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HANDBOOK OF PH'iSIOLGGV 



NEUROPHYSIOLOGY I 



rather slow rates of temperature change. The response 

 soon fades away when the temperature is kept below 

 20°C and is thus always of a phasic character in con- 

 trast to the steady paradoxical discharge of cold fibers 

 when exposed to constant temperatures above 45°C. 

 The fact that the latency of the warm fiber response to 

 sudden cooling is only about a third of the shortest 

 latency of its discharge to warming induced Dodt & 

 Zotterman (23) to consider whether the former re- 

 sponse of the warm fibers is due to an excitation of the 

 warm receptor or to its nerve fiber. As was originally 

 shown by Bernhard & Granit (7) rapid cooling of a 

 nerve trunk excites class A fibers directly and such a 

 rapid cooling also stimulates the endings of mechano- 

 ceptive fibers (53). This nerve fiber discharge in every 

 respect is of the same character as the discharge of 

 warm fibers to cooling. It has a relatively short 

 latency, the length of which varies with the rate of 

 cooling; the discharge is phasic, i.e. there is no steady 

 discharge. When, however, the thermode was placed 

 on the other side of the tongue where the more central 

 part of the lingual nerve runs closely under the sur- 

 face, no discharge of the warm fibers could be seen 

 when this surface was suddenly cooled from 45° to 

 25°C. 



An old question in the field of sensory physiology 

 is whether sensation is in part due to direct stimula- 

 tion of the .sensory nerve fibers as well as of the recep- 

 tors. In the function of thermoreception this question 

 is, as we have seen, of particular importance. For that 

 reason Dodt (18) made a thorough study of thermo- 

 sensitivity of A fibers in the lingual nerve and com- 

 pared the responses in specific cold, warm and 

 mechanoceptive fibers upon thermal stimulation of 

 the surface of tongue and of the lingual nerve, re- 

 spectively. He found that all three types of afferent 

 fibers were phasically excited by local cooling of the 

 nerve trunk. In mechanoceptive fibers this occurred 

 whenever the temperature drop was of a sufficient 

 magnitude, regardless of the final value, the effect 

 being optimally elicited when the nerve was at an 

 initial temperature between 35° to 40 °C. Cold fibers, 

 however, were excited only when the nerve trunk was 

 cooled to below a certain threshold value of about 

 20°C. Warm fibers were mixed in their reactions, 

 some responding like mechanoceptive fibers, others 

 having a distinct threshold. Warming of the nerve 

 trunk never led to excitation of sensory A fibers. 



The cold and the mechanoceptive fibers could be 

 blocked by low and high temperatures, the cold fibers 

 being blocked below 16° to 25°C and above 50° to 



52 °C, whereas the thresholds in mechanoceptors were 

 in both types of block 5° to 8°C lower. 



Cold fibers, excited by cooling the nerve to tem- 

 peratures insufficient to cause blocking, show follow- 

 ing the phasic excitation an impulse-free interval, the 

 duration of which varies directly with the length of 

 the excitatory burst and inversely with the back- 

 ground frequency of impulses coming from the recep- 

 tor. 



These findings suggest that under physiological 

 conditions normal and paradoxical sensations of cold 

 are due to the stimulation of the thermal receptors or 

 the nerve fibers included in the end organ and never 

 to a direct stimulation of their myelinated nerve 

 fibers. 



PAR.^DOXic.'^L DISCHARGES. When the temperature of 

 the tongue is raised above 45 °C, a steady discharge of 

 cold fibers is produced (24). This impulse activity 

 increases slowly and attains a level corresponding to 

 the prevailing temperature. This paradoxical dis- 

 charge begins at 45 °C and a maximum frequency of 

 7 to 7.5 impulses per sec. is attained at 5o°C. The 

 lower threshold temperature of this paradoxical dis- 

 charge lies about 5°C above the upper limit of the 

 usual range of temperature within which the cold 

 receptors display a steady discharge (fig. 22). Para- 

 doxical excitation of cold receptors at temperatures 

 below 45°C does not occur. Thus the cold sensation 

 which appears after a rise in the skin temperature 

 from 20° to 35°C as described by Thunberg (88) is 

 not due to any paradoxical excitation but to a reap- 

 pearance of the usual steady discharge of cold fibers 

 when the temperature approaches the final value of 



FIG. 22. Graphs showing impulse frequency of the steady 

 discharge of a single cold fiber iopen circles) and of a single warm 

 fiber QfiUed circles') as a function of the temperature of the recep- 

 tors within the range of 10 to 50°C. [From Zotterman (99).] 



