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University Department of Clinical Neurology, Radcliffe Infirmary, Oxford, UK.

1. The voluntary discharge characteristics, short term synchronization, coherence and responses to transcranial magnetic stimulation of a sample of twenty-two pairs of simultaneously recorded low threshold motor units in the right human first dorsal interosseus muscle have been determined. 2. Peristimulus time histograms and cumulative sums (cusums) of motor unit discharge showed either excitatory or inhibitory responses to cortical stimuli. Over the whole motor unit sample, the primary excitatory response had a mean onset of 24.6 ms and the inhibitory response 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 the other was inhibited. 3. Most pairs of motor units showed short term synchrony in their voluntary discharge, but the same motor units driven by magnetic cortical stimuli showed little tendency to discharge together more frequently than would be predicted from their independent behaviour. This held true for discharges in both the early primary excitatory peak and in the later secondary peak in peristimulus time histograms. 4. Series of magnetic stimuli, causing either excitation or inhibition of individual tonically active motor units, had no effect on the size of the central peak of the cross-correlogram of the motor unit pair. However, frequency analysis of pairs of motor unit spike trains showed an increase in coherence in the 16-32 Hz band during magnetic stimulation. 5. It is concluded that, whilst many corticospinal fibres branch extensively within motoneurone pools, there are also fast conducting corticomotoneuronal fibres allowing the independent monosynaptic activation of individual motoneurones. 6. It is also suggested that during sustained voluntary tonic activity, magnetic stimuli applied to the brain can increase the synchrony in common input fibres to pairs of motoneurones.