PATTERNED ACTIVITIES 379 



discharge frequency were found to be distributed in between two extreme 

 values : 16°C and 31°C. In Fig. 1 are given examples of two cells with 

 such extreme (9^^,. In these, starting from 22°C, any limited increase in 

 the temperature caused a decrease in the frequency of the low 0^^^ nerve 

 cell ('* cold " neuron) but an increase in the high d^p^ nerve cell (" warm " 

 neuron). Conversely, coohng brought about a high frequency discharge 

 in the nerve cell of low O^pf. Changes in frequency were controlled by 

 changes in the cell membrane potential (MP) which, in "cold" neurons, 

 was increased by warming and decreased by cooling (within certain thermal 

 limits). 



Experiments indicated that such MP changes were due to local effects 

 of the temperature in differentiated areas of the cell membrane, spatial 

 gradients driving intrinsic generator currents. The latter may be enhanced 

 or abolished by appropriately directed transmembrane currents (through a 

 second internal microelectrode). If a change in MP due to changes in 

 other factors (e.g. partial pressure of CO.,, O2, etc.) can, to some extent, 

 be additive to that due to a change in temperature, one could have at least 

 one simple interpretation of why thermal optima are dependent upon 

 other environmental conditions. 



In addition, other identifiable nerve cells display more elaborate patterned 

 transitions in their activity, when submitted to temperature changes, than 

 those referred to above (Fig. 2). The Br-iypo, cell, for example, which at 

 normal temperatures is autoactive in bursts occurring on rhythmic slow 

 waves, undergoes with hyperthermia a marked hyperpolarization with 

 superimposed repeating " inhibitory " potentials of an uncommonly large 

 amplitude. At the same time the autoactivity of the Ge/7-type cell, con- 

 tinuously regular at normal temperatures, gradually turns, on warming, 

 into a discontinuous type : the membrane potential displays square- 

 shaped changes jumping from a hyperpolarized to a depolarized level. 

 During the latter, high frequency spikes or oscillatory potentials are 

 elicited. Thus the Gen-iypQ cell mimics, at high temperature, the patterned 

 behaviour specific to the i5/*-type cell at lower temperatures. 



It appears to us from the above, that in addition to mere changes in 

 spike frequency, more elaborate modal transitions in the activity of specific 

 cells may be invoked as possible mechanisms for discrimination and 

 supplying information at the level of primary receptor units. 



REFERENCES 



Arvanitaki, a. and Chalazonitis, N. 1957. Pointes et potentiels positifs du soma 

 neuronique en fonction de la temperature. C. R. Acad. Sci. 245, 1079-1081. 



Arvanitaki, A. and Chalazonitis, N. 1958. Configurations modales de Tactivite, 

 propres a differents neurones d'un meme centre. J. Physiol. Paris, 50, 122-125. 



