Changes in motor unit synchronization following central nervous lesions in man.

1. Single motor unit spike trains have been recorded during voluntary isometric contraction of the affected intrinsic hand muscles of patients with unilateral central nervous lesions. These have been compared with similar recordings made from the patients' unaffected hand muscles and with recordings made from the hand muscles of healthy subjects. 2. Cross-correlation analysis was performed between the times of occurrence of the motor unit spike trains. The time course of central cross-correlogram peaks constructed for normal subjects and stroke patients was used to infer properties of the underlying common EPSPs and the impulse-generating properties of the motoneurones. The results of this analysis were compared between the two groups. In addition, the size and time course of cross-correlogram peaks obtained from the patients were related both to the patients' clinical state and to their hand and fine finger function. 3. Central nervous lesions were found to result in either a narrowing or broadening of the time course of motor unit synchronization. These changes were attributed either to an increase in the size of common EPSPs with respect to synaptic noise, or to the effects of presynaptic synchronization of motoneurone inputs. 4. Longitudinal studies of motor unit discharges in the year following the stroke demonstrated, in some patients, differences in the level of motor unit synchronization. These paralleled improvements in the patients' fine motor control. Pooled data from patients with varying deficits of fine motor control confirmed that loss or reduction of motor unit synchronization was associated with a corresponding slowing in the performance of rapidly alternating finger movements. 5. The results of the present study suggest that the branched common presynaptic inputs that generate motor unit synchronization are either of corticospinal tract origin or are intimately dependent on its function. Differences in the strength and time course of motor unit synchronization are demonstrated that may reflect the altered behaviour of presynaptic inputs to motoneurones following central nervous damage in man.