![]() ![]() These conclusions are not based on actual D and I wave recordings but rather on the effect that D and I waves may have on individual motoneurons. These studies have estimated that spinal motoneurons supplying hand muscles receive only I waves ( Day et al., 1989), whereas spinal motoneurons supplying the tibialis anterior muscle (TA) receive D and I waves ( Priori et al., 1993 Awiszus and Feistner, 1994). The effect of D and I waves on muscle responses evoked by TCMS has been estimated in awake humans using peristimulus time histograms (PSTHs) of motor unit firing ( Day et al., 1989 Priori et al., 1993 Awiszus and Feistner, 1994). If the spinal motoneuron potential is already near threshold, then it is more likely that an earlier wave in the SCEP will cause it to fire ( Day et al., 1987 Mills, 1991). The D and I waves are thought to descend in the corticospinal tracts and generate a sequence of EPSPs at the spinal motoneurons causing them to fire by temporal summation ( Mills, 1991 Taylor et al., 1993). ![]() The D wave is thought to result from direct activation of corticospinal neurons (probably at the initial segment or at the first bend of the axon), and the I waves are thought to result from indirect activation of corticospinal neurons via interneurons excited by the stimulus ( Patton and Amassian, 1954 Amassian et al., 1990, 1992 Edgley et al., 1990, 1992 Berardelli et al., 1991). Transcranial magnetic stimulation (TCMS) in humans and subhuman primates evokes a descending spinal cord-evoked potential (SCEP) that contains a direct (D) wave followed by several indirect (I) waves ( Amassian et al., 1990 Edgley et al., 1990 Thompson et al., 1991 Burke et al., 1993 Kitigawa and Moller, 1994 Houlden et al., 1996 Kaneko et al., 1996 Nakamura et al., 1996). Leg muscle responses were usually contingent on the SCEP containing at least four waves (D, I 1, I 2, and I 3). 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. More waves were recruited as the stimulus intensity increased. In the remaining five subjects, only the I 1 wave was recruited first. ![]() In one subject, the D, I 1, and I 2 waves were recruited simultaneously, and in another subject, the I 1 and I 2 waves were recruited simultaneously. In three subjects, SCEP threshold (T) stimulus intensities recruited both the D wave (direct activation of corticospinal neurons) and the first I wave (I 1, indirect activation of corticospinal neurons). In this report we provide evidence of direct and indirect activation of corticospinal neurons after TCMS. 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. 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. ![]()
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