MECHANICAL RESPONSE PROPERTIES OF NOCICEPTORS INNERVATING FELINE HAIRY SKIN

1. The responses of feline cutaneous nociceptors were examined in vivo by systematically manipulating the intensive and spatial dimensions o f mechanical stimulation. A computer-controlled motor was used to appl y prescribed forces (5-90 g) to a nociceptor's receptive field, with f lat-tipped, cylindrical probes of various sizes (contact areas: 0.1-5. 0 mm(2)). The stimulating device and protocols were similar to those p reviously used to evaluate human perception, thus allowing for compari sons of the two data sets. 2. With a ramp-and-hold stimulus of control led force, most nociceptors showed a slowly adapting (SA) response thr oughout the stimulus. In this way, nociceptors resembled low-threshold SA mechanoreceptors. However, in contrast to SA mechanoreceptors, noc iceptors failed to exhibit an onset burst of activity associated with the stimulus ramp. Nineteen percent (6 of 31) of the nociceptors often showed the opposite trend during the stimulus, e.g., a gradually incr easing firing rate. Most of these nociceptors (5 of 6) had particularl y high mechanical thresholds. 3. With 30 stimuli repeated at short int ervals (6-8 s), 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 min, there was no apparent trend of reduced responsiveness between series. 4. Overall, nociceptors respond ed in an orderly way to variations in force and probe size. For a give n probe size, larger forces produced greater responses; for a given fo rce, smaller probes produced greater responses. The relationship betwe en probe size and force was best described as an even tradeoff between force and a linear dimension of the probe (i.e., probe perimeter), ra ther than the area of the probe. Thus a given pressure (force/area) di d not evoke the same response from nociceptors as probe size was varie d. 5. There were two significant differences in the mechanical respons iveness between A fiber and C fiber nociceptors. First, for a given se t of stimuli, A fiber nociceptors exhibited a greater response rate th an the C fiber nociceptors. Second, the A fiber nociceptors exhibited a greater differential response related to probe size than the C fiber nociceptors. On the basis of these two features, the A fiber nocicept ors' response profiles showed a closer parallel with previously report ed human pain thresholds than the C fiber nociceptors did. 6. When the nociceptors were subdivided as to their mechanical threshold, those w ith lower thresholds [mechanically sensitive afferents (MSAs)] showed a response saturation with the more intense stimuli. On average, the s timulus levels at which saturation occurred were close to human pain t hreshold. Those nociceptors with higher thresholds [mechanically insen sitive afferents (MIAs)] did not show such saturation. Thus only the M IAs appeared to have the capacity to unambiguously encode mechanical s timulus intensities above pain threshold. The MSAs, on the other hand, exhibited their greatest dynamic response range near the threshold fo r nonpainful sharpness. Thus the group of afferents commonly defined a s nociceptors exhibit a heterogeneity of mechanical response propertie s, which may serve functionally different roles for perception.