EXCITATORY AND INHIBITORY PROCESSES 197 



EXCITATION OR INHIBITION PROCESSES 

 INDUCED BY LIGHT 



An Artificial Photoreceptor Cell 



A Sepia giant axon stained with vital dyes such as methylene blue or 

 neutral red behaves as a good photoreceptor cell (Chalazonitis, 1954). Our 

 attention will be limited to several main general features displayed in this 

 model only //; aerobic conditions. 



The excitability and membrane resistance changes induced by the light. 

 Faintly stained with methylene blue, continuously stimulated by 10/sec 

 pulses of equal subliminal intensities, the giant axon of Sepia responds in the 

 dark by local damped prepotentials of equal amplitudes. 



In the presence of oxygen, the light being turned on, the amplitude of the 

 prepotentials (initiated by the pulses of equal intensities) rapidly increases 

 until a spike is initiated after 2-5 sec of illumination. This may be termed an 

 increase in the excitability induced by light. Simultaneously the response 

 acquires a marked undamped shape. It is noteworthy that in the presence of 

 nitrogen the light induces a reverse effect, a decrease of the excitability. 



Using the square pulse technique, it is possible to demonstrate moreover 

 that in the presence of oxygen light induces an immediate increase in the 

 membrane conductance. This is also substantiated by the oscillatory be- 

 haviour of the response under illumination, thus indicating that in the 

 analogous circuit representative of the electrical properties of the membrane, 

 the resistance undergoes a decrease (Arvanitaki, 1939; Cole. 1941)- 



The immediate graded depolarization and early miniature potentials initiated 

 by light. The facts are schematic in the axon stained with neutral red (Fig. 1). 

 At "on", the membrane depolarizes without delay and continues to do so 

 at an increasing rate. At the same time, uncoordinated bumps of miniature 

 potentials are surging. These significantly reveal the recruitment in a quasi- 

 quantal manner of multiple active foci or patches in the illuminated area of 

 the membrane. They recall the miniature responses described by Tasaki and 

 Spyropoulos (1958) and Spyropoulos (1959) in the squid axon membrane 

 under voltage clamp. 



The initiation of oscillatory potentials and spiking. As the depolarization 

 of the membrane proceeds, the miniature potentials appear to synchronize 

 into regular oscillations of increasing amplitudes, preluding the spiking 

 (Fig. 1). The kinetics of this generator depolarization is a definite function 

 of the light intensity. It may be shown that the maximum rate and the 

 maximum amplitude of the generator potential, as well as the reverse of the 

 latency of the spikes' initiation, increase roughly as a linear function of the 

 logarithm of the light intensity. 



Such bioelectrical reactions, considered as a whole, are by far supra- 

 maximal responses. They may be elicited by flashes of durations shorter 



