180 N. CHALAZONITIS 



(Arvanitaki and Chalazonitis, 1957, 1958). (For recent information on in- 

 hibition mechanisms relating to nerve cells see Eccles, 1957; Arvanitaki and 

 Chalazonitis, 1957; Bullock, 1958; Fessard. 1959; Grundfest, 1959; Kuffler, 

 1959.) 



1. THE FUNCTIONALLY DIFFERENTIATED NEURONS 



Half a dozen of the giant nerve cells of the visceral ganglion of ApJysia 

 fasciata are easily identifiable according to their size and their location relative 

 to the nerve trunks (see Fig. 1). Such cells may be divided into three main 

 types, depending upon the patterns of their spontaneous, or synaptically 

 induced electrical activity (Arvanitaki and Chalazonitis, 1958; Chalazonitis, 

 1959; Tauc, 1960). While the ganglion is bathed in sea water saturated with 

 oxygen the autoactivity observed is long lasting (more than 2 hr) and is 

 observed in 10% of the nerve cells examined. 



Fig. 1 . Dorsal face of Aplysia fasciata visceral ganglion. 



A, Br, B, Gen, and A', identifiable giant somata. 1, 2, 3, 4, 5, medium-sized 



somata (100-250// diameter) identifiable owing to their vicinities with the 



former. ii.A., pleurobranchial nerve; n.Br., branchial nerve; n.sp., spermathecal 



nerve; n.an., anal nerve; n.gen., genital nerve; ii.per., pericardial nerve; n.B.. 



pleurogenital nerve; v.^. ganglionic vessel. 



The three types of activity are as follows: The first type is seen ii. the A cell 

 i^see Fig. 2). The spikes are of constant, relatively low frequency (0-5- 

 1/sec at 20 °C). 



The second type is seen in the Br cell (see Fig. 2). It is characterized by trains 

 of spikes or slow waves. The duration of the trains is several seconds. The 

 mean spike frequency on each wave is about 4/sec. 



The third type is seen in B or Gen and sometimes in p.c.r. cells. It is charac- 



