More conditioning stimuli enhance synaptic plasticity in the human spinal cord

- Paired corticospinal-motoneuronal stimulation (PCMS) induces spinal plasticity.
- More PCMS produces longer lasting, more reliable facilitation.
- PCMS has potential to improve motor output in patients with insufficient descending drive.

ObjectiveTo examine whether more paired corticospinal-motoneuronal stimulation (PCMS) is more effective at inducing spinal level plasticity.MethodsTo produce facilitation, corticospinal volleys evoked by motor cortical transcranial magnetic stimulation (TMS) were timed to arrive at corticospinal-motoneuronal synapses prior to antidromic potentials evoked in motoneurones by electrical brachial plexus stimulation. Paired stimuli were delivered repeatedly. 50-pair conditioning (50-PCMS) was compared to 100 pairs in single block (100-PCMSsingle) and spaced (2 blocks of 50, 15-min break; 100-PCMSspaced) patterns, and to 50 single, unpaired TMS (50-TMS). Biceps responses to cervicomedullary stimulation (cervicomedullary motor evoked potentials, CMEPs) and TMS (motor evoked potentials, MEPs) were measured before and for 1 h after conditioning (recorded each 5 min).ResultsAfter 100-PCMS, average CMEP areas were increased by 46 ± 55% (mean ± SD; n = 10; 100-PCMSsingle) and 71 ± 99% (100-PCMSspaced). 50-PCMS produced a non-significant 6 ± 40% increase. After 100-PCMSsingle and 100-PCMSspaced, CMEPs were larger than those after 50-TMS from 0 to 60 min (p < 0.05). 100-PCMSsingle and 100-PCMSspaced produced more reliable changes than 50-PCMS. Overall, MEPs were larger at 35-60 min; however there were no differences between conditioning protocols.ConclusionsMore PCMS produces more reliable enhancement of corticospinal transmission.SignificanceThis technique has therapeutic potential to improve muscle control in patients with reduced descending drive.