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The Journal of Neurophysiology Vol. 83 No. 3 March 2000, pp. 1426-1434
Copyright ©2000 by the American Physiological Society
Division of Neurology and The Toronto Western Hospital Research Institute, University Health Network, University of Toronto, Toronto, Ontario M5T 2S8, Canada
Chen, Robert and
Rami Garg.
Facilitatory I Wave Interaction in Proximal Arm and Lower Limb
Muscle Representations of the Human Motor Cortex. J. Neurophysiol. 83: 1426-1434, 2000. Transcranial
magnetic stimulation (TMS) of the human motor cortex elicits direct and
indirect (I) waves in the corticospinal tract. Facilitatory I wave
interaction has been demonstrated with a suprathreshold first stimulus
(S1) followed by a subthreshold to threshold second stimulus (S2).
Intracortical inhibition (ICI) and intracortical facilitation (ICF) can
be studied by another paired TMS paradigm with a subthreshold
conditioning stimulus (CS) followed by a suprathreshold test stimulus.
Facilitatory I wave interaction in motor representations other than the
hand area and its relationship to ICI and ICF has not been studied. We
studied I wave interaction, ICI and ICF in an intrinsic hand muscle
(abductor pollicis brevis, APB), in a proximal arm muscle (biceps
brachii, BB) and in a lower limb muscle (tibialis anterior, TA) in 11 normal subjects. I wave facilitation was studied by paired TMS at 24 interstimulus intervals (ISIs) from 0.5 to 5.1 ms. For APB and TA,
facilitation occurred in three distinct peaks at ISIs of 0.9-1.7,
2.5-3.5, and 4.1-5.1 ms. For BB, facilitation was significant for the
first two peaks. The latencies of the peaks were similar among
different muscles, but the magnitude of facilitation was much greater
for APB and TA compared with BB. For all three muscles, changing the S2
to transcranial electrical stimulation (TES) resulted in much less
facilitation of the first peak. For APB, there was significant I wave
facilitation with S2 at 72% motor threshold (MT). The same stimulus
used as the CS did not elicit ICF at ISI of 15 ms, suggesting that the
threshold for eliciting I wave facilitation is lower than that for ICF. For BB and TA, there was no I wave facilitation with S2 at 90% of APB
MT, and the same stimulus used as CS led to ICI. Thus in BB and TA the
threshold for eliciting ICI is lower than that for I wave facilitation.
We conclude that the circuits that mediate I wave interactions are
present in the proximal arm and lower limb representations of the motor
cortex. I wave facilitation occurs predominately in the cortex and may
be primarily related to the monosynaptic corticomotoneuronal (CM)
system. The reduced I wave facilitation for BB compared with APB and TA
may be related to less extensive CM projection and involvement of other
polysynaptic descending pathways. I wave facilitation, ICI, and ICF
appears to be mediated by different neuronal circuits.
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