448 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



10 r- Jmp/iec 



5 - 



from 40 - 20 C 

 .. 20 - 40 C 



25 



10 



i5 



•C ♦<? 



FIG. 15. Impulse frequency of the steady discharge of a single 

 cold fiber as a function of the temperature of the tongue surface. 

 The frequencies were first measured at temperature steps from 

 40° to '20°C and then again in reverse order. [From Hensel & 

 Zotterman (54).] 



was recorded once again when the temperature was 

 raised in the same steps as before. It is seen that the 

 points lie on the same curve. Even after hours of 

 experiments exposing the receptors to widely difTering 

 temperatures, the steady di.scharge appears with the 

 same frequency when the temperature of the tongue 

 is restored to the initial value. The impulse frequency 

 of the steady discharge of the cold receptors thus 

 depends entirely on the temperature. 



The steady discharge of cold fibers shows a maxi- 

 mum frequency of about 10 impulses per sec. The 

 site of this inaximum on the temperature scale varies 

 in different fibers between 20° and 34°C, while the 

 upper and lower temperature liinits in extreme cases 

 reach 10° and 41 °C. The total frequency of cold 

 impulses in the nerve, which is the sum of the dis- 

 charges from the single cold fibers, reaches its inaxi- 

 mum (fig. 16) at a temperature between 15 and 20 °C 



(54)- 



Warm fibers. Judging from the relative spike height 



the fibers inediating warmth were conceived to be 

 somewhat larger in diameter than the cold fibers (96). 

 Preparations containing only warm fibers display a 

 steady discharge to constant temperatures between 

 20° and 47°C. In single filler preparations the fre- 

 quency of this steady discharge varies in a consistent 

 manner with the temperature, although the maxi- 

 mum discharge as well as the upper and lower tem- 

 perature limits vary somewhat (fig. 17). The max- 

 imum usually was found between 37.5° and 40°C. 

 At higher temperatures the steady discharge falls ofT 



rather steeply. Above 47°C and below 20°C no steady 

 discharge was noticed. The maximum frequency 

 varied in single warm fiber units between 1.5 and 3.7 

 impulses per sec. (23). The discharge was never as 

 regular as that of the cold fibers which may depend on 

 the comparatively low frequency and also on the 

 possibility that the warm fibers may divide peripher- 

 ally to supply more than one end organ. 



The low sensitivity of warm receptors to tempera- 

 tures between 20° and 30°C aho has an important 

 bearing on the interpretation of Weber's phenomenon 

 of persisting cold sensation (cf. page 443). When the 

 cold object is removed from the skin there is a distinct 

 pause in the cold sensation due to the postexcitatory 

 depression of the cold receptors. When the cold sensa- 

 tion then slowly reappears, although the temperature 

 of the skin is gradually rising, there will be very little 

 interference from the scattered warm receptors. Thus 

 the steady discharge of impulses from the cold recep- 

 tors which display their maximum sensitivity in just 

 this temperature range 25° to 30°C will stand out still 

 more conspicuously. 



Jmplc,ec 

 100 r- 



— o— Single fiber 

 — •- 4-5 fibers 

 Itl 10-20 fibers 



55 'C ^ 



FIG. 16. Total impulse frequency of the steady discharge in 

 different preparations of the cat lingua! nerve as a function of 

 the temperature of the tongue surface. [From Hensel & Zotter- 

 man (54).] 



