Some patients with spinal cord injuries later experience a substantial recovery of movement, and a new study in monkeys may explain why this is. The findings may lead to better ways to treat patients with spinal cord injures.
The researchers found certain nerve fibers that were not damaged when the spinal cord was injured spontaneously grew, or sprouted, and compensated for the severed connections, allowing the monkeys to gain back much sensation and movement.
This type of repair likely occurs only in cases of mild spinal cord injury — severe cases result in more permanent paralysis. However, the researchers said the findings might still have implications for those with more serious spinal injuries.
"If we can understand how this growth is naturally occurring, how this compensatory sprouting is naturally occurring, then we can potentially develop new treatments to elicit the same growth, or enhance the same growth in humans" with severe spinal cord injuries, said study researcher Ephron Rosenzweig of the Department of Neurosciences at the University of California, San Diego.
The study was published today (Nov. 14) in the journal Nature Neuroscience.
Injury and rehabilitation
The corticospinal tract is a bundle of nerve fibers that connects the brain's cortex with the spinal cord. These nerve fibers are critical for fine movements, such as hand manipulation, Rosenzweig said, and are very resistant to re-growth after an inury.
In the study, Rosenzweig and his colleagues made precise cuts to the spinal cords of 14 rhesus monkeys, severing only connections on the right side of their corticospinal tract. The monkeys lost the ability to make fine movements with their right hands. [Related: Read more about the ethics of animal research .]
The animals then underwent rehabilitation exercises, and the researchers tested their ability to retrieve food rewards from a platform.
After 24 weeks, the monkeys showed substantial recovery of their hand motor function. On autopsy, those who had the greatest recovery in their ability to move their right hands also had the greatest growth of nerve fibers within the corticospinal tract that had been spared during the initial injury.
This growth was able to restore 60 percent of the original spinal cord connections, the researchers said.
"I think it's incredibly important work," said Jason Carmel, of the Columbia College of Physicians and Surgeons, who has researched spinal cord injury recovery in rats and was not involved in the new study. The fact that the study demonstrates a change not only in the spinal cord anatomy, but also in the monkey's behavior is exciting, he said.
But he noted the study only shows a correlation, and the researchers don't know yet whether the nerve growth they observed actually caused the behavioral improvements.
"The next step is to be able to, in some way, prove the change in anatomy is really responsible for the behavior," he said.
Monkeys vs. rats
The sprouting of nerves the researchers saw was much more extensive than the growth that has been observed in previous work conducted in rats, Rosenzweig said. "We would not have expected just how plastic the corticospinal tract can be," he said.
The findings highlight key differences between primates and rodents, and provide "an example of how some of those differences can hide critical pieces of information," Rosenzweig said.
The corticospinal tract is much more important for movement in people than in rats, he said. But now that they know what occurs in primates, they can go back and adjust rodent models in order to carry out further experiments.
Carmel said rats have also shown robust growth of nerve fibers after spinal injuries, but it's difficult to compare the two models because of their anatomical differences.
He noted, "the monkey is a lot closer to the patient we want to recover function for."