CHAPTER XXVIII 



The photoreceptor process in vision' 



GEORGE WALD [ Biological Laboratories, Harvard University, Cambridge, Massachusetts 



CHAPTER CONTENTS 



Chemistry of Visual Excitation 



Rhodopsin 



Porphyiopsin 



lodopsin 



Cyanopsin 



Recapitulation 



Role of Opsin in Visual Excitation 

 Physiological Correlations 



Absorption Spectra and Spectral Sensitivity : Purkinje 

 Phenomenon 



Visual Adaptation and the Bleaching and Synthesis of 

 Visual Pigments 



Vitamin A Deficiency and Night Blindness 



Nicotinamide 



CHEMISTRY OF VISUAL EXCITATION 



LIGHT INITIATES a ncrvous excitation in the outer 

 segments of the rods and cones which, transmitted 

 from one neuron to another to centers in the brain, 

 ends in exciting visual sensations. To achieve this 

 resuh probably the whole apparatus must be thrown 

 into activity; yet all of it waits upon and, to a degree, 

 retains the impress of the primary processes of ex- 

 citation in the receptor cells. 



The general arrangement of these processes is clear 

 from first principles. Light to have any effect, chemi- 

 cal or physical, must be absorbed. The rods and 

 cones must therefore contain substances which absorb 

 visible light — hence pigments — and are changed 

 thereby so as to yield a nervous excitation. The photo- 



' The investigations from this laboratory were supported in 

 part by the Rockefeller Foundation, the Office of Naval Re- 

 search, and the Public Health Service. The author wishes to 

 thank Dr. Ruth Hubbard for help with the preparation of 

 this manuscript. 



.sensitive pigments must be continuously restored, 

 or vision would cease soon after a light went on. 

 The excitatory state must also be rapidly removed, 

 or vision would continue long after a light went off. 

 It would aid the economy of such a system if these re- 

 actions were coupled so as to complete a cycle but 

 this, though an advantage, is not essential. All 

 photoreceptor processes may therefore be formulated 

 as follows: 



» Photosensitive pigment 



-Excitatory product^ 



light 



This is not only the basic arrangement for photo- 

 reception but, generalized to include stimuli other 

 than light, it must also be the form of all neural 

 excitation. Every irritable tissue must contain similar 

 arrangements for reacting with the stimulus, for 

 removing its effects and for restoring the original 

 system. One may therefore expect to meet the same 

 fundamental pattern of reactions at every level of the 

 \isual pathway; and the entire process of visual 

 excitation from rods and cones to cerebral cortex 

 may be conceived as a chain of such .systems. The 

 peculiar importance of the photoreceptor systems 

 rests, therefore, not on their intrinsic form but on 

 their unique sensitivity to light and their initial 

 position in the chain, by virtue of which certain of 

 their properties are imposed on the entire visual 

 response. 



Four visual pigments are known : rhodopsin and 

 porphyropsin in rods, and iodopsin and cyanopsin 

 in cones. All of them are built upon a common plan. 

 They are all carotenoid-proteins — proteins bearing 

 carotenoid chromophores to which they owe their 

 color and sensitivity to light. The rhodopsin system 

 will be described in some detail since it provides the 



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