Activity-dependent slowing of conduction differentiates functional subtypes of C fibres innervating human skin

1. The effects of impulse activity on conduction in cutaneous C fibres have been examined in 46 microneurographic recordings from 11 normal subjects a nd 11 diabetic patients with normal nerve conduction. A tungsten microelect rode was inserted into a cutaneous nerve, usually the superficial peroneal close to the ankle, and intraneural microstimulation was used to identify a n area of skin innervated. Three minute trains of 0.25 ms stimuli at 1, 2 a nd 4 Hz were then delivered to the surface of the skin, separated by interv als of 6 min with stimulation at 0.25 Hz. Slowing and block of conduction w ere measured from the nerve responses for up to set-en C units per stimulat ion sequence. 2. Three types of C unit were distinguished by their responses to repetitiv e stimulation: type 1 units slowed progressively during the 3 min trains; s lowing of type 2 units reached a plateau within 1 min; while type 3 units h ardly slowed at all. Data from normal and diabetic subjects did not differ and were pooled. After 3 min at 2 Hz, the percentage increases in latency w ere for type 1, 28.3 +/- 9.7 (n = 63 units, mean +/- S.D.); for type 2, 5.2 +/- 1.6 (n = 14); and for type 3, 0.8 +/- 0.5 (n = 5), with no overlap. Af ter 3 min am 4 Hz, 58% of type 1 units (but no type 2 or 3 units) blocked i ntermittently Recovery of latency after stimulation was faster for type 2 t han for type 1 units, but conduction velocities of the three types were sim ilar. 3. Type 1 units were identified as nociceptors and 7 type 2 units were iden tified as 'cold' fibres, activated by non-noxious cold, with no overlap in modality. None of the units tested was activated by weak mechanical stimuli or reflex sympathetic activation. 4. Spike waveforms were averaged for 18 type 1, 10 type 2 and 6 type 3 unit s. All units had predominantly triphasic action potentials with a major neg ative peak, but those of type 3 units were on average both smaller and brie fer than those of type 1 and type 2 units. 5. It is concluded that repetitive electrical stimulation reliably differen tiates nociceptive from cold-specific C fibres innervating human hairy skin , as has previously been shown for the rat. Cold fibres can propagate impul ses continuously at much higher rates than nociceptive fibres. The nature o f the type 3 units is unclear.