Changes in motoneuron excitability during voluntary muscle activity in humans with spinal cord injury

Abstract

The excitability of resting motoneurons increases following spinal cord injury (SCI). The extent to which motoneuron excitability changes during voluntary muscle activity in humans with SCI, however, remains poorly understood. To address this question, we measured F waves by using supramaximal electrical stimulation of the ulnar nerve at the wrist and cervicomedullary motor-evoked potentials (CMEPs) by using high-current electrical stimulation over the cervicomedullary junction in the first dorsal interosseous muscle at rest and during 5 and 30% of maximal voluntary contraction into index finger abduction in individuals with chronic cervical incomplete SCI and aged-matched control participants. We found higher persistence (number of F waves present in each set) and amplitude of F waves at rest in SCI compared with control participants. With increasing levels of voluntary contraction, the amplitude, but not the persistence, of F waves increased in both groups but to a lesser extent in SCI compared with control participants. Similarly, the CMEP amplitude increased in both groups but to a lesser extent in SCI compared with controls. These results were also found at matched absolutely levels of electromyographic activity, suggesting that these changes were not related to decreases in voluntary motor output after SCI. F-wave and CMEP amplitudes were positively correlated across conditions in both groups. These results support the hypothesis that the responsiveness of the motoneuron pool during voluntary activity decreases following SCI, which could alter the generation and strength of voluntary muscle contractions. NEW & NOTEWORTHY How the excitability of motoneurons changes during voluntary muscle activity in humans with spinal cord injury (SCI) remains poorly understood. We found that F-wave and cervicomedullary motor-evoked potential amplitude, outcomes reflecting motoneuronal excitability, increased during voluntary activity compared with rest in SCI participants but to a lesser extent that in controls. These results suggest that the responsiveness of motoneurons during voluntary activity decreases following SCI, which might affect functionally relevant plasticity after the injury.

Posteroanterior Cervical Transcutaneous Spinal Cord Stimulation: Interactions with Cortical and Peripheral Nerve Stimulation

“…We also observed higher F-wave amplitude and persistence in SCI participants than in AB participants. This suggests an increased state of spinal motor neuron excitability after SCI, as has been noted by others [41], especially in the context of lesions in the rostral cervical cord [42]-of the 13 participants with SCI in our study, 6 had SCI at C3, and 2 had SCI at C4. Conversely, we observed a non-significantly higher unpaired H-reflex amplitude in AB participants than in SCI participants, which was surprising given that H-reflexes are usually increased below the level of SCI.…”

Cortical and Subcortical Effects of Transcutaneous Spinal Cord Stimulation in Humans with Tetraplegia

“…Some aspects of the timing of the application of TESS relative to performing the motor task, however, might be important to consider. The responsiveness of the motoneuron pool during voluntary activity also decreases following SCI (Vastano and Perez, 2020), which might benefit from extra inputs recruited by TESS. Our results showing effects on muscles innervated by segments distal to the applied stimulation site suggest that TESS effects extend below targeted segments.…”

The Skeletal Muscle Emerges as a New Disease Target in Amyotrophic Lateral Sclerosis

“…The relationship between MN degeneration and NMJ denervation in ALS remains elusive, but recent advances have demonstrated that NMJ functionality could play a crucial role in ALS development [ 21 ]. The homeostatic balance of MNs is especially refined due to their ATP-dependent axonal transport rate, axon length [ 22 ], and excitability pattern [ 23 ], demanding high support of adjacent cell populations under physiological and pathological conditions. Non-neuronal cells of NMJ (i.e., terminal Schwann cells and kranocytes) supply this nutritional, metabolic, and trophic supply, playing a key role in commandeering MN function and health (discussed in the NMJ section).…”