Cortical mechanisms of unilateral voluntary motor inhibition in humans

Abstract

While motor control is very often a goal-oriented event, little is known about the mechanisms underlying the termination of motor performance. To investigate what type of cortical activation underlies the muscle relaxation required to terminate the act, we performed single- and double-pulse transcranial magnetic stimulation (TMS) studies during voluntary muscle relaxation in nine normal volunteers. Subjects maintained a weak isometric contraction of the right first dorsal interosseous muscle (FDI), and either increased the level of contraction (Contraction), terminated the contraction (Relaxation), or maintained it (No-go) depending on a visual cue. Motor evoked potentials (MEP) and the silent period (SP) were recorded from the FDI during motor activity. To measure intra-cortical inhibition (ICI), we also performed double-pulse TMS, applying subthreshold conditioning stimuli at interstimulus intervals of 2 ms. When single-pulse TMS was given just prior to muscle relaxation (-21 to -70 ms), the MEP was reduced while the SP was unchanged. Intra-cortical inhibition was smaller just prior to the muscle relaxation. Unilateral voluntary muscle relaxation may not be associated with activation of the intracortical inhibitory system, but rather with the possible excitation of the corticospinal system, which can inhibit motoneurons disynaptically. These findings suggest that multiple inhibitory mechanisms act in diverse ways to achieve motor inhibition.

Group‐level variations in motor representation areas of thenar and anterior tibial muscles: Navigated Transcranial Magnetic Stimulation Study

“…The specificity of individual optimal stimulation sites, however, may vary even in healthy subjects, e.g. due to varying level of alertness during the measurement session [Salih et al, 2005], fluctuating relaxation level in the target muscle [Begum et al, 2005], or errors caused by the measurement system. Commonly the reproducibility of the mapped location of the representation area of muscles is good from session-to-session using the eXimia navigation system [Danner et al, 2008;Julkunen et al, 2009].…”

Contribution of intracortical inhibition in voluntary muscle relaxation

“…Both studies found changes in SICI approximately 20-70 ms prior to the termination of first dorsal interosseous (FDI) muscle activity. However, Buccolieri et al (2004a) found that SICI increased prior to FDI relaxation, whereas Begum et al (2005) found that SICI decreased prior to relaxation.…”

“…Specifically, muscle relaxation may be mediated by increased intracortical inhibition (Buccolieri et al 2004a), leading to withdrawal of corticospinal input (Kamper et al 2003;Rothwell et al 1998). Alternatively, muscle relaxation may be mediated by corticospinal activation of spinal inhibitory interneurons Crenna 1984, 1985;Schieppati et al 1986;Begum et al 2005).…”

“…Two previous studies examining the role of shortinterval intracortical inhibition (SICI) in muscle relaxation have reported conflicting results (Begum et al 2005;Buccolieri et al 2004a). Both studies found changes in SICI approximately 20-70 ms prior to the termination of first dorsal interosseous (FDI) muscle activity.…”

“…Thus, changes in SICI observed before, during, and after muscle relaxation in Buccolieri et al (2004a) may have been due to changes in the level of muscle activity and may not represent modulation of SICI specific to a person's intention to relax. In Begum et al (2005), the level of muscle activity during relaxation just prior to stimulation or during baseline SICI measurement is unknown and thus incomparable. As for unilateral versus bilateral relaxation, simultaneous contralateral muscle activity has been shown to be associated with increased interhemispheric inhibition (IHI) and thus suppressed SICI for the ipsilateral hand muscle (Yedimenko and Perez 2010).…”