ROD- 



-CONE 



<?4> (h jp 



OPTIC NERVE 

 FIBER 



1 



Fig. 23-6. Photoreceptors and associated nerve cells 

 in the retina of the human eye. 



eral areas of the retina, and they are em- 

 ployed mainly in "twilight vision." The rods 

 are activated by relatively weak light, but 

 they are not capable of distinguishing colors 

 and sharp outlines — which accounts for the 

 grayness and haziness of vision when the light 

 is dim. 



The Chemistry of Visual Excitation. Black- 

 white, or rod vision, is somewhat different 

 from color, or cone vision. Moreover, we have 

 a better understanding of the chemistry of 

 rod excitation, owing to the early (1925-1945) 

 studies of Selig Hecht at Columbia Uni- 

 versity and a great many more recent investi- 

 gations, pioneered most actively, perhaps, by 

 George Wald at Harvard University. 



The rods of a dark-adapted eye are almost 



Responses of Higher Animals: The Receptors - 423 



unbelievably sensitive to light. Excitations, 

 detected in the optic nerve, have been elicited 

 by exceedingly brief flashes, enduring for 

 only one microsecond (.000001 second). In 

 fact, one or more of the rod cells may be ex- 

 cited by less than 10 quanta of light energy. 



The light-absorbing agency of the rods has 

 been identified as rhodopsin; prior to its 

 chemical identification, it was called visual 

 purple. Rhodopsin is a conjugated protein, 

 formed by union between a pigment, re- 

 tinene, and a protein, opsin. Essentially, light 

 absorption by retinene and the consequent 

 alteration in the molecular structure of 

 retinene triggers excitation. Retinene is an 

 aldehyde compound derived by action of a 

 dehydrogenase upon vitamin A. This ac- 

 counts, of course, for the well-known defi- 

 ciency effect of vitamin A, namely night- 

 blindness (p. 350). 



The absorption of light alters the intra- 

 molecular structure of retinene. Retinene is 

 changed from one stereoisomeric form (the 

 cw-configuration) to another (the trans-con- 

 figuration). 1 7Y«n.s-retinene, apparently, is the 

 actual excitatory substance. Its presence in 

 the rod cell leads to the generation of a vol- 

 ley of excitations in the axon of the rod 

 (Fig. 23-6), and these excitations are trans- 

 mitted to the serially arranged neurons of 

 the system. The excitation does not persist, 

 however. The £r<7r?s-retinene-opsin complex, 

 which has been called lumirhodopsin, is un- 

 stable. Soon <r<7n.s-retinene breaks (by hy- 

 drolysis) off from the protein. Subsequently, 

 a re-formation of active rhodopsin, available 

 for further excitations, requires the expendi- 

 ture of metabolic energy — to reverse the iso- 



1 Cis-trans changes in form depend upon the pres- 

 ence of a double bond between two carbon atoms in 

 a molecule and upon a nonidentity between the two 

 atoms or radicals that are in direct connection with 

 each of these carbon atoms. For example, we may in- 

 dicate the two forms of dichlorethylene as follows: 



CI 



CI CI 



