Motor cortical plasticity in extrinsic hand muscles is determined by the resting thresholds of overlapping representations

Abstract

Knowledge of the properties that govern the effectiveness of transcranial magnetic stimulation (TMS) interventions is critical to clinical application. Extrapolation to clinical populations has been limited by high inter-subject variability and a focus on intrinsic muscles of the hand in healthy populations. Therefore, the current study assessed variability of continuous theta burst stimulation (cTBS), a patterned TMS protocol, across an agonist–antagonist pair of extrinsic muscles of the hand. Secondarily, we assessed whether concurrent agonist contraction could enhance the efficacy of cTBS. Motor evoked potentials (MEP) were simultaneously recorded from the agonist flexor (FCR) and antagonist extensor (ECR) carpi radialis before and after cTBS over the FCR hotspot. cTBS was delivered with the FCR relaxed (cTBS-Relax) or during isometric wrist flexion (cTBS-Contract). cTBS-Relax suppressed FCR MEPs evoked from the FCR hotspot. However, the extent of FCR MEP suppression was strongly correlated with the relative difference between FCR and ECR resting motor thresholds. cTBS-Contract decreased FCR suppression but increased suppression of ECR MEPs elicited from the FCR hotspot. The magnitude of ECR MEP suppression following cTBS-Contract was independent of the threshold-amplitude relationships observed with cTBS-Relax. Contraction alone had no effect confirming the effect of cTBS-Contract was driven by the interaction between neuromuscular activity and cTBS. Interactions across muscle representations should be taken into account when predicting cTBS outcomes in healthy and clinical populations. Contraction during cTBS may be a useful means of focusing aftereffects when differences in baseline excitability across overlapping agonist–antagonist cortical representations may mitigate the inhibitory effect of cTBS.

Interindividual Variability of Lower-Limb Motor Cortical Plasticity Induced by Theta Burst Stimulation

“…Furthermore, we did not assess the cortical excitability of the soleus muscle in the current work. Previous studies reported that the overlapping cortical representations with proximal antagonist muscle affect each TBS (Mirdamadi et al, 2015(Mirdamadi et al, , 2016. Similarly, TBS to the motor cortex area of soleus muscleareas that overlaps with TA muscle may affect our results.…”

“…Considering both lower-limb motor cortices are proximally located, it is difficult to apply TBS to the motor cortex of one hemisphere without stimulating the other, especially at higher stimulus intensity (Huang et al, 2018;Lin et al, 2019). Additionally, the interaction between synergistic and antagonist cortical representations of arm muscles affects the plasticity induced by TBS (Mirdamadi et al, 2015(Mirdamadi et al, , 2016. Therefore, the inducibility of LTP-like or LTD-like synaptic plasticity is different in different motor cortical representations.…”

The Role of Transcranial Magnetic Stimulation as a Diagnostic Tool for Multiple Sclerosis

“…The TMS measures most commonly used in the analysis of motor data are resting motor threshold (MT; the minimal TMS intensity required to evoke an MEP), latency (the time from triggering the pulse to the start of the MEP), amplitude (baseline to peak or peak to peak value) and area (measured as root mean squared, or as the area under the line of the rectified signal) [57]. MT is the lowest stimulus intensity of TMS that elicits a recordable MEP in the target muscle [58].…”

Photogenetics Inhibits the Activity of Glutamate Neurons Projected from the Motor Cortex to the Ventroposterolateral Nucleus of the Thalamus to Alleviates Central Post-Stroke Pain in Rats

“…Currently, common neuromodulation therapies for CPSP in clinical practice include repetitive transcranial stimulation, motor cortex electrical stimulation, and deep brain stimulation (Mirdamadi et al 2016;Liu et al 2020). Our research explored optogenetics as an alternative neuromodulation approach.…”