Physiological Basis of Transcranial Magnetic Stimulation

Abstract

At present, transcranial magnetic stimulation (TMS) is the most widely used technique that allows us lo examine transmission in the corticospinal pathway, primary motor cortex, and cortical areas projecting to primary motor cortex noninvasively and with minimal discomfort in human subjects. A suprathreshold TMS stimulus results in multiple descending waves as recorded from epidural electrodes placed over the spinal cord. A short-latency direct wave (D wave) is followed by several longer latency indirect waves (1 waves). The D wave is thought to result from direct depolarization of the initial axon segment of corticospinal neurons and is most effectively activated in human subjects by using high-intensity TMS or by using transcranial electrical stimulation. I waves are thought to be generated through the depolarization of an axon synapsing directly onto a corticospinal neuron (i.e., monosynaptically, 11), and the following 1 waves (12 and later) may require local polysynaptic circuits. The use of different coil designs has expanded the range of TMS protocols, giving flexibility to administer more focal stimulation paradigms targeting different intracortical circuits (1 waves). Furthermore, multiple pulse protocols have been used to examine the role of TMS as an adjunct therapy for individuals with various psychological and motor disorders. The results from these studies are still inconclusive, and the current data suggest that rTMS after effects are complex, and there is a need of a more indepth characterization of the stimulation parameters used in individual patients. The development of tailored TMS and rTMS protocols in humans with and without motor disorders may represent an avenue to further improve the therapeutic use of this widely used neurophysiological tool in human subjects. (PsycInfo Database Record (c) 2023 APA, all rights reserved)