Effect of Transcranial Magnetic Stimulation on Demyelinated Neuron Populations

Transcranial magnetic stimulation (TMS) is non-invasive neuromodulation therapy which uses time-varying magnetic fields to induce electric fields within the patient's brain, thus allowing for neural stimulation of the targeted region. While past studies have used finite-element analysis (FEA) to model the effects of stimulation on brain tissue, there have been limited studies which analyze the effects of the same stimulation on the neuron responses. We use a python package called NEST to model the populations of neurons which are healthy as well as those that have diminished or absent myelin sheath. We model diminished myelin sheath by increasing the capacitance of the neuron. We study the effects of TMS on the synaptic activity of these populations by utilizing clinical parameters specific to TMS. Furthermore, we compare our results to the models of brain tissue stimulation using the FEA software Sim4Life. Our results indicate that all neuron populations, regardless of their myelination state, retain some stimulation threshold which increases discretely as the myelin sheath diminishes. Using tissue analysis, we also computed the range of TMS current necessary to reach these stimulation thresholds for demyelinated populations. Furthermore, we find that the maximum-induced E-field on the cortical surface does not exceed 220 V/m for stimulation of highly demyelinated neuron populations. Therefore our study finds that although demyelinated neurons exhibit much lower nominal synaptic activity than healthy neurons, they are nevertheless responsive to TMS, and these stimulation thresholds can be reached without inducing an unsafe maximum E-field on the cortex.