Modulating Motor Cortex Plasticity via Cortical and Peripheral Somatosensory Stimulation

The interaction between the motor and somatosensory systems is essential for effective motor control, with evidence indicating that somatosensory stimulation influences the excitability of the primary motor cortex (M1). However, the mechanisms by which repetitive stimulation of both cortical and peripheral somatosensory systems affects M1 plasticity are not well understood. To investigate this, we examined the effects of continuous theta-burst stimulation (cTBS) applied to the primary somatosensory cortex (S1) and transcutaneous electrical nerve stimulation (TENS) of the median nerve on various measures of corticospinal excitability and M1 intracortical circuits. Specifically, we assessed motor-evoked potentials (MEPs), short-latency intracortical inhibition (SICI), intracortical facilitation (ICF), and short-latency afferent inhibition (SAI) before and after administering cTBS and TENS. Our results demonstrated that cTBS increased MEPs for at least 50 minutes, whereas TENS increased MEPs for 10 minutes. Neither cTBS nor TENS had an impact on SICI and ICF. However, cTBS decreased SAI, while TENS did not affect SAI. The sham procedures for both cTBS and TENS did not produce significant changes in MEPs, SICI, ICF, or SAI. These findings suggest that both cortical and peripheral somatosensory stimulation modulate corticospinal excitability, with the effects of cortical stimulation being more prolonged. Neither type of stimulation influences inhibitory and excitatory intracortical neural circuitry within M1. Notably, cortical somatosensory stimulation modulates the interaction between M1 and S1, whereas peripheral somatosensory stimulation does not. This study elucidates distinct mechanisms through which cortical and peripheral somatosensory stimulation influence M1 plasticity.