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Sex-related differences in motoneuron firing behaviour during typical development.

Alex Yacyshyn,Ghazaleh Mohammadalinejad,B. Afsharipour,Jennifer Duchcherer,Jack Bashuk,David J Bennett,Francesco Negro,K. Quinlan,M. Gorassini

Published 2025 in Journal of Neurophysiology

ABSTRACT

Sex-related differences have been documented for intrinsic motoneuron excitability and firing rates in young adults, with females exhibiting higher excitability than males; however, it is unknown if these parameters are affected by sex earlier in development. We compared the sex-related differences in firing behaviour of single tibialis anterior (TA) motor units decomposed from high-density surface EMG in young (7-17 years) and older (18-28 years) developmental males and females during triangular dorsiflexion contractions at 10%, 20%, and 30% of their maximal voluntary contraction (MVC) torque. The amount of motoneuron self-sustained firing at estimated synaptic inputs below that needed for recruitment was measured (i.e., ΔF), along with start, maximal, and end firing rates and the interspike interval variability across the entire contraction. When taking the recruitment threshold of the motor units into account, both young and older development females had larger ΔF values compared to males, which functionally allows more sustained firing of TA motoneurons in females. Young development females also had faster firing rates compared to males and equivalent firing variability, an effect that disappeared in the older development group, where males exhibited higher start rates and less firing variability. Overall, while female motor unit firing rates became similar to males as they matured, they were more variable, slower at recruitment, and continued to exhibit evidence of greater persistent inward currents that sustained firing (ΔF), the latter indicating that sex-related differences in intrinsic motoneuron excitability are established early during development.

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