Non-Invasive Stimulation for Improving Motor Function

Recovery of motor function continues to be a problem following Spinal Cord Injury. Non-invasive brain stimulation techniques, targeting cortical areas, have been shown to enhance the excitability in the human motor cortex, and these changes in the motor cortex may be of significance for the rehabilitation of brain injured patients. However, little is known about the adaptational changes in the excitability/plasticity of spinal neural circuits in spinal cord injury patients. The purpose of this study is to investigate the excitability of cortical and spinal inhibitory and excitatory mechanisms before and following a period of repetitive and synchronized dual peripheral nerve and brain stimulation. Repetitive, paired brain and peripheral nerve stimulation as a neuromodulatory tool, paired associative stimulation (PAS), has been well described. In this technique, stimuli are timed such that afferent and efferent volleys interact at the level of the cortex, that lead to a temporary enhancement of Motor Evoked Potential (MEP) amplitude in target muscles, and when applied repeatedly, lead to a sustained effect, outlasting the intervention period. This repetitive technique has been done in healthy subjects and patients with neurological diseases. By modifying the time between paired stimuli, the investigators will generate afferent/efferent interactions in the spinal cord. The working hypothesis of this study is that the acute facilitation of the H-reflex during Paired TMS and peripheral nerve stimulation, may be harnessed to modulate spinal excitability (sustained increase in the MEP amplitude). That is, the investigators will test if similar to PAS, a change in excitability outlasting the stimulation/intervention period may occur with afferent/efferent interactions, although at the level of the spinal cord rather than the cortex, and be useful to strengthen residual pathways after damage to the spinal cord.