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 contributions from slowly adapting afferents supplying muscles involved in the generation of grip forces and from digital joint afferents. Impulses 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 between the receptor-related digit (index finger, middle finger, ring finger or thumb) and an opposing digit (thumb or index finger). Ramp-and-hold 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 the distal (pulling) and the proximal (pushing) directions. Afferents from the long flexors of the digits (n=19), regardless of their muscle-spindle or tendon-organ origin, did not respond to the load forces before the onset of the automatic grip response, even with the fastest ramp rates. Their peak discharge closely followed the peak rate of increase in grip force. During the hold phase of the load stimulus, the afferents sustained a tonic discharge. The discharge rates were significantly 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 forces 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. Furthermore, the ensemble pattern from the joint afferents (n=6) revealed no significant 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 awarded a significant role in triggering the automatic changes in grip force. Rather, their responses appeared to reflect the reactive forces generated 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 scaling an appropriate change in grip force in response to an imposed change in load force, but that muscle and joint afferents may provide information related to the reactive forces produced by the subject.