Abstract
It is demonstrated in cats and monkeys that disynaptic pyramidal excitation in forelimb motoneurons can be mediated via propriospinal neurons (PNs) located in the C3-C4 segments. In contrast, it is still under debate whether there are indirect corticosmotoneuronal (CM) excitatory projections to forelimb motoneurons in humans. In the current study, we reassessed the pathways in humans, using peri-stimulus time histogram (PSTH) technique of single motor unit (MU) firing probability following stimulation of the pyramidal tract. In 9 subjects, who all gave informed consent, activities of a MU were recorded from the biceps brachii (BB) or the first dorsal interosseous (FDI) muscle during weak cocontraction of the muscles. Pyramidal stimulation was achieved by transcranial magnetic or electrical stimulation (TMS or TES) of the contralateral primary motor cortex, or electrical or magnetic cervicomedullary stimulation (CMS). PSTHs were constructed with a bin width of 0.1 ms. TMS produced multiple peaks at short latencies in the PSTH of BB (50/57 MUs), whose total duration (4.5+2.0 ms) was longer than the corresponding value of FDI. TES or CMS could also produce multiple peaks in BB (31/60 MUs; mean duration: 1.9+1.2 ms). We conclude that stimulation of the pyramidal tract could activate indirect CM inputs also in humans, especially in an arm muscle. Results of CMS suggest that the inputs are partly mediated by spinal neurons, including PNs. [J Physiol Sci. 2007;57 Suppl:S157]
