A SPINAL injury is like a permanent set of roadworks on a highway, forever ensnaring nerve signals in a traffic jam. So it makes sense to try to ease the congestion by building a bypass.
The section of spinal cord below an injury is usually intact and capable of responding to signals. However, because so few signals can pass through the injured region of the spine, it is often left isolated from the brain. 鈥淲hat we want to do is plug in new connections to bypass the damaged region,鈥� says John Martin at Columbia University in New York.
Martin suspected that, with a little surgical assistance, spinal cord nerves above an injury might be capable of making such connections with nerves lower down the spine. 鈥淲e know the nerves can make new connections to muscle, so we asked whether it鈥檚 possible for them to also connect with spinal cord neurons isolated through injury.鈥�
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Taking rats with spinal injuries, Martin鈥檚 team selected a motor nerve branching from the healthy cord above the injury and cut it away from the abdominal muscle it normally communicates with. They then stretched the free end across the injured section of spinal cord and used a protein 鈥済lue鈥� to fix it to the spinal cord below the injury.
When the team examined the nerve under a microscope two weeks later, they found it had sprouted new extensions which had begun to form connections, or synapses, with the motor nerves in the isolated lower spine. Zapping the spinal cord above the injury made the lower limbs of the rats twitch, showing that motor signals had begun once again to pass along the entire length of the spine. They note that removing the nerve from the abdominal muscle did not appear to cause any major side effects, and suggest this is because nearby nerves pick up the slack.
鈥淭he nerve sprouted new extensions which began to form connections with the previously isolated lower spine鈥�
鈥淚t鈥檚 amazing that the nerve bridge makes functional synapses and regenerates to the state it does,鈥� says Marie Filbin of the City University of New York. However, she cautions that it may not be possible to 鈥渞eprogram鈥� a nerve that normally connects to an abdominal muscle to transmit the sophisticated signals needed to produce fine, controlled movements.
Reggie Edgerton at the University of California, Los Angeles, says Martin鈥檚 approach may have considerable clinical potential, but agrees it鈥檚 too early to tell whether it will be clinically useful. 鈥淣ow we know that we can re-establish a functional connection across the lesion, one of the next problems to resolve is whether it will be possible for the animal to control the signals passed down the cord,鈥� he says. Success hinges on activating the appropriate neurons in the lower spinal cord, Edgerton says.
His own research suggests that interneurons in the lower spine coordinate the controlled movement of lower limbs without any input from the brain, if they are electronically stimulated.
Martin鈥檚 team鈥檚 findings were presented at the New York State Spinal Cord Injury Research Program Symposium in New York last month.
The Human Brain 鈥� With one hundred billion nerve cells, the complexity is mind-boggling. Learn more in our cutting edge special report.
