Onion skin is not a universal firing pattern for spinal motoneurons: simulation study

Area of Science:

• Neuroscience
• Motor Control
• Computational Physiology

Background:

• Muscle force regulation relies on motor unit (MU) recruitment and firing rates.
• Contrasting models, 'onion skin' and 'afterhyperpolarization' (AHP), describe MU firing-recruitment relationships.
• Previous research often imposed one scheme, lacking a unified explanation.

Purpose of the Study:

• To investigate the coexistence and conditions governing the 'onion skin' and AHP firing-recruitment schemes.
• To resolve the longstanding dichotomy in motor unit firing patterns using a computational approach.

Main Methods:

• Utilized a high-fidelity computational model of motoneurons, emphasizing physiological fidelity and heterogeneity.
• Simulated various motoneuron pool activation levels and synaptic input rates.
• Analyzed firing patterns across different motor unit types (slow, fast-fatigue resistant, fast-fatigable).

Main Results:

• The 'onion skin' and AHP schemes are not mutually exclusive and can coexist.
• Scheme prevalence depends on motoneuron pool activation, synaptic input rates, and MU type.
• 'Onion skin' is observed in unsaturated motoneurons, while AHP dominates in saturated motoneurons, particularly slow-type MUs.

Conclusions:

• Provides a unified explanation for the observed dichotomy in motor unit firing patterns.
• Identifies key factors influencing the emergence of different firing-recruitment schemes.
• Highlights the context-dependent nature of motor unit control strategies.