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Mice Walk Again After Spinal Cord Injury

Spinal cord injuries sever the connection between the brain and body. Researchers have long thought that, to restore movement, the long nerve fibers that run from the brain to the lower spinal cord had to be regrown. A new study in mice showed that nerves within the spinal cord can rearrange and restore those connections. The finding could lead to new therapies for the estimated 250,000 Americans living with spinal cord injuries.

Spinal cord injuries damage the nerves that allow the brain to send and receive messages from throughout the body, controlling everything from our muscles and breathing to digestion. Researchers assumed that the reason some people recover from spinal cord injuries is that these long nerve fibers from the brain haven't been completely severed. However, scattered evidence over the past several decades has suggested that other shorter nerve cells (or neurons) within the spinal cord may also be involved. The question of which neurons contribute to recovery is an important one, since the answer will guide the development of future therapies.

Dr. Michael Sofroniew at the University of California at Los Angeles led a team that set out to better understand how the body can recover from spinal cord injuries. The team's work was supported by NIH's National Institute of Neurological Disorders and Stroke (NINDS) and 3 private foundations. Using a mouse model, the researchers cut half of the long nerve fibers on 1 side of the spinal cord and then the half on the other side at a higher point. One group of mice had the second cut done immediately after the first, while the other had the second cut done 10 weeks later. The cuts all left many shorter nerves within the spinal cord untouched.

In the January 2008 edition of Nature Medicine, the researchers reported that mice regained the ability to control their legs within 8 weeks when the cuts were made at different times. The mice walked more slowly and less confidently than before their injury, but still recovered mobility. Those mice whose cuts were made at the same time, however, didn't recover. Recovery, the researchers believe, may depend on nerve cells within the spinal cord having the time to reorganize and form a sort of bypass around the injury sites.

To confirm that the short nerves in the center of the spinal cord were responsible for recovery, the researchers blocked those neurons and found that the animals' paralysis returned.

“Imagine the long nerve fibers that run between the cells in the brain and lower spinal cord as major freeways,” Sofroniew said. “When there's a traffic accident on the freeway, what do drivers do? They take shorter surface streets. These detours aren't as fast or direct, but still allow drivers to reach their destination.”

These findings add to a growing body of research showing that the nervous system can reorganize after injury. The researchers say that their next step will be to learn how to encourage nerve cells in the spinal cord to grow and form these new pathways.

—by Harrison Wein, Ph.D.