EXCITATORY AND INHIBITORY PROCESSES 209 



mic Stimuli via the afferent nerve. Functional presynaptic potential changes 

 are known to occur (Lloyd, 1949; Wall, 1958; Wall and Johnson, 1958). 

 Furthermore, it is not to be disregarded that presynaptic or synaptic structures 

 might be endowed with some mode of autoactivity. 



Thus the question arises whether the generator depolarization is built up 

 by a massive recruitment of e.p.s.p. activities. As generator depolarization 

 has been demonstrated in the axon membrane, however, there is no reason to 

 disregard such a generator potential as having possible intrinsic origins; in 

 addition, an eventual contribution of the activities of the synaptic structures 

 is not excluded. 



INHIBITIONS BY LIGHT 

 IN THE GEN TYPE CELLS 



Contrasting to the above, other well identifiable nerve cells of the Gen or 

 of the B type always hyperpolarize when illuminated (Arvanitaki and Chala- 

 zonitis, 1958a, 1960). The hyperpolarization is established without appreciable 

 delay and grows to reach from 10 to 20 mV. Simultaneously, series of re- 

 peated postsynaptic inhibitory potentials are superimposed at increasing 

 frequencies, and any emission of spikes is consequently inhibited. The ampli- 

 tude of the hyperpolarization and the frequency of the i.p.s.p.'s increase with 

 the intensity of the activating light. At "off" the membrane repolarizes to- 

 wards the initial level, the i.p.s.p.'s emission subsides and that of the spikes 

 is re-established. 



In an attempt to search for an explanation of these inhibitory actions 

 initiated by light, simultaneous recordings of the electrical activities (Arvani- 

 taki and Chalazonitis, 1959; Chalazonitis and Arvanitaki, 1958) have been 

 systematically performed in many nerve cells contiguous to the cell which 

 was illuminated. Three main features may be distinguished. It happened 

 that the inhibition of the illuminated nerve cell was not accompanied by any 

 obvious transition in the activity of the neighbouring cells. This was a rather 

 rare case which occurred with higher probability at low light intensities. 



Secondly, synergic or concurrent inhibition was manifest in neighbouring 

 cells: in many instances the neighbouring cells showed a mere decrease in 

 the frequency of spikes, accompanied by a more or less noticeable repolari- 

 zation of the membrane, without any detectable superimposed i.p.s.p.'s 

 (Fig. 10). In others, numerous i.p.s.p.'s appeared as well. Time relations 

 between the latter and the i.p.s.p.'s of the illuminated soma were never- 

 theless not conspicuous (Fig. 1 1). 



Finally, reciprocal excitation-inhibition patterns were repeatedly record- 

 ed in illuminated contiguous nerve cells, as illustrated in Fig. 12. At 

 "on", hyperpolarization, i.p.s.p.'s and inhibition of the spiking develop in 

 the Gen cell while depolarization and an increase in the spikes' fre- 

 quency evolve in a nearby smaller, densely pigmented cell. We have been 



