450 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



sec 50 



FIG. i8. Impulse frequency of a single cold fiber in the cat 

 lingual nerve during induction of rapid temperature changes 

 in the tongue. A, From 32° to 30°C; B, from 40° to 38°C; C, 

 from 42° to 40°C; D, from 44° to 42 °C. Cooling starts at time 

 zero. [From Hensel & Zotterman (54).] 



of the cold fibers is far smaller than the surface of the 

 thermode in contact with the tongue. The cold recep- 

 tors involved can thus be influenced only by the 

 temperature changes below the thermode and not at 

 all bv any spread of cooling outside the thermode. 



]\'arm fibers. The discharge of the warm fibers in 

 response to thermal stimuli displays essentially the 

 same characteristic features as does that of the cold 

 fibers but in the reverse order. Thus rapid warming 

 to a constant level produces an overshooting discharge 

 after which the discharge adjusts itself fairly quickly 

 into an irregular steady rhythm. The initial volley 

 (fig. 20) appears after latencies varying from 0.15 to 

 0.55 sec. which was taken as indicating that the 

 warm receptors were situated at varying depths in the 

 tongue. Short latencies were observed particularly 

 for temperature rises from ;57° to 40°C, whereas 

 latencies for corresponding rises in the range of 25° 

 to 35°C were considerably longer. The discharge of a 

 single warm receptor displays a much higher initial 

 frequency compared with that of a cold receptor 

 exposed to a corresponding drop in temperature, and 

 the sequence of impulses thereafter is interrupted by 

 sudden pauses in the discharge. The mean value of 



the frequency falls, however, in an exponential way 

 as does the discharge of the cold receptors. 



Figure 21 shows simultaneous records from a 

 strand of the chorda tympani (above) and from a 

 strand of the lingual ner\-e (below). The former 

 preparation contained warm fibers only while the 

 lingual strand possessed cold fibers and one touch 

 fiber. At a constant temperature of 23 °C there is a 

 steady discharge of cold fibers in the lingual prepara- 

 tion (below) while hardly any spikes are recorded 

 from the chorda strand. When the temperature was 

 raised to 38.5°C, a discharge of the warm fibers 

 occurred while the steady discharge of the cold fibers 

 disappeared. A quick return of the temperature to 

 23°C produced a small oflf-efTect from the warm 

 fibers at the same time as the cold fibers started their 

 firing. When this procedure was repeated after 6 sec. 

 (fig. 21), hardly any change in the response could be 

 noticed. If however the temperature was raised to 

 44.8°C, a certain difiference in the warm fiber response 

 was noticed. First of all, the sudden rise elicits a much 

 stronger initial frequency of warm impulses. Secondly, 

 when the temperature dropped there appeared a 



Time t 



FIG. 19. Graphs showing .-J, impulse frequency of the steady 

 discharge of a single cold fiber ; B, frequency of a sudden tem- 

 perature drop at different temperatures. [From Hensel (48).] 



