Functional Study of Motor Neurons After Spinal Cord Injury

Background Introduction

Spinal Cord Injury (SCI) is a severe neurological disorder that often leads to the loss of motor function in patients. Although patients may lose voluntary control of limb movements after SCI, studies have shown that motor neurons below the injury level may still retain some functionality. However, the specific behavior of these motor neurons after injury and the mechanisms underlying their functional recovery remain largely unknown. To better understand the changes in motor neurons after SCI, researchers analyzed the discharge characteristics of motor units in both SCI patients and healthy controls during attempted hand movements using High-Density Surface Electromyography (HDsEMG) and ultrasound imaging.

This study aims to reveal the neural and spatial properties of motor neurons after SCI, explore how these neurons adapt post-injury, and analyze potential mechanisms for functional recovery. By comparing motor unit activity between SCI patients and healthy controls, the researchers hope to provide new insights for future neural interface technologies and rehabilitation therapies.

Source of the Paper

This paper was co-authored by Daniela Souza de Oliveira, Marco Carbonaro, Brent James Raiteri, Alberto Botter, Matthias Ponfick, and Alessandro Del Vecchio. The research team is affiliated with multiple institutions, including the Department of Artificial Intelligence in Biomedical Engineering at Friedrich-Alexander-Universität Erlangen-Nürnberg in Germany, the Laboratory for Engineering of the Neuromuscular System (LISiN) at Politecnico di Torino in Italy, the Faculty of Sport Science at Ruhr University Bochum in Germany, and the Spinal Cord Injury Center at Krankenhaus Rummelsberg GmbH in Germany. The paper was first published on December 20, 2024, in the Journal of Neurophysiology, with the DOI 10.1152/jn.00389.2024.

Research Process

Study Participants and Experimental Design

The study recruited eight participants with chronic motor complete SCI (SCI group) and twelve healthy controls (control group). The SCI group consisted of patients with injury levels between C4 and C6, who had no voluntary hand movement for at least one year. The control group comprised healthy adults aged 27.1±3.4 years. All participants provided informed consent, and the study was approved by the ethics committee.

During the experiment, researchers placed high-density surface electromyography electrode grids on the participants’ forearm muscles to record muscle activity during attempted hand movements. Participants watched videos of a virtual hand performing movements and attempted to mimic finger flexion and extension tasks. The tasks included individual flexion and extension of five fingers, finger opening and closing, and two- and three-finger pinches, all performed at a frequency of 0.5 Hz for 42 seconds.

Data Collection and Analysis

Researchers used HDsEMG to record muscle activity and applied blind source separation algorithms to decompose the signals and extract the discharge patterns of individual motor units. To further analyze the spatial properties of motor units, ultrasound imaging was used to observe muscle tissue displacement.

For data analysis, the researchers employed Non-negative Matrix Factorization (NNMF) to extract common input patterns of motor units. By calculating the phase difference between motor unit discharge patterns and virtual hand kinematics, motor units were classified as task-modulated or nonmodulated, and their discharge characteristics were analyzed.

Experimental Results

The study found that the discharge patterns of motor units in both the SCI and control groups could be divided into two main modes, corresponding to finger flexion and extension movements. Although the correlation between motor unit discharge patterns and virtual hand kinematics was lower in the SCI group, the time delays were similar between the two groups, suggesting that common input to motor neurons is preserved after SCI.

Additionally, the proportion of nonmodulated motor units was significantly higher in the SCI group, indicating that some motor units exhibited irregular or tonic firing patterns after SCI. Researchers also found that the active areas of motor units were larger in the SCI group, which may be related to neural remodeling and muscle fiber reinnervation after injury.

By combining HDsEMG and ultrasound imaging, researchers observed localized displacements in paralyzed muscles of SCI patients during attempted movements, indicating the presence of functional motor units in these muscles.

Conclusions and Significance

This study reveals changes in the discharge characteristics and spatial distribution of motor neurons after SCI, demonstrating that despite the inability to voluntarily control limb movements, motor neurons in SCI patients retain some functionality. The findings provide important insights for developing high-performance brain-machine interface systems based on motor neuron activity, particularly in decoding movement intentions from paralyzed muscles.

Furthermore, the study highlights the diversity of motor unit behavior after SCI, suggesting that future rehabilitation therapies should consider individual differences and develop personalized treatment plans based on specific motor unit properties.

Research Highlights

1. Novel Experimental Methods: The study combined HDsEMG and ultrasound imaging to simultaneously record neural activity of motor units and muscle tissue displacement in SCI patients for the first time.
2. Key Findings: The study found that common input to motor neurons is preserved after SCI, and some motor units can still respond to movement intentions, offering new possibilities for neural interface technologies.
3. Clinical Applications: The results suggest that real-time decoding of movement intentions from paralyzed muscles is achievable, providing new directions for rehabilitation therapies and assistive device development for SCI patients.

Additional Valuable Information

The research team plans to further explore long-term changes in motor units after SCI and optimize motor unit control through closed-loop training and real-time feedback. Additionally, the public availability of the study data will provide valuable resources for other researchers, advancing the field of spinal cord injury research.