Thesis (M. Res. (Med.))--University of Hong Kong, 2005. / Also available in print.

Thesis (Ph. D.)--University of Washington, 1999. / Vita. Includes bibliographical references (leaves 127-130).

Thesis (Ph. D.)--University of Queensland, 2003. / Includes bibliographical references.

Thesis (M. Res. (Med.))--University of Hong Kong, 2005.

Thesis (Ph. D.)--University of Hong Kong, 2010. / Includes bibliographical references (p. 159-190). Also available in print.

In deeply anesthetized mammals, as typified by the adult cat, there is limited evidence that the firing-rate response of spinal motor neurons to sustained simulation usually features a progressive reduction in firing rate, termed late adaptation, that begins 1-2 s after the onset of sustained stimulation. The fullest description of late adaptation has been provided by Kernell & Monster (1982a,b) who evoked repetitive firing in spiral motor neurons of deeply anesthetized cats by the conventional procedure of an intracellular injection of a sustained depolarizing current. The main purpose of the present study was to extend on the results of their work. The first hypothesis tested was: Sustained depolarizing extracellular stimulation of motor neurons is more effective in maintaining repetitive discharge than sustained depolarizing intracellular stimulation. Investigations pioneered by Kernell & Monster (1982a,b) tested the association between late adaptation and other type (size)-related properties of motor neurons. Such analyses are within the rubric of Henneman's (1957, 1977) Size Principle, one component of which proposes that the properties of motor neurons and the muscle fibers they innervate are tightly coupled. The second hypothesis was proposed to continue this inquiry. It stated that: Late adaptation (during both sustained and intermittent stimulation), and other discharge-related properties of motor neurons are associated with other type (size)-related properties of these cells and their motor units. For both hypotheses, there was an emphasis on providing a quantitative description of late-adaptation. In the present study, the duration of repetitive firing in response to sustained stimulation significantly exceeded that in the Kernell & Monster (1982a,b) study, thereby providing evidence in support of the first hypothesis. For sustained stimulation, significant associations were found between the time constant of late adaptation and three neuromechanical properties of the cell's motor unit: axonal conduction velocity; twitch contraction time; and, peak tetanic force. Similarly, significant associations were found between the peak firing rate and these neuromechanical properties for both sustained and intermittent stimulation. Significant associations were also found between the extent of between-train adaptation during intermittent stimulation and two of the neuromechanical properties: axonal conduction velocity and peak tetanic force. These results provided evidence in support of the second hypothesis. In summary, the present work has provided a new opening in the study of the active (firing) properties of motor neurons, by quantitating late adaptation during sustained stimulation, and between-train adaptation during intermittent stimulation. This information provides new insights into the fundamental properties of motor neurons and adds important new firing-rate parameters to the continuing evaluation of Henneman's Size Principle.

Direct cortico-motoneuronal (CM) connections of corticospinal tract neurons are a distinctive feature of the primate motor system which are known to be important for the capacity to perform independent finger movements. However, it is still unclear how the appropriate combinations of CM cells are recruited to produce the selective (fractionated) control over muscles of the upper limb that is necessary for independent finger movements. I have investigated whether GABAergic intracortical inhibitory (ICI) circuits in human motor cortex contribute to the selection of the appropriate CM cells during a motor task requiring selective activation of one of several intrinsic hand muscles. Behaviour of ICI circuits during voluntary contraction was compared for the dominant and non-dominant hemisphere of right-handed subjects, as hemispheric differences in ICI may contribute to preferential use of the right hand for fine motor tasks. Finally, I investigated the range of forces over which ICI contributes to selective activation of a hand muscle. Neurologically normal adult human subjects were recruited for all experiments. Surface electrodes recorded electromyographic activity of abductor pollicis brevis (APB), first dorsal interosseous and abductor digiti minimi muscles during controlled isometric contractions of APB at different force levels while subjects attempted to keep the other two muscles relaxed using visual feedback of EMG. Paired-pulse transcranial magnetic stimulation (TMS) was used to assess ICI at rest and during selective activation of a hand muscle. TMS intensity and interstimulus interval were varied in different trials. Data were compared for two different directions of induced current in the brain; posteriorly directed current (PA stimulation) and anteriorly directed current (AP stimulation). ICI is suppressed for corticospinal neurons controlling the muscle targeted for selective activation; no change in ICI was seen for corticospinal neurons controlling the muscles required to be relaxed. This indicates that differential modulation of ICI in human motor cortex contributes to selective activation of a hand muscle. The direction of current flow induced in the brain proved to be critical for demonstrating this effect. It was observed with AP stimulation but not PA stimulation. I argue that this is due to preferential activation by PA stimulation of interneurons producing I1 waves in corticospinal neurons. These interneurons are not acted upon by ICI circuits. This problem makes the conventional PA paired-pulse TMS technique unreliable for the assessment of ICI during voluntary contraction. With AP stimulation it was demonstrated that ICI is not modulated during weak selective activation of a hand muscle (&lt5percent of maximal voluntary contraction), but ICI effects on CM cells controlling the target muscle are progressively suppressed at higher levels of activation. The present study is the first to examine hemispheric differences in ICI during selective isometric contraction of an intrinsic hand muscle. No hemispheric differences were observed. These studies have demonstrated a functional role for ICI in fractionation of hand muscle activity in normal subjects. It also provides an improved basis for investigating the changes in ICI with TMS in various neurological conditions in which it has been reported that GABAergic inhibition is abnormal. / Thesis (Ph.D.)--School of Molecular and Biochemical Science, 2004.

Thesis (M. Sc.)--York University, 2002. Graduate Programme in Kinesiology and Health Science. / Typescript. Includes bibliographical references. Also available on the Internet. MODE OF ACCESS via web browser by entering the following URL: http://wwwlib.umi.com/cr/yorku/fullcit?pMQ71630.

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Thesis (Ph. D. in Neuroscience)--Vanderbilt University, May 2009. / Title from title screen. Includes bibliographical references.