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

Pulling or pushing forces applied to an object gripped between finger and thumb excite tactile afferents in the digits in a manner awarding these afferents probable roles in triggering the reactive increases in grip force and in scaling the changes in grip force to the changes in applied load-force. In the present study we assessed the possible con tributions from slowly adapting afferents supplying muscles involved i n the generation of grip forces and from digital joint afferents. Impu lses were recorded from single afferents via tungsten microelectrodes inserted percutaneously into the median or ulnar nerves of awake human subjects. The subject held a manipulandum with a precision grip betwe en the receptor related digit (index finger, middle finger, ring finge r or thumb) and an opposing digit (thumb or index finger). Ramp-and-ho ld load forces of various amplitudes (0.5-2.0 N) and ramp rates (2-32 N/s) were delivered tangential to the parallel grip surfaces in both t he distal (pulling) and the proximal (pushing) directions. Afferents f rom the long flexors of the digits (n=19), regardless of their muscle- spindle or tendon-organ origin, did not respond to the load forces bef ore the onset of the automatic grip response, even with the fastest ra mp rates. Their peak discharge closely followed the peak rate of incre ase in grip force. During the hold phase of the load stimulus, the aff erents sustained a tonic discharge. The discharge rates were significa ntly lower with proximally directed loads despite the mean grip-force being similar in the two directions. This disparity could be explained by the differing contributions of these muscles to the finger-tip for ces necessary to restrain the manipulandum in the two directions. Most afferents from the short flexors of the digits (n=17), including the lumbricals, dorsal interossei, opponens pollicis, and flexor pollicis brevis, did not respond at all, even with the fastest ramps. Furthermo re, the ensemble pattern from the joint afferents (n=6) revealed no si gnificant encoding of changes in finger-tip forces before the onset of the increase in grip force. We conclude that mechanoreceptors in the flexors of the digits and in the interphalangeal joints cannot be awar ded a significant role in triggering the automatic changes in grip for ce. Rather, their responses appeared to reflect the reactive forces ge nerated by the muscles to restrain the object. Hence, it appears that tactile afferents of the skin in contact with the object are the only species of receptor in the hand capable of triggering and initially sc aling an appropriate change in grip force in response to an imposed ch ange in load force, but that muscle and joint afferents may provide in formation related to the reactive forces produced by the subject.