Each year spinal cord injuries rob some 10,000 Americans of the freedom to move. Now there is a glimmer of hope that victims of severe injuries may some day walk again. Last July, researchers Lars Olson, Henrich Cheng, and Yihao Cao at the Karolinska Institute in Sweden announced that they had succeeded in restoring some muscle function to rats with severed spinal cords.
When we move a muscle voluntarily, a brain neuron sends a signal down its long extension, the axon, to the spinal cord. Brain axons sheathed in myelin, an insulating substance, make up the spinal cord’s white matter, which forms a cylinder around a core of myelin-free motor and sensory neurons--the gray matter. At various points along the spine, axons from the brain leave the white matter and plunge into the gray matter to make contact with motor neurons, which relay signals to the muscles.
The myelin sheath allows those signals to travel faster from the brain, but it has a downside: it contains chemicals that prevent axons from regrowing if they’re severed. Gray matter, on the other hand, is free of the inhibitory proteins. Olson, Cheng, and Cao reasoned that severed axons might regrow and form the appropriate connections if they were somehow coaxed to grow into gray matter.
To find out, Cheng severed the rats’ spinal cords at chest level, leaving a quarter-inch gap. From the rats’ chest muscles, he removed bits of nerve fibers--axon conduits--to create tunnels across the gap; muscle nerve fibers contain specialized cells that actually stimulate rather than inhibit axonal growth. Cheng anchored the fiber tunnels in place with thin metal wires and a natural adhesive called fibrin.
Three weeks later, a few of the rats began to show signs of recovery. Apparently the severed axons from the brain grew through the tunnels into the gray matter and reestablished connections with enough motor neurons to restore some limited movement. By the following year, the animals, while not cured, were able to support their weight and hobble around their cages.
Much research, Olson cautions, is needed before the technique can be tested on humans. One formidable obstacle is that most spinal cord injuries involve crushed neurons, not neatly severed ones. And injuries to the neck, such as the one actor Christopher Reeve suffered last spring, will be particularly difficult to overcome because they affect breathing. But Olson, who devoted himself to spinal cord research after a friend became paralyzed in a diving accident 30 years ago, is encouraged. Watching the rats move, he says, was the happiest moment of my scientific life.
