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Estimulação magnética transcraniana:estudo da localização das populações celulares estimuladas num modelo heterogéneo realista do cortéx cerebral
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Nesta tese de doutoramento são apresentadas simulações numéricas da distribuição do campoeléctrico induzido por Estimulação Magnética Transcraniana (TMS) no córtex cerebral e tecidosadjacentes. Este trabalho visa melhorar os cálculos já existentes na literatura e proporestimativas da localização das populações celulares estimuladas.A localização das células estimuladas foi estimada sob duas abordagens. Na primeira, o campoeléctrico induzido foi usado para calcular os mecanismos de estimulação associados asegmentos neuronais rectos e longos e a terminações e dobras axonais. A amplitude dosmecanismos e o limiar de estimulação celular foram estimados considerando ainda os diâmetrosdos axónios e a duração do estímulo. Os resultados sugerem que durante a estimulação docórtex motor com impulso monofásico PA e em condições de limiar motor, poderá ocorrer orecrutamento de células de diâmetro médio em toda a coroa da circunvolução (interneurónios) eno lábio da circunvolução (interneurónios e células piramidais), enquanto que as célulaspiramidais de Betz poderão ser recrutadas ao longo de quase toda a profundidade do sulco. Osresultados sugerem ainda a importância das heterogeneidades para o recrutamento de algumascélulas.Na segunda abordagem ao problema, foi estudada a resposta de modelos neuronais ao campoeléctrico calculado no primeiro trabalho. À distribuição espacial do campo foram adicionadasvariações temporais de impulsos monofásicos (PA e AP) e bifásicos (AP-PA e PA-AP). Osresultados aqui obtidos reforçam a importância das dobras e das terminações axonais naestimulação cortical. No que concerne o recrutamento de células piramidais, as estimativasrespeitantes à estimulação por impulso PA estão em maior conformidade com os resultados daliteratura do que as obtidas com a primeira abordagem ao problema.Apesar das limitações, este trabalho de modelação é uma metodologia útil na compreensão dosmecanismos de estimulação celular e na determinação das regiões corticais estimuladas emTMS.
In this doctorate thesis I present numerical simulations of the distribution of the electric fieldinduced during Transcranial Magnetic Stimulation (TMS) of the cerebral cortex and adjacenttissues. This work aims to improve previous calculations and propose estimates of thelocalization of the cellular populations stimulated.The localization of the stimulated cells was estimated using two approaches. In the first one, theinduced electric field was used to calculate the stimulation mechanisms related to straight andlong neuronal segments, axonal terminations and axonal bends. The magnitude of themechanisms and the stimulation threshold were estimated considering axonal diameters and theduration of the stimulus. Results suggest that during monophasic PA stimulation of the motorcortex, and under motor threshold conditions, recruitment of medium sized cells may occuralong the whole crown of the gyrus (interneurons) and in the lip of the sulcus (interneurons andpyramidal cells), while recruitment of Betz cells may occur approximately along the wholedepth of the sulcus. Results suggest also that heterogeneities are important for the recruitment ofsome cells.In the second approach to the problem, the response of neuronal models to the electric fieldcalculated in the first simulations was studied. Time variations corresponding to monophasic(AP and PA) and biphasic (AP-PA and PA-AP) stimulus where added to the spatial distributionof the electric field. The results reinforce the importance of bends and axonal terminations incortical stimulation under TMS. Concerning the recruitment of pyramidal cells, estimatesrelated to PA monophasic stimulation are in better agreement with experimental results reportedin the literature than the estimates obtained under the first approach to the problem.In spite of the limitations, this modelling work is a useful methodology for understanding themechanisms of cellular stimulation and for the assessment of the cortical regions stimulatedduring TMS.
In this doctorate thesis I present numerical simulations of the distribution of the electric fieldinduced during Transcranial Magnetic Stimulation (TMS) of the cerebral cortex and adjacenttissues. This work aims to improve previous calculations and propose estimates of thelocalization of the cellular populations stimulated.The localization of the stimulated cells was estimated using two approaches. In the first one, theinduced electric field was used to calculate the stimulation mechanisms related to straight andlong neuronal segments, axonal terminations and axonal bends. The magnitude of themechanisms and the stimulation threshold were estimated considering axonal diameters and theduration of the stimulus. Results suggest that during monophasic PA stimulation of the motorcortex, and under motor threshold conditions, recruitment of medium sized cells may occuralong the whole crown of the gyrus (interneurons) and in the lip of the sulcus (interneurons andpyramidal cells), while recruitment of Betz cells may occur approximately along the wholedepth of the sulcus. Results suggest also that heterogeneities are important for the recruitment ofsome cells.In the second approach to the problem, the response of neuronal models to the electric fieldcalculated in the first simulations was studied. Time variations corresponding to monophasic(AP and PA) and biphasic (AP-PA and PA-AP) stimulus where added to the spatial distributionof the electric field. The results reinforce the importance of bends and axonal terminations incortical stimulation under TMS. Concerning the recruitment of pyramidal cells, estimatesrelated to PA monophasic stimulation are in better agreement with experimental results reportedin the literature than the estimates obtained under the first approach to the problem.In spite of the limitations, this modelling work is a useful methodology for understanding themechanisms of cellular stimulation and for the assessment of the cortical regions stimulatedduring TMS.
Descrição
Tese de doutoramento, (Engenharia Biomédica e Biofísica), 2010, Universidade de Lisboa, Faculdade de Ciências
Palavras-chave
Campo electromagnético Modelacao numérica Estimulação magnética transcraniana Estimulação neuronal Córtex cerebral Engenharia biomédica Teses de doutoramento