title: Plasticity of motor threshold and motor-evoked potential amplitude--a model of intrinsic and synaptic plasticity in human motor cortex?
abstract: BACKGROUND
  Neuronal plasticity is the physiological correlate of learning and memory. In animal experiments, synaptic (i.e. long-term potentiation (LTP) and depression (LTD)) and intrinsic plasticity are distinguished. In human motor cortex, cortical plasticity can be demonstrated using transcranial magnetic stimulation (TMS). Changes in motor-evoked potential (MEP) amplitudes most likely represent synaptic plasticity and are thus termed LTP-like and LTD-like plasticity.
  
  
  OBJECTIVE
  We investigated the role of changes of motor threshold and their relation to changes of MEP amplitudes.
  
  
  METHODS
  We induced plasticity by paired associative stimulation (PAS) with 25 ms or 10 ms inter-stimulus interval or by motor practice (MP) in 64 healthy subjects aged 18-31 years (median 24.0).
  
  
  RESULTS
  We observed changes of MEP amplitudes and motor threshold after PAS[25], PAS[10] and MP. In all three protocols, long-term individual changes in MEP amplitude were inversely correlated to changes in motor threshold (PAS[25]: P = .003, n = 36; PAS[10]: P = .038, n = 19; MP: P = .041, n = 19).
  
  
  CONCLUSIONS
  We conclude that changes of MEP amplitudes and MT represent two indices of motor cortex plasticity. Whereas increases and decreases in MEP amplitude are assumed to represent LTP-like or LTD-like synaptic plasticity of motor cortex output neurons, changes of MT may be considered as a correlate of intrinsic plasticity.
authors.affiliation: Division of Neuropaediatrics and Muscular Disorders, Department of Paediatrics and Adolescent Medicine, University Medical Centre, Freiburg, Germany.
authors.forename: Igor
authors.initials: I
authors.lastname: Delvendahl
authors.forename: Nikolai H
authors.initials: NH
authors.lastname: Jung
authors.forename: Nicola G
authors.initials: NG
authors.lastname: Kuhnke
authors.forename: Ulf
authors.initials: U
authors.lastname: Ziemann
authors.forename: Volker
authors.initials: V
authors.lastname: Mall
mesh_terms: D000293:Adolescent; D000328:Adult; D004558:Electric Stimulation; D004576:Electromyography; D019054:Evoked Potentials, Motor / Q000502:physiology*; D005260:Female; D006801:Humans; D008297:Male; D008475:Median Nerve / Q000502:physiology; D009044:Motor Cortex / Q000502:physiology*; D018482:Muscle, Skeletal / Q000502:physiology; D009473:Neuronal Plasticity / Q000502:physiology*; D011930:Reaction Time / Q000502:physiology; D050781:Transcranial Magnetic Stimulation