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Session: Motor Control I
Session starts: Friday 25 January, 10:30
Presentation starts: 11:15
Room: Lecture room 558

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Matthijs Perenboom (TU Delft)
Mark van de Ruit (University of Birmingham)
Jurriaan de Groot (LUMC)
Alfred Schouten (TU Delft)
Carel Meskers (LUMC)

Abstract:
Understanding mechanisms of reflex control provides insight into movement disorders following supraspinal nerve lesions, for example after stroke. To assess cortical involvement in reflex modulation, subthreshold transcranial magnetic stimulation (TMS) is used to alter motoneuron pool excitability. Subthreshold TMS does not elicit direct muscle response, thereby allowing to define the effect of supraspinal input without afferent contributions. This study combined mechanically applied ramp-and-hold perturbations (stretch duration: 40 ms) with TMS (Magstim Rapid2 with figure-8 coil, Magstim Co, Whitland, UK) at interstimulus intervals (ISI) ranging from 35-80 ms, with random non-TMS and non-reflex trials mixed in. Eleven participants (48±13yrs (M±SD), 3 female) had to maintain a target torque of 10% maximum voluntary torque. Subthreshold TMS pulses (97% of active motor threshold) were applied over the primary motor cortex, at the individually assessed hotspot for m. flexor carpi radialis using neuronavigation (ANT, Enschede, The Netherlands). Electromyographic (EMG) muscle response was measured using high-density EMG (TMSi, Enschede, The Netherlands), with an afterwards constructed bipolar bar electrode configuration. Magnitude of resulting EMG peak activity, i.e. short (20-50 ms) and long (55-100) latency reflex responses were calculated by taking mean of 40% highest samples per peak time slot at all ISIs and compared to non-TMS trials. The effect of ISI on reflex response was tested using a linear mixed model (SPSS Version 17, alpha = 0.05). Subthreshold magnetic stimulation was found to nearly double the stretch evoked EMG response (p < 0.001) when TMS pulses were timed to arrive at the muscle in the time slot of the long latency response. Involvement of the primary motor cortex in peripheral stretch reflex loop was demonstrated for the human wrist flexor muscle. This involvement indicates the existence of supraspinal pathways in the long latency reflex component.