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

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 whether these parameters are affected by sex earlier in development. We compared the sex-related differences in firing behavior of single tibialis anterior (TA) motor units decomposed from high-density surface electromyography (EMG) in young (7-17 yr) and older (18-28 yr) 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., self-sustained firing (Δ)], 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 Δ values compared with males, which functionally allows more sustained firing of TA motoneurons in females. Young development females also had faster firing rates compared with 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, although 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 (Δ), the latter indicating that sex-related differences in intrinsic motoneuron excitability are established early during development. The intrinsic excitability of tibialis anterior (TA) motoneurons was compared between males and females before (7-17 yr) and during (18-28 yr) adulthood. Female motor units exhibited greater self-sustained firing compared with males in both the pre-adult and adult groups, suggesting the intrinsic excitability of motoneurons is established early in life and may not be tightly dependent on the level of sex hormones, unlike the number of decomposed motor units that reflect muscle size.