CHEMORECEPTOR MECHANISMS 



17 



0.125 



14 



16 



2 4 6 8 10 12 



TIME -SECONDS 

 Fig. 16. Change in reaction time with temperature and concentration after stimu- 

 lation of the ovipositor of the cricket Gryllus with NaCl. 



increase in temperature of the entire mouthpart. In this last experiment the 

 cell body of the sensory neuron was subjected to the change in temperature. 

 In all of the experiments conducted in this laboratory by Dethier and by Arab, 

 in which the temperature of the stimulus, and presumably the receptor surface 

 at the tip of the hair, was changed while the cell body and the afferent nerve 

 fibers remained at room temperature, no significant alteration of threshold was 

 detectable. Absence of a change of threshold under these conditions means that 

 at all temperatures a given concentration of stimulus results in a given fre- 

 quency of impulses in the afferent nerve. Furthermore, since the effect of the 

 temperature change is at the receptor site some 300-400 microns distant from 

 the cell body where the impulses are presumed to be generated, it seems reason- 

 able to assume that the temperature effect, if any, would be acting on either or 

 all of three processes and no others, namely: combination of sugar with receptor 

 substance, removal of sugar, and depolarization. The absence of a demonstrable 

 temperature effect suggests either (1) that the limiting process is temperature 

 independent (or has a temperature coefficient approximating 1) or (2) that the 

 net temperature effect of a reaction with a positive temperature coefficient and 



