Sensitization of myelinated nociceptive afferents that innervate monkey hand

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Abstract
Myelinated primary afferents (44) innervating the monkey [Macaca mulatta, M. fascicularis] hand and sensitive to both mechanical and heat stimuli (AMH) were studied by the method of single-unit recording. The thermal response properties were studied with use of a non-contact laser thermal stimulator that provided step increases in skin temperature with rise times of 200-250 ms and temperature control within 0.1.degree. C. Mechanical response properties were studied with use of a stimulator that provided step increases in force. The AMH had a mean conduction velocity of 31.1 .+-. 1.5 (SE) m/s (range, 5.2-53.3 m/s), a mean receptive-field area of 37 .+-. 4 mm2 (range, 2.3-121 mm2), a mean pressure threshold of 3.5 .+-. 0.3 bars (Von Frey technique; range, 1.6-9.4 bars), no response to cooling stimuli, and no spontaneous activity prior to stimulation. Each of the AMH became sensitized to heat after repeated exposure of its receptive field to high intensities of heat. Before sensitization, 88% (37 of 42) had heat thresholds > 49.degree. C. After sensitization the heat thresholds were lower for all fibers tested (n = 30) and 67% had thresholds < 45.degree. C. The response properties of AMH were compared with those of polymodal nociceptive C-fibers (CPN) that innervated the monkey hand. The receptive fields of 12 CPN were exposed to 53.degree. C stimuli of 3 s duration delivered every 28 s. All but 1 CPN showed response depression as a function of iterative stimulation. The response properties of AMH were compared with those of 8 slowly adapting mechanoreceptive afferents (SAM) that innervated the monkey hand. The pressure threshold (.hivin.x, 1.09 bars) was significantly less and the conduction velocity (.hivin.x, 42.6 m/s) and receptive-field area (.hivin.x, 65.8 mm2) were significantly greater than corresponding values for the AMH. SAM responded only during the cooling phase of heat stimuli. This thermal response increased with iterative heating, and thus is considered a special form of sensitization. AMH are a distinct population of mechanically sensitive cutaneous nociceptive afferents that develop a prolonged increased sensitivity to heat following noxious heating of their receptive fields. Hyperalgesia that follows a cutaneous burn may be mediated, at least in part, by activity in AMH.

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