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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



pulses which in the lime, /, arrive at the central organ 

 also is a function of these three factors, 



^3 0(e,de/d/,F) 



i.e. the value of n/t becomes greater when a) the 

 temperature is lower, A) the rate of cooling is greater 

 and c) when the receptive field is enlarged — i.e. when 

 the number of stimulated cold receptors is increased. 

 The rate of «// is nothing else but the central threshold 

 which thus can be written, 



E3^ 



c 



The results of the sen.sory-physiological studies are 

 thus in very good accordance with those obtained 

 from electrophysiological investigations on the specific 

 thermal fibers. 



The declining impulse frequency at constant tem- 

 perature is the so-called "physiological adaptation' of 

 the thermal sense recorded objectively. As Hensel 

 (42) concluded from his sensory-physiological studies, 

 it should be more correct to avoid the use of the partic- 

 ular words, adaptation or change of excitability in 

 order to express the temporal decrease of the excita- 

 tion under a constant stimulus, as these expressions 

 lead to a conception of a specific process separated 

 from excitation. Adaptation is then assumed when a 

 temporal change of the excitation occurs while the 

 stimulus is kept constant. But this depends upon the 

 definition of stimulation. When as in the thermal 

 sense the temperature (9) is the stimulus, adaptation 

 appears at constant stimulation. If, however, the 

 stimulus is the rate of temperature change, dS d<, 

 there will be no adaptation during constant stimula- 

 tion. According to the usual definition, adaptation is 

 therefore nothing else than an indirect description of 

 the time factor of a sense organ based on its response 

 to a specific mode of stimulation. 



At constant temperature of the skin, the magnitude 

 of nit is dependent upon the temperature and the 

 area of the skin. If the thermal receptors were evenly 

 distributed, the thermosensible tonus would thus be a 

 direct function of the integral skin temperature. This 

 is, however, not the case as some parts, especially the 

 trigeminal area, display a much greater density of 

 thermal receptors and are thus likely to exert a more 

 dominant influence upon the thermoregulation of the 

 body. It is very likely that the central threshold of 

 conscious cold sensations lies at a higher level than the 

 threshold of the thermal receptor discharge (34, 54), 



which implies that a certain part of the afferent 

 thermoregulatory inflow occurs below the threshold 

 of our consciousness. 



Excitation Alecfianism of 1 hernial Receptors 



The fact that there is a distinct discharge of im- 

 pulses from thermal receptors when there is a com- 

 plete temperature equilibrium between the two sides 

 of the receptor layer, i.e. when the spatial as well as 

 the temporal temperature gradient is zero, shows that 

 this activity does not depend upon any exchange of 

 thermal energy. Thus there must occur in the recep- 

 tors, processes — probably of a chemical nature — 

 which are governed by temperature without any 

 external exchange of energy in the skin. 



For this reason it is not practicable to express the 

 thresholds of the temperature sense — in analogy with 

 the eye and the ear — in terms of a thermal energy. 



The course of the receptor discharge at constant 

 temperatures and particularly the effect of tempera- 

 ture changes suggests that we have to deal with at 

 least two interacting processes, one exciting and one 

 inhibiting. We can thus, according to Sand (77), 

 assume that the frequency of the steady discharge of 

 the cold receptor, n, is dependent upon the difference 

 between two temperature dependent processes, E and 

 /. The difference between these should give the im- 

 pulse frequency, n (45, 54). 



FIG. 26. Graphs illustrating discharging mechanism of a 

 cold receptor. Abscissae, skin temperatures; ordinates, rates of 

 impulse discharge. In the lower left is a plot of the steady 

 discharge of a cold receptor assuming that the frequency of dis- 

 charge (n) is a function of the difference between two tempera- 

 ture dependent processes E and 7 (jibove'). On the right is illus- 

 trated the time course of the effect of sudden cooling from a 

 temperature of fli to one of ^2 and back to ffi. The intersection of 

 the curves of E and / gives the upper threshold temperature 

 9o of the cold receptor. [From Zotterman (100).] 



