Modulation of inhibitory and excitatory neural circuits in the primary motor cortex following theta-burst rTMS to area 5
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Subcortical and cortical loci interact with the primary motor cortex (M1) and influence the neural circuitry mediating hand movement. Area 5 located within the superior parietal lobule has direct connectivity with M1, is largely dedicated to the representation of the hand, and is considered important for thumb opposition movements. The present study examined the modulation of inhibitory and excitatory neural circuits within bilateral M1 before and after continuous (cTBS), intermittent (iTBS), and sham theta-burst stimulation (TBS) over left-hemisphere area 5. Two experiments were performed to address the influence of area 5 on neural circuitry within M1. Specifically, inhibitory circuitry (short interval intracortical inhibition (SICI)) and excitatory circuitry (motor evoked potentials (MEPs), intracortical facilitation (ICF)), were examined for the representation of the first dorsal interosseous (FDI) muscle within bilateral M1. MEPs, SICI, and ICF were measured bilaterally before and at 5-20 minutes, 25-40 minutes, and 45-60 minutes after TBS cessation. The order for right versus left M1 recordings for MEPs, SICI, and ICF recordings were kept constant within subjects across each time block and this order of cortex stimulated (right, left) was randomized across subjects. The results of Experiment 1 and 2 demonstrate that area 5 selectively influences M1 circuitry such that MEPs are increased bilaterally following area 5 cTBS and increased in the right FDI following area 5 iTBS. Area 5 TBS does not modulate ICF or SICI. The novel findings from the Master’s thesis suggest area 5 is a cortical loci that influences M1 excitatory circuitry and possibly motor control of the hand.