The responses of afferent fibres from the glabrous skin of the hand during voluntary finger movements in man.

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Abstract
Afferent activity of 111 single units from the glabrous skin area was recorded percutaneously in the median nerve of human subjects using W electrodes. The majority of the units (103) were classified as low-threshold mechanosensitive units belonging to 1 of the 4 categories: rapidly adapting with small receptive fields (RA), rapidly adapting with large receptive fields (PC, presumed Pacinian corpuscle units), slowly adapting with small fields (SA I) and slowly adapting with large fields (SA II). The size of the responses (in number of impulses) to indentation and stretching of the skin was compared with the responses elicited during voluntary isotonic finger movements, which avoided trivial excitation of the units by direct touch. All 4 types of units, and 77% of the single units, were activated by isotonic movements. The decreasing order of responsiveness was PC, SA II, SA I, RA. Almost all responsive units were excited during the dynamic phase of ramp and smooth oscillatory movements. Static responses occurred only with 50% of the slowly adapting units, corresponding to a 3rd of the total sample (SA II, 81%; SA I, 17%). For all 4 types of units the dynamic responses to movements were of similar size as the responses to localized skin indentation with a von Frey hair at 5 times threshold. The possible implications for kinaesthesia and motor control are discussed.