title: The suppression of the long-latency stretch reflex in the human tibialis anterior muscle by transcranial magnetic stimulation.
abstract: The purpose of this study was to investigate the transcortical nature of the long-latency stretch reflex (M3) in the human tibialis anterior muscle. This was achieved by applying a single pulse of subthreshold (90% motor threshold) transcortical magnetic stimulation (subTMS) at the site of the motor cortex. Such a stimulus is able to activate intracortical inhibitory circuits and thereby depress motor cortical output. We hypothesized that it would also suppress a transcortical reflex loop. The stretch reflex was elicited using a pedal attached to an electric motor. SubTMS was applied at several intervals prior to M3. Recordings were repeated 20-40 times. The reflex components were quantified using 20-ms windows in the averaged rectified electromyogram (EMG). SubTMS evoked significantly larger depression of M3 than of the background EMG in the same time frame when applied 55-85 ms prior to M3 ( P<0.05, n=10). Furthermore, the effect on M3 was significantly larger than the effect on the spinal M2 ( P<0.01, n=7). Our results provide evidence that the long-latency stretch reflex in the tibialis anterior muscle is at least partly transcortical.
authors.affiliation: Center for Sensory-Motor Interation, Aalborg University, Fredrik Bajers Vej 7 D3, DK-9220, Aalborg, Denmark. jvd@smi.auc.dk
authors.forename: Johan
authors.initials: J
authors.lastname: van Doornik
authors.forename: Yoshihisa
authors.initials: Y
authors.lastname: Masakado
authors.forename: Thomas
authors.initials: T
authors.lastname: Sinkjaer
authors.forename: Jens Bo
authors.initials: JB
authors.lastname: Nielsen
mesh_terms: D000328:Adult; D004558:Electric Stimulation; D004576:Electromyography; D006801:Humans; D007866:Leg / Q000294:innervation / Q000502:physiology*; D009044:Motor Cortex / Q000502:physiology*; D009119:Muscle Contraction / Q000502:physiology; D018482:Muscle, Skeletal / Q000294:innervation / Q000502:physiology*; D009433:Neural Inhibition / Q000502:physiology*; D009434:Neural Pathways / Q000502:physiology; D011930:Reaction Time / Q000502:physiology*; D012026:Reflex, Stretch / Q000502:physiology*; D013997:Time Factors; D050781:Transcranial Magnetic Stimulation