Exercise XX 



ELECTRICAL ACTIVITY OF A SENSE ORGAN 105 



o 



W-cis retinene, the chromophore of visual pigments 



CH3 



I 

 C 



H 

 C 



H2C 



H2C 



\ 



C 



1/ 



c 



c 



CH3 H 



light 



CH3 



I 



c 



H 

 C 



CH3 



I 



c 



H 

 C 



\ / \ / \ / \ 



c 



H 



C 



H 



C 

 H 



O 



C 

 H2 



CH3 



all-?ra/u retinene, C19H27CHO 



something of the action of hght on the receptor. 

 All photoreceptors so far examined contain light- 

 sensitive pigments, substances which are changed 

 on absorbing visible light so as somehow to in- 

 duce a nervous excitation. Each such visual pig- 

 ment is composed of a colorless protein, called 

 an opsin, to which is attached as chromophore 

 or color group the yellow, fat-soluble substance, 

 retinene (vitamin A aldehyde, C19H27CHO). It 

 is for this reason that vision depends upon vita- 

 min A: the first symptom of vitamin A deficiency 

 in man and other animals is the failure of vision 

 called night-blindness. 



Retinene itself is very light yellow in color. 

 The visual pigment of the Limulus eye that is 

 formed by the attachment of retinene to opsin is 

 red in color. It is this red pigment that absorbs 

 the light which is effective in vision. Molecules 

 of retinene come in a variety of shapes, cis-trans 

 isomers of one another. A special, unstable, bent 

 and twisted shape of retinene (the l\-cis isomer) 

 is the only one that can join with opsin to form a 



visual pigment. When a quantum of light is ab- 

 sorbed by the visual pigment, the effect is to 

 straighten out the retinene to the aW-trans 

 isomer. Somehow, perhaps by exposing an ac- 

 tive site on the opsin which had been covered 

 before, this leads to the depolarization and 

 nervous excitation. (1 1-m retinene is both bent, 

 as are all cis molecules, and twisted, owing to 

 the — H on Cio running into the — CH3 on C13, 

 which keeps the molecule from lying flat.) 



Before a molecule that has responded in this 

 way can participate again in excitation, the 

 retinene must be rebent and retwisted back into 

 the active shape that can recombine with opsin 

 to regenerate the visual pigment. That is, the 

 effect of light on the visual pigment is both to 

 excite vision and to inactivate temporarily the 

 pigment. The result is a temporary decline of 

 visual sensitivity, called light adaptation. Then 

 some time must elapse during which the visual 

 pigment is regenerated, and the eye regains its 

 maximal sensitivity. This is dark-adaptation. 



