Biophysics of magnetic stimulation

Abstract Magnetic stimulators work by inducing brief currents in the brain which excite nerve cells. Electrical energy stored in a capacitor is released into a coil held over the scalp. A brief surge of current flows through the coil to produce a transient magnetic field. This field passes freely into the surrounding medium and induces in it an electric field which impedes the magnetic field. This is Faraday’s law. If the electric field falls in a conductor, then current will flow. The ability of this current to excite nerve cells depends upon its time course, magnitude and direction. The basic magnetic stimulator has a simple design (Barker 1991). A capacitor which can hold a high voltage is charged up and the stored energy is released through a switch to cause current to flow through a coil (Fig. 2.1). The rate at which the power supply can charge the capacitor is limited and sets the maximal repetition rate which the stimulator can achieve. The switch clearly must be able to sustain the large current surges involved.