Response properties of thin myelinated (A-delta) fibers in human skin nerves.

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Abstract
The receptive properties of A-.delta.-fibers were studied in young healthy volunteers by single-fiber recording from the cutaneous branch of the radial nerve. Mechanical stimulation was performed with a set of von Frey hairs. Response to cooling was tested with ether or ice. A feedback-controlled radiant-heat stimulator delivered heat pulses at different temperatures, ranging from 37 to 46.5.degree. C. A paint-removing substance, containing methylene chloride in methanol, was used as a chemical irritant. The evoked sensation was registered by asking the subjects to report their sensations and by cross-modality matching. The conduction velocity was computed in 140 A-.delta.-fibers (mean .+-. SD, 19.2 .+-. 7.2 m/s). The observed values corresponded well with the diameter distribution of thin myelinated fibers in the radial nerve of man. Mechanical threshold measurements in 66 A-.delta.-fibers revealed a low-threshold group (.ltoreq. 8.8 mN), which has some characteristics in common with the down hair receptors found in animal experiments. The other fibers had high mechanical thresholds (.gtoreq. 22.5 mN) in the C polymodal nociceptor range. Of those mechanoreceptive receptors, 21% were activated also by radiant heat. Responsiveness to heat usually seemed not to be a consequence of sensitization. The discharge frequency to radiant heat was higher in some A-.delta.-fibers than in C-fibers. A-.delta.-Fibers behaved differently from C polymodal nociceptors, since not all A-.delta.-fibers activated by chemical stimulation were responsive to radiant heat. Comparison of reported sensation and neural activity indicated that activation of an A-.delta.-fiber did not always coincide with a pain sensation. Human A-.delta.-fibers are similar to those described in other mammalian species. A-.delta.-fibers with high mechanical threshold show a higher receptor specificity than C polymodal nociceptors. The firing frequency on noxious stimulation is often higher in A-.delta.-fibers than in C-fibers.