Synaptic inputs to motor neurons underlying muscle coactivation for functionally different tasks have different...

Area of Science:

• Neuroscience
• Motor Control
• Biophysics

Background:

• The central nervous system (CNS) can achieve similar movement outcomes using varied muscle activation patterns.
• Co-contraction of antagonist muscles, while not directly generating torque, plays a crucial role in modulating joint mechanical stiffness.
• Existing theories propose separate neural pathways for force generation and stiffness modulation, but the synaptic input differences remain uncharacterized.

Purpose of the Study:

• To investigate and differentiate the synaptic inputs to motor neurons (MNs) during co-contraction for stiffness modulation versus force generation.
• To identify distinct neural origins and characteristics of motor neuron activation patterns for these two functional tasks.

Main Methods:

• Participants performed upper-limb co-contraction tasks (biceps brachii and triceps brachii) for either stiffness modulation or force generation.
• Motor neuron spike trains were identified via decomposition of high-density electromyography (EMG) signals.
• Cross-correlogram and coherence analyses (cross-muscle and within-muscle) were employed to examine MN synchronization and synaptic input characteristics.

Main Results:

• Higher synchronization was observed between MNs when modulating stiffness compared to force generation.
• Cross-muscle coherence analysis revealed β-band peaks (suggesting cortical origin) during stiffness modulation, which were absent during force generation.
• Within-muscle coherence analysis identified distinct subsets of MNs recruited specifically for force generation or stiffness regulation.

Conclusions:

• The findings suggest separate cortical inputs and pathways for stiffness modulation, distinct from those controlling force generation.
• Stiffness modulation appears to be driven by specific cortical inputs targeting a separate set of motor neurons.
• This study provides novel insights into the neural strategies for muscle recruitment, highlighting specific spectral characteristics of synaptic inputs for different motor tasks.