Corticospinal motoneuronal synaptic plasticity induction can modulate the speed of learning ballistic finger movements: Possible use in rehabilitation

Paired corticospinal motoneuronal stimulation (PCMS) applying transcranial magnetic stimulation (TMS) over the primary motor cortex paired with peripheral electrical nerve stimulation (PNS) can induce bidirectional associative plasticity at the corticospinal motoneuronal synapses. Direction of the induced plasticity depends on the interstimulus interval (ISI) between TMS and PNS, and the voluntary motor output is modulated in the same manner. Here we investigated the effects of PCMS on the motor learning of finger movements. Excitability at the corticospinal motoneuronal synapses was evaluated by cervicomedullary motor-evoked potential (CMEP) amplitudes from 8 subjects to find the optimal ISIs. PCMS resulted in either long-term potentiation (LTP)-like plasticity (PCMS_LTP) at an ISI of 0 ms (i.e., presynaptic and postsynaptic volleys arriving at the motoneurons simultaneously) or long-term depression (LTD)-like effect (PCMS_LTD) at ISIs of +9 and +20 ms: that is, with postsynaptic inputs by PNS reaching the motoneurons at 9 and 20 ms later than presynaptic volleys. Next, motor learning performance was evaluated by acceleration (ACC) of ballistic abduction movements of the index finger in 11 subjects. PCMS_LTP strongly increased ACC, whereas PCMS_LTD decreased it. Changes in ACC were positively correlated with changes in CMEP amplitudes induced by PCMS. Synaptic plasticity induction at the corticospinal motoneurons modulates movement speed in parallel with direction of the induced plasticity. These findings clarify that synaptic plasticity induction at the corticospinal motoneurons using noninvasive brain simulation can modulate motor performance in humans and suggest that this intervention could be useful in rehabilitating patients with spinal cord lesions or movement disorders.