SHORT-TERM SYNCHRONIZATION OF HUMAN MOTOR UNITS AND THEIR RESPONSES TO TRANSCRANIAL MAGNETIC STIMULATION

1. The voluntary discharge characteristics, short term synchronization , coherence and responses to transcranial magnetic stimulation of a sa mple of twenty-two pairs of simultaneously recorded low threshold moto r units in the right human first dorsal interosseus muscle have been d etermined. 2. Peristimulus time histograms and cumulative sums (cusums ) of motor unit discharge showed either excitatory or inhibitory respo nses to cortical stimuli. Over the whole motor unit sample, the primar y excitatory response had a mean onset of 24.6 ms and the inhibitory r esponse a mean onset of 31.0 ms. Responses of pairs of motor units to magnetic stimulation could be dissimilar; at some stimulus intensities one of the motor unit pair could be excited by the stimulus whilst th e other was inhibited. 3. Most pairs of motor units showed short term synchrony in their voluntary discharge, but the same motor units drive n by magnetic cortical stimuli showed little tendency to discharge tog ether more frequently than would be predicted from their independent b ehaviour. This held true for discharges in both the early primary exci tatory peak and in the later secondary peak in peristimulus time histo grams. 4. Series of magnetic stimuli, causing: either excitation or in hibition of individual tonically active motor units, had no effect on the size of the central peak of the cross-correlogram of the motor uni t pair. However, frequency analysis of pairs of motor unit spike train s showed an increase in coherence in the 16-32 Hz band during magnetic stimulation. 5. It is concluded that, whilst many corticospinal fibre s branch extensively within motoneurone pools, there are also fast con ducting corticomotoneuronal fibres allowing the independent monosynapt ic activation of individual motoneurones. 6. It is also suggested that during sustained voluntary tonic activity, magnetic stimuli applied t o the brain can increase the synchrony in common input fibres to pairs of motoneurones.