EXCITATORY AND INHIBITORY PROCESSES 195 



lies in the action of light being here analysed in single cells, by methods per- 

 mitting time resolution as well as site resolution of the events. 



In this regard pioneer experiments already demonstrating photosensitivity 

 in unstained preparations, but performed on the scale of an organ, muscle 

 or nerve centres (d'Arsonval, 1891 ; Prosser, 1934), did not afford any analysis 

 at the unitary scale.* 



In our approach, we have concentrated on three aspects of the problem: 



(a) The cytostructural aspect, involving the identification of the cellular 

 location of the molecules that absorb the light energy, in other words, the 

 site of action of the stimulus. 



(b) The biophysical aspect, involving the identification of the intracellular 

 pigments, and allowing some information on the probable biophysical and 

 biochemical transitions initiated by the light. 



(c) The electrophysiological aspect, involving the identification of the bio- 

 electrical changes initiated by light and the study of relationships between the 

 incident light energy and the kinetics of the bioelectric processes as recorded 

 by intracellular microelectrodes. 



In this venture, we have employed primarily naturally highly pigmented 

 cells (such as cardiac fibres, central nerve cells, plant cells, etc.) (Table 1). 

 Although the pigments found in these cells are mainly involved in such 

 functions as respiration, photosynthesis, etc., they were found to be capable 

 of acting as photoreceptor structures leading to the generation of bioelectric 

 signals. 



This review will be limited to the aspects of excitation and inhibition 

 processes induced by light on pigmented nerve cells and to the inhibitory 

 effects determined by radiations of the near infra-red. 



It is worthwhile, nevertheless, to quote first fundamental data displayed 

 by experiments on an experimental model of a photoreceptor cell, the isolated 

 giant axon of Sepia stained with vital dyes (Chalazonitis, 1954). 



* On the other hand, the well known photodynamic effects (see Blum, 1941; Lippay, 

 1929, 1930; Kosmann, 1938; Kosmann and Lillie, 1935; Sandow and Isaacson, 1960) 

 imply sensitization of biological systems to light by dyes (which seldom are viial) allowing 

 photochemical reactions in which molecular oxygen takes part. But these reactions have 

 nothing in common with normal oxygen metabolism of living systems. They are irreversible 

 oxidations of cellular structural components, which do not take place thermally in living 

 conditions. The observed and measured effects in photodynamic actions are in general 

 long latency injury effects. 



In contradistinction, the photoactivation effects considered here are essentially reversible 

 processes of short latencies, in which on\y functional reactions are implied. In these, mole- 

 cular oxygen which takes part is only involved through the respiratory pathway as a result 

 of the increase of the velocity constants of the chain under the action of light (Arvanitaki 

 and Chalazonitis, 1960; Chalazonitis, 1954). 



