Anatomical Plasticity of Rostrally Terminating Axons as a Possible Bridging Substrate across a Spinal Injury

JOURNAL OF NEUROTRAUMA, 2020 · DOI: 10.1089/neu.2018.6193 · Published: March 15, 2020

Spinal Cord Injury Neurology Neuroplasticity

Simple Explanation

Following a spinal cord injury (SCI), the body attempts to compensate for the lost connections by reorganizing the remaining neural circuits. This study investigates how nerve fibers, specifically those that normally terminate above the injury site, might contribute to this reorganization by sprouting new connections that bypass the damaged area. The research focuses on propriospinal neurons (PSNs), which are local spinal cord neurons, and the rubrospinal tract (RST), a pathway from the brain that influences movement. The study examines how these systems adapt after a spinal cord hemisection (a partial cut) in cats. The findings suggest that some nerve fibers that typically end above the injury can grow new branches to connect with areas below the injury, potentially forming a 'bridge' across the damaged region. This could be a target for future rehabilitation strategies.

Study Duration

16 weeks

Participants

22 adult female cats

Evidence Level

Not specified

Key Findings

1
The number of long propriospinal neurons (PSNs) with axons extending below the lesion was significantly and permanently decreased after injury.
2
The numbers of short PSNs and rubrospinal tract (RST) neurons significantly increased, suggesting their ability to develop new connections below the lesion.
3
Non-axotomized T8 short PSNs and ipsilateral RST neurons showed particularly striking contributions, suggesting recruitment of neurons that normally terminate more rostrally.

Research Summary

This study assessed the contributions of long and short propriospinal neurons (PSNs) and rubrospinal tract (RST) neurons to axonal projections caudal to a thoracic hemisection in cats. Results showed a significant and permanent decrease in long PSNs with axons below the lesion, but significant increases in short PSNs and RST neurons. The study suggests that uninjured neurons, which normally terminate above the lesion, can sprout and extend to more caudal segments below the lesion, contributing to functional recovery.

Practical Implications

Novel Target for Rehabilitation

The rostral population represents a major component of the bridging substrate seen and may represent an important anatomical target for evolving rehabilitation approaches.

Reinterpretation of RST Studies

RST studies compare contralateral RN (axotomized) with the ipsilateral RN (non-axotomized), assuming that the number of labeled neurons in the ipsilateral RN is equivalent to the number in the normal animal. However, our results clearly show that this may not be the case.

Understanding Plasticity Mechanisms

The current study shows that another mechanism for plasticity exists— the ability of uninjured neurons, which normally terminate above the level of the lesion, to develop collaterals capable of bridging an injury site and potentially innervate segments caudal to the lesion.

Study Limitations

1
The retrograde tracer approach used in the current study cannot address aspects of plasticity of spared long PSNs.
2
Aberrant sprouting cannot be ruled out as a contributor to the neuronal counts.
3
The study was conducted on cats, and the results may not be directly applicable to humans.

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