Spinal Interneuron Excitability in ALS

ALS

Pathophysiology, Spinal interneurons, Electrophysiology, EMG, transcranial magnetic stimulation (TMS)

Amyotrophic lateral sclerosis (ALS) is due to neurodegeneration of upper and lower motor neurons, leading to muscle atrophy, paralysis and death. However, there is growing evidence that interneurons involved in the gain regulation of spinal motoneuron (lower motor neurons) and in sensorimotor integration may participate in the pathogenesis of ALS. While sensory afferents in the peripheral nerve are traditionally thought to be unaffected at the beginning of the disease, diffusion MRI has revealed degeneration and demyelination of the posterior columns in the spinal cord of patients recently diagnosed with ALS, and there are sporadic reports of sensory involvement. Early alteration of the sensorimotor integration could participate to the degeneration of motor neurons and interneurons. The goal of the project is to further investigate sensorimotor integration at spinal level in human patients recently diagnosed with ALS, and to study whether an interneuron pathology could participate in ALS pathogenesis. Our project has first an interest for the fundamental research aiming at increasing basic knowledge of pathophysiology of ALS, and specifically on the functional effects of the underlying neurodegenerative mechanisms. By testing the excitability of spinal interneurons in patients recently diagnosed, and by doing so for clinically uninvolved muscles, we will be able to evaluate whether an interneuron pathology could be involved in ALS. Our results will help to understand better the chain reactions in the neurodegenerative processes that dramatically evolve until the death of all motor neurons. Our project has also an interest for the development of therapeutic approaches for ALS. Indeed, our methods will help to determine specific electrophysiological biomarkers that will help to evaluate quantitatively spinal and corticospinal neural processes: their changes during the course of the disease (follow-up study), the effect of therapeutic agents and/or rehabilitation methods on their excitability, and their repercussions on motor neuron activity (evaluation of therapeutics). Lastly, our methods could be tested in other neuromuscular diseases to determine possible differences in spinal neural activity. Indeed, the motor dysfunction common to several neuromuscular diseases can make it difficult to make a definitive diagnosis. The development of specific biomarkers is crucial for an early diagnosis, and to evaluate the best treatment for the patients as rapidly as possible.

Given the all-or-none properties of neuromuscular junctions, the electromyogram (EMG) reflects the activity of spinal motoneurons. These neurons are the last order neurons in all neural pathways involved in motor control. Modification of the afferent neural activity, whatever its level (spinal or supraspinal), will affect the excitability of spinal motoneurons and thus EMG activity. Combined with electrical and/or magnetic stimulation of peripheral nerve and/or cortical structures, it is possible to test indirectly the excitability of corticospinal and spinal reflex pathways involved in human motor control. One of the novelties is that we will investigate the interneurons that are activated by motor axons or afferents inputs from affected muscles (distal musculature: hand/wrist or foot/ankle muscles), which control clinically unaffected muscles (proximal musculature: elbow or knee muscles). This has a threefold interest i) functional exploration of clinically unaffected muscles that are never evaluated in ALS patients, ii) to better interpret the EMG signal and to better elucidate the pathophysiological mechanisms underlying the change in EMG activity, and iii) to determine if interneuron pathology manifests before detectable change in the motoneuron. The conditioned EMG and threshold tracking methods will be used to evaluate the excitability of spinal interneurons in ALS patients, as compared to sex and age-matched healthy subjects.

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