EXCITATORY AND INHIBITORY PROCESSES 



203 



rate of the depolarization, and is. in the second record, much lower than in 

 the third. The maximum amplitude of the generator potential is, nevertheless, 

 equal in the two. 



Fig. 4. The monochromatic activation of the heme-protein (579 mi^) is of higher 

 efficiency than the monochromatic activation of the carotene-protein (490 ma). 

 An A cell initially inactive in the dark is here submitted to 3 steady monochromatic 

 illuminations of equal incident intensities (6 10 « cal g mm ^sec i), alternatively 

 from top to bottom: A 490 miji; A 579 mi^; A 490 mix. The discharges mitiated at 

 "on" (signalled by arrow) are of higher frequency by the 579 mi^ activation. 

 Calibrations: 80 mV; 0-5 sec. 



The generator potential subsides if the light is removed after the maximum 

 depolarization has been attained. If the light is turned off before that time, 

 the rate of the depolarization decreases and a maximum is reached, although 

 later and lower. Thus, with intensities high enough, it is quite possible to 

 elicit bioelectrical responses by flashes of duration as short as some few 

 milliseconds. 



According to the intensity of the flashes, graded generator depolarization, 

 with or without spiking, may be initiated (Fig. 7). Again, the maximum rate 

 of the generator depolarization, as well as the maximum frequency of the 

 spikes in the discharge, increase linearly with the logarithm of the intensity 

 of the flash (Fig. 9). 



The possibility of eliciting bioelectrical responses by such short flashes is 

 of great theoretical bearing. 



In the sequences of reactions initiated by light in the precise cellular site 

 where pigments are located, hence, where light is absorbed, the bioelectrical 

 phenomena start immediately, close to the primary photophysical events. 

 The latter is quantitatively related to the generator depolarization which is 

 the fundamental photobioelectrical reaction at the cellular level. 



This notion decreases the value of the bioelectrical reaction threshold 

 towards a much lower order: in a previous work, a 10^ quanta579 threshold 



