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Static, Low-Frequency, and Pulsed Magnetic Fields in Biological Systems
Published in James C. Lin, Electromagnetic Fields in Biological Systems, 2016
The spatial extent of the effects of TMS on neural tissue is only coarsely understood. One key problem is the realistic calculation of the electric field induced in the brain, which proves difficult due to the complex gyral folding pattern that results in an inhomogeneous conductivity distribution within the skull. Thielscher, Opitz, and Windhoff (2011) estimated the electric field induced in the brain using the finite element method (FEM) together with a high-resolution volume mesh of the human head to better characterize the field induced in cortical gray matter (GM). The volume mesh was constructed from T1-weighted structural MRI to ensure an anatomically accurate modeling of the gyrification pattern. Five tissue types were taken into account, corresponding to skin, skull, cerebrospinal fluid (CSF) including the ventricles, cortical GM, and cortical white matter. The authors characterized the effect of current direction on the electric field distribution in GM. The field strength in GM was increased by up to 51% when the induced currents were perpendicular to the local gyrus orientation. This effect was mainly restricted to gyral crowns and lips, and did not extend into the sulcal walls. As a result, the focality of the fields induced in GM was increased. The authors speculated that this enhancement effect might in part explain the dependency of stimulation thresholds on coil orientation, which is commonly observed in TMS motor cortex studies. In contrast to the clear-cut effects of the gyrification pattern on induced field strength, current directions were predominantly influenced by the CSF–skull boundary.
Advanced 4D-bioprinting technologies for brain tissue modeling and study
Published in International Journal of Smart and Nano Materials, 2019
Timothy J. Esworthy, Shida Miao, Se-Jun Lee, Xuan Zhou, Haitao Cui, Yi Y. Zuo, Lijie Grace Zhang
Due to the complex and highly interconnected nature of the brain’s tissues, its physiology and development are exceedingly difficult to model both computationally and in vitro [45]. Arguably, one of the most challenging facets of neurodevelopment to simulate is the mechanical folding of the cortical tissues. Namely, there are two cortical regions of the brain; the cortex of the cerebrum, known as the cerebral cortex, and the cortex of the cerebellum, known as the cerebellar cortex. The process of cortical folding of the cerebral cortex is known as ‘gyrification’, whereas the analogous process of cortical folding in the cerebellum is known simply as ‘foliation’. The actual processes which guide the folding patterns of these two cortical tissues are different in some respects, but share many biomechanical similarities. In order to better discuss the processes of the cortical folding across the cerebrum and the cerebellum, it is useful to first consider a few key aspects of the brain’s gross anatomy as well as fundamental cytoarchitectural elements involved in neurodevelopment.