Analysis of magnetic stimulation of a concentric axon in a nerve bundle.

In this paper, we present an analysis of magnetic stimulation of an axon located at the center of a nerve bundle. A three-dimensional axisymmetric volume conductor model is used to determine the transmembrane potential response along an axon due to induced electric fields produced by a toroidal coil. We evaluate four such models of an axon located in: 1) an isotropic nerve bundle with no perineurium, 2) an anisotropic nerve bundle without a perineurium, 3) an isotropic nerve bundle surrounded by a perineurium, and 4) an anisotropic nerve bundle surrounded by a perineurium. The transmembrane polarization computed along an axon for the above four models is compared to that for an axon located in an infinite homogeneous medium. These calculations indicate that a nerve bundle with no sheath has little effect on the transmembrane potential. However, the presence of a perinerium around the nerve bundle and anisotropy in the bundle significantly affects the shape of the transmembrane response. Therefore, during magnetic stimulation, nerve bundle anisotropy and the presence of perineurium must be taken into account for calculation of stimulus intensities for threshold excitation.