Modulation of Corticospinal Output to the Hand via Somatosensory Afferent Input

The primary somatosensory cortex (SI) and primary motor cortex (M1) receive somatosensory afferent input to influence motor hand circuitry and control. Much of the literature has investigated this relationship extensively using animal models. In contrast, much of these relationships and neural mechanisms are still not well understood in humans. The present work investigated homosynaptic and hetersynaptic protocol's modulatory effects on SI and M1 sensorimotor circuitry. Experiment 1 used the homosynaptic protocol continuous theta-burst stimulation (cTBS) over SI and M1 and measured motor evoked potentials (MEP) and short-latency afferent inhibition (SAI). CTBS over M1 suppressed MEPs and did not alter SAI. In contrast, cTBS over SI facilitated MEPs and decreased median and digital nerve evoked SAI. Experiment 2 used the heterosynaptic protocol rapid-rate paired associative stimulation (rPAS) on SI and M1. SAI and MEPs were measured to investigate the sensorimotor changes following rPAS. Results indicated minimal decreases in SAI but increases in MEPs following SI rPAS. However, M1 rPAS lead to significant reductions in SAI and increased MEPs. The findings from this thesis highlight the selective modulation of sensorimotor circuitry through the use of various stimulation protocols. iv ACKNOWLEDGEMENTS