title: What Makes the Muscle Twitch: Motor System Connectivity and TMS-Induced Activity.
abstract: Transcranial magnetic stimulation (TMS) of the primary motor cortex (M1) evokes several volleys of corticospinal activity. While the earliest wave (D-wave) originates from axonal activation of cortico-spinal neurons (CSN), later waves (I-waves) result from activation of mono- and polysynaptic inputs to CSNs. Different coil orientations preferentially stimulate cortical elements evoking different outputs: latero-medial-induced current (LM) elicits D-waves and short-latency electromyographic responses (MEPs); posterior-anterior current (PA) evokes early I-waves. Anterior-posterior current (AP) is more variable and tends to recruit later I-waves, featuring longer onset latencies compared with PA-TMS. We tested whether the variability in response to AP-TMS was related to functional connectivity of the stimulated M1 in 20 right-handed healthy subjects who underwent functional magnetic resonance imaging while performing an isometric contraction task. The MEP-latency after AP-TMS (relative to LM-TMS) was strongly correlated with functional connectivity between the stimulated M1 and a network involving cortical premotor areas. This indicates that stronger premotor-M1 connectivity increases the probability that AP-TMS recruits shorter latency input to CSNs. In conclusion, our data strongly support the hypothesis that TMS of M1 activates distinct neuronal pathways depending on the orientation of the stimulation coil. Particularly, AP currents seem to recruit short latency cortico-cortical projections from premotor areas.
authors.affiliation: Max Planck Institute for Neurological Research, 50931 Cologne, Germany Department of Neurology, University Hospital Cologne, 50937 Cologne, Germany.
authors.forename: Lukas J
authors.initials: LJ
authors.lastname: Volz
authors.affiliation: Sobell Department of Motor Neuroscience and Movement Disorders, UCL Institute of Neurology, London WC1N 3BG, UK Department of Neurology, Graduate School of Medicine, The University of Tokyo, Tokyo 113-0033, Japan.
authors.forename: Masashi
authors.initials: M
authors.lastname: Hamada
authors.affiliation: Sobell Department of Motor Neuroscience and Movement Disorders, UCL Institute of Neurology, London WC1N 3BG, UK.
authors.forename: John C
authors.initials: JC
authors.lastname: Rothwell
authors.affiliation: Max Planck Institute for Neurological Research, 50931 Cologne, Germany Department of Neurology, University Hospital Cologne, 50937 Cologne, Germany Institute of Neuroscience and Medicine (INM-3), Juelich Research Centre, Germany.
authors.forename: Christian
authors.initials: C
authors.lastname: Grefkes
mesh_terms: D000328:Adult; D000344:Afferent Pathways / Q000098:blood supply / Q000502:physiology*; D001703:Biophysics; D003463:Cues; D004576:Electromyography; D019054:Evoked Potentials, Motor / Q000502:physiology*; D005260:Female; D006225:Hand / Q000294:innervation; D006801:Humans; D007091:Image Processing, Computer-Assisted; D008279:Magnetic Resonance Imaging; D008297:Male; D009044:Motor Cortex / Q000098:blood supply / Q000502:physiology*; D018482:Muscle, Skeletal / Q000502:physiology*; D010100:Oxygen / Q000097:blood; D010775:Photic Stimulation; D011930:Reaction Time / Q000502:physiology; D050781:Transcranial Magnetic Stimulation; D055815:Young Adult
chemical_list: D010100:Oxygen
keywords: I-waves; TMS; fMRI; functional connectivity