Regaining Limb Movement Despite many years of intensive research, there is still an urgent need for novel treatments to help patients restore motor function after spinal cord injuries. van den Brand et al. (p. 1182 ) produced left and right hemisections at different levels of the rat thoracic spinal cord to cause complete hind limb paralysis mimicking the situation in humans with spinal cord injury. Systemic application of pharmacological agents, combined with a multisystem rehabilitation program including a robotic postural neuroprosthesis, restored voluntary movements of both hind limbs.

Severe spinal cord contusions interrupt nearly all brain projections to lumbar circuits producing leg movement. Failure of these projections to reorganize leads to permanent paralysis. Here we modeled these injuries in rodents. A severe contusion abolished all motor cortex projections below injury. However, the motor cortex immediately regained adaptive control over the paralyzed legs during electrochemical neuromodulation of lumbar circuits. Glutamatergic reticulospinal neurons with residual projections below the injury relayed the cortical command downstream. Gravity-assisted rehabilitation enabled by the neuromodulation therapy reinforced these reticulospinal projections, rerouting cortical information through this pathway. This circuit reorganization mediated a motor cortex-dependent recovery of natural walking and swimming without requiring neuromodulation. Cortico-reticulo-spinal circuit reorganization may also improve recovery in humans.

Experimental and clinical studies suggest that primate species exhibit greater recovery after lateralized compared to symmetrical spinal cord injuries. Although this observation has major implications for designing clinical trials and translational therapies, advantages in recovery of nonhuman primates over other species has not been shown statistically to date, nor have the associated repair mechanisms been identified. We monitored recovery in more than 400 quadriplegic patients and found that that functional gains increased with the laterality of spinal cord damage. Electrophysiological analyses suggested that corticospinal tract reorganization contributes to the greater recovery after lateralized compared with symmetrical injuries. To investigate underlying mechanisms, we modeled lateralized injuries in rats and monkeys using a lateral hemisection, and compared anatomical and functional outcomes with patients who suffered similar lesions. Standardized assessments revealed that monkeys and humans showed greater recovery of locomotion and hand function than rats. Recovery correlated with the formation of corticospinal detour circuits below the injury, which were extensive in monkeys, but nearly absent in rats. Our results uncover pronounced inter-species differences in the nature and extent of spinal cord repair mechanisms, likely resulting from fundamental differences in the anatomical and functional characteristics of the motor systems in primates versus rodents. Although rodents remain essential for advancing regenerative therapies, the unique response of the primate corticospinal tract after injury re-emphasizes the importance of primate models for designing clinically relevant treatments.

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