Discharge patterns of afferent cutaneous nerve fibers from the rat's tail during prolonged noxious mechanical stimulation.

Feedback controlled constant force stimuli of 4, 6 and 8 N intensities and of 120 s duration were applied to the receptive fields of cutaneous afferent fibers in the rat's tail. Two types of nociceptive units showed sustained discharges during these stimuli: "polymodal" unmyelinated C-units (MH-C units, N = 18, c.v. 0.5-0.9 m/s) and high-threshold mechanoreceptive A-delta-units (HTM-units, N = 10, c.v. 1.9-11.2 m/s). In addition two classes of sensitive low threshold mechanoreceptors, SA I (N = 6) and SA II (N = 5) units, responded to the prolonged mechanical stimuli. At the onset of a noxious pressure, 11 of the 18 polymodal nociceptors exhibited dynamic responses (lasting about 10 s) which were followed by slowly adapting tonic discharges that lasted for the duration of the stimuli. The remaining polymodal C-fiber units (8/18) did not show dynamic discharges at 4 and 6 N. Phasic and tonic discharges were positively correlated with stimulus strength. The HTM-units encoded stimulation intensity mainly by their dynamic discharges. The tonic discharges of these units displayed faster adaptation rates with stronger mechanical stimuli, i.e. encoding of stimulation intensity became progressively weaker during the tonic phase. The discharges of sensitive SA I and SA II units with A beta axons were not positively correlated with the strength of noxious pressure stimuli. Tonic discharge rates of SA I units were negatively correlated to stimulus strength, whereas SA II units usually stopped firing in the course of a stimulus and became reversibly irresponsive to mechanical stimulation. Possible afferent mechanisms underlying the induction of pain by sustained noxious mechanical stimulation are discussed.