THERMAL SENSATIONS 



449 



^'"Plsec 



— o— Single fibers 

 _o— Two fibers 



50 °C 



Fig. 17. Graph showing frequency of the steady discharge of different single and dual warm 

 fiber preparations as a function of the temperature of the tongue surface. [From Dodt & Zotterman 



(23)-] 



RESPONSE OF THERMAL RECEPTORS TO TEMPERATURE 



CHANGES. For the investigation of the influence of 

 temporal changes in temperature on the impulse fre- 

 quency, rapid changes from one constant temperature 

 level to another constant level were used. A purely 

 rectangular shape of the temperature rise curve could 

 not be obtained as the equalization of the receptor 

 temperature takes a certain time, but a very rapid 

 adjustment to a constant value was ensured. 



Cold fibers. Sudden cooling of the tongue produces, 

 as is seen in figure 18, a rapid discharge of the cold 

 fibers which quickly declines to the final value of the 

 steady discharge characteristic of the prevalent tem- 

 perature. The maximum response of a .single cold 

 fiber seen when applying a very rapid cooling from 

 40° to 2°C was 140 impulses per .sec. which is about 

 15 times as high as the maximum frequency recorded 

 at a constant temperature. 



In contrast to the steady discharge, the frequency 

 of which is determined solely by the temperature, the 

 maximum frequency at temperature changes is not 

 so much dependent on the initial or final temperature 

 as on the rate of the temperature change (dd/di). 

 Rapid cooling can thus produce a discharge from cold 

 receptors even in a rather warm temperature region 

 above the upper temperature limit of the steady dis- 

 charge as is shown in figure 18. 



The maximum rate of discharge of the cold fibers 

 in response to rapid cooling is, however, not exclu- 

 sively determined by the rate of cooling (dd/di), as 



is evident from figure igi?. Applying equal tempera- 

 ture drops of 2°C at various initial temperatures, it 

 was found that identical intracutaneous temperature 

 changes elicited different grades of excitation in the 

 cold receptors depending upon the range of tempera- 

 ture within which the change occurred. 



Rapid warming of the cold receptors to a constant 

 temperature leads to an immediate cessation of the 

 steady discharge. If this temperature lies below the 

 upper temperature limit of the steady discharge, the 

 impulses reappear and adjust themselves at a fre- 

 quency corresponding to the prevailing temperature 

 (fig. 19). The length of this pause caused by warming 

 the cold receptors depends upon the rate of warming 

 and the range of temperature. Thus, while rapid 

 cooling leads to an 'overshooting' excitation of the 

 cold receptors, rapid warming of these receptors 

 produces an 'overshooting' inhibition. 



If the warming is small or follows at a relatively 

 low rate, the cold impulses may not disappear at all 

 but occur only at another frequency. Thus cold im- 

 pulses were shown to appear even during warming of 

 the cold receptors. This offers a ready explanation of 

 Weber's 'persisting cold' sensation (cf page 443) 

 that below a certain temperature cold sensations may 

 occur even when the temperature of the receptor 

 layer of the skin is rising. The objection ba.sed on the 

 spread of the cooling to surrounding parts of the skin 

 was quite pointless in these experiments since, when 

 thin nerve preparations are used, the receptive field 



