Motor unit discharge characteristics during voluntary contraction in patients with incomplete spinal cord injury

Synchronisation of motor unit discharges is commonly seen in hand muscles o f normal man but is absent following neurologically complete spinal cord in jury and reduced after stroke. These findings support the notion that some corticospinal inputs to motoneurones are shared and contribute to the obser ved synchrony of discharge. In this study we have examined motor unit disch arge in hand muscles below the level of an incomplete spinal cord injury in an attempt to relate strength of synchrony to the integrity of the cortico spinal tract. Eight patients with incomplete spinal cord injury (neurologic al level C3-C7) and eight control subjects took part in the study. The pati ents had sustained injury 14-191 weeks prior to the recordings and had sinc e regained good motor function in their hands. Two concentric needle electr odes were inserted into the first dorsal interosseus muscle which subjects were instructed to contract weakly so that potentials from individual motor units could be reliably identified on both recordings. Synchrony was detec ted by constructing cross-correlograms between the discharges of pairs of i ndividual motor units. The amount of synchronous firing was determined from the magnitude of any peak in the cross-correlogram, as the probability abo ve chance (XP) of one motor unit firing with respect to the other and vice versa. The degree of synchrony was lower (P < 0.05) in the patient group (m ean XP 0.06) than in the control group (mean XP 0.09). The incidence of sig nificant synchrony was lower in the patient group (41.8 %) than in the cont rol group (92.9 %). The mean (+/- S.E.M.) frequency of motor unit discharge was slightly lower (P < 0.05) in patients (9.7 +/- 0.4 impulses s(-1)) tha n controls (10.8 +/- 0.5 impulses s(-1)). The mean width of synchrony peaks was narrower (P < 0.05) in patients (11.4 +/- 1.1 ms) than controls (13.2 +/- 0.6 ms). We conclude that the weaker synchrony of motor unit discharge in incomplete spinal cord injury may reflect permanent damage to some corti cospinal axons.