Mechanisms of peripheral nervous system stimulation using the magnetic coil.

We measured the distribution of the electric fields induced within isotropic and anisotropic volume conductors by figure '8' and round magnetic coils (MCs). For various MC orientations maximal electric fields were induced parallel to the inner surface of the volume conductor. Predictions from physical models roughly approximating human forearm and vertebral column were then tested by MC stimulation of distal median nerve at the wrist and nerve roots at the cervical spine, respectively. Predictions that were confirmed included: (1) Peripheral nerve is most easily excited by a tangential-edge round MC in which current is induced parallel to the long axis of the nerve. As a corollary, peripheral nerve is least excited when the round MC is geometrically bisected by the long axis of the nerve. (2) Peripheral nerve excitation most likely occurs at locations corresponding to the first spatial derivative of the electric field; i.e. where the electrical field is changing most rapidly over distance. For the figure '8' MC, this occurs distally when the anterior divergence of the junction is directed distally. (3) Nerve roots are excited at a relatively constant latency, low threshold location. This site most likely corresponds to the vicinity of the neuroforamina where the induced electric field is most intense and the first spatial derivatives peak. By contrast, the low value of the first derivative in the longitudinal axis of the vertebral canal implies that long tracts in the spinal cord are not likely to be excited using commercially available MCs. Our study also indicates that intracranial stimulation of facial nerve occurs at a selected low threshold location, probably reflecting focusing of induced current.