Electrical stimulation of human corticospinal axons at the level of the lumbar spinal segments

Abstract

Electrical stimulation over the mastoids or thoracic spinous processes has been used to assess subcortical contribution to corticospinal excitability, but responses are difficult to evoke in the resting lower limbs or are limited to only a few muscle groups. This might be mitigated by delivering the stimuli lower on the spinal column, where the descending tracts contain a greater relative density of motoneurons projecting to lower limb muscles. We investigated activation of the corticospinal axons innervating tibialis anterior (TA) and rectus femoris (RF) by applying a single electrical stimulus over the first lumbar spinous process (LS). LS was paired with transcranial magnetic stimulation (TMS) at interstimulus intervals (ISIs) of −16 (TMS before LS) to 14 ms (LS before TMS). The relationship between muscle contraction strength (10%–100% maximal) and the amplitude of single‐pulse TMS and LS responses was also investigated. Compared to the responses to TMS alone, responses to paired stimulation were significantly occluded in both muscles for ISIs ≥−8 ms (p ≤ 0.035), consistent with collision of descending volleys from TMS with antidromic volleys originating from LS. This suggests that TMS and LS activate some of the same corticospinal axons. Additionally, the amplitude of TMS and LS responses increased with increasing contraction strengths with no change in onset latency, suggesting responses to LS are evoked transsynaptically and have a monosynaptic component. Taken together, these experiments provide evidence that LS is an alternative method that could be used to discern segmental changes in the corticospinal tract when targeting lower limb muscles.

Corticospinal responses during passive shortening and lengthening of tibialis anterior and soleus in older compared to younger adults

“…The variability of responses (CV%) was comparable to published data in lower leg musculature at rest (e.g. Lewis et al., ; Škarabot et al., ), for both older (SOL: 26 ± 10, 27 ± 10 and 32 ± 16%; TA: 36 ± 18, 42 ± 21 and 31 ± 19% during static position, passive shortening and lengthening, respectively) and younger (SOL: 32 ± 12, 37 ± 14 and 30 ± 10%; TA: 39 ± 15, 35 ± 17 and 41 ± 15% during static position, passive shortening and lengthening, respectively) individuals.…”

“…The firing of muscle spindle afferents increases proportionally to the magnitude of the muscle stretch, but remains low during muscle shortening (Matthews, 2011). This behaviour influences corticospinal responses; in both upper and lower limb musculature, it has been shown that passive shortening and lengthening are accompanied by increased and decreased corticospinal excitability, respectively (Lewis & Byblow, 2002;Lewis, Byblow, & Carson, 2001;Škarabot et al, c 2019 The Authors. Experimental Physiology c 2019 The Physiological Society 2019a).…”

Sex differences in fatigability following exercise normalised to the power–duration relationship

“…1995), corresponding to the level of the eighth thoracic spinous process (T8) as this stimulating site has recently been shown to activate corticospinal axons at the level of lumbar spinal segments (Škarabot et al . 2019 a ). The pre‐exercise LEP was standardised to 15–25% of M max .…”

Corticospinal excitability of tibialis anterior and soleus differs during passive ankle movement

“…The anode (2.5 cm 2 ) was placed 5 cm above the upper edge of the cathode (Ugawa et al 1995), corresponding to the level of the eighth thoracic spinous process ( T 8 ). This stimulating site has recently been shown to activate corticospinal axons at the level of lumbar spinal segments (Škarabot et al 2018).…”