The Problem of Visual Excitation*

Abstract

This paper attempts to come to grips with the problem, how the action of light on a visual pigment results in a nervous excitation. The only action of light in vision is to isomerize retinene, the chromophore of the visual pigments, from the 11-cis to the all-trans configuration. This change triggers a progressive opening-up of the protein structure, exposing new reactive groups. Since the absorption of one photon by one molecule of visual pigment may stimulate a dark-adapted rod, some large amplification process is needed between the act of absorption and the response. This may be an enzymatic catalysis, or the consequence of puncturing a critical membrane. A microspectrophotometric study of retinas and single rods shows the outer segment to have a quasi-crystalline structure, in which the visual pigments are almost perfectly oriented, and even “free” molecules capable of diffusion maintain a degree of orientation. Examination of mud puppy retinas in the electron microscope has revealed several new aspects of structure: (1) A system of cytoplasmic filaments (“dendrites”) springing from the inner segments of the rods and cones and standing like palisades around the outer segments. These may facilitate exchanges of material between the inner and outer segments. (2) Systems of particles in the membranes of the dendrites and pigment epithelium processes, which may be involved in interchanges of material with the outer segments. (3) A system of particles in crystalline array in the rod lamellae, which may contain the visual pigment. If so, measurements of the visual pigment in situ show that each particle should contain about 50 molecules of pigment. Such typically solid-state processes as exciton migration and photoconduction probably have at most very limited scope in the outer segments of rods and cones. The seat of excitation is probably the plasma membrane which envelops rod outer segments and composes also the lamellae in cones.