CONTROL OF GRIP FORCE DURING RESTRAINT OF AN OBJECT HELD BETWEEN FINGER AND THUMB - RESPONSES OF CUTANEOUS AFFERENTS FROM THE DIGITS

Unexpected pulling and pushing loads exerted by an object held with a precision grip evoke automatic and graded increases in the grip force (normal to the grip surfaces) that prevent escape of the object; unloa ding elicits a decrease in grip force. Anesthesia of the digital nerve s has shown that these grip reactions depend on sensory signals from t he digits. In the present study we assessed the capacity of tactile af ferents from the digits to trigger and scale the evoked grip responses . Using tungsten microelectrodes inserted percutaneously into the medi an nerve of awake human subjects, unitary recordings were made from te n FA I and 13 FA II rapidly adapting afferents, and 12 SA I and 18 SA II slowly adapting afferents. While the subject held a manipulandum be tween a finger and the thumb, tangential load forces were applied to t he receptor-bearing digit (index, middle, or ring finger or thumb) as trapezoidal load-force profiles with a plateau amplitude of 0.5 - 2.0 N and rates of loading and unloading at 2 - 8 N/s, or as ''step-loads' ' of 0.5 N delivered at 32 N/s. Such load trials were delivered in bot h the distal (pulling) and proximal (pushing) direction. FA I afferent s responded consistently to the load forces, being recruited during th e loading and unloading phases. During the loading ramp the ensemble d ischarge of the FA I afferents reflected the first time-derivative of the load force (i.e., the load-force rate). These afferents were relat ively insensitive to the subject's grip force responses. However, high static finger forces appeared to suppress excitation of these afferen ts during the unloading phase. The FA II afferents were largely insens itive to the load trials: only with the step-loads did some afferents respond. Both classes of SA afferents were sensitive to load force and grip force, and discharge rates were graded by the rate of loading. T he firing of the SA I afferents appeared to be relatively more influen ced by the subject's grip-force response than the discharge of the SA LI afferents, which were more influenced by the load-force stimulus. T he direction in which the tangential load force was applied to the ski n influenced the firing of most afferents and in par ticular the SA II afferents. Individual afferents within each class (except for the FA IIs) responded to the loading ramp before the onset of the subject's g rip response and may thus be responsible for initiating the automatic increase in grip force. However, nearly half of the FA I afferents rec ruited by the load trials responded to the loading phase early enough to trigger the subject's grip-force response, whereas only ca. one-fif th of the SA Is and SA IIs did so. These observations, together with t he high density of FA I receptors in the digits, might place the FA I afferents in a unique position to convey the information required to i nitiate and scale the reactive grip-force responses to the imposed loa d forces.