Investigation of mood disorders by transcranial magnetic stimulation
TMS utilizes the principle of electromagnetic induction, which was first discovered by Michael Faraday in 1831. It involves the discharge of a large current (peak current: approximately 5000 amps) from a capacitor through a copper-wire coil. A rapid time-varying magnetic field is induced (rise time, approximately 0.05 msec; field strength: approximately 2 T) at the level of the coil. When the coil is held to the head of a subject, the magnetic field pulse penetrates the scalp and skull and induces a small current parallel to the plane of the coil in the adjacent brain. When the induced current is sufficient (several milliamperes per square centimeter), depolarization of neuronal membranes occurs and hence action potentials are generated. When the coil is held tangentially to the scalp, the induced current flows parallel to the surface of the brain surface, thereby preferentially activating interneuronal elements that are oriented horizontally to the surface of the brain [10]. In the case of the hand area within the primary motor cortex, TMS is thought to predominantly activate the pyramidal cells transynaptically through excitatory interneuronal elements [10-13]. This hypothesis was supported by several studies in humans [10,12,13] and monkeys [11] that showed that the difference in latency between electromyographic (EMG) responses or corticospinal volleys evoked by electrical and magnetic stimulation was due to synaptic transmission time in cortical circuits. This became known as the direct (D) and indirect (I) wave hypothesis, based on the pattern of waveforms recorded at the level of the spinal cord. The D wave represents the first volley of the multiple descending volleys in the spinal cord evoked by transcranial stimulation and is believed to be evoked by direct excitation of pyramidal tract neurons. I waves are the subsequent volleys which appear to be generated by indirect excitation of the pyramidal tract neurons via cortical interneurons. Transsynaptic activation of pyramidal cells seems to be the most likely mechanism provided that the stimulation intensity is low and the induced current is in a direction anterior and perpendicular to the central sulcus [14-19].
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