214 A. ARVANITAKI AND N. CHALAZONITIS 



The stained axon as a detector of infra-red radiations. At low temperatures 

 the autoactive axon, methylene blue stained, exhibits in the dark an oscillatory 

 behaviour with periodic spike emission. On such a preparation, visible radia- 

 tions elicit depolarization and increase in the frequency of the spikes 

 (Fig. 13). In contrast, on the same axon, a beam of infra-red radia- 

 tions induces hyperpolarization, damping of the pre-existing oscillatory 

 activity, and inhibition of spikes emission (Fig. 13a). The effect reverses readily 

 upon the cessation of the infra-red. 



In the neutral red stained axon, following the splitting of the repolarization 

 phase of the spike into three components and the uncoupling of the third 

 (see Fig. 4), the membrane potential settles for a long time at a plateau of 

 depolarized level (Arvanitaki and Chalazonitis, 1955b). Tasaki and Hagiwara 

 (1957) have demonstrated in squid giant axon under TEA the existence of 

 such an upper depolarized stable state. If an infra-red beam of threshold 

 intensity is applied during the plateau, the repolarization process is readily 

 initiated (Fig. 14). 



The jump in the membrane potential may be 40 mV or more. A similar 

 effect of infra-red injection has been demonstrated by Spyropoulos 

 (1961) in the nodal membrane. The infra-red repolarizing response may be 

 considered in relation to the all-or-none repolarizing response elicited by 

 a "threshold" inward current in the potassium treated squid axon (Segal, 

 1958) and analysed by Tasaki (1959) on nerve cells in the dorsal root ganglion 

 of the toad. 



Inhibitory reactions of the Aplysia somata to infra-red. While the response 

 to light of the different identifiable somata may be amphoteric, only in- 

 hibitory reactions to infra-red radiations have been as yet recorded. Depend- 

 ing on the identity of the cell, one of the two following inhibitory patterns 



may be seen: 



A mere increase of the membrane potential: This starts without latency at 

 "on" and increases with time to reach a maximum value which is a function 

 of the intensity of the irradiation. The discharge at "off" may be simply 

 considered as a post inhibitory rebound (Fig. 15). As a rule this pattern of 

 inhibition has been recorded in the A type cells. 



Initially autoactive cells repolarize at "on", and the emission of spikes is 

 stopped during the irradiation, the length of the pause increasing as a function 

 of the time and the intensity. As expected, autoactive somata, being the 

 more sensitive to the displacement of their membrane potential, react readily 

 to the infra-red radiations. A beam of 10'^ cal g mm"'^ sec'i intensity during 

 0-2 sec is sufficient to elicit a conspicuous inhibition. 



On the immediate increase of the membrane potential, superimposed series 

 ofi.p.s.p.'s: This pattern, illustrated in Fig. 17 and Fig. 18, has been mainly 

 recorded in the Gen type cells. Soon after the initiation of the membrane 

 potential increase at "on", a series of i.p.s.p.'s of uniform shape appear 



