THE MECHANICAL RESPONSE PROPERTIES OF NOCICEPTORS INNERVATING FELINE HAIRY SKIN. Garell, P. Charles, Sandra L. B. McGillis, and Joel D. Greenspan. Depts. of Neurosurgery and Physiology, and Program in Neuroscience, SUNY Health Science Center, Syracuse, NY 13210 USA.
APStracts 2:0331N, 1995.
SUMMARY AND CONCLUSIONS
1. The responses of feline cutaneous nociceptors were examined in vivo by systematically manipulating the intensive and spatial dimensions of mechanical stimulation. A computer-controlled motor was used to apply prescribed forces (5-90g) to a nociceptor's receptive field (RF), with flat-tipped, cylindrical probes of various sizes (contact areas: 0.1-5.0mm 2 ). The stimulating device and protocols were similar to those previously used to evaluate human perception, thus allowing for comparisons of the two data sets. 2. With a ramp-and-hold stimulus of controlled force, most nociceptors showed a slowly adapting response throughout the stimulus. In this way, nociceptors resembled low threshold slowly adapting (SA) mechanoreceptors. However, in contrast to SA mechanoreceptors, nociceptors failed to exhibit an onset burst of activity associated with the stimulus ramp. Nineteen percent (6/31) of the nociceptors often showed the opposite trend during the stimulus, e.g., a gradually increasing firing rate. Most of these nociceptors (5/6) had particularly high mechanical thresholds. 3. With 30 stimuli repeated at short intervals (6-8s), response rates tended to decrease across trials. This phenomenon was most evident with more intense stimuli. When two series of stimuli were separated by 4-5 minutes, there was no apparent trend of reduced responsiveness between series. 4. Overall, nociceptors responded in an orderly way to variations in force and probe size. For a given probe size, larger forces produced greater responses; for a given force, smaller probes produced greater responses. The relationship between probe size and force was best described as an even trade off between force and a linear dimension of the probe (i.e., probe perimeter), rather than the area of the probe. Thus, a given pressure (force/area) did not evoke the same response from nociceptors as probe size was varied. 5. There were two significant differences in the mechanical responsiveness between A- fiber and C-fiber nociceptors. First, for a given set of stimuli, A-fiber nociceptors exhibited a greater response rate than the C-fiber nociceptors. Secondly, the A-fiber nociceptors exhibited a greater differential response related to probe size than the C-fiber nociceptors. Based on these two features, the A-fiber nociceptors' response profiles showed a closer parallel with previously reported human pain thresholds than the C-fiber nociceptors did. 6. When the nociceptors were subdivided as to their mechanical threshold, those with lower thresholds (MSA -- mechanically sensitive afferents) showed a response saturation with the more intense stimuli. On average, the stimulus levels at which saturation occurred were close to human pain threshold. Those nociceptors with higher thresholds (MIA - mechanically insensitive afferents) did not show such saturation. Thus, only the MIAs appeared to have the capacity to unambiguously encode mechanical stimulus intensities above pain threshold. The MSAs, on the other hand, exhibited their greatest dynamic response range near the threshold for non-painful sharpness. Thus, the group of afferents commonly defined as nociceptors exhibit a heterogeneity of mechanical response properties, which may serve functionally different roles for perception. 

Received 17 April 1995; accepted in final form 13 October 1995.
APS Manuscript Number J258-5.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 30 November 95