672 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



model for all the others. Once this system is under- 

 stood, the others emerge as simple variants upon a 

 common theme. 



Rhudo/isiii 



Franz Boll discovered the red pigment rhodopsin 

 in the rods of frogs in 1876 (5). It is characteristic 

 of the rods of marine fishes and land \ertebrates 

 (62). There is no evidence (hat it is ever found in 

 cones. 



Some years ago rhodopsin was shown to par- 

 ticipate in a cycle of the following skeletal form (56) : 



Rhodopsin 



/■ 



Vitamin A + opsin < 

 ('visual white') 



\Ught 

 \ 



(orange intermediates) 



\ 



Retinene + opsin 



('visual yellow') 



Rhodopsin bleaches in the light through orange 

 intermediates to a mixture of the yellow carotenoid, 

 retinene, and the colorless protein, opsin (fig. i). 

 The retinene is then converted to colorless vitamin 

 A. Rhodopsin is resynthesized on the one hand from 

 retinene and opsin, on the other from vitamin A 

 and opsin. 



Morton has shown that retinene is vitamin A 

 aldehvde (3): 



CH, 



CH;, 



C CH, CH,, 



y \ hh|hhh|hh 

 HjC C— C=C— C=C— 0=C— C=C— C— OH 



I II H 



\ / 



c 



Vitamin A 



FIG. I. .Absorption spectra of bullfrog 

 rhodopsin and of the product of its 

 bleaching in aqueous digitonin solution, 

 pH 5.55- Rhodopsin possesses three ab- 

 sorption maxima: the a-band, mainly 

 responsible for the spectral sensitivity of 

 rod vision; the ^-band, which, like a-, 

 belongs to the prosthetic group; and 

 the 7-band, due to the protein opsin. 

 On bleaching, the a- and (3-bands are 

 replaced by the retinene band at about 

 385 m/i; the opsin band remains un- 

 changed. [From Wald (63).] 



2 Z 



2.0 



1.8 



1.6 



14 



l.i 



1.0 



0.8 - 



0.6 - 



0.4 - 



0.2 



Rhodops in 



B ie ached product 



200 



400 SOO 



600 



