208 



A. ARVANITAKI AND N. CHALAZONITlS 



I 



Fig. 9. Maximum increase determined by light in the spikes frequency as function 



of intensity. 



Effects of Hght intensity on the maximum frequency of firing evoked by the Hght on 

 initially inactive cells (curves I, II), or the maximum increase of spikes frequency 

 determined by the light on initially autoactive cells (curves 111, IV). For curve I, 

 the duration of the illumination was I sec; for curve II, 20 msec; for curves III 

 and IV, steady illuminations. Data of curves I, II, 111, were obtained from cells of 

 150-200 ijL diameter. Data of curve IV was from a giant A cell (500 [x). Abscissa: 

 logio incident intensity of light; Ordinate: maximum increase of the spikes 



frequency. 



amplitude and 10 msec duration supervened. The hypothesis that they could 

 be miniature excitatory post synaptic potentials was considered. In effect, 

 some full grown e.p.s.p.'s with an easily identifiable shape are sometimes also 

 superimposed on the subliminal generator depolarization. Yet it is difficult 

 to conclude in favour of a presynaptic bombardment due to the photoactiva- 

 tion of a second soma, because such e.p.s.p.'s may occur even if the illuminat- 

 ing microbeam was carefully restricted to the impaled soma. It is nevertheless 

 possible to imagine that the light would also be adequate to activate directly 

 either the presynaptic terminals or the postsynaptic membrane. 



As a matter of fact, the excitability of one or both of these structures is 

 exhausted by illumination, as may be shown by inter-injections of orthodro- 



