P05 Effects of postural challenge on responses to cortical stimulation: a potential probe of extrapyramidal tract function?
Russell CL[1, 2]*, Difford NB[1, 2]*, Stamenkovic A[1,2], Arpel C[1,2], Stapley PJ[1,2], Walsh JA[1,2], McAndrew DJ[1,2] and Shemmell J[1,2]
*Equal author contributions
- School of Medicine, University of Wollongong, Wollongong, Australia
- Illawarra Health and Medical Research Institute, Wollongong, Australia
The reticulospinal tract is known to play a key role in postural control yet is difficult to directly assess in humans due to its subcortical origins (1). However, cortico-reticulospinal contributions to responses generated by transcranial magnetic stimulation (TMS) over the motor cortex are shown to occur ~10 ms later than those driven by corticospinal input in monkeys (2). We hypothesised that the area of late responses (10-20 ms after response onset) in the lower limb would be greater than early responses (0-10 ms after response onset) when posture was challenged.
Two experimental protocols targeted 1) the triceps brachii or 2) medial gastrocnemius with single-pulse TMS. Bilateral muscle activity was recorded during isometric contractions (5% MVC upper limb; 20% MVC lower limb). Motor evoked potentials were produced and response area quantified across four levels of postural challenge: seated, bilateral and uni-lateral limb support (right or left limb). During the upper limb tasks, balance also involved using both or one arm.
Preliminary analysis (n = 5) shows that late responses were significantly larger than early responses across conditions (p = 0.045). A small (but ns) increase in the area of the late response was observed when the contralateral arm was supporting posture. In the lower limb, a greater late response (vs. early) was observed in the postural support limb.
These results suggest that postural challenge modulates TMS-induced muscle responses in the upper and lower limbs, with the modulation being primarily restricted to late portions of the response.
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- Fisher KM, Zaaimi B, Baker SN (2012) Reticular formation responses to magnetic brain stimulation of the primary motor cortex. J Physiol 590(16), 4045-4060