Decreased input to the motor cortex increases motor cortical excitability.
OBJECTIVE
To investigate whether a short-duration reduction of input to the motor cortex affects excitability in the hand region of the motor cortex.


METHODS
Subjects (n=10) received sets of transcranial magnetic stimulation of the motor cortex (TMS) and peripheral ulnar nerve stimulation. Stimuli were delivered before and after 20 min of inactivity of the test hand. The evoked compound muscle action potentials were recorded in two relaxed intrinsic hand muscles using surface EMG.


RESULTS
Motor evoked potential size (MEP; expressed relative to the maximal M-wave) increased by approximately 30-40 in both hand muscles (P=0.012) following inactivity. The enlarged MEP was not associated with changes in F-wave size, a marker of motoneurone excitability, or changes in intracortical inhibition and facilitation measured with paired-pulse TMS.


CONCLUSIONS
MEP growth most likely reflects an increase in motor cortical excitability. The increased excitability appears to be more associated with reduced voluntary drive to and from the motor cortex rather than reduced afferent input from the periphery.


CONCLUSIONS
These results have important implications for any investigation of motor cortical excitability in relaxed subjects. The outcome of an experimental intervention is the net result of the intervention itself and alterations in cortical excitability produced by the subjects' inactivity.
Todd
Gabrielle
G
Discipline of Physiology, School of Molecular and Biomedical Science, The University of Adelaide, Adelaide 5005, Australia.
Butler
Jane E
JE
Gandevia
S C
SC
Taylor
Janet L
JL
D000328:Adult; D001143:Arousal / Q000502:physiology; D004558:Electric Stimulation; D004576:Electromyography; D005071:Evoked Potentials / Q000502:physiology; D005260:Female; D006225:Hand / Q000294:innervation / Q000502:physiology; D006801:Humans; D008297:Male; D009044:Motor Cortex / Q000502:physiology*; D009068:Movement / Q000502:physiology; D009434:Neural Pathways / Q000502:physiology; D012684:Sensory Thresholds / Q000502:physiology; D012881:Skin Temperature / Q000502:physiology; D050781:Transcranial Magnetic Stimulation; D014459:Ulnar Nerve / Q000502:physiology