Sensitization of nociceptive cutaneous nerve fibers from the rat's tail by noxious mechanical stimulation.
This single fiber study on rat tail nerve afferents attempts to establish a peripheral neural correlate for the hyperalgesia to mechanical stimulation which follows injury to the skin. Mechano-heat sensitive C fibers (MH-C or "polymodal" nociceptors) and high-threshold mechanoreceptive A delta fibers (HTM-A delta) were examined with a series of constant noxious pressure stimulations (4-6-8-4 N on 25 mm2, 120 s each, 5 min intervals). These injurious stimuli were either directed to the most sensitive spot of the receptive fields (central stimulation) or closely outside their borders (1-5 mm). With this protocol no clear sensitization was seen in MH-C fibers apart from a stronger dynamic response to central stimulation in some of them. In contrast, most HTM-A delta units, irrespective of the site of noxious stimulation, developed spontaneous activity, lowering of their von Frey thresholds and expansion of their receptive fields. All HTM-A delta units responded to outside stimulation: upon the first stimulus (4 N) there was a delayed discharge of continuously increasing frequency ("recruited response"), but the onset of the last stimulation (4 N repeated) evoked vigorous dynamic responses in many fibers. The recruitment of HTM-A delta nociceptor activity may contribute to post-injury hyperalgesia to mechanical stimulation and it may counteract adaptation of the single afferent fiber during prolonged noxious influence.
D000818:Animals; D004056:Differential Threshold; D006197:Hair; D009412:Nerve Fibers / Q000503:physiopathology*; D009619:Nociceptors / Q000503:physiopathology*; D010146:Pain / Q000503:physiopathology*; D010812:Physical Stimulation; D011312:Pressure; D051381:Rats; D011919:Rats, Inbred Strains; D011999:Recruitment, Neurophysiological; D012867:Skin / Q000294:innervation*; D013623:Tail