Fisher, Rebecca Jane; (2002) Inhibition and oscillatory activity in human motor cortex. Doctoral thesis (Ph.D.), University College London (United Kingdom).

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Abstract

Using transcranial magnetic stimulation (TMS) important information can be obtained about the function of motor cortical circuitry during performance of voluntary movements by conscious human subjects. In particular, pairs of TMS pulses can probe inhibitory pathways projecting onto corticospinal neurones, which themselves project to motoneurones innervating hand muscles. This allows investigation of inhibitory circuitry involved in the performance of specific motor tasks, such as the precision grip. Previous studies have shown that pronounced synchronous oscillatory activity within the hand motor system is present at both cortical and muscular level when subjects maintain steady grasp of an object in a precision grip. The origin of this synchronous activity is unknown. However modelling studies have suggested that inhibitory pathways are likely to play an important role in the generation of cortical oscillations, and therefore TMS was used in this Thesis to investigate the origin of synchrony present during the precision grip task. In the first study, parameters of the paired-pulse test used to measure intracortical inhibition were examined. It was found that by modifying the intensities of the stimuli, and the interval between the paired-pulses, different phases of inhibition could be measured. This enabled specific use of TMS to investigate inhibitory pathways. Both single and paired-pulse TMS were then delivered to the motor cortex of subjects performing a precision grip task. It was found that low intensity TMS could reset the phase of muscle oscillatory activity, consistent with corticospinal neurones being part of the circuitry that generates the oscillatory rhythm. When, in the paired-pulse test, a low intensity stimulus was followed a few milliseconds later with a larger TMS stimulus, in the paired-pulse test, strong intracortical inhibition could be measured. This suggested that inhibitory interneurones activated by low intensity TMS could play an important role in the rhythm-generating network. An additional study looked at the importance of cutaneous receptor feedback on synchrony, by studying the effects of local anaesthesia of the index finger and thumb. Whereas low intensity TMS was shown to enhance synchronous activity between muscle pairs, suppression of cutaneous feedback from the digits reduced it. Results in this Thesis suggest that inhibitory interneurones within the motor cortex are important in the generation of synchronous activity within the hand motor system. This synchrony is also under the influence of cutaneous afferent input.

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