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Abstract
Neural plasticity can be induced in humans using non-invasive stimulation techniques. The aim of this thesis was to further characterise non-invasive stimulation techniques for the induction of plasticity in the motor pathways at a spinal level in humans. One technique shown to induce plasticity at a spinal level is paired corticospinal-motoneuronal stimulation (PCMS), which involves the repeated pairing of stimuli to both corticospinal axons and motor axons to elicit modifications at corticospinal-motoneuronal synapses. The first study examined whether PCMS-induced facilitation was enhanced when more stimulus pairs were used, with results showing that 100 pairs produced better, more reliable facilitation than 50 pairs. The second study looked at the mechanisms involved in PCMS-induced facilitation, revealing a dependence on N-methyl-D-aspartate receptors (NMDARs). Transcutaneous spinal direct current stimulation (tsDCS), which consists of the delivery of weak, non-invasive currents through surface electrodes placed over the spine, is another technique that induces plasticity at a spinal level, although its effect on the descending motor pathway is unclear. Hence, the third study involved the use of tsDCS applied to the cervical spinal cord, to determine whether motor pathway plasticity could be induced. No effect of cervical tsDCS on motor pathways supplying the upper limb was found. As this study looked for changes after a period of tsDCS, the fourth and final study looked at whether any acute changes occurred during stimulation. This study revealed bidirectional modifications of muscle responses that were found to be caused by a non-physiological interaction of two electrical stimuli in close proximity, rather than being a real, physiological effect. These studies suggest that PCMS is a more promising technique than cervical tsDCS for inducing motor pathway plasticity at a cervical spinal level in humans. PCMS, and possibly cervical tsDCS if an effective protocol were to be found, may have therapeutic potential for enhancing descending drive to upper limb muscles by improving corticospinal transmission at a spinal level. This may have clinical relevance for those with reduced neural drive to upper limb muscles, as is the case in incomplete spinal cord injury, stroke and other disorders of the motor pathway.