title: Mechano-insensitive nociceptors encode pain evoked by tonic pressure to human skin.
abstract: Single unmyelinated axons in the superficial branch of the peroneal nerve of human subjects were recorded (microneurography) and the response patterns during tonic pressure stimulation (14N at 30 mm(2); 120 s) were assessed using the previously described "marking technique". It was found that tonic pressure stimuli induced augmenting pain responses which were matched by the discharges of initially mechano-insensitive ("silent") C-units, whereas mechano- and heat-responsive "polymodal" C-nociceptors showed a response pattern incompatible with the stimulus-induced perceptions, namely strong initial excitation, followed by adaptation. Eighteen mechano- and heat-responsive "polymodal" C-fibers and 11 mechano-insensitive units were studied. The former had von Frey thresholds <160 mN, the latter were not excited by a von Frey filament of 750 mN (six of them responded to radiant heat stimulation). However, in the course of pressure stimulation, nine of the 11 mechano-insensitive units were activated after more than 20s. A second, identical pressure stimulus induced a stronger response in mechano-insensitive and a weaker response in mechano-responsive units. The stronger response, indicating sensitization, matched the more intense pain perception during the second pressure stimulus. It is concluded that mechano-insensitive C-nociceptors encode pressure-induced pain in human hairy skin and that they play an important role in static mechanical hyperalgesia.
authors.affiliation: Department of Clinical Neurophysiology, University of Uppsala, Uppsala, Sweden.
authors.forename: R
authors.initials: R
authors.lastname: Schmidt
authors.forename: M
authors.initials: M
authors.lastname: Schmelz
authors.forename: H E
authors.initials: HE
authors.lastname: Torebjörk
authors.forename: H O
authors.initials: HO
authors.lastname: Handwerker
mesh_terms: D000328:Adult; D005260:Female; D006801:Humans; D008297:Male; D008465:Mechanoreceptors / Q000502:physiology*; D009619:Nociceptors / Q000502:physiology*; D010147:Pain Measurement / Q000379:methods* / Q000523:psychology; D010543:Peroneal Nerve / Q000502:physiology; D010812:Physical Stimulation / Q000379:methods; D011312:Pressure; D011930:Reaction Time / Q000502:physiology*; D012879:Skin Physiological Phenomena; D018709:Statistics, Nonparametric