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William M. Sauvé, MD, is Medical Director, TMS NeuroHealth Centers, Richmond. Lawrence J. Crowther, MEng, is Graduate Research Assistant, Department of Electrical and Computer Engineering, Iowa State University.
Disclosure: Dr. Sauvé is employed by TMS NeuroHealth Centers. Mr. Crowther has no relevant financial relationships to disclose.
Since the days of Pliny, humans have been curious about the effects of electrical stimulation on the body, and since Michael Faraday famously developed the concept of electromagnetic induction, it has been understood that changing magnetic fields can cause current to flow in conductive material,1 including the brain. Although the ability to stimulate the brain using magnetic pulses was established by Jacques dâ[euro](TM)Arsonval in 1896,2 this phenomenon has only recently been used as a therapeutic modality in psychiatry. Known as repetitive transcranial magnetic stimulation (rTMS), it was approved for the treatment of major depressive disorder by the U.S. Food and Drug Administration in 2008. What follows is a very basic overview of the underlying physics of TMS, how it impacts the neurons in the brain, some of the methods of application, and the various stimulation devices in use.
<bold>Faradayâ[euro](TM)s Law of Electromagnetic Induction</bold>
Both Michael Faraday and Joseph Henry independently discovered the concept of electromagnetic induction in 1831, but Faraday was the first to publish his findings. Simply put, a magnetic field that is in motion relative to a conductor brings about a current in said conductor. Hence, a changing magnetic field induces a flow of electric current in nearby conductors that, for the purposes of this article, include human tissue.3 The most commonly used form of expression for this concept is the Maxwell-Faraday equation, also referred to as Faradayâ[euro](TM)s Law.
Electromagnetic induction is the key principle in transcranial magnetic stimulation (TMS), taking advantage of the fact that every electric current has a magnetic field surrounding it, with alternating currents bringing about fluctuating magnetic fields. Fluctuating magnetic fields in turn cause electric current to flow in conductors placed within them; the conductors in the case of TMS being neurons in the brain, thus allowing for electrical stimulation of neurons within the brain in a non-invasive fashion.
<bold>Direct Neuronal Effects of TMS</bold>
It has been demonstrated that a magnetic field pulsed adjacent to a volume conductor (such as the brain)...