P216 Do repetitive spinal motor neuron discharges underlie the facilitation and inhibition in paired-pulse paradigm?

Abstract

Objective To explore whether repetitive Motorneuron Discharges (repMDs) contribute to intracortical facilitation (ICF) and short intracortical inhibition (SICI). Background Transcranial magnetic stimulation (TMS) leads to repetitive spinal motor neuron discharges (repMNDs). The paired-pulse TMS (PP-TMS) paradigm allows the exploration of the motor cortex physiology, but the precise mechanisms remain undetermined. The triple stimulation technique (TST) and an extended TST-technique including a 4th and 5th stimulation, Quadruple (QuadS) and Quintuple (QuintS) stimulation, respectively, allow a more precise exploration of the central motor conduction and of repMNDs. Design/methods We explored the PP TMS paradigms of short intracortical inhibition (SICI) with an inter-stimulus interval (ISI) of 2 ms and intracortical facilitation (ICF) with an ISI of 10 ms in the conventional way (TMS), combined with the TST, the QuadS and QuintS in a randomized design in 20 healthy volunteers. Results TMS and TST paradigms confirm the ICF and SICI and they appear to be associated with more repMNDs in ICF than in SICI. About half of the subjects have repMND following a single pulse TMS in the QuadS and QuintS condition (60% and 40%, respectively) and generally more in the QuadS than in the QuintS condition. There appear more repMNDs in the PP-TMS paradigm of ICF than with a single pulse TMS and less than latter in SICI. Conclusions Our preliminary results suggest a possible contribution of repetitive spinal motor neuron discharges in the conditioned responses of PP-TMS, but there is an inter- and intra-individual variability which needs to be further explored.