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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



change in temperature by conduction from ambient 

 media and mechanical deformation of the sensory 

 membrane, do stimulate. It is concluded that these 

 are minor or incidental in some fibers. Direct measure- 

 ment of the change in temperature of water flowing 

 over the membrane necessary to elicit a response gave 

 values of 0.003 to o.oo5°C. 



Thus the nerves of the facial pit organs of rattle- 

 snakes are composed of an essentially homogeneous 

 population of warm fibers behaving principally as do 

 mammalian warm fibers. But for the receptors of the 

 pit organ the normal stimulus is chiefly radiant and 

 not conducted heat, and several anatomical properties 

 adapt it to a high sensitivity in terms of caloric flux. 



Dodt (19) describes discharges from the glosso- 

 pharyngeal nerve of the frog in response to tempera- 

 ture changes in the tongue of more than 3°C even in a 

 temperature range below I5°C. This response appears 

 only to warming, never to cooling. The response of 

 these fibers to heating the tongue resembles in many 

 ways that of mammalian warm fibers as well as that 

 of the pit organ of the rattlesnake. Further experi- 

 mental analysis is, however, necessary to decide 

 whether this response to heating the tongue of the 

 frog is due to the stimulation of nociceptive fibers 

 or of more or less specific warm fibers. 



Qiiantitative Relations Belwem Temperature 

 Movements and Nerve Fiber Discharge 



By use of well defined and thermoelectrically con- 

 trolled thermal stimuli applied to the tongue of the 

 cat, it has been possible to work out the fundamental 

 relationships between the temperature and the activ- 

 itv of the thermal fibers. This work has principally 

 been carried out in the writer's laboratory in a series 

 of investigations by Hen.sel, Dodt and co-workers. 



METHODS. For quantitative studies of cold receptors 

 we have used fine strands of the cat's lingual nerve 

 containing only one or a few cold fibers. Preparations 

 containing single or a few warm fibers are best ob- 

 tained from the chorda tympani of the cat. [For the 

 operative technique see Zotterman (96), Hensel & 

 Zotterman (53) and Dodt & Zotterman (22).] For 

 thermal stimulation we used a metal thermode, open 

 at the top, which had a free outflow on one side (fig. 

 12). The thermode had a gold-plated silver i)ottom of 

 20 X 30 mm and a thickness of o. i mm. From above, 

 two constantly flowing jets of water at diff^erent 

 temperatures were directed on to the bottom of the 

 thermode in such a way that the jets could suddenly 



FIG. 12. .\ppaiatus for applying rapid temperature changes 

 to the surface of the tongue. Th, thermode; li, silver bottom; 

 n'l and Ti'2, water jets of different temperatures; 0\ and O2, 

 outflows; S, switch; arrow, movement of switch; A, axis of 

 switch; Ti and To, thermocouple wires; J, junction in bottom 

 of thermode; /., lead strip. [From Hensel et al. (51)-] 



be interrupted. In this way it was possible to produce 

 very rapid and exact temperature changes of the 

 gold-plated silver foil. Soldered on the thermode 

 bottom was a thermocouple with a diameter of 0.05 

 mm which enabled us to record the true temperature 

 changes of the silver foil. Because of the rapid temper- 

 ature change, which could exceed 300°C per sec, 

 the temperature was recorded either by a micro- 

 galvanometer of Moll or by the second beam of the 

 double beam cathode-ray oscillograph which was 

 used for recording the action potentials. The thermode 

 was adjusted on the tip of the tongue which rested on 

 a cork plate. It can easily be shown both mathemati- 

 cally and experimentally that a constant temperature 

 is reached in the receptor layer of the skin only 

 negligibly later than at the surface of the tongue. In 

 many experiments thermocouples were inserted to 

 different depths into the tongue in order to record the 

 temperature within the mucous membrane. 



DISCHARGE .\T CONST.ANT TEMPERATURE. Cold fibers. 



When the thermode is adjusted at a constant tempera- 

 ture the frequency of the cold spikes attains a constant 

 final value after a short interval. A record from a 

 nerve preparation containing two cold fibers (one 

 giving diphasic, the other monophasic spikes) will be 

 seen in figure 13. The thermode was previously kept 

 for a long time at a constant temperature of 34°C. 

 Even at this temperature there was present a steady 

 discharge of the monophasic fiber at a rate of 9 im- 



