EXCITATORY AND INHIBITORY PROCESSES 223 



Considering the particular case of the generator depolarizations induced 

 by hght, the present study reveals them as endowed with strikingly analogous 

 characteristics, whether elicited in highly specialized cells as visual photo- 

 receptors (see Fuortes, 1959), or in cells such as Aplysia ganglion somata 

 completely deprived of any photoreceptor function. It has been suggested 

 that the mechanism mediating the generator potential proceeds in the photo- 

 receptor through the release of some chemical transmitter (Fuortes, 1959; 

 Grundfest, 1958). If the same hypothesis could also be adopted for the 

 mechanism of the generator depolarization induced by light in the Aplvsia 

 soma, we would expect to find in both cells some common, possibly 

 synaptic, fundamental function. But generator depolarization is also elicited 

 by light in the simple stained Sepia axon itself, wherein no synaptic structure 

 is implied. 



Thus, irrespective of the proper functions of the various cells considered 

 (see above, p. 209), generator potentials may be triggered by stimuh of various 

 nature, by synaptic bombardment, or even by intramural determinants acting 

 in the pacemaker sites of the cell. Hence, in the building up of the generator 

 depolarization, some link common to the different cells and activable by the 

 various kinds of energy, must be implied. 



The particular case of the light stimulus is but an aspect in one of the 

 most fascinating and most intensively investigated fields in biology, that 

 which concerns transitions from light to life. 



As electrophysiologists, our problem is to situate in time and space the 

 bioelectrical transitions of which the cell membrane is the site, in the long 

 sequence of events initiated on injection of light into the cell. Whatever be 

 the case, the first act is the absorption of a photon by a given molecule in a 

 given cellular site, i.e. the raising of an electron to a higher energy level. In 

 the reactions to light considered here, the input of the electromagnetic energy 

 is on definite cytostructures where the pigment molecules are ordered 

 (granules and lamellae in lipochondria {Aplysia nerve cells), cristae in mito- 

 chondria (cardiac cells), lamellae and grana in chloroplasts (plant cells), 

 lamellae in the retinal photoreceptors' outer segment), or where the dye 

 molecules are stacked, as in the stained axon. 



An impressive amount of sound information concerning the above sequence 

 is available from works on photosynthesis. Fortunately, on illumination, 

 generator depolarization and spike discharge have been also demonstrated 

 at the cell membrane output, in a suitable plant cell, the Nitella (Arvanitaki 

 and Chalazonitis, 1949c, 1950). Thus, several mechanisms tracked in the 

 processes of plant cells' photoactivation might be suggested as fitting to our 

 problem. 



The concentration of absorbing molecules (cytochromes in the mito- 

 chondria, intraneuronal heme-protein and carotene protein in the Aplvsia 

 hpochondria, stacked dye molecules in the stained axon membrane, visual 



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