Sensitivity of cutaneous cold fibers to noxious heat: paradoxical cold discharge

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Abstract
Sensitivity to heat was examined in 457 single cold fibers which were isolated in the median and ulnar nerves of macaque monkeys. Controlled changes in rectal temperature revealed a striking relation between the proportion of cold fibers which responded to a given noxious thermal stimulus and the animal''s body temperature. A paradoxical response to a 20-s 53.degree. C test stimulus was observed in only 18.6% of 59 cold fibers sampled in animals maintained at 37.degree. C rectal temperature, but in 89.3% of 93 cold fibers in animals maintained at 39.degree. C. Further investigation of the presence or absence of responses to other stimulus temperatures above and below 53.degree. C revealed that the heat thresholds of individual cold fibers increase or decrease in an inverse relationship with the body temperature. The effect of elevating the rectal temperature could be reproduced in a hypothermic animal by acute section of the nerve at a level proximal to the recording site and by administration of an .alpha.-adrenergic blocking agent, phenoxybenzamine. The sensitivity to heat of cutaneous cold fibers is related inversely to the activity in some population of .alpha.-adrenergic efferents suppressed by hyperthermia. The quantitative responses evoked in individual cold fibers were correlated with the parameters which specify the delivery of thermal square-waves to a constant surface area. The response to a suprathreshold temperature pulse into the noxious range accelerated nonlinearly with increasing stimulus intensity and duration. Moreover, the repetition of a constant stimulus led to an increase in the discharge evoked by successive stimuli without a change in the heat threshold. The rate of this increase and the maximum level of response attained were dependent on the stimulus intensity and the interstimulus interval. The response returned rapidly to the initial level if iterative stimulation was interrupted for only a few minutes. Both the number of active cold fibers and the activity evoked in individual cold fibers increase as the destructiveness of the noxious thermal stimulation increases.

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