Myelinated afferent fibres innervating the primate skin and their response to noxious stimuli

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Abstract
The functional characteristics of cutaneous receptors in the squirrel monkey were determined by recording discharges of single myelinated afferent fibers in peripheral nerves with micro-electrodes or from fine filaments prepared by dissection. One hundred and sixty-nine fibers of the posterior femoral cutaneous nerve and 209 of the superficial radial nerve with conduction velocities between 4 and 88 m/sec were classified according to the nature of the most effective stimulus, discharge characteristics, adaptation rate and organization of the receptive field. Twenty per cent of the fibers innervating either hairy or glabrous skin required strong mechanical stimuli for activation; thresholds ranged from moderate to ovenly damaging pressures. This class showed little or no sensitivity to thermal changes including noxious heat. Their receptive fields consisted of numerous, mechanically-excitable points or spots. All such fibers gave higher impulse frequencies to noxious than to innocuous mechanical stimuli and a large fraction were considered to be nociceptors because they responded only to noxious deformation. The conduction velocities of nociceptors were distributed between 5 and 28 m/sec. AH but a few of the other fibers encountered responded vigorously to innocuous stimuli and were readily identified as corresponding to one of the receptor types known to exist in either the primate or the cat. These sensitive receptors were systematically tested by intense cutaneous stimuli; their response to injurious stimuli always could be mimicked by innocuous ones. Therefore, in the primate a particular class of slowly-con ducting myelinated fibres is partially responsible for signalling mechanically-induced cutaneous damage. The probable relation between such afferent fibers and certain kinds of cutaneous pain is explored.