202 A. ARVANITAKI AND N. CHALAZONITIS 



On one hand, the higher the oxygen pressure, the higher the rate of 

 increment of the frequency (or the yielding of the photoactivation process). 

 On the other hand, the efficiency of the activation by the 579 m/x mono- 

 chromatic beam is higher than that by the whole visible spectrum of equal 

 incident energy. 



This fact was consistent with the idea that part of the wave lengths in the 

 whole visible spectrum are inactive, or even inhibitory. In fact, by external 

 microelectrodes in the Aplysia nerve cells, earher data showed that inhibition 

 might occur by A 490 m/x, which is absorbed by the carotenoid molecules 

 (Arvanitaki and Chalazonitis, 1949b). 



Using now the intracellular microelectrode technique (Arvanitaki and 

 Chalazonitis, 1955a, 1958a, b, 1960), it is found that activation of the A type 

 giant soma by a A 490 m/j. beam only occasionally, and with very low efficiency, 

 elicits an "on" response (see Fig. 4). Most frequently it elicits an "off" 

 response, and, if the soma was initially autoactive, it provokes an inhibition 

 of the spiking at "on", its re-establishment at "off" (Fig. 5). In the A type 

 cells, inhibition by 490 mjn is observed mainly in units which on inspection 

 appear rather yellowish, indicating a high concentration of the intracellular 

 carotenoid molecules. 



It is also to be noted that the primary immediate effect in this kind of 

 inhibition observed in the A cell is an increase of the membrane potential. 

 Unhke the inhibitory processes to be examined below, no conspicuous in- 

 hibitory post synaptic potentials are here superimposed (Fig. 5). 



The generator depolarization. Early kinetics of the excitatory process initiated 

 by light. The excitatory effects of the light are fundamentally related to the 

 graded generator depolarization of the cell membrane. This initial bio- 

 electrical response to light is fairly conspicuous in certain nerve cells very 

 densely pigmented, of medium size (diameter 100-200 /x), contiguous 

 to the A cell or to the Gen cell, and extremely sensitive to monochromatic 

 A 579 m^ light. 



Recordings at high amplification showed that initiation of the generator 

 depolarization is fairly synchronous to the incidence of the light stimulus 

 (Fig. 6). This depolarization grows at an increasing rate as the illumination 

 proceeds. The maximum rate is roughly a linear function of the logarithm of 

 the light intensity. By the square pulse technique, it has been established, 

 moreover, that an increase in the membrane conductance is concomitant with 

 the generator depolarization. 



When the rate of the generator depolarization reaches a threshold value, 

 spiking is initiated. The reciprocal of the latency of the spiking, as well as the 

 maximum frequency of the spikes in the discharge, are linear functions of the 

 logarithm of the light intensity. Hence, they vary directly with the value of 

 the maximum rate of the generator depolarization. This is well shown in 

 Fig. 6; the maximum frequency of the spiking varies as does the maximum 



