198 



A. ARVANITAKI AND N. CHALAZONITIS 



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Fig. I. Photobioelectrical reactions in the stained nerve fibre. 

 Sepia axon of 300 [x diameter, isolated and stained with neutral red; initially at 

 rest in the dark. Local illumination in the presence of oxygen; 5'C. A narrow 

 beam (0-3 mm width) of light (whole visible. 700-400 nvj.) strikes locally the axon 

 at the times signalled by the arrows. Recording microelectrode, 0-5 mm from the 

 illuminated spot. Three successive steady illuminations at increasing intensities, 



from top to bottom: 1-2, 1-8 and 5-4 10 ^ cal g mm - sec i. 

 On the exponentially growing generator potential are superimposed irregular 

 miniature potentials, then oscillatory potentials initiating the discharge of spikes. 

 Only the lower portions of the impulses are seen at this high amplification. 

 Following the first impulses of the discharge the membrane potential stabilizes at 

 an upper depolarized state, not seen at this amplification (but see Fig. 3). 



than the latency of spikes (ahhough the rate of growth of the generator 

 potential decreases when the light is turned off (Fig. 2) and consequently the 

 initiation of the spikes is delayed). 



Obviously, the maximum development of the generator potential and the 

 initiation of the spiking are late secondary reactions, as compared to the 

 primary transitions triggered by the light. In an attempt to determine a 

 threshold value of the photic stimulus, the light energy just sufficient to elicit 



