Changes in motor cortex excitability during ipsilateral hand muscle activation in humans

Objectives: To test whether unilateral hand muscle activation involves chan ges in ipsilateral primary motor cortex (M1) excitability. Methods: Single- and paired-pulse transcranial magnetic stimulation (TMS) o f the right hemisphere was used to evoke motor evoked potentials (MEPs) fro m the resting left abductor pollicis brevis (APB) in 9 normal volunteers. W e monitored changes in motor threshold (MT), MEP recruitment, intracortical inhibition (ICI) and intracortical facilitation (ICF) while the ipsilatera l right APE was either at rest or voluntarily activated. Spinal motoneuron excitability was assessed using F-wave recording procedures. Results: Voluntary muscle activation of the ipsilateral APE significantly f acilitated the MEPs and F-waves recorded from the contralateral APE. Facili tation was observed with muscle activation >50% of the maximum voluntary fo rce and with stimulus intensities >20% above the individual resting motor t hreshold. Intracortical inhibition significantly decreased in the ipsilater al M1, while there was no significant change in intracortical facilitation during this maneuver. Conclusions: Unilateral hand muscle activation changes the excitability of homotopic hand muscle representations in both the ipsilateral M1 and the co ntralateral spinal cord. While the large proportion of MEP facilitation mos t likely occurred at a spinal level, involvement of the ipsilateral hemisph ere may have contributed to the enlargement of magnetic responses. (C) 2000 Elsevier Science Ireland Ltd. All rights reserved.