444 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



increases in frequency as the temperature slowly rises 

 from 15 to '25°C. 



Sensation iij 'Hot' 



Alrutz (2) suggested that the sensation of hot was a 

 mixed sensation of warmth and 'paradoxical' cold 

 although it is a subjectively simple sensation not 

 divisible by introspective analysis and is qualitatixely 

 different from the sensations of cold and warmth. 

 Thunberg (87), Kiesow (62) as well as Trotter & 

 Davies (90) criticized this theory of Alrutz, main- 

 taining that the paradoxical cold sensation can 

 readily be apprehended and that the applied heat 

 even stimulates other sensory fibers in the skin. Hacker 

 (35) observed, however, in an experiment on him.self 

 in a traumatized region of the skin where no cold 

 spots but numerous warm spots were found that no 

 sensation of hotness was obtained but only of warmth. 

 Goldscheider (32), however, rejected Alrutz' inter- 

 pretation because the sensation of hot is felt most 

 strongly in regions where the warmth sensibility 

 is particularly good and not in regions where the 

 cold sensibility is comparatively stronger than that 

 of warmth. 



Kaila (61) described an experiment on thermal 

 receptors of the penis in man which greatly strengthens 

 the original view of Alrutz (2). Usually the tip of the 

 penis does not possess any sensibility of warmth while 

 cold and pain is easily evoked. When the tip of the 

 penis is dipped into water of 40°C, the subject experi- 

 ences a rather unplea.sant painful sensation; this 

 temperature does not act on the cold receptors. If, 

 however, the temperature is raised to 45°C, an intense 

 sensation of cold is produced as this temperature 

 stimulates the cold fibers. The sensation is, however, 

 not really painful. When now a greater part of the 

 penis is dipped into the water at 45 °C, warm recep- 

 tors are also stimulated and a specific .sensation of 

 pleasant heat appears. 



This simple experiment is an example showing 

 how simultaneous stimulation of different receptors 

 can evoke the sensation of a specific quality in which 

 it is not possible to recognize the elementary sensa- 

 tions which each of the specific receptors involved 

 produce. Thus, when we speak of 'hot' as an elemen- 

 tary sen.sation, as Alrutz did, or as a 'fusion' the dis- 

 tinction is fictitious because the integration ma\' ise 

 effected deeply below the 'threshold of consciousness." 

 Head (40) maintained that this occurred as early as 

 the first synapse. For further analysis see page 452. 



ELECTROPHYSIOLOGY OF THERM.\L NERVE FIBERS 



In recent years the function of the thermal recep- 

 tors has been subjected to more objective investigation 

 by the comijination of effective methods for recording 

 the temperature and the action potentials from the 

 nerve fibers involved. 



Specificity of .Nerve Fibers in Mammals 



The first recording of the specific electric response 

 to thermal stimulation was made by this writer in 

 1936. He was generally able to see leading off from a 

 fine branch of the lingual nerve a number of small 

 action potentials with spike heights I3 to ifo of that 

 of the largest spike potentials elicited by touching the 

 tongue (fig. 10). When the tongue is washed with 

 warm water these small spikes disappear to return 

 shortly if the tongue is laid free in the air at room 

 temperature. A faint draft over the tongue increases 

 the number of impulses, and a sudden fine stream of 

 air from a syringe on the receptive field elicits a dis- 





■■^W^"**^*V*^*^^<^ 



FIG. 10. ^4. Microphotograph of lingual nerve preparation. 

 Magnification, 685. Largest fibers measure 10 ^ in diameter. 

 .\lsheimer-Mann stain. B. Record from the same preparation 

 showing the ratio between the spike heights of cold and touch 

 impulses. The irregular discharge of the small cold spikes is 

 due to the exposure of the tongue to air. The four large spikes 

 were elicited by touching the tongue with a brush. [From Zot- 

 terman (96).] 



