Short-term synchrony of motor unit discharge during weak isometric contraction in Parkinson's disease

Short-term synchrony between the discharges of motor units has been assessed in Parkinson's disease (PD) and normal man. The discharges of single motor units were recorded in the extensor digitorum communis (EDC) muscle of the forearm or the tibialis anterior (TA) muscle of the leg during weak, voluntary isometric contraction. Short-term synchrony was defined as a narrow peak (total width less than 25 ms) in cross-correlograms constructed from the discharges of pairs of motor units. There was no difference in the incidence of short-term synchrony between PD and normal age-matched subjects for either the EDC or TA muscle. On average, 60% of pairs of motor units exhibited synchrony, but this varied between 0% and 100% for both groups. The amount of short-term synchrony was assessed as the probability (above chance) of discharge of one motor unit with respect to the other. In TA, but not EDC, this index was greater for PD than for normal subjects. The high indices of synchrony in TA in PD were not related to lower discharge rates of motor units. Parkinson's disease subjects, but not normal subjects, also showed broad correlations that were invariably associated with periodic discharges in the range 4-6 Hz. In some instances, a peak of short-term synchrony was observed superimposed on the broad correlation. The periodic correlograms were often associated with overt tremor which accompanied the contraction. Motor units occasionally discharged paired impulses (doublet discharges) with short interspike intervals of 5-15 ms (normal and PD) or, as a more persistent feature in PD, longer interspike doublets (20-60 ms) associated with periodic synchrony (4-6 Hz). The abnormal discharge characteristics of motor units are discussed in relation to the bulbospinal control of presynaptic drive to motor neurons in PD.