Afferent-driven modulation of spinal interneuron circuits across disease stages in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease that progressively disrupts voluntary motor command through combined cortical and spinal motor neuron degeneration. However, how spinal circuits reorganize during the disease remains poorly understood, particularly in humans. This study examined the function of excitatory and inhibitory spinal interneuron circuits that control upper and lower limb movements, using non-invasive electrophysiological techniques targeting specific afferent-motoneuron pathways at cervical and lumbar levels. These segments are clinically relevant, as spinal-onset forms constitute the predominant clinical presentation of ALS. We compared patients with ALS at different stages of functional impairment to healthy individuals. Spinal circuits predominantly driven by muscle spindle afferents (group Ia and II) showed, at the group level, a marked reduction in inhibition together with enhanced propriospinal excitation. In contrast, pathways mediated by tendon afferents (group Ib) and cutaneous inputs appeared preserved in unstratified analyses. However, when accounting for disease stage, inhibitory dysfunction emerged as an early feature, whereas excitation increased progressively with functional impairment, and modulations also became detectable in Ib- and cutaneous-driven responses. These findings reveal an afferent- and stage-dependent hierarchy of spinal dysfunction, following a reproducible sequence from early disinhibition to maladaptive excitation. This dynamic pattern mirrors the organization observed in preclinical spinal models and aligns with cortical pathophysiology, where widespread loss of inhibition precedes selective increases in excitation. Together, these results refine the mechanistic understanding of motor network disorganization in ALS and identify inhibitory interneurons as potential therapeutic targets to stabilize spinal network function.