Responses in glabrous skin mechanoreceptors during precision grip in humans

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Abstract
Impulses in single tactile units innervating the human glabrous skin were recorded percutaneously from the median nerve using tungsten electrodes. The units were classified as belonging to one of the four categories: fast adapting with small receptive fields (FA I), fast adapting with large receptive fields (FA II), slowly adapting with small fields (SA I), and slowly adapting with large fields (SA II). A small test object was lifted, positioned in space and replaced using the precision grip between fingers and thumb. The grip force, the load force (vertical lifting force), the vertical movements of the object and vibrations (accelerations) in the object were recorded. After being virtually silent between lifts, the FA I units whose fields contacted the object became highly active during the initial period of grip force increase (initial response). This was also true for most SA I units. Accordingly, most of the skin deformation changes took place at low grip forces (below ca. 1 N). Later, while the load and grip forces increased in parallel during isometric conditions, the FA I and SA I units continued firing but generally at declining impulse rates. As long as the object was held in the air, the SA I units generally maintained firing with a tendency to adaptation. A minority of the FA I unit also discharged, especially during periods of pronounced physiological muscle tremor. The SA I units usually became silent when the grip and load forces in parallel declined to zero during isometric conditions after the object had contacted the table. However, during the very release of the grip the FA I units and some SA I units showed brief burst discharges (release response). The FA II units responded distinctly to the mechanical transients associated with the start of the vertical movement and especially with the sudden cessation of movement at the terminal table contact. FA II units whose end organs were remotely located in relation to the skin areas in contact with the object also responded. Most FA II units also discharged at the initial touch and at the release of the object, albeit less reliably than the type I units. In addition to weak dynamic responses during the phase of isometric force increase, the SA II units showed comparatively strong tonic responses while the object was held during static conditions. High firing rates also were maintained during long-lasting lifts. Moreover, it was established that the signals in SA II afferents were related to the three dimensional force profile in the grip. The results are discussed with regard to the possible implications for the control of precise manipulative movements.