Paralysis after severe spinal cord injuries is permanent, but after milder injuries patients often experience substantial recovery. New work in monkeys, published online in Nature Neuroscience, suggests that such recovery is due to strong growth of new nerve fibers, not to regeneration of the severed original connections. This finding may pave the way for more studies in primate models, to unravel the mechanisms that enable post-injury sprouting growth that might eventually be applied to improve recovery in human patients.
Mark Tuszynski and colleagues cut the right corticospinal tract (CST) — a nerve fiber bundle in spinal cord that in primates is crucial for voluntary movement on the right side — and observed that right hand and leg function in adult rhesus monkeys began to recover 4-8 weeks after the injury. At autopsy, the authors found strong growth of new nerve fibers originating in the uninjured left CST, crossing the spinal cord midline and forming new connections with motor neurons in the right spinal cord. This allowed activation of muscle fibers that had been cut off from the nervous system by the injury.
This study reveals an important difference between the post-injury responses in monkeys and rodents, which have been the preferred models for spinal cord injury research. Nothing resembling the ‘repair’ of a lesioned CST by its contralateral counterpart has been observed in rats or mice. The authors propose that more studies in primate models will be necessary in order to unravel the mechanisms that enable post-injury sprouting, with the hope of eventually applying such insight to improve recovery in human patients.