• Christian Leukel, Wolfgang Taube, Jörn Rittweger, Albert Gollhofer, Michel Ducos, Tobias Weber, and Jesper Lundbye-Jensen.
• Department of Medicine, Movement and Sport Science, University of Fribourg, Switzerland. Electronic address: christian.leukel@unifr.ch.
• Clin Neurophysiol. 2015 Jan 1;126(1):131-9.

ObjectivesJoint immobilization has previously been shown to modulate corticospinal excitability. The present study investigated changes in the excitability of distinct fractions of the corticospinal pathway by means of conditioning the H-reflex with transcranial magnetic stimulation (TMS) of the primary motor cortex (Hcond). This method allows assessment of transmission in fast (monosynaptic) and slow(er) (polysynaptic) corticospinal pathways.Methods9 subjects underwent 8weeks of unilateral ankle joint immobilization during daytime, 7 subjects served as controls. The measures obtained before and after immobilization included stretch- and H-reflexes assessing excitability of the spinal reflex circuitries, TMS recruitment curves estimating overall changes in corticospinal excitability, and Hcond.ResultsTMS recruitment curves showed an overall increase in corticospinal excitability following immobilization. Importantly, Hcond revealed significant facilitation of conditioned reflexes, but only for longer conditioning intervals, suggesting that immobilization increased excitability only of slower, indirect corticospinal pathways. No changes were observed in the control group. Immobilization had no significant effects on spinal reflex measures.Conclusions8weeks of ankle joint immobilization was accompanied by pathway-specific modulation of corticospinal transmission.SignificanceIt is particularly interesting that fast corticospinal projections were unaffected as these are involved in controlling many, if not most, movements in humans.Copyright © 2014 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved.

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