EXCITATORY AND INHIBITORY PROCESSES 217 



with a fairly regular evolution in their frequency as the irradiation proceeds. 

 However, paradoxically, the amplitude of these i.p.s.p.'s does not decrease 

 while the underlying membrane potential increases by 10-15 mV. 



As the irradiation is prolonged and the membrane potential increased, a 

 sudden hyperpolarizing jump towards a lower stable state supervenes (Fig. 

 17). recalling that described above in the axon. At this level the amplitude of 

 the superimposed high frequency i.p.s.p.'s falls to zero. At "off" the mem- 

 brane redepolarizes towards its initial value, the spiking is re-established 

 with a post inhibitory rebound, and the emission of the i.p.s.p.'s is stopped. 



Before the occurrence of the hyperpolarizing jump, the amplitude of the 

 I.p.s.p.'s does not decrease even if the membrane potential surpasses the 

 equilibrium level as marked out by the initial spike's post-hyperpolarization 

 peak. Moreover, this equilibrium level shifts towards a lower value in the 

 post inhibitory discharge spikes. 



If the infra-red action is studied with the soma membrane potential pre- 

 viously "fixed" at different levels (Fig. 18), it is seen that only a 5 mV positive 

 displacement is sufficient to determine a relevant decrease in the i.p.s.p.'s 

 amplitude. This is in accordance with previously known data in the spinal 

 motoneuron (Coombs, Eccles and Fatt, 1955), in inhibitory neuromuscular 

 junctions (Fatt and Katz, 1953; Hoyle and Wiersma, 1958a and 1958b; 

 Grundfest, Reuben and Rickles, 1959), in receptor crustacean neurons 

 (Kuffler, 1958; Kuffler and Eyzaguirre, 1955). The displacement of the 

 membrane potential was here brought about merely by varying the tempera- 

 ture of the preparation (Chalazonitis and Arvanitaki, 1957; Chalazonitis, 

 1961b). 



Sensitivity of the above cells to infra-red detection. The change of the tissue's 

 temperature by infra-red may be measured with some accuracy (Chalazonitis, 

 1954). The surface of the irradiated cell being approximately 10 ^ mm^, the 

 threshold intensity lO^'* cal g mm"-' sec^^ applied for 0-2 sec, the threshold 

 energy is of the order of 10"^^ cal g and the temperature elevation of the soma 

 has been estimated to be about 0-01 6°C (Arvanitaki and Chalazonitis, 1960). 



Such evaluations may be useful in comparing the sensitivity to infra-red 

 of these common ganglion nerve cells to that of specialized sense organs. The 

 anatomy and function of the sense organ in the facial pit of Pit Vipers have 

 been thoroughly studied by Bullock and co-workers. The sensory endings 

 were found to be sensitive to a change in temperature of the order of 

 0-00 rC (Bullock and Diecke, 1956; Bullock and Fox, 1957). This is close 

 to the value found for human thermoreception. Moreover giant synapses of 

 invertebrates are temperature sensitive, as described by Bullock (1956), and 

 isolated invertebrate ganglia, as well as excised nerves, react to rapid warming 

 and cooling (Kerkut and Taylor, 1956; Granit and Skoglund, 1945; Bernhard 

 and Granit, 1946). 



