Modular organization of locomotor networks in people with severe spinal cord injury

Frontiers in Neuroscience, 2022 · DOI: 10.3389/fnins.2022.1041015 · Published: December 7, 2022

Spinal Cord Injury Neurology Rehabilitation

Simple Explanation

Locomotion requires complex coordination of muscles in the body. It is a general consensus that the central nervous system (CNS) often controls the coordination by activating groups of muscles via networks of interneurons in the spinal cord, rather than controlling each of the muscles individually. We here characterize the modular structure of the locomotor network expressed by the human spinal cord after SCI, and relate the modular structures to healthy patterns. We also explore if chronicity of the injury that allows extensive neuroplastic changes of the spinal circuitry aﬀects the modular organization of the locomotor circuitry. Using non-negative matrix factorization (NMF) and independent component analysis (ICA), we investigate synergy proﬁles of leg muscle activity observed during stepping with manual facilitation and body weight support in people with SCI and non-disabled individuals.

Study Duration

Not specified

Participants

73 individuals with SCI and 7 non-disabled individuals

Evidence Level

Original Research

Key Findings

1
Results for both NMF and ICA indicated that the subjects with SCI were similar among themselves, but expressed a greater variability in the number of synergies for criterion variance capture compared to NDs, and weaker correlation to NDs.
2
Further, the clinical characteristics of SCI subjects and chronicity did not predict any signiﬁcant differences in the spatial synergy structures despite any neuroplastic changes.
3
Further, post-training synergies did not become closer to ND synergies in two individuals.

Research Summary

In this study, we have characterized the modular structure of the locomotor network that the spinal cord is capable of producing. We identiﬁed modularity in response to the step-related sensory feedback received during assisted stepping in subjects with severe SCI, and able-bodied locomotion (NDs). Our results indicated that the numbers of synergy patterns identiﬁed in SCIs were more variable than those in NDs. Further, neither the severity of the injury, the expected strength of supraspinal inputs, nor the post-injury time available for neuroplastic changes of the spinal circuitry was related to diﬀerences in the synergy structures in the SCIs. Lastly, our results in two successfully rehabilitated individuals indicated no discernable patterns that were consistent between them, indicating that the recovery of walking function with epidural stimulation is not dependent on the normalization of the spinal generated synergy patterns.

Practical Implications

Rehabilitation Strategies

The consistent synergy patterns in SCI subjects, and their variations from healthy patterns, must impact the design and assessment of future rehabilitation strategies.

Neuromodulation Techniques

Further investigation is needed on how neuromodulatory techniques like epidural and transcutaneous electrical stimulation impact modular control of locomotor circuitry and synergy pattern generation.

Predictors for Functional Improvement

Future research should explore whether features of spinal-generated modular patterns, such as the number of modules or specific synergy patterns, serve as constraints or predictors for functional improvement.

Study Limitations

1
The excitation that the selected step speed and BWL provided to the spinal cord may not be identical to the excitation in NDs, who did not require manual facilitation during stepping.
2
The purpose of manual facilitation during stepping was to provide stepping-related sensory feedback that provides excitation to the spinal circuitry that is otherwise silent, and to optimize body kinematics.
3
It remains conceivable that if the complex and technically diﬃcult experiment were to be performed in which the initial untrained SCI kinematics and loading in stepping were somehow precisely matched to some variant of identical ND kinematics and loading, the patterns in stepping and the synergy production of each would converge to identical outputs.

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