204 A. ARVANITAKI AND N. CHALAZONITIS 



energy to elicit an "on" response composed of a single spike was tentatively 

 evaluated (Arvanitaki and Chalazonitis, 1960). But the genesis of a spike 

 marks the attainment of a liminal rate of the generator depolarization, and 

 represents the achievement of a tremendous bioelectric response. Thus, it is 

 now conceivable that a minute detectable depolarization threshold is to be 

 found at lower and lower quantal values. 



Fig. 5. Inhibition by monochromatic 490 mij. irradiation in certain A somata of 

 high carotene-protein content. 



An orange-yellow A soma (concentration of the carotene-protein higher than 

 that of the heme-protein), initially autoactive in the dark, is here submitted to 

 three steady illuminations of diflFerent spectral compositions, but of equal incident 

 intensities (6 10 '• cal g mm - sec i). While the activation of the heme-protein 

 (579 miji) determines an increase of the spikes frequency (excitation), that of the 

 carotene-protein (490 mij.) determines a decrease (inhibition), without any 

 detectable sign of inhibitory post-synaptic potentials. The response to the activation 

 by the whole visible (L.T.) determines an immediate transitory increase of the 

 frequency, followed by the cessation of the spikes. This is an example of interfering 

 excitatory and inhibitory effects. 



Calibrations: 60 mV, and 0-4 sec. 



On the nature of the generator depolarization. The generator depolarization might 

 now be recognized as a common process in excitation. It is triggered in different sensory 

 cells by appropriate stimuli. One of the clearest demonstrations of this phenomenon 

 has been in the Crayfish stretch receptor cell (Eyzaguirre and Kuffler, 1955a, 1955b; 

 Kuffler, 1959), whose microanatomy has been studied by Florey and Florey (1955). 

 Generator potentials have been found in the Pacinian corpuscle (Loewenstein and 

 Rathkamp, 1958), in olfactory cells (Ottoson, 1956; Macleod, 1959), and above all 

 in visual photoreceptors, (MacNichol, Wagner and Hartline, 1953; Svaetichin, 1956; 

 Fuortes, 1958 and 1959;MacNichol and Svaetichin, 1958; etc.). 



Moreover, generator depolarization has been recorded by external or internal 

 microelectrodes in various central nerve cells: Aplysia somata (Arvanitaki and Chala- 

 zonitis, 1949d, 1955a, 1958b), crustacean heart ganglia cells (Hagiwara and Bullock, 

 1957; Watanabe and Bullock, 1959), cortical neurons (Li and Jasper, 1953), and spinal 

 interneurons and motoneurons (Kolmodin and Skoglund, 1958), finally in cardiac 

 pacemaker cells (Dudel and Trautwein, 1958). The association of graded, relatively 

 slow depolarization with discharge of impulses was known as a general pheno- 

 menon from previous data on spinal motoneurons (Barron and Matthews, 1938), and 

 on invertebrate nerve fibers and cardiac cells (Arvanitaki, 1938). 



