstract

Retinal rods and cones, which are the front-end light detectors in the eye, achieve wonders together by being able to signal single-photon absorption and yet also able to adjust their function to brightness changes spanning 10(9)-fold. How these cells detect light is now quite well understood. Not surprising for almost any biological process, the intial step of seeing reveals a rich complexity as the probing goes deeper. The odyssey continues, but the knowledge gained so far is already nothing short of remarkable in qualitative and quantitative detail. It has also indirectly opened up the mystery of odorant sensing. Basic science aside, clinical ophthalmology has benefited tremendously from this endeavor as well. This article begins by recapitulating the key developments in this understanding from the mid-1960s to the late 1980s, during which period the advances were particularly rapid and fit for an intricate detective story. It then highlights some details discovered more recently, followed by a comparison between rods and cones.

Fig. 1.

Phototransduction mechanism in rods. (A) Schema showing the reaction pathways in the outer segment. GCAP, guanylate–cyclase-activating protein; hν, photon; Rh, rhodopsin; Rh*, photoactivated rhodopsin; Rh*∼P, phosphorylated form of rhodopsin; GAP (GTPase-activating protein) complex, composed of (not shown) RGS-9 (regulator of G protein-signaling isoform 9), PDEγ (inhibitory subunit of the phosphodiesterase), R9AP (RGS-9 anchoring protein), and G_β5 (an orphan G_β subunit). +, stimulation or positive modulation; −, inhibition or negative modulation. (Modified with permission from refs. and , based on an original figure in ref. .) (B) Flow chart showing the sequence of events triggered by light in phototransduction. (Modified from ref. , based on an original figure in ref. .)