Effects of stimulus parameters and tissue inhomogeneity on nerve excitation processes in magnetic stimulation of the brain: A simulation study

In this study, we used a computer simulation to investigate the nerve excitation processes of the nerve axon in an inhomogeneous volume conductor in magnetic stimulation. We assumed that the nerve axon was located in an inhomogeneous conducting medium with two regions having different conductivities that simulate different tissue types. The distribution of induced electric fields was calculated with the finite element method. The nerve fiber was modeled after equivalent electrical circuits having active nodes of Ranvier. We observed the excitation threshold when the coil current waveforms and direction are changed with varying the electrical properties of the tissue. The simulation results show that the threshold is lower when biphasic waveforms are used and that the optimal current direction depends on tissue conductivity. The results also suggest that the nerve is excited even when the coil current flow is perpendicular to the axon in inhomogeneous conducting media. The results in this study give useful information to explain the experimental results in magnetic stimulation of the brain.