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J Neurophysiol (March 1, 2003). 10.1152/jn.00950.2002
Submitted on Submitted 21 June 2002; accepted in final form 29 October 2002
Division of Neurology, Krembil Neuroscience Center and Toronto Western Research Institute, University Health Network, University of Toronto, Toronto, Ontario M5T 2S8, Canada
Chen, Robert,
Derek Yung, and
Jie-Yuan Li.
Organization of Ipsilateral Excitatory and Inhibitory
Pathways in the Human Motor Cortex. J. Neurophysiol. 89: 1256-1264, 2003. Motor cortex stimulation has both
excitatory and inhibitory effects on ipsilateral muscles. Excitatory
effects can be assessed by ipsilateral motor-evoked potentials (iMEPs).
Inhibitory effects include an interruption of ipsilateral voluntary
muscle activity known as the silent period (iSP) and a reduction in
corticospinal excitability evoked by conditioning stimulation of the
contralateral motor cortex (interhemispheric inhibition, IHI). Both iSP
and IHI may be mediated by transcallosal pathways. Their relationship to the contralateral corticospinal projection and whether iSP and IHI
represent the same phenomenon remain unclear. The neuronal population
activated by transcranial magnetic stimulation (TMS) is highly
dependent on the direction of the induced current in the brain. We
examined the relationship among iMEP, iSP, IHI, and the contralateral
corticospinal system by examining the effects of different stimulus
intensities and current directions. Surface electromyography (EMG) was
recorded from both first dorsal interosseous (FDI) muscles. The iSP in
the right FDI muscle was obtained by right motor cortex stimulation
during voluntary muscle contraction. IHI was examined by conditioning
stimulation of the right motor cortex followed by test stimulation of
the left motor cortex at interstimulus intervals (ISIs) of 2-80 ms.
The induced current directions tested in the right motor cortex were
anterior medial (AM), posterior medial (PM), posterior lateral, and
anterior lateral (AL). Contralateral MEPs (cMEPs) had the lowest
threshold with the AM direction and the shortest latency with the PM
direction. iMEPs were present in 8 of 10 subjects. Both iMEP and IHI
did not show significant directional preference. iSP was observed in
all subjects with the highest threshold for the AL direction and the
longest duration for the AM direction. cMEP, iSP, and IHI all increased
with stimulus intensity up to ~75% stimulator output. Target muscle
activation decreased IHI at 8-ms ISI but had little effect on IHI at
40-ms ISI. iSP and IHI at 8-ms ISI did not correlate at any stimulus
intensities and current directions tested, and factor analysis showed
that they are explained by different factors. However, active IHI at
40-ms ISI was explained by the same factor as iSP. The different
directional preference for cMEP compared with iMEP and IHI suggests
that these ipsilateral effects are mediated by populations of cortical
neurons that are different from those activating the corticospinal
neurons. iSP and IHI do not represent the same phenomenon and should be
considered complementary measures of ipsilateral inhibition.
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