Electromagnetic mechanism of magnetic nerve stimulation

Rabbit sciatic nerves were stimulated by pulsed magnetic fields and nerve responses were analyzed on the basis of electromagnetic theory to understand the dominant factors in magnetic stimulation. Some conducting substance surrounding the nerve is required to induce the currents exciting it. The strength of a magnetic stimulus is evaluated by the rate of change in the vector potential at the nerve, dA/dt, which equals the magnitude of the induced electric field E. The minimum strength of dA/dt for exciting the nerve is 18 Wb/ms (18 V/m) in the agar with a conductivity of 0.6 S/m. The induced current density of 10 A/m2 is comparable to that used in the electric stimulation of peripheral nerves. The component of the vector potential parallel to the nerve is more effective in stimulating the nerve than the component perpendicular to it.