Visual Receptors as Biological Transducers 



E. F. MacNichol, Jr. 



The Thomas C. Jenkins Department of Biophysics, The Johns Hopkins Uni- 

 versity, Baltimore, Maryland 



Receptor organs may be considered to be transducers whereby partic- 

 ular forms of energy coming from the environment are, to use con- 

 L. cepts borrowed from the communications engineering field, filtered, 

 amplified, compressed, and encoded in a form suitable for transmission to 

 the central nervous system. 



The function of filtering seems to be largely determined by accessory struc- 

 tures such as the mechanical and hydraulic linkages in the ear and the dioptric 

 system of the eye. Additional filtering is undoubtedly done in the receptor cells 

 themselves as, for example, by the specific absorption of a narrow region of 

 electromagnetic spectrum by the visual pigments. The photoreceptors are 

 literally tuned to a narrow band of wavelength occupying about an octave of 

 electromagnetic spectrum. 



The biochemistry of the pigments responsible for the highly selective absorp- 

 tion of energy has been worked out in detail, especially by Wald and his col- 

 leagues. 



How the energy thus absorbed is amplified and encoded into the nerve mes- 

 sage is much less well understood. Since the pioneering work of Adrian (1935) 

 and Bronk many receptor physiologists have been engaged in a form of wire 

 tapping which has, fortunately, not yet brought us to the attention of con- 

 gressional committees. It has been possible to intercept the messages in single 

 fibers of sensory nerves by electrical means so that they can be listened to and 

 recorded oscillographically. Thus the codes produced by many kinds of recep- 

 tors have largely been broken. 



Much of the work that has been done in this field has been admirably re- 

 viewed by Granit in his recent book (1955). Study of this material leads to the 

 conclusion that in coded messages are basically the same for all receptors so far 

 studied. This is not surprising since the information must in all cases be in a 

 form suitable for transmission by nerve fibers. The well known "all or nothing" 

 properties of nerve fibers require that the messages be transmitted in a 

 form known in the communications field as pulse time or pulse frequency 

 modulation. The only variations that are allowable are time relationships be- 

 tween impulses or groups of impulses, the frequencies of trains of impulses, or 



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