State- and identity-dependent motor neuron excitability shapes cutaneous long-latency reflexes
Finck Y, Soteropoulos DS, Del Vecchio A (2026)
Publication Type: Journal article
Publication year: 2026
Journal
Book Volume: 136
Pages Range: 1528-1543
Journal Issue: 4
Abstract
Neuromuscular reflexes elicited by sensory nerve stimulation provide valuable insights into neural motor control pathways. Analysis at the level of individual motor units (MUs) is feasible via electromyographic decomposition, but the factors shaping MU-specific reflex responses remain poorly understood. We investigated long-latency responses to cutaneous electrical stimulation in a large population of tibialis anterior MUs from nine healthy subjects during isometric ankle dorsiflexion at 10–30% of maximum voluntary contraction. Individual MU reflex responses differed markedly. Using 1,000 stimulation pulses per trial, substantially more than the 150–300 typically reported in previous studies, provided more reliable estimates of cutaneous reflex characteristics. Across the motor pool, reflex magnitude increased with force level (P < 0.001) while excitation probability correlated significantly with MU recruitment threshold in 78% of subjects (P ¼ 0.012). Furthermore, excitation probability increased systematically with contraction intensity (P < 0.001) for individually tracked MUs. Postexcitatory depression (PED) magnitude correlated significantly with excitation probability (r ¼ 0.50, P < 0.001) of individual MUs. A targeted reflex-removal analysis, validated by MU simulations incorporating realistic excitation probabilities into ordinary firing patterns, reduced the PED by 84.2% in simulated data but only by 34.7% in recorded units. These findings suggest that the PED is a complex, hybrid phenomenon, resulting from synchronization-induced discharge resetting and additional independent inhibitory components. These findings demonstrate that MU-level reflex excitability to somatosensory input is influenced by state- and identity-dependent motor neuron characteristics, underscoring the importance of using sufficient stimulation pulses for reliable reflex measures and MU population analysis. NEW & NOTEWORTHY We show that cutaneous long-latency reflex excitability at the single motor unit level scales with force via two mechanisms: high-threshold units show comparable reflex gain despite biophysical disadvantage, while active units increase excitation probability as drive rises. We further show postexcitatory depression is a hybrid phenomenon combining synchronization-induced spike resetting and, presumably, centrally mediated inhibition. Estimating single-unit reflex characteristics with 1,000 stimuli also proved significantly more reliable than in previous work using at most 300 repetitions.
Authors with CRIS profile
Yannick Finck
Professur für Neuromuscular Physiology and Neural Interfacing
Alessandro Del Vecchio
Department Artificial Intelligence in Biomedical Engineering (AIBE)
Involved external institutions
United Kingdom (GB)How to cite
APA:
Finck, Y., Soteropoulos, D.S., & Del Vecchio, A. (2026). State- and identity-dependent motor neuron excitability shapes cutaneous long-latency reflexes. Journal of Neurophysiology, 136(4), 1528-1543. https://doi.org/10.1152/jn.00225.2026
MLA:
Finck, Yannick, Demetris S. Soteropoulos, and Alessandro Del Vecchio. "State- and identity-dependent motor neuron excitability shapes cutaneous long-latency reflexes." Journal of Neurophysiology 136.4 (2026): 1528-1543.
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