Peripheral neural correlates of cutaneous anaesthesia induced by skin cooling in man

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Abstract
The effect of local skin cooling on the behavior of low- and high-threshold mechanoreceptive afferents innervating glabrous and non-glabrous skin was studied in microneurographic recordings on awake human subjects. Cooling with ice or ethyl chloride to a skin surface temperature below 10 degrees C caused a reduction of receptor sensitivity in 49 out of 52 studied low-threshold afferents. This effect was reversible upon warning but some reduction often persisted for a few minutes after normal skin temperature had been reached. The subjects' sensations of application and removal of von Frey hair stimuli were more resistant than had reappeared before the sensation of sustained pressure. This could be explained by shorter recovery times for fast than for slowly adapting units and by a relative preservation of the dynamic responses of the slowly adapting units. During the recovery phase some low-threshold mechanoreceptive afferents exhibited a transient 'spontaneous' discharge in the absence of external mechanical stimulation. The suppression of afferent C-fibre responses to needle strokes was more pronounced and long-lasting than the effect on A-fibre responses and largely paralleled the recovery of sensation of pain. It is concluded that the local anaesthetic effect of skin cooling is to a large extent explicable in terms of receptor desensitization although other mechanisms may contribute.