Progressive suppression of intracortical inhibition during graded isometric contraction of a hand muscle is not influenced by hand preference |
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Authors: | Maryam Zoghi Michael A Nordstrom |
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Institution: | (1) Discipline of Physiology and Research Centre for Human Movement Control, School of Molecular and Biomedical Science, The University of Adelaide, Adelaide, SA, 5005, Australia |
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Abstract: | GABAergic intracortical inhibition (ICI) in human motor cortex (M1) assists fractionated activation of muscles, and it has
been suggested that hemispheric differences in ICI may contribute to hand preference. Previous studies of this issue have
all been conducted at rest, with conflicting results. Testing during voluntary activation may reveal functionally relevant
differences. In normal subjects, we assessed (1) operation of ICI circuits during selective activation of an intrinsic hand
muscle at different forces, and (2) whether this differs between right and left hemispheres. Surface EMG was recorded bilaterally
from abductor pollicis brevis (APB), first dorsal interosseous (FDI) and abductor digiti minimi (ADM) muscles in eleven right-handed
subjects. A circular coil applied paired transcranial magnetic stimulation (TMS) with posteriorly directed current in the
brain. Conditioning intensity was 0.8 × active threshold and interstimulus interval was 3 ms. TMS was applied to right or
left M1 while subjects were at rest or performing isometric thumb abduction at different forces (0.5, 1, 2, 3, 5 and 10 N)
with the contralateral hand. Conditioning TMS was less effective at suppressing the muscle evoked potential in APB during
2–10 N thumb abduction (P < 0.0001) versus rest, but not with lower target forces (0.5, 1 N). Conditioning TMS was less effective for FDI and ADM only
during 10 N thumb abduction. We conclude that differential modulation of ICI in M1 during selective muscle activation is a
function of target isometric force level. At low forces (<5% MVC), ICI is not modulated for the corticospinal neurons controlling
the active or inactive muscles. There is a progressive reduction of ICI effects on corticospinal neurons at higher forces,
which is largely restricted to corticospinal neurons controlling the muscle targeted for activation over the range of forces
tested (up to ∼25% MVC). The pattern of ICI modulation with selective voluntary muscle contraction was similar in left and
right hemispheres during this relatively simple static task. If hemispheric differences in operation of M1 ICI circuits contribute
to hand preference, a more challenging finger movement protocol may be needed to demonstrate this asymmetry. |
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Keywords: | Motor cortex Transcranial magnetic stimulation Finger Force GABA Handedness |
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