210 A. ARVANITAKI AND N. CHALAZONlTlS 



unsuccessful as yet in attempts to demonstrate consistent time relation 

 between the spikes' emission in the excited cell and the i.p.s.p.'s occurrence 

 in the contiguous inhibited cell. In spite of this failure, a synaptic origin of 

 the inhibition is suggested in these cases. Such patterns already recorded 

 under orthodromic activation in Aplysia neighbouring cells (Arvanitaki and 

 Chalazonitis, 1959) reveal subtle interactions between the responding units 

 and draw attention to an underlying elaborate anatomical organization. 



Excitatory and inhibitory interactions have been demonstrated in the vertebrate eye 

 and ascribed to the complex organization of the retina (Granit, 1947, 1955). In the 

 Limulus lateral eye conspicuous inhibitions are also present and have been attributed 

 to interactions possibly owing to an elaborate organization supplied by a "plexus" 

 which is an extensive system of cross-connecting strands of nerve fibers, so as to form 

 a three-dimensional network closely associated to the axons of the retinula and 

 eccentric cells (Hartline, 1938, 1941, 1949; Hartline, Wagner and MacNichol, 1952; 

 Hartline, Wagner and Ratliff, 1956). It is to be noted that anatomical investigations 

 in the Aplysia ganglia have shown, lying under the cortical giant nerve cells, some 

 analogous network of fine strands of fibres which might perhaps supply similar 

 functions, as above. It also recalls networks as described in other fields by Horridge 

 (I960) and Szentagothai (1960). 



As regards the inhibitory processes elicited by radiations in the Aplysia 

 cells, one must first consider the hypothesis that in some of the ganglion 

 cells synaptic inhibitions through odd excitation of some interneurons would 

 be imphed. The hodological mediation of some underlying organizer 

 plexus might also be inferred. However, besides the above, other inhibitory 

 processes are elicited by light which might suggest other mechanisms, possibly 

 of primary inhibition. For example, there are conspicuous occurrences with- 

 out latency, when hght is "on", of maintained repolarizations free of i.p.s.p.'s. 

 There is also in given cells the permutation from the excitatory type to the 

 inhibitory type of response by merely changing the wave length of the acti- 

 vating radiation without introducing any topological variance. In fact, in 

 the Sepia stained axon wherein any means of secondary inhibition is out of 

 the question, excitation may be also turned into inhibition by changing the 

 wave length of the irradiation. To similar inhibitions by light pertains the 

 off response studied by Kennedy (1958) in excised segments of the naturally 

 pigmented pallial nerve of Mactra. Such facts suggest that a direct primary 

 inhibition mechanism might in addition be operative through given wave 

 lengths in Aplysia nerve cells. This implies that the activated subcellular 

 stru -^ures would be certain intermediary ones, or, indeed, those which 

 funci onally mediate the development of inhibitory processes. 



GENERAL INHIBITORY EFFECTS 

 OF INFRA-RED RADIATIONS 



Inhibitory effects of infra-red radiations are noticeable in the Sepia axon 

 as well as in the Aplysia nerve cells. 



