Rehabilitation Strategies after Spinal Cord Injury: Inquiry into the Mechanisms of Success and Failure

Abstract

Body-weight supported locomotor training (BWST) promotes recovery of load-bearing stepping in lower mammals, but its efficacy in individuals with a spinal cord injury (SCI) is limited and highly dependent on injury severity. While animal models with complete spinal transections recover stepping with step-training, motor complete SCI individuals do not, despite similarly intensive training. In this review, we examine the significant differences between humans and animal models that may explain this discrepancy in the results obtained with BWST. We also summarize the known effects of SCI and locomotor training on the muscular, motoneuronal, interneuronal, and supraspinal systems in human and non-human models of SCI and address the potential causes for failure to translate to the clinic. The evidence points to a deficiency in neuronal activation as the mechanism of failure, rather than muscular insufficiency. While motoneuronal and interneuronal systems cannot be directly probed in humans, the changes brought upon by step-training in SCI animal models suggest a beneficial re-organization of the systems' responsiveness to descending and afferent feedback that support locomotor recovery. The literature on partial lesions in humans and animal models clearly demonstrate a greater dependency on supraspinal input to the lumbar cord in humans than in non-human mammals for locomotion. Recent results with epidural stimulation that activates the lumbar interneuronal networks and/or increases the overall excitability of the locomotor centers suggest that these centers are much more dependent on the supraspinal tonic drive in humans. Sensory feedback shapes the locomotor output in animal models but does not appear to be sufficient to drive it in humans.

Keywords: locomotor function; neuroplasticity; rehabilitation; spinal cord injury.

Conflict of interest statement

No competing financial interests exist.

Figures

FIG. 1.

Locomotor training induced plasticity in animals and humans. Summary of the effects of step-training on various aspects of the locomotor circuitry in animal models (all panels, left) and in humans (all panels, right). Electromyographic (EMG) data for complete and incomplete spinal cord injury (SCI) individuals provided courtesy of Dr. Maria Knikou. AIS, American Spinal Injury Association Impairment Scale; ES, electrical stimulation; NA, noradrenalin; VLF, ventrolateral funiculus; PICs, persistent inward currents.