MOLECULAR ASPECTS OF VISUAL EXCITATION *

The main purpose of this contribution is to examine a t the molecular level the manner in which the absorption of light acts upon the photosensitive pigments of the eye. However, we should like to go further and begin to understand how these initial changes are translated into a nervous excitation. The present status of this situation has recently been reviewed by Wald.' Unfortunately, nothing is as yet known about the molecular mechanisms involved in the excitation of any end organ. We know, however, that all such excitations result eventually in a nervous impulse that involves electric fluctuations brought about by movements of ions. A number of contributions to this monograph deal with these phenomena. We shall therefore direct our attention to the manner in which the molecular changes induced by light might produce such ion currents. In order to approach these problems, it helps to be acquainted with the structure, as close to the molecular level as possible, of the apparatus within which the process of excitation takes place. In the case of visual excitation these are the rods and cones of the retina. Each of these consists of an inner and outer segment. The inner segment contains the nucleus and carries on the vegetative functions of the cell; the outer segment contains the visual pigments and is specialized for photoreception. The outer segment of a vertebrate rod or cone consists of a stack of one to several thousand disks, regularly spaced, and each made up of 1 or 2 layers or membranes. Each membrane is almut 40 to 160 A thick, depending upon the species of animaL2 On the basis of their properties in polarized light, W. J. Schmidt showed, twenty years ago, that vertebrate rods and cones contain alternate layers of protein and lipid,3 and it is likely that the membranes made visible in the electron microscope2 represent one or the other of these components. Within this highly organized, regular structure, the molecules of visual pigment are oriented in a definite geometry. Their chromophores lie in planes perpendicular to the long axis of the that is, within or parallel to the membranes. A rhodopsin molecu!e of molecular weight about 40,000, if spherical, has a diameter of about 40 A. Its dimensions are therefore of the same order of magnitude as the thickness of the rod membranes. I t is important to realize, further, that rhodopsin is a major structural component of the rod outer limb, constituting about 14 per cent of its dry weight in cattle and about