THERMAL SENSATIONS 



453 



35°C. This phenomenon is only another example of 

 'persisting cold sensation' (cf. page 443). The low 

 sensitivity of the warm receptors and the relatively 

 high sensitivity of the cold receptors between 20° and 

 30°C has an important bearing on our interpretation 

 of the Weber phenomenon of persisting cold sensation. 

 When a cold object is removed from the skin, there is 

 an obvious pause in the cold sensation due to the 

 postexcitatory depression of the cold receptors. When 

 the cold sensation then slowly reappears, although the 

 temperature of the skin is gradually rising, there will 

 be very little interference from the rather scattered 

 warm receptors. Thus the steady discharge of im- 

 pulses from the cold receptors which display their 

 ma,\imum sensitivity in just this temperature region, 

 25° to 30°C, will stand out still more conspicuously. 



The question concerning the real existence of a 

 paradoxical warmth sensation brought about by cool- 

 ing has been the subject of much discussion C72). The 

 reason for this is now quite obvious. We have to con- 

 sider not only that cooling of the skin stimulates 

 numerous cold receptors and that the ensuing cold 

 sensation thus will mask the paradoxical warmth 

 sensation ijut also the fact that the 'paradoxical' 

 response of the warm fibers is of phasic character and 

 soon fades away. 



Thus we can conclude that the paradoxical sensa- 

 tion of cold experienced when the skin is heated to a 

 temperature between 45° and 50°C has its ph)'siologi- 

 cal analogy in a steady discharge of specific cold 

 fibers. 



The paradoxical warmth sensation which generalh 

 is masked by an intense cold sensation has its counter- 

 part in a phasic discharge of specific warm fibers to 

 the cooling. 



Effect of temperatures above 47°c. The fact that 

 the steady discharge of the warm receptors generalh 

 disappears at a temperature above 47°C must mean 

 that above this temperature the quality of sensation 

 which generally is described as hot has little to do 

 with the feeling of warmth (cf. page 444). Alrutz' 

 (2) suggestion that the sensation of heat was a mixed 

 sensation of warmth and paradoxical cold must be 

 revised to some degree. When the skin is suddenly 

 heated from 35° to 50°C, there occurs first a sudden 

 transient discharge of warm fibers accompanied by a 

 paradoxical cold fiber discharge which continues as 

 long as the temperature is kept at this level. To this 

 paradoxical discharge of cold fibers a discharge of 

 pain fibers is gradually added (96, 97). Skouby (So) 



has recently found that the subjective pain threshold 

 lies at temperatures of 47. i ° to 48.5°C. Thus it can be 

 concluded that, when temperatures of above 47 °C are 

 applied and after the temperature change in the skin 

 has ceased, the sensation of heat then experienced is 

 the resultant of a mixed inflow of paradoxical cold 

 and pain impulses. This sensation is thus initiated only 

 by warm and paradoxical cold impulses and to the 

 persisting paradoxical cold discharge, pain impulses 

 are gradually added as the temperature is kept at a 

 constant value above 47 °C. At still higher tempera- 

 tures the heat will destroy the fibers. Heating the skin 

 to more than 50°C very quickly not only inactivates 

 the mechanoceptive fibers in the tongue (96) but 

 also causes the steady paradoxical discharge of the 

 cold fiber to disappear leaving the signalling duty 

 entirely to pain fibers. This course of events was fully 

 confirmed by recent experiments of Dodt (19). 



intracut.anequs gradient. In order to investigate 

 the importance of the intracutaneous temperature 

 gradient for the stimulation of the thermoceptors 

 Hensel & Zotterman (55) recorded the action poten- 

 tials from the cold fibers of the lingual nerve of the cat 

 when cold stimuli were applied to the tongue so as to 

 cause negative or positive intracutaneous temperature 

 gradients (cf. page 446). The nerve preparations 

 chosen were those containing cold fibers supplying 

 only the upper surface of the tip of the tongue. In order 

 to produce negative or positive temperature gradients, 

 the tongue was cooled from either the upper or the 

 lower surface, respectively, the temperature on both 

 sides of the tongue being recorded thermoelectrically. 



The cooling of the upper surface immediately gave 

 rise to a strong discharge of cold spikes which instantly 

 disappeared on rewarming (fig. 23). On cooling of 

 the lower side no impulses appeared at first ijut within 

 1.5 to 3 sec, when the cold had penetrated the tongue 

 and reached the upper surface, cold impulses appeared 

 with increasing frequency. On rewarming of the lower 

 surface the cold impulses persisted at first until the 

 upper surface was also warmed again. In some experi- 

 ments the cooling of the receptor laser was produced 

 by injecting cold solutions into the lingual artery. 

 This way of cooling produced the same cold receptor 

 discharges as cooling the surface. The participation 

 of deep thennoreceptors could be entirely excluded 

 in these experiments, and the arrest of the blood flow 

 in the tongue had no primary influence on these 

 findings. 



These experiments demonstrate that the stimulation 



