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The Journal of Neuroscience, July 22, 2009, 29(29):9370-9379; doi:10.1523/JNEUROSCI.0143-09.2009

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Behavioral/Systems/Cognitive
Slowly Adapting Mechanoreceptors in the Borders of the Human Fingernail Encode Fingertip Forces

Ingvars Birznieks,¹ Vaughan G. Macefield,^1,2 Göran Westling,³ and Roland S. Johansson³

¹Prince of Wales Medical Research Institute, Sydney, New South Wales 2031, Australia, ²School of Medicine, University of Western Sydney, Sydney, New South Wales 1797, Australia, and ³Department of Integrative Medical Biology, Physiology Section, Umeå University, SE-901 87 Umeå, Sweden

Correspondence should be addressed to Ingvars Birznieks, Prince of Wales Medical Research Institute, Randwick, Sydney, NSW 2031, Australia. Email: i.birznieks{at}powmri.edu.au

There are clusters of slowly adapting (SA) mechanoreceptors in the skin folds bordering the nail. These "SA-IInail" afferents, which constitute nearly one fifth of the tactile afferents innervating the fingertip, possess the general discharge characteristics of slowly adapting type II (SA-II) tactile afferents located elsewhere in the glabrous skin of the human hand. Little is known about the signals in the SA-IInail afferents when the fingertips interact with objects. Here we show that SA-IInail afferents reliably respond to fingertip forces comparable to those arising in everyday manipulations. Using a flat stimulus surface, we applied forces to the finger pad while recording impulse activity in 17 SA-IInail afferents. Ramp-and-hold forces (amplitude 4 N, rate 10 N/s) were applied normal to the skin, and at 10, 20, or 30° from the normal in eight radial directions with reference to the primary site of contact (25 force directions in total). All afferents responded to the force stimuli, and the responsiveness of all but one afferents was broadly tuned to a preferred direction of force. The preferred directions among afferents were distributed all around the angular space, suggesting that the population of SA-IInail afferents could encode force direction. We conclude that signals in the population of SA-IInail afferents terminating in the nail walls contain vectorial information about fingertip forces. The particular tactile features of contacted surfaces would less influence force-related signals in SA-IInail afferents than force-related signals present in afferents terminating in the volar skin areas that directly contact objects.

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Received Dec. 3, 2008; revised May 12, 2009; accepted June 24, 2009.

Correspondence should be addressed to Ingvars Birznieks, Prince of Wales Medical Research Institute, Randwick, Sydney, NSW 2031, Australia. Email: i.birznieks{at}powmri.edu.au

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