Effect of stimulus intensity and voluntary contraction on corticospinal potentials following transcranial magnetic stimulation.

Following magnetic transcranial stimulation, motor-evoked potentials (MEPs) from the abductor digiti minimi muscle, and evoked spinal cord potentials (ESCPs) from the cervical epidural space were recorded simultaneously in 9 subjects in the awake and anesthetized condition. In the awake condition, during voluntary contraction, one (n = 5) or two (n = 4) components of the ESCPs were elicited at the threshold stimulus intensity of the MEPs. As the stimulus intensity increased, an early response (n = 7) and multiple late components were recorded. The first component at high stimulus output (average 80%) preceded the small potentials elicited at threshold stimulus intensity. The latency of each component of the ESCPs during voluntary contraction was the same as that during the resting condition. In addition, the enhancement of amplitude of the ESCPs during voluntary contraction was not significant compared with that recorded at rest. During general anesthesia with volatile anesthetics, the first component of the ESCPs could be elicited at high stimulus intensity, but later components were markedly attenuated. In paired transcranial magnetic stimulation, the amplitude of this first potential following the test stimulus completely recovered within the 2 ms interstimulus interval. From these results, we hypothesized that the first component was generated non-synaptically (D-wave), but later components were generated transsynaptically (I-waves). Compound muscle action potentials (CMAPs) and F-waves also were recorded following supramaximal ulnar nerve stimulation at the wrist. Peripheral conduction time, which included synaptic delay in spinal motor neurons, was measured as follows (latency of CMAPs+ latency of F-wave + 1)/2 (ms). The central motor conduction time (CMCT) was measured by subtracting the peripheral conduction time from the onset latency of the MEP at high stimulus intensity in the awake state. During voluntary contraction, the calculated CMCT (4.9 +/- 1.0 ms) was the same as the onset latency of the second component of the ESCPs (I-wave, 4.3 +/- 0.2 ms) recorded from the C6-C6/7 epidural space. These results suggest that transcranial magnetic stimulation generates I-waves preferentially when the stimulus intensity was set at just the threshold level of the MEPs during voluntary contraction in the awake condition. At high stimulus intensity, transcranial magnetic stimulation can elicit both D- and I-waves, but most spinal cells require I-wave activation to fire. Facilitatory effects of voluntary contraction on the muscle response following transcranial magnetic stimulation mainly originates at a spinal level.