THERMAL SENSATIONS 



455 



'Mill iMIlllllllllllMMIIIIIIIIIIlllllllinillllllllllllllllhllllllllllllMlllllllllllllMllUlllinilllllllll 



*oi'c Menthol 1 10.000 



tei4yiUitiiiiiiitiiiiiiiiiiliiiiiiililuijiiii>iiiiiiiii 





iiiiiiiiiiitiiiiiiiilliiniiii 



'<: Menthol 1 10.000 ,, , , , i , , 



iyil MUm iiiJ.4 11 1 11 i U iJ " l i oUL 



iiiiiiiiiiiiiiiiiMiiiiiiiiiiiiMiiiiiiiiiiiiiiiiiiiniiiiiiiiiiiiiiiuiinihiiiiiiiiiiiiiiiiiiiiiHiiiiiiiiiihiiiiiiiii 



•wr-f Menthol 1 10.000 



f"- 1 1 -[-M 1 ' ■■^'^^^"'^ i.ii.iii>iiiy.y.ti 



lillllllillinliMilinliii 





FIG. 25. Action potentials from cold and warm fibers in a 

 thin strand of the cat lingual nerve after the application of 

 menthol solution to the tongue. Under the action of menthol 

 (i : 10,000) there is at 40°C strong discharge of cold libers which 

 disappears on warming, to be followed by discharge from a 

 warm fiber. [From Hensel & Zottcrman (56).] 



substances into the skin produced an increased num- 

 ber of cold spots, Dodt el al. (21) investigated the 

 effect on single thermal fibers. Minute amounts of 

 acetylcholine shift the temperature range of the steady 

 discharge of cold fibers towards the warm side and 

 increase definitely the rate of the steady discharge of 

 the receptors inside the normal range of temperature. 

 Larger amounts produce a depression of the steady 

 discharge and a narrowing of the temperature range 

 recorded. Corresponding results were found with 

 warm fiber preparations. 



Dodt (20) has recently investigated the influence 

 of carbon dio.xide on the thermal receptors. An in- 

 crease of the pClOa reduced the rate of the steady dis- 

 charge of cold receptors, whereas it caused an in- 

 crease of the steady discharge of warm receptors. The 

 regulating structures will thus, under the action of 

 carbon dioxide, receive a false picture of the actual 

 thermal conditions in the periphery which will lead 

 to a fall of the rectal temperature without any sub- 

 jective discomfort. 



Aqueous menthol solutions of 1:10,000 lead to 

 strong steady discharge of the tongue cold receptors 

 at constant warm temperatures at which without 

 menthol there is no discharge (fig. 25). At lower 

 temperatures at which the cold receptors are steadily 

 discharging without menthol, this substance produces 

 a great increase of the steady cold impulse frequency. 

 Further studies of Dodt el al. (21) of the effect of 

 menthol on single fibers showed that menthol exerts 

 an effect, not only on the cold fiber activity in the 

 usual temperature range between about 10° and 38°C, 

 but also on the paradoxical cold fiber discharge 

 between 45° and 5o°C. In agreement with Gold- 

 scheider these authors observed that inenthol sensi- 

 tizes the warm fibers also. 



The effect of menthol on the cold and warm fibers 

 can be completely compensated for by sudden heating 

 and cooling, respectively, or by keeping the tongue 

 at a constant higher and lower temperature, respec- 

 tively. These measures can cause the cold and warm 

 impulses provoked by menthol to disappear entirely. 

 Thus it is not simply the question of a chemical 

 'inadequate' stimulation of the thermal receptors but 

 of a sensitization of the thermal effect. The threshold 

 of the menthol effect lies between the concentrations 

 of I : 1 ,000,000 and 1 : 500,000. 



Following the finding of Bing & Skouby (8) that 

 the introduction of small amounts of cholinergic 



THEORETICAL CONSIDERATIONS 



Cenlral Threshold 



From the sensory physiological studies it appeared 

 that three factors are governing the occurrence of a 

 thermal sensation: a) the absolute intracutaneous 

 temperature, d, 6) the rate of change of the intra- 

 cutaneous temperature, dd/dl, and the area F, the 

 extension of the stimulated field. 



So far as the conditions for the occurrence of a 

 thermal sensation can jje expressed in physical- 

 thermal terms, the excitation mechanism can be repre- 

 sented by a three-dimensional system of thermal-spa- 

 tial-temporal factors which arc mutually dependent 

 on each other and to a great extent exchangeable. 

 The threshold condition can thus be expressed as fol- 

 lows: 



E 3 i(e,dS/dt, F) 



where E is the abstraction class of the sensation, 3 

 the implication sign of a probability implication (75). 

 A sensation of cold, for example, would thus occur 

 when a) d is low, b~) the rate of cooling, dO/dl, is 

 sufficient, and c) the receptive field has a certain area 



(42). 



The recordings of the action potential from periph- 

 eral cold fibers show that the total number, n, of im- 



