Cassandra Russell – Abstract

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

  1. School of Medicine, University of Wollongong, Wollongong, Australia
  2. 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.

  1. Petersen BW (1979) Reticulospinal projections to spinal motor nuclei. Ann Rev Physiol 41, 127-140
  2. 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