THERMAL SENSATIONS 



433 



The principle of the method consists of determining 

 by means of small rectangular temperature steps the 

 threshold temperature change, d„ for the cold recep- 

 tors. A large well-defined cold pulse was applied then 

 to the surface by means of a special thermode de- 

 scribed in figure 14; then the lapse of time, /, from 

 the beginning of the pulse until the appearance of the 

 first cold fiber spike was measured. The tiine, t, is 

 composed of the "thermal latency,' te, the time which 

 the cold needs to penetrate into the receptor layer, 

 and two constants: the nerve conduction time, <„, of 

 the cold fibers and the physiological latency, /r, of the 

 cold receptor. For the thermal latency, te, from which 

 the depth of the receptor can be calculated, we 

 obtain 



te 



tr- 



When the thermal diffusion coefficient, a, of the living 

 skin is known, it is possible to calculate to what depth 

 the threshold temperature change, d„ has proceeded 

 within the time, te- This depth is the depth of the 

 receptor. By means of a double beam cathode ray 

 oscillograph for simultaneous recording of the electric 

 response from the cold fibers and the temperature of 

 the surface of the tongue, the beginning of the tem- 

 perature course was easily determined with an ac- 

 curacy of ±0.002 sec. 



Figure i shows a record of the discharge of cold 

 spikes from a strand of the lingual nerve of the cat in 

 response to a sudden cooling from 38 to I5°C and 

 rewarming. Simultaneously the temperature of the 

 silver bottom of a thermode on the tongue was re- 

 corded by the second beam. After an interval, / = 

 0.023 sec., from the beginning of the cooling, the first 

 action potential from the cold fibers appeared. On 

 rewarming, the last cold fiber spike disappeared after 



an interval of 0.027 ^^c. from the beginning of the 

 rewarming. The determinations of / were repeated 

 several times for each preparation. As was shown by 

 Zotterman (96) the shorter the interval, /, the larger 

 the temperature steps. For the preparation of figure 

 I, for example, values of / between 0.015 ^^^- (for 

 steps from 38 to 5°C) and 0.07 sec. (for steps from 38 

 to 34°C) were obtained. For the sum of the two con- 

 stants /„ and tr an interval of about 0.006 sec. was 

 computed. The latency of the cold receptors, about 

 0.003 to 0.005 sec, is obtained by comparing the 

 intervals, /, at large and small temperature steps. 

 Using this value of /r we obtained exactly the same 

 depth at all temperature steps; at larger values of tr 

 the values of the depth were too small compared to 

 the values computed when using medium or small 

 temperature steps. From about 70 separate measure- 

 ments on six cats the following values were obtained : 



Relatise 



threshold - - 

 Average depth 



of receptors 

 Dispersion 



0-5 



o. It mm 



0.18 mm 



0.20 mm 



±0.015 n^"^ ±0.015 "^"^ ±0.018 mm 



The physiological depth determinations of the cold 

 receptors are in good accordance with the histological 

 observations made on serial slides from the same area 

 of the cat tongue. The epithelium of the papillae has 

 a height of 0.05 to 0.08 mm. The musculature of the 

 tongue starts with a rather sharply defined border 

 line at a depth of about 0.3 mm. Closely above the 

 musculature of the tongue there is a well-developed 

 net of blood vessels. Thus, according to these deter- 

 minations, the cold receptors are situated subepi- 

 thelially partly in the papillae and particularly at their 

 base or just beneath them. 



FIG. I . Simultaneous records of cold potentials in a fine strand of cat lingual nerve and of tempera- 

 ture of silver bottom of thermode on tongue during sudden cooling from about 40° to I5°C and 

 rewarming. Lejt temperature scale for cooling, right scale for rewarming. Time, 50 cps. [From Hensel 



