RESPONSIVENESS IN VERTEBRATES 



rhodopsin, sometimes called visual purple; rhodopsin absorbs light rays and is 

 bleached in the process (Fig. 4.16). In this way, the receptor cell is stimu- 

 lated, and impulses are conducted to the central nervous system by way of the 

 optic nerve. Complete resynthesis of rhodopsin can occur only in dim light or 

 complete darkness. If there is a deficiency in vitamin A, rhodopsin cannot be 

 reformed and night blindness results. In the rods of fresh-water vertebrates 

 there is a purple light-sensitive pigment called porphyropsin, the synthesis of 

 which depends on a supply of as vitamin A2, a molecule with a structure 

 slightly different from that of vitamin A; the reaction cycle is like that of 

 rhodopsin. The cones of land vertebrates contain a light-sensitive violet 

 pigment, iodopsin, which has a reaction cycle like that of rhodopsin except 

 that the protein involved is photopsin, not scotopsin. Cones of fresh-water 

 vertebrates contain a light-sensitive blue pigment named cyanopsin and the 

 reaction cycle is like that of porphyropsin except that the protein concerned 

 is photopsin. Cones function under conditions of good illtimination and make 

 possible appreciation of fine detail and of color. Certain defects iji cones lead 

 to color blindness. 



Fig. 4.15. Visual cells of vertebrates; 

 xlOOO. A, common rod of leopard 

 frog; B, common cone of leopard frog; 

 C, rod of man; and D, cone of man. 



d, oil droplet, typically embedded in 



e, the ellipsoid, which is probably a 

 light-concentrating region; /, the 

 footpiece which makes contact with 

 processes of nerve cells in the outer 

 plexiform layer (cf. Fig. 4.14); /, posi- 

 tion of external limiting membrane; 

 TO, myoid, a contractile region not 

 present in human visual cells; n, 

 nucleus; 0, outer segment or light- 

 sensitive portion. (From G. L. Walls, 

 The Vertebrate Eye, copyright 1942 by 

 Cranbrook Institute of Science, re- 

 printed by permission.) 



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