title: Direct corticospinal pathways contribute to neuromuscular control of perturbed stance.
abstract: The antigravity soleus muscle (Sol) is crucial for compensation of stance perturbation. A corticospinal contribution to the compensatory response of the Sol is under debate. The present study assessed spinal, corticospinal, and cortical excitability at the peaks of short- (SLR), medium- (MLR), and long-latency responses (LLR) after posterior translation of the feet. Transcranial magnetic stimulation (TMS) and peripheral nerve stimulation were individually adjusted so that the peaks of either motor evoked potential (MEP) or H reflex coincided with peaks of SLR, MLR, and LLR, respectively. The influence of specific, presumably direct, corticospinal pathways was investigated by H-reflex conditioning. When TMS was triggered so that the MEP arrived in the Sol at the same time as the peaks of SLR and MLR, EMG remained unaffected. Enhanced EMG was observed when the MEP coincided with the LLR peak (P < 0.001). Similarly, conditioning of the H reflex by subthreshold TMS facilitated H reflexes only at LLR (P < 0.001). The earliest facilitation after perturbation occurred after 86 ms. The TMS-induced H-reflex facilitation at LLR suggests that increased cortical excitability contributes to the augmentation of the LLR peaks. This provides evidence that the LLR in the Sol muscle is at least partly transcortical, involving direct corticospinal pathways. Additionally, these results demonstrate that approximately 86 ms after perturbation, postural compensatory responses are cortically mediated.
authors.affiliation: Univ. of Freiburg, Dept. of Sport Science, Schwarzwaldstr. 175, 79117 Freiburg i.Br., Germany. wolfgang.taube@sport.uni-freiburg.de
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Taube
authors.forename: Martin
authors.initials: M
authors.lastname: Schubert
authors.forename: Markus
authors.initials: M
authors.lastname: Gruber
authors.forename: Sandra
authors.initials: S
authors.lastname: Beck
authors.forename: Michael
authors.initials: M
authors.lastname: Faist
authors.forename: Albert
authors.initials: A
authors.lastname: Gollhofer
mesh_terms: D000328:Adult; D004558:Electric Stimulation; D004576:Electromyography; D019054:Evoked Potentials, Motor / Q000502:physiology*; D005260:Female; D006181:H-Reflex / Q000502:physiology; D006801:Humans; D008297:Male; D009046:Motor Neurons / Q000502:physiology; D018482:Muscle, Skeletal / Q000294:innervation* / Q000502:physiology*; D009469:Neuromuscular Junction / Q000502:physiology*; D004856:Postural Balance / Q000502:physiology*; D011712:Pyramidal Tracts / Q000502:physiology*; D011930:Reaction Time / Q000502:physiology; D050781:Transcranial Magnetic Stimulation