The operative deformation mechanisms which include both dislocation slip and twinning have a significant impact on the mechanical response of hexagonal close-packed (HCP) metals. Twinning plays an important role in accommodating plastic deformation due to the limited number of independent slip systems in HCP metals. The objective of this research is to study the deformation mechanisms associated with twinning in HCP metals (magnesium and zirconium alloys). Heat treatments are often involved in the manufacturing of zirconium alloys. These alloys exhibit a strong thermal anisotropy with a thermal expansion coefficient along the c-axis nearly two times of that along a-direction. Therefore, residual stresses/strains are generated during the heat treatment process which influences the mechanical response (e.g. lattice evolution) under subsequent loading. The elastic viscoplastic self-consistent (EVPSC) model has been improved which includes thermal strain to study the behavior of a Zircaloy-2 slab under moderately large strains. Various self-consistent schemes (SCSs) of the EVPSC model are evaluated in terms of the deformation behavior of the material under different uniaxial strain paths. Numerical results show that the Affine and Meff=0.1 self-consistent models give much better performance for the Zircaloy-2 slab than the Secant and Tangent models. The EVPSC-TDT model has been employed to mimic the twinning and detwinning behavior of extruded Mg alloy ZK60A under monotonic and cyclic loading. The model differentiates between the stress required to initiate twinning and that required to grow (thicken) existing twins. This enables the model to simulate the unusual sharp yielding behavior during twinning as well as the gradual yielding associated with detwinning. It is demonstrated that this model gives a good prediction of the strength anisotropy, strength asymmetry, and strain hardening behavior along different directions, for cases in which the contribution of twinning is large, small and intermediate. For the first time, the lattice strain evolution is well predicted in an extruded magnesium alloy under cyclic loading which involves twinning and detwinning. In all polycrystal models, an empirical equation for the termination of twinning in a grain is required. A new physics-based empirical equation for describing this phenomenon in magnesium alloys has been proposed in this study. It should be noted that the popular empirical equation currently used in all polycrystal models is applied at the grain level, while the new empirical equation is introduced at the twinning system level. The new description is represented by a single parameter while the commonly used empirical equation depends on two parameters. It is demonstrated that the proposed empirical equation is easily calibrated with the single parameter and is able to accurately simulate the experimentally observed rapid hardening associated with twinning exhaustion. / Thesis / Doctor of Philosophy (PhD)

Learning induces functional and structural plasticity. This thesis used a range of neuroimaging approaches in both humans and rodents to address three main questions: (1) Can we predict learning performance using baseline imaging measures? (2) To what extent do performance outcomes or training amount determine experience-dependent plastic changes? (3) What biological mechanisms underlie white matter plasticity detected using MRI? Effects of performance and amount of practice on brain structure were studied by varying the amount of juggling practice. Brain structure was found to predict performance on a complex juggling task before learning acquisition. Both performance and practice were found to affect brain structure after learning. Overall, participants that achieved higher performances had higher grey matter (GM) and WM matter change. Also, participants that trained juggling for longer had higher positive brain changes than participants that practiced less. The effects of juggling performance and practice in functional connectivity and GABA levels as measured by MR spectroscopy (MRS) were also investigated. High intensity training was found to decrease the motor resting-state network strength while lower intensity increased the network strength. The increase in strength was associated with a decrease in GABA concentration. A correlation was also found between motor resting-state strength change and GABA concentration change after learning. Finally, since WM plasticity has not been thoroughly investigated and to understand which cellular events underlie WM change, an animal model of motor learning was combined with diffusion tensor imaging (DTI) and immunohistochemistry. Learning a novel motor task increased WM fractional anisotropy, an indirect measure of WM microstructure, in the contralateral hemisphere to the used paw. Immunohistochemistry staining with myelin basic protein (MBP) antibody of this region revealed higher myelin stain intensity for the learning group that correlated with performance in the task.

published_or_final_version / Mechanical Engineering / Doctoral / Doctor of Philosophy

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Plasticity of synapses is not static across the lifespan. As the brain matures and ages, the ability of neurons to undergo structural and functional change becomes more limited. Further, there are a number of modulatory factors that influence the expression of synaptic plasticity. Here, three approaches were taken to examine and manipulate plasticity in the auditory thalamocortical system of rats. Using an in vivo preparation, long-term potentiation (LTP) and paired pulse (PP) responses were used as measures of long- and short-term plasticity, respectively. First, the effect of intracortical zinc application in the primary auditory cortex (A1) on LTP was examined. Following theta burst stimulation (TBS) of the medial geniculate nucleus (MGN), juvenile and middle-age rats, but not young adults, showed greater levels of LTP with zinc application relative to age-matched control animals. Next, PP responses were examined between rats reared in unaltered acoustic conditions and those reared in continuous white noise (WN) from postnatal day (PD) 5 to PD 50-60 (i.e., subjected to patterned sound deprivation). Rats reared in WN demonstrated less PP depression relative to controls, indicating that WN rearing alters short-term thalamocortical synaptic responses. Furthermore, control males showed no change in PP response following LTP induction, indicating a postsynaptic locus of LTP, whereas increased PP depression following LTP induction was seen in WN animals, suggestive of a presynaptic involvement in LTP. Finally, differences in plasticity between male and female rats were investigated, and the result of early WN exposure on both sexes was examined. Males and females did not show consistent differences in LTP expression; however WN exposure appeared to affect LTP of females less than their male counterparts. PP responses were then compared between WN-reared males and females, and no difference was found. This indicates that short-term plastic properties of auditory thalamocortical synapses between the sexes do not differ, even though plasticity on a longer time scale following sensory deprivation does indicate some difference. Together, the experiments summarized here identify some of the important factors that contribute to the regulation of short- and long-term synaptic plasticity in the central auditory system of the mammalian brain. / Thesis (Master, Neuroscience Studies) -- Queen's University, 2012-10-30 16:01:28.796

Terminally differentiated Schwann cells (SCs), the glial cells in the adult peripheral nerves, display a remarkable plasticity by adopting a de-differentiated phenotype following injury and becoming specialised to repair-type cells for promoting nerve regeneration. Adult SC plasticity is also hijacked by leprosy-causing Mycobacterium leprae during peripheral nerve infection, which make SCs susceptible to reprogramming and generation of progenitor/stem-like cells for bacterial advantage. Interestingly, de-differentiated SCs generated during nerve injury and infection reactivated stem cell transcription factor Sox2, which is essential for maintaining pluripotency in embryonic stem cells (ESCs). In this study we address what role Sox2 plays and how it is involved in adult SC plasticity. We identified that Sox2 binds to a network of gene targets in de-differentiated adult SCs across the mouse genome. This Sox2 target network is distinct from Sox2 target genes in core ESC pluripotency, and appears to be modulated by SC microenvironmental changes and pathological conditions, as nerve crush injury and infection-induced reprogramming expanded Sox2 binding to target genes. In vivo knockdown by shRNA of Sox2 in wild type adult nerves demonstrated reduction in SC de-differentiation. Mutant mice defective in natural nerve degeneration, de-differentiation and regeneration (Wallerian degeneration slow mice; Wlds) not only show impaired Sox2 binding to its target genes but also a delay in Sox2 and target gene expression after nerve crush injury. Together, these in vivo data reveal an impact of Sox2 and its target network on SC plasticity. Furthermore, altered expression of many of these target genes after Sox2 knockdown in wild type adult Schwann cells in vitro and in vivo as well as in injured Wlds nerves suggests a functional role of a Sox2 target network in nerve injury-repair processes. This includes Sox2 target genes such as Megf10, Btc, Atf3 and Nestin. By acting on these genes Sox2 may coordinate relevant gene functions ranging from phagocytosis/clearance, proliferation, transcription and cytoskeletal dynamics. Thus, this study proposes a novel concept of how reactivation of an embryonic stem cell regulator like Sox2 in adult tissues coordinates a gene network regulating Schwann cell plasticity and multiple biological functions facilitating the nerve injury-repair process. These findings may aid in developing strategies towards promoting nerve regeneration, or designing treatments for neuropathies in which deregulation of Schwann cell de-differentiation contributes to pathogenesis.