Study on the Discharge Characteristics of Motor Neurons Innervating Paralyzed Muscles after Spinal Cord Injury

Academic Background

Spinal Cord Injury (SCI) is a severe neurological injury that typically leads to the loss of motor and sensory functions below the injury level. Although patients clinically diagnosed with complete SCI are considered unable to voluntarily control muscles below the injury level, previous studies have shown that some patients retain functional motor neurons below the injury level, which may be controlled through residual neural pathways. However, research on the behavioral characteristics of these spared motor neurons after SCI and their adaptation to injury remains insufficient.

Understanding the properties of these spared motor neurons is crucial for developing neural interface-based assistive devices. For example, by decoding signals from these motor neurons, Brain-Computer Interface (BCI) systems could be developed to help paralyzed patients regain partial motor function. However, current research primarily focuses on animal models, with limited systematic studies on human motor neuron characteristics after SCI.

This study aims to analyze the activity patterns and spatial properties of motor neurons in SCI patients and healthy controls during attempted hand movements using High-Density Surface Electromyography (HD-sEMG) and ultrasound imaging, thereby revealing changes in motor neurons after SCI and their mechanisms of functional preservation.

Source of the Paper

This research was conducted by Daniela Souza de Oliveira, Marco Carbonaro, Brent James Raiteri, and collaborators. The authors are affiliated with Friedrich-Alexander-Universität Erlangen-Nürnberg (Germany), Politecnico di Torino (Italy), Ruhr University Bochum (Germany), and other institutions. The paper was published on December 20, 2024, in the Journal of Neurophysiology, with the DOI 10.1152/jn.00389.2024.

Research Process and Results

1. Study Subjects and Experimental Design

The study recruited 8 patients with chronic complete SCI (SCI group) and 12 healthy controls. Participants in the SCI group were aged between 18 and 60 years, with injury levels located between C4 and C6. During the experiment, participants attempted finger flexion and extension movements (including thumb, index, middle, ring, and little fingers, as well as finger pinching and opening-closing movements) by watching videos of a virtual hand.

High-Density Surface Electromyography (HD-sEMG) was used to record forearm muscle activity, and ultrasound imaging was employed to observe muscle displacement. To analyze motor neuron activity patterns, the research team applied the Non-negative Matrix Factorization (NMF) algorithm to extract common input patterns of motor neurons and classified them into task-modulated and non-modulated (i.e., tonic or irregularly firing) based on their discharge patterns.

2. Data Analysis and Key Findings

2.1 Motor Neuron Mode Analysis
Using the NMF algorithm, the research team successfully extracted two main motor neuron modes corresponding to finger flexion and extension movements. These two modes accounted for most of the variance in motor neuron activity in both the SCI and control groups (SCI group: 78.1%, control group: 74.0%), indicating that common input patterns to motor neurons are preserved after SCI.

2.2 Motor Neuron Classification
Based on phase difference analysis, the study found that the proportion of non-modulated motor neurons was significantly higher in the SCI group than in the control group (SCI group: 53.4%, control group: 46.2%), while the proportion of task-modulated motor neurons was lower (SCI group: 20.8%, control group: 26.2%). This suggests that, although motor neurons can still respond to common inputs after SCI, their control over discharge patterns is significantly reduced.

2.3 Muscle Ultrasound Imaging Results
Using ultrasound imaging, the research team observed localized displacements in the forearm muscles of SCI patients during attempted hand movements, closely related to motor neuron discharge activity. This indicates that, although patients cannot physically move their fingers, their forearm muscles can still respond to neural inputs.

2.4 Motor Neuron Spatial Properties
The study also found that the motor unit action potential area was significantly larger in the SCI group than in the control group (SCI group: 560.0 mm², control group: 448.0 mm²), suggesting that reinnervation may occur after SCI, leading to a more widespread distribution of motor unit muscle fibers within the muscles.

3. Conclusions and Significance

This study demonstrates that, even in patients with complete SCI, spared motor neurons can still respond to common inputs and retain some functionality. Although the proportion of non-modulated motor neurons is higher, decoding the activity of these motor neurons can still enable the recognition of movement intentions. This provides an important theoretical basis for developing neural interface-based assistive devices.

Additionally, by combining HD-sEMG and ultrasound imaging, the research team achieved synchronous observation of motor neuron activity and muscle displacement in human SCI patients for the first time, offering a new experimental method for future neurorehabilitation research.

Research Highlights

1. Innovative Methodology: This study combined HD-sEMG and ultrasound imaging to achieve synchronous recording and analysis of motor neuron activity and muscle displacement in complete SCI patients for the first time.
   
2. Key Findings: The study revealed that, even in complete SCI patients, spared motor neurons can still respond to common inputs, and their activity patterns are similar to those of healthy controls.
   
3. Application Value: Decoding the activity of these spared motor neurons provides critical theoretical support for developing neural interface-based assistive devices, helping paralyzed patients regain partial motor function.
   

This research was partially funded by the European Research Council (ERC) and the German Federal Ministry of Education and Research (BMBF), providing essential support for the successful completion of the study.