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This Article Full Text Full Text (PDF) All Versions of this Article: 584/2/651    most recent jphysiol.2007.134205v1 Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Butler, J. E. Articles by Petersen, N. T. Search for Related Content PubMed PubMed Citation Articles by Butler, J. E. Articles by Petersen, N. T. Related Collections Integrative Related Article

¹ Department of Exercise and Sport Sciences, and Department of Neuroscience and Pharmacology, University of Copenhagen, Denmark
² Prince of Wales Medical Research Institute and University of New South Wales, Randwick, NSW, Australia 2031

The properties of the human motor cortex can be studied non-invasively using transcranial magnetic stimulation (TMS). Stimulation at high intensity excites corticospinal cells with fast conducting axons that make direct connections to motoneurones of human upper limb muscles, while low-intensity stimulation can suppress ongoing EMG. To assess whether these cells are used in normal voluntary contractions, we used TMS at very low intensities to suppress the firing of single motor units in biceps brachii (n = 14) and first dorsal interosseous (FDI, n = 6). Their discharge was recorded with intramuscular electrodes and cortical stimulation was delivered at multiple intensities at appropriate times during sustained voluntary firing at 10 Hz. For biceps, high-intensity stimulation produced facilitation at 17.1 ± 2.1 ms (lasting 2.4 ± 0.9 ms), while low-intensity stimulation (below motor threshold) produced suppression (without facilitation) at 20.2 ± 2.1 ms (lasting 7.6 ± 2.2 ms). For FDI, high-intensity stimulation produced facilitation at 23.3 ± 1.2 ms (lasting 1.8 ± 0.4 ms), with suppression produced by low-intensity stimulation at 25.2 ± 2.6 ms (lasting 7.5 ± 2.6 ms). The difference between the onsets of facilitation and suppression was short: 3.1 ± 1.2 ms for biceps and 2.0 ± 1.5 ms for FDI. This latency difference is much less than that previously reported using surface EMG recordings (10 ms). These data suggest that low-intensity cortical stimulation inhibits ongoing activity in fast-conducting corticospinal axons through an oligosynaptic (possibly disynaptic) path, and that this activity is normally contributing to drive the motoneurones during voluntary contractions.

(Received 10 April 2007; accepted after revision 9 August 2007; first published online 16 August 2007)
Corresponding author N. Petersen: Department of Exercise and Sport Sciences, University of Copenhagen, Blegdamvej 3, 2200 Copenhagen, Denmark. Email: npetersen{at}ifi.ku.dk

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