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The Journal of Neuroscience, March 1, 1999, 19(5):1855-1862

Spinal Cord-Evoked Potentials and Muscle Responses Evoked by Transcranial Magnetic Stimulation in 10 Awake Human Subjects

David A. Houlden¹, Michael L. Schwartz¹, Charles H. Tator², Peter Ashby³, and William A. MacKay⁴

¹ Division of Neurosurgery, Sunnybrook Health Science Centre, Toronto, Ontario, Canada, ² Division of Neurosurgery and ³ Division of Neurology, Toronto Hospital, Western Division, Toronto, Ontario, Canada, and ⁴ Department of Physiology, University of Toronto, Toronto, Ontario, Canada

Transcranial magnetic stimulation (TCMS) causes leg muscle contractions, but the neural structures in the brain that are activated by TCMS and their relationship to these leg muscle responses are not clearly understood. To elucidate this, we concomitantly recorded leg muscle responses and thoracic spinal cord-evoked potentials (SCEPs) after TCMS for the first time in 10 awake, neurologically intact human subjects. In this report we provide evidence of direct and indirect activation of corticospinal neurons after TCMS. In three subjects, SCEP threshold (T) stimulus intensities recruited both the D wave (direct activation of corticospinal neurons) and the first I wave (I₁, indirect activation of corticospinal neurons). In one subject, the D, I₁, and I₂ waves were recruited simultaneously, and in another subject, the I₁ and I₂ waves were recruited simultaneously. In the remaining five subjects, only the I₁ wave was recruited first. More waves were recruited as the stimulus intensity increased. The presence of D and I waves in all subjects at low stimulus intensities verified that TCMS directly and indirectly activated corticospinal neurons supplying the lower extremities. Leg muscle responses were usually contingent on the SCEP containing at least four waves (D, I₁, I₂, and I₃).

Key words: magnetic stimulation; motor-evoked potentials; spinal cord-evoked potentials; corticospinal tract; anesthesia; motor cortex (human)

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Copyright © 1999 Society for Neuroscience  0270-6474/99/1951855-08$05.00/0

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