Significance of cutaneous input for precise hand movements.

During precision manipulation of small objects a refined coordination of forces exerted on the object by the tips of the fingers and thumb is required. An essential feature of this coordination is that the grip forces change in parallel with the load forces produced to overcome various forces counteracting the intended manipulation. The balance between the grip forces and load forces tending to cause slips is adapted to the friction between the skin and the object, providing a relatively small safety margin to prevent slips. This adaptation, which is dependent on tactile afferent input from the fingers, works highly automatically. Tactile afferent signals account for an initial adjustment of the force balance to the current frictional condition, taking place soon after the object is initially touched. Moreover, brief bursts of action potentials elicited in dynamically sensitive units by small slips can trigger a change in this balance, resulting in an increased safety margin for the prevention of further slips. This change includes a rapid reflex response (latency 60-80 msec) together with updating of a sensorimotor memory maintaining the new force balance.