[Paper Review] Common Synaptic Input, Synergies, and Size Principle: Control of Spinal Motor Neurons for Movement Generation

François Hug, Simon Avrillon+2 more|arXiv (Cornell University)|Jul 27, 2022

EEG and Brain-Computer Interfaces33 citations

TL;DR

This review introduces a framework where motor neurons are grouped into functional clusters based on common inputs; these clusters may span muscles and differ from traditional motor neuron pools, reducing control dimensionality and implying limited selective control within a cluster.

ABSTRACT

Understanding how movement is controlled by the central nervous system remains a major challenge, with ongoing debate about basic features underlying this control. In this review, we introduce a new conceptual framework for the distribution of common input to spinal motor neurons. Specifically, this framework is based on the following assumptions: 1) motor neurons are grouped into functional groups (clusters) based on the common inputs they receive; 2) clusters may significantly differ from the classical definition of motor neuron pools, such that they may span across muscles and/or involve only a portion of a muscle; 3) clusters represent functional modules used by the central nervous system to reduce the dimensionality of the control; and 4) selective volitional control of single motor neurons within a cluster receiving common inputs cannot be achieved. We discuss this framework and its underlying theoretical and experimental evidence.

Motivation & Objective

• Motivate a new framework for distributing common input to spinal motor neurons.
• Propose that motor neurons are organized into functional clusters rather than fixed pools.
• Argue that clusters can span muscles or involve partial muscle innervation, altering control strategies.
• Suggest that clusters act as modules to reduce control dimensionality for movement generation.

Proposed method

• Define a conceptual framework based on common input to spinal motor neurons.
• Outline assumptions about clustering, cross-muscle span, and functional modularity.
• Summarize theoretical and experimental evidence supporting the framework.
• Discuss implications for motor control theories such as the size principle and synergies.

Experimental results

Research questions

• RQ1 What is the nature of common input distributions among spinal motor neurons?
• RQ2 Do motor neurons form functional clusters that span muscles and differ from classical pools?
• RQ3 How do these clusters affect the dimensionality of neural control required for movement?
• RQ4 Is selective volitional control of single motor neurons within a cluster feasible?
• RQ5 What theoretical and experimental evidence supports a cluster-based control framework?

Key findings

• A conceptual framework is proposed where motor neurons are grouped into functional clusters based on shared inputs.
• Clusters may extend across muscles or involve only portions of a muscle, challenging traditional motor neuron pool definitions.
• Clusters are presented as functional modules that reduce the dimensionality of neural control needed for movement generation.
• The authors discuss both theoretical arguments and experimental evidence supporting the cluster-based organization.
• The framework implies limitations on selectively controlling individual motor neurons within a cluster receiving common inputs.