title: Interhemispheric plasticity in humans.
abstract: BACKGROUND
  Chronic unimanual motor practice increases the motor output not only in the trained but also in the nonexercised homologous muscle in the opposite limb. We examined the hypothesis that adaptations in motor cortical excitability of the nontrained primary motor cortex (iM1) and in interhemispheric inhibition from the trained to the nontrained M1 mediate this interlimb cross education.
  
  
  METHODS
  Healthy, young volunteers (n=12) performed 1000 submaximal voluntary contractions (MVC) of the right first dorsal interosseus (FDI) at 80% MVC during 20 sessions.
  
  
  RESULTS
  Trained FDI's MVC increased 49.9%, and the untrained FDI's MVC increased 28.1%. Although corticospinal excitability in iM1, measured with transcranial magnetic stimulation (TMS) before and after every fifth session, increased 6% at rest, these changes, as those in intracortical inhibition and facilitation, did not correlate with cross education. When weak and strong TMS of iM1 were delivered on a background of a weak and strong muscle contraction, respectively, of the right FDI, excitability of iM1 increased dramatically after 20 sessions. Interhemispheric inhibition decreased 8.9% acutely within sessions and 30.9% chronically during 20 sessions and these chronic reductions progressively became more strongly associated with cross education. There were no changes in force or TMS measures in the trained group's left abductor minimi digiti and there were no changes in the nonexercising control group (n=8).
  
  
  CONCLUSIONS
  The findings provide the first evidence for plasticity of interhemispheric connections to mediate cross education produced by a simple motor task.
authors.affiliation: Department of Exercise and Sport Science, East Carolina University, Greenville, NC 27858, USA. hortobagyit@ecu.edu
authors.forename: Tibor
authors.initials: T
authors.lastname: Hortobágyi
authors.forename: Sarah Pirio
authors.initials: SP
authors.lastname: Richardson
authors.forename: Mikhael
authors.initials: M
authors.lastname: Lomarev
authors.forename: Ejaz
authors.initials: E
authors.lastname: Shamim
authors.forename: Sabine
authors.initials: S
authors.lastname: Meunier
authors.forename: Heike
authors.initials: H
authors.lastname: Russman
authors.forename: Nguyet
authors.initials: N
authors.lastname: Dang
authors.forename: Mark
authors.initials: M
authors.lastname: Hallett
mesh_terms: D000222:Adaptation, Physiological; D000328:Adult; D054022:Cerebrum / Q000502:physiology*; D019054:Evoked Potentials, Motor / Q000502:physiology; D005260:Female; D007839:Functional Laterality / Q000502:physiology; D006225:Hand / Q000502:physiology; D006801:Humans; D008297:Male; D009043:Motor Activity / Q000502:physiology*; D009044:Motor Cortex / Q000502:physiology; D009119:Muscle Contraction / Q000502:physiology; D009473:Neuronal Plasticity / Q000502:physiology*; D050781:Transcranial Magnetic Stimulation