Nocifensive responses to cutaneous thermal stimuli in the cat: stimulus-response profiles, latencies, and afferent activity

Abstract

Rapid-onset thermal pulses (38 degrees C X s-1) ranging from 43 to 60 degrees C (adapting temperature, 38 degrees C) were applied by contact thermodes to the shaved outer thighs of 29 partially restrained cats while they were eating (coincident stimuli) or between eating periods (noncoincident stimuli). The stimulus-associated probability and latency of three putative nocifensor responses were determined: interruption of eating or of exploration for food, hindlimb movement, and vocalization. These behavioral responses terminated the stimulus. An analysis of response trends and latencies supports the interpretation that these are unlearned escape responses to noxious stimuli. Response threshold was defined as the lowest temperature at which the average response probabilities at the test temperature and at 38 degrees C were outside one another's 95% confidence limits. Average threshold varies with the type of response and the behavior at the time of stimulation. Noncoincident stimuli elicit both interrupt and movement responses at 47 degrees C and vocalization at 53 degrees C. Coincident stimuli also elicit interrupt responses at 47 degrees C but are associated with an increased hindlimb movement threshold (51 degrees C) and vocalization threshold (55 degrees C). The lowest threshold determined for an individual cat was 43 degrees C. Response probability increases as a nonmonotonic function of stimulus intensity. Maximum sensitivity to increases in temperature occurs at stimulus intensities between 50 and 55 degrees C. Above 55 degrees C, some cats show an attenuation of response sensitivity. Special tests failed to reveal any evidence that receptor sensitization or suppression affected the stimulus-response profiles. The average response latency for all behaviors of all cats is 2.3 +/- 0.12 (SE)s from the onset of the stimulus pulse plateau. Most responses occurred within 2.5 s. No significant relationship was found between response latency and type of response or stimulus intensity. The composition and time of onset of heat-evoked afferent activity in the lateral femoral cutaneous nerve was investigated in six cats by colliding antidromic A-delta- and C-fiber volleys with the orthodromic discharge evoked by the thermodes used in the behavioral studies. There was no evidence that noxious thermal pulses evoked activity in A-delta-afferents, but C-fibers could be shown to become active at approximately 46 degrees C, at or before the onset of the thermal pulse plateau.(ABSTRACT TRUNCATED AT 400 WORDS)