EVIDENCE FOR BILATERAL INNERVATION OF CERTAIN HOMOLOGOUS MOTONEURON POOLS IN MAN

1. Surface EMG recordings were made from left and right homologous mus cle pairs in healthy adults. During each recording session subjects we re requested to maintain a weak isometric contraction of both the left and right muscle. 2.. Cross-correlation analysis of the two multiunit EMG recordings from each pair of muscles was performed. Central peaks of short duration (mean durations, 11.3-13.0 ms) were seen in correlo grams constructed from multiunit EMG recordings obtained from left and right diaphragm, rectus abdominis and masseter muscles. No central pe aks were seen in correlograms constructed from the multiunit EMG recor dings from left and right upper limb muscles. 3. To investigate descen ding pathways to the homologous muscle pairs, the dominant motor corte x was stimulated using a focal magnetic brain stimulator whilst record ing from homologous muscle pairs. 4. Following magnetic stimulation of the dominant motor cortex, a response was recorded from both right an d left diaphragm, rectus abdominis and masseter muscles. In contrast, when recording from homologous upper limb muscles, a response was only seen contralateral to the side of stimulation. 5. The finding of shor t duration central peaks in the cross-correlograms constructed from mu ltiunit recordings from left and right diaphragm, rectus abdominis and masseter, suggests that muscles such as these, that are normally co-a ctivated, share a common drive. The mechanism is discussed and it is a rgued that the time course of the central correlogram peaks is consist ent with the hypothesis that they could be produced by a common drive that arises from activity in last-order branched presynaptic fibres al though presynaptic synchronization of last-order inputs is also likely to be involved. 6. The results of the magnetic stimulation experiment s suggest that this common drive may involve the corticospinal tract. 7. We saw no evidence for a common drive to left and right homologous muscle pairs that may be voluntarily co-activated but often act indepe ndently.