CONTROL OF MOTOR UNITS IN HUMAN FLEXOR DIGITORUM PROFUNDUS UNDER DIFFERENT PROPRIOCEPTIVE CONDITIONS

1. Changing the posture of the human fingers can functionally 'disenga ge' the deep finger flexor muscle from its normal action on the termin al phalanx of the fourth (or third) finger. This enables the activity of the muscle to be studied both with and without its normal proprioce ptive inputs. 2. Spike trains of long duration from pairs of concurren tly active motor units in this muscle were recorded in both the engage d and disengaged hand postures. Subjects voluntarily kept one of the m otor units (the 'controlled' unit) discharging at the same target freq uency in both postures. The strength of short-term synchrony, the stre ngth of common drive, and the variability of discharge of these pairs of motor units were determined in both postures. 3. All subjects repor ted that the effort required to activate the motor units in the diseng aged hand posture was substantially greater than in the normal engaged posture. 4. Short-term synchrony, which is a function of common corti cospinal inputs to pairs of motor units, was similar in both hand post ures. However, the strength of common drive was significantly decrease d when the muscle was disengaged. Although the neural substrate for co mmon drive is not known, this observation suggests that proprioceptive feedback is involved either directly or indirectly. 5. Although the d ischarge rate of the 'uncontrolled' motor units increased when the mus cle was disengaged, the variability of discharge of these and the 'con trolled' motor units increased significantly This supports the idea th at the precision with which fine motor tasks can be performed is impro ved when proprioceptive feedback is intact.