200 A. ARVANJTAKI AND N. CHALAZONITIS 



the first detectable bioelectrical response would have to be defined. The latter 

 might be the response to activation of a single patch of the membrane. In 

 such a case, the light threshold energy would approach a few quanta. 



Another important feature is the delay of the least detectable photo- 

 bioelectrical reaction. As the rate of development of the generator potential 

 increases exponentially with time, it should be expected that the least measur- 

 able delay would tend to a lower and lower value as the sensitivity of the 

 recording device is improved. 



Pigmented Nerve Cells — Natural Photoreceptor Cells 



The giant nerve cells of Aplysia ganglia share with other pigmented photo- 

 activable cells (see Table 1) the common important cytostructural feature of 

 localization of the absorbing molecules in intracellular organelles where 

 they are tightly ordered on fine substructures. 



These giant nerve cells have been thoroughly studied for two reasons: 



First, they are very densely pigmented. Among the intracellular pigments 

 found in these somata, heme-protein with three main absorption bands at 

 579, 542 and 418 m^i (in the oxygenated state) and carotene-proteins absorb- 

 ing mainly at 490 and 463 mix, have been identified. 



The pigments are located in intracellular organelles of approximately 

 0-7 /x diameter, the lipochondria (or "grains"), (see Chalazonitis. 1961a). 

 The "grains" are organized under the cell membrane into layers and islets 

 at the borders of the highly basophilic deep somatoplasm surrounding the 

 nucleus, as is well seen on sections through the cells. The lipochondria may 

 be isolated by careful microdissection of the cell membrane, or in larger 

 amounts by centrifugation methods (Arvanitaki and Chalazonitis, 1960; 

 Chalazonitis and Arvanitaki, 1951, 1956; Chalazonitis and Lenoir, 1959). 

 The number of lipochondria in a giant cell has been tentatively calculated as 

 being of the order of 10*' (Arvanitaki and Chalazonitis, 1960). 



The second reason was the striking cytological differentiations which 

 allow the recognition of the different cells under the microscope and 

 the experimentation on given identifiable somata (see Plate 1). These cells are 

 identifiable not only on account of their constant location, size and shape, 

 but also by the characteristic patterns of their activity (Arvanitaki and 

 Chalazonitis, 1955a, 1958b). 



Taking advantage of such a situation, photoactivations have been brought 

 about on given differentiated types of cells: the so-called type A cell, the 

 type Gen cell, and the smaller cells immediately contiguous to these two types. 



ASPECTS OF THE RESPONSES TO LIGHT 

 OF THE A NERVE CELLS 

 The large type A nerve cells may in the dark be either autoactive by spikes 

 at constant frequencies, or in the resting state. Generally these cells respond 



