Sex-related differences in motoneuron firing behavior during typical development

Area of Science:

• Neuroscience
• Motor Control
• Developmental Biology

Background:

• Sex-related differences in motoneuron excitability are known in adults.
• It remains unclear if these differences emerge early in development.

Purpose of the Study:

• To compare sex-related differences in tibialis anterior (TA) motor unit firing behavior in young and older developmental groups.
• To determine if sex differences in motoneuron excitability are established early in life.

Main Methods:

• High-density surface electromyography (EMG) was used to decompose single motor unit activity from the TA muscle.
• Motor unit firing rates and self-sustained firing (ΔF) were analyzed during triangular dorsiflexion contractions at varying intensities (10-30% MVC) in males and females aged 7-28 years.

Main Results:

• Females exhibited greater self-sustained firing (ΔF) than males in both young (7-17 yr) and older (18-28 yr) groups.
• Young females had faster firing rates than young males, but this difference diminished in older groups.
• Older males showed higher start firing rates and less firing variability compared to older females.

Conclusions:

• Sex-related differences in intrinsic motoneuron excitability, specifically greater persistent inward currents sustaining firing in females, are established early in development.
• These early-established differences suggest motoneuron excitability is not solely dependent on adult sex hormone levels.