Inhibition in the human motor cortex is reduced just before a voluntary contraction.
OBJECTIVE
To examine inhibition in the human motor cortex before and during voluntary movements.


METHODS
The balance between the excitation and inhibition of corticospinal neurons in the human motor cortex was tested by conditioning the motor evoked potentials (MEP) evoked in forearm muscles by transcranial magnetic stimulation with a preceding subthreshold stimulus delivered through the same coil.


RESULTS
When normal individuals (n = 9) made a tonic wrist extension, inhibition of the forearm extensor MEP decreased, whereas that of the forearm flexors was unchanged. When these individuals made a tonic wrist flexion, inhibition of the forearm flexor MEP diminished, whereas that of the forearm extensors was unchanged. When normal individuals (n = 10) made a phasic wrist extension in response to an auditory signal, inhibition of the extensor MEP began to decline about 95 msec before the onset of the agonist EMG activity.


CONCLUSIONS
The changes in balance of excitation and inhibition of corticospinal neurons associated with a voluntary movement precede the movement and are directed at the corticospinal neurons projecting to the agonists. These changes may help to select the population of cortical neurons responsible for the movement.
Reynolds
C
C
The Toronto Hospital, Western Division, University of Toronto, Ontario, Canada.
Ashby
P
P
D000200:Action Potentials / Q000502:physiology; D004558:Electric Stimulation; D004576:Electromyography; D019054:Evoked Potentials, Motor / Q000502:physiology*; D006801:Humans; D009044:Motor Cortex / Q000502:physiology*; D009119:Muscle Contraction / Q000502:physiology*; D009431:Neural Conduction / Q000502:physiology; D012016:Reference Values; D014953:Wrist / Q000502:physiology*