Unresponsive afferent nerve fibres in the sural nerve of the rat

Handwerker, H. O.; Kilo, S.; Reeh, P. W.

doi:10.1113/jphysiol.1991.sp018507

The proportion of primary afferent nerve fibres in a skin nerve of the rat that responded tor failed to respond to mechanical or thermal stimulation of the skin in the noxious and non-noxious range was analyzed. Activity of afferent nerve fibres was recorded from the dorsal roots. Units projecting into the sural nerve were selected using supramaximal electrical stimulation of the nerve stem. All other hindleg nerves were cut. The receptive fields were searched by carefully examining the hindleg skin with noxious and innocuous mechanical, cooling and warming stimuli. Probing of the intrinsic foot muscles and manipulation of the ankle and toe joints was employed to recruit units projecting to deeper tissues. In a first series of twenty-two experiments, eighty-nine rapidly conducting myelinated A.beta. units, thirty slowly conducting myelinated A.delta. units and 101 unmyelinated C units were investigated. Most units were identified as belonging to one of the established classes of cutaneous sensory units and this was also ascertained by a collision tests. Two A.beta., eight A.delta. and forty-six C fibres did not respond to any one of the stimuli. Electrical thresholds and conduction velocities of the unresponsive C fibres were not significantly different from those of the units responding to natural stimulation of their receptive fields. In the A.delta. group unresponsive and high threshold mechanoreceptive units were preferentially found among the units with the slowest conduction velocities. In a second series of seven experiments, one single nerve filament containing responsive and unresponsive C fibres was tested repetitively at 30 min intevals. Twenty unresponsive units and seven units responding to noxious mechanical and/or heat stimuli were studied. Ten of the twenty initially unresponsive units became activated by mechanical and/or heat stimuli after observation times of 30-150 min. Some of these units had mechanical thresholds as low as 64mN (tested with calibrated von Frey hairs), or thermal thresholds down to 42.degree. C. Two of the ten C units which became responsive in the course of an experiment later lost their responsiveness again. On the other hand, two of the C units which were initially responsive to noxious heat and/or noxious mechanical stimuli became completely unresponsive after repetitive stimulation, whereas one unit initially only responding to noxious heat became responsive to mechanical stimuli, suggesting that mechanical and heat responsivneess may be separately gained or lost by sensory C fibres. We conclude that a large proportion of afferent C and A.delta. units innervating the hairy skin of rat are insensitive to acute physical stimuli under physiological conditions. At least some of these units may be sensitized under more long-lasting perturbations of the tissue, notably in inflammation, and thus may contribute to inflammatory pain.