ABSTRACT
The effects of electrical stimulation on nerve, muscle, and other structures have been studied for over 200 years, starting with Galvani’s frog experiments. Faraday allegedly experimented with the use of rapidly changing magnetic fields to induce a flow of current in tissue (1), and in recent years, there has been a particular interest in magnetic field stimulation to evaluate brain dysfunction. Devices generally use aircore simulators to induce cranial stimulation for diagnostic purposes (2). The use of specially fabricated magnetizable cores can significantly improve the efficiency of this procedure because less power is required, there is more flexibility in frequency selection, and a more precise focus can be directed to the targeted area. This proprietary enhanced containment structure allows us to stimulate muscle groups at frequencies up to nearly 50 Hz in a stimulus cycle having a period of 2 to 10 s. By using multiple cores, a smaller section area or muscle group can be specifically stimulated thus reducing effects on adjacent tissues. It is generally held that magnetic fields cannot have biological effects unless there is motion of or within the field. The static magnetic fields emanating from the poles of permanent magnets are motionless. By contrast, time-varying magnetic fields induce an electrical field whose magnitude is proportional to its rate of change. These induced electrical fields circulate around magnetic field lines. Unlike the fields produced by functional electrical stimulation procedures, they require no electrodes and have no discretely defined input and exit points. Nevertheless, they perform in an identical fashion with respect to transmitting a charge through a neuron boundary, thus increasing the potential inside that boundary. When that increase in potential exceeds the threshold necessary for an action potential, the nerve fires. Unlike heat or x-rays, magnetic fields are unaffected by bone or body tissues, which is one of the advantages of magnetic resonance imaging as a diagnostic aid, particularly with respect to intracranial pathology. Pulsed magnetic field stimulation therapy provides similar advantages because its ability to freely penetrate bone and other structures make it a convenient and safe way to penetrate internal sites.
