NONDIGITAL AFFERENT INPUT IN REACTIVE CONTROL OF FINGERTIP FORCES DURING PRECISION GRIP

Sensory inputs from the digits are important in initiating and scaling automatic reactive grip responses that help prevent frictional slips when grasped objects are subjected to destabilizing load forces. In th e present study we analyzed the contribution to grip-force control fro m mechanoreceptors located proximal to the digits when subjects held a small manipulandum between the tips of the thumb and index finger. Lo ads of various controlled amplitudes and rates were delivered tangenti al to the grip surfaces at unpredictable times. Grip forces (normal to the grip surfaces) and the position of the manipulandum were recorded . In addition, movements of hand and arm segments were assessed by rec ording the position of markers placed at critical points. Subjects per formed test series during normal digital sensibility and during local anesthesia of the index finger and thumb. To grade the size of movemen ts of tissues proximal to the digits caused by the loadings, three dif ferent conditions of arm and hand support were used; (1) in the hand-s upport condition the subjects used the three ulnar fingers to grasp a vertical dowel support and the forearm was supported in a vacuum cast; (2) in the forearm-support condition only the forearm was supported; finally, (3) in the no-support condition the arm was free. With normal digital sensibility the size of the movements proximal to the digits had small effects on the grip-force control. In contrast, the grip con trol was markedly influenced by the extent of such movements during di gital anesthesia. The poorest control was observed in the hand-support condition, allowing essentially only digital movements. The grip resp onses were either absent or attenuated, with greatly prolonged onset l atencies. In the forearm and no-support conditions, when marked wrist movements took place, both the frequency and the strength of grip-forc e responses were higher, and the grip response latencies were shorter. However, the performance never approached normal. It is concluded tha t sensory inputs from the digits are dominant in reactive grip control . However, nondigital sensory input may be used for some grip control during impaired digital sensibility. Furthermore, the quality of the c ontrol during impaired sensibility depends on the extent of movements evoked by the load in the distal, unanesthetized parts of the arm. The origin of these useful sensory signals is discussed.