686 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



flectance was detected on illuminating such rod-free 

 areas as the fovea or the optic disc. On light-adapting 

 areas known to contain rods, increases in the re- 

 flectance of blue-green light were recorded, ap- 

 parently caused by bleaching rhodopsin. The vari- 

 ation in magnitude of this effect along the horizontal 

 meridian, from nasal to temporal, can be correlated 

 with the distribution of rod density (fig. 17). 



When the eye is exposed to light, the rhodopsin 

 content falls exponentially to a steady state level at 

 which the rate of bleaching is balanced by the 

 regeneration rate. As might be expected, the rho- 

 dopsin content at the steady state decreases as the 

 level of illumination is raised (fig. 18). The time 

 course of bleaching roughly parallels the course of 

 light adaptation of human rod vision (cf. 73). Fol- 

 lowing light adaptation, the rhodopsin concentra- 

 tion rises regularly in the dark (fig. 18) and ap- 

 proaches a maximum value in about thirty minutes 

 (50), in good agreement with the time required for 

 human rod dark adaptation (fig. 19). 



The course of bleaching and rcsynthesis of rhodopsin 

 in the human retina, measured in this way, agrees 

 with the course of human light and dark adaptation 

 only when the latter is plotted in terms of log sen- 

 sitivity. It is the logarithm of the visual sensitivity 

 that rises and falls with time much as does the con- 

 centration of rhodopsin. A theory has been proposed 

 which accounts for this relationship (65, 72). The 

 rod is viewed as a compartmented structure. Each 

 compartment contains a large quantity of rhodopsin 

 and is discharged by the absorption of a first quantum 

 of light. The residual rhodopsin of a discharged 



160 



'■120 



80 



50 



-fO 30 

 •Nasal 



10 10 

 Degrees 



20 30 40 

 — Temporal 



FIG. 17. Distribution of rhoclop.sin den.sity in the human 

 retina. Circles: measurements of rhodopsin density at tlie points 

 shown along the horizontal meridian. Line: rod density per 

 mm'' in the same region. [From Campbell & Rushton (8).] 



i 



10 

 Time (mm) 



FIG. 18. Bleaching and resynthesis of rhodopsin in the 

 human retina 15 degrees temporal to the fovea. Open circles: 

 On exposing the eye successively to lights of increasing bright- 

 ness (i, 5 and 100 units, where i unit = 20,000 trolands), 

 the rhodopsin content falls each time to a new steady-state 

 level at which the rate of bleaching is balanced by the regenera 

 tion rate. Filled circles: In the dark, rhodopsin regenerates. 

 Complete recovery (not shown in figure) takes about 30 min. 

 (50). [From Campbell & Rushton (8).] 



compartment continues to absorb light and to 

 bleach but can no longer contribute to excitation. 

 A rod is rendered wholly inexcitable when each of 

 its compartments has absorbed at least one quantum 

 of light, i.e. when in each of its compartments at 

 least one molecule of rhodopsin has been bleached. 

 In this way the bleaching of very little rhodopsin 

 can lead to a high state of light adaptation^. This 

 hypothesis, pursued mathematically, leads to the 

 expectation that the logarithm of the visual sensitivity 

 should be appro.ximately proportional to the con- 

 centration of visual pigment (72). 



The same relationships appear to hold for cones. 

 Rushton (49) has recently modified his method to 

 measure cone pigments in the human fovea. He 

 finds that in the dark, following exposure to a bright 

 light, cone visual pigment is resynthesized much more 

 rapidly than rhodopsin (fig. 20). The course of 

 synthesis parallels human cone dark adaptation (fig. 

 19). It has long been known that in man and many 

 other animals the cones dark-adapt much more 

 rapidly than the rods. In the human eye the dark 



^ The term bleach is here used loosely to in\oK'e the entire 

 chain of eflfects that follows the absorption of light by rho- 

 dopsin. The first such effect is the production of lumi-rhodopsin; 

 then by thermal reactions meta-rhodopsin (still without literal 

 bleaching); and finally a mixture of all-/ran.f retinene and opsin. 

 The excitation process probably depends upon the first of 

 these steps, the change to lumi- or at most mcta-rhodopsln. 



