Theta Burst Stimulation of Human Primary Motor Cortex Degrades Selective Muscle Activation in the Ipsilateral Arm

Abstract

Bradnam LV, Stinear CM, Byblow WD. Theta burst stimulation of human primary motor cortex degrades selective muscle activation in the ipsilateral arm. J Neurophysiol 104: 2594 -2602, 2010. First published September 15, 2010 doi:10.1152/jn.00365.2010. This study investigated whether repetitive transcranial magnetic stimulation (TMS) delivered as continuous theta burst stimulation (cTBS) to left M1 degraded selective muscle activation in the contralateral and ipsilateral upper limb in healthy participants. Contralateral motorevoked potentials (cMEPs) were elicited in left and right biceps brachii (BB) before either elbow flexion or forearm pronation. A neurophysiological index, the excitability ratio (ER), was computed from the relative size of BB cMEPs before each type of movement. Short interval intracortical inhibition (SICI) was assessed in cMEPs of right BB with paired-pulse TMS of left M1. Ipsilateral MEPs (iMEPs) and silent periods (iSPs) were measured in left BB with single-pulse TMS of left M1. Low-intensity cTBS was expected to suppress corticospinal output from left M1. A sham condition was also included. Real but not sham cTBS caused increases in BB ER bilaterally. In the right arm, ER increased because BB cMEPs before flexion were less facilitated, whereas cMEPs in the pronation task were unaffected. This was accompanied by an increase in left M1 SICI. In the left arm, ER increased because BB cMEPs before pronation were facilitated but were unaffected in the flexion task. There was also facilitation of left BB iMEPs. These changes in the left arm are consistent with inappropriate facilitation of left BB ␣-motoneurons (␣MNs) before pronation. This is the first demonstration that cTBS of M1 can alter excitability of neurons controlling ipsilateral proximal musculature and degrade ipsilateral upper limb motor control, providing evidence that ipsilateral and contralateral M1 shape the spatial and temporal characteristics of proximal muscle activation appropriate for the task at hand.

Anodal Direct Current Stimulation of the Cerebellum Reduces Cerebellar Brain Inhibition but Does Not Influence Afferent Input from the Hand or Face in Healthy Adults

“…Each participant completed a screening questionnaire, which was reviewed by a study doctor prior to participation, to ensure safety for TMS and DCS. In each study, participants attended two sessions, separated by at least 5 days as per previous protocols in healthy adults [8, 36,37]. Participants underwent anodal or sham DCS, in counterbalanced order, and all were blinded to the intervention at each session.…”

Cathodal transcranial direct current stimulation suppresses ipsilateral projections to presumed propriospinal neurons of the proximal upper limb

“…There is growing evidence for a functionally important role of ipsilateral M1 in upper limb control in healthy humans (Bradnam et al 2010a;Davare et al 2007;Duque et al 2008;Muellbacher et al 2000;Perez and Cohen 2008). For example, ipsilateral M1 may contribute to task-specific selective muscle activation alongside contralateral M1 by maintaining the balance of excitation and inhibition over presumed PNs and ␣MNs (Bradnam et al 2010a).…”

“…For example, ipsilateral M1 may contribute to task-specific selective muscle activation alongside contralateral M1 by maintaining the balance of excitation and inhibition over presumed PNs and ␣MNs (Bradnam et al 2010a). This is particularly relevant in proximal muscles for which there are dense ipsilateral projections between M1 and spinal ␣MNs (Kuypers 1964;Lemon 2008).…”

Theta burst TMS increases cerebral blood flow in the primary motor cortex during motor performance as assessed by arterial spin labeling (ASL)

“…TBS effects have consistently been displayed by electrophysiological measurements in terms of modifications in motor output, i.e. motor evoked potentials (Bradnam et al, 2010;Gentner et al, 2008;Huang et al, 2005Huang et al, , 2008Stefan et al, 2008;Talelli et al, 2007). Hence, TBS effects may also be reflected in motor behavioral changes (Agostino et al, 2007).…”