Bypassing the Spinal Cord to Overcome Paralysis

From a mundane scratch to a graceful leap, every move we make is directed by a neural chain of command. Orders to move originate in the motor cortex, which gives the green light to movement plans coming from other parts of the brain. These commands then travel down to the spinal cord, which houses neurons that connect to muscles.

This chain is broken when a person is paralyzed.

Scientists have been searching for a way to reconnect this chain in those with paralysis to enable movement. Using complicated equations, massive computing power, and lengths of wires, some have tried to decode the language of the brain's messages to control devices like robotic arms.

But a recent report provides a simpler solution: Bypass the spinal cord by feeding the brain's movement commands directly to muscles. Then step back and let the brain do its work.

Bridging the gap with brain control

To simulate a spinal cord injury, scientists at the University of Washington temporarily paralyzed the wrist of a monkey with a local anesthetic that blocked impulses in a nerve. Then, using electrodes, a video game, and biofeedback, they tried to restore movement in this wrist.

First the researchers inserted a fine wire electrode—half the width of a single human hair—into the motor cortex to eavesdrop on movement commands in the brain while the monkey moved its wrist to control a cursor on a computer screen.

Once the nerve block kicked in, these movement commands reached a dead end, and the monkey's wrist no longer moved. That's when scientists rerouted the commands directly into the wrist muscles. Electricity is electricity after all, and what works for the brain also contracts muscle.

But just zapping a muscle with brain signals is not enough to move the wrist back and forth. The monkey had to learn how to control the electronic chatter coming from the brain cell that had been wired to the muscle. This was no easy task considering that ordinarily an entire nation of neurons gives a command; in this experiment, only one neuron was tasked with figuring out how to move the wrist.

Through biofeedback, in less than an hour the monkey learned—through trial and error—to exert conscious control over the activity of the lonely brain cell. It could dial the activity up or down, at will, to move the cursor in one direction or another.

"We really credit this to the flexibility of the brain," says Chet Moritz, Ph.D., a senior fellow and lead author of the study.

Teaching a neuron a new—and amazing—trick

We don't know what this particular monkey was thinking, but when we humans imagine a movement, it creates signals in the brain similar to those used to execute the movement. This feature of the brain has already allowed paralyzed people to use brain control with some success. Matthew Nagle, a quadriplegic, learned to control the signals coming out of his brain to move a cursor on a computer screen, which enabled him to open e-mail, draw a circle and play Pong.

But the most surprising part of the new study was the revelation of just how flexible the brain is. Normally there is a division of labor in the motor cortex, with different neurons in charge of different limbs. But Moritz and his colleagues found that the signals used to direct wrist movement didn't have to come from "wrist-specialist" neurons. Other neurons, say an index finger-specialist, could step in and learn to move the wrist.

Moritz says that it could be a decade or more before this technique is ready for controlling more complicated movements, like grasping a cup.

But finding that the brain can reprogram its neurons to take on new jobs may make the mission to restore movement in paralyzed limbs a bit easier.

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After spending 15 years in the lab doing her own neuroscience research, Michele Solis is now putting her Ph.D. to work as a science writer. Her work covers a variety of topics including autism, linguistics, and animal communication. She contributes regularly to the Autism Speaks, Simons Foundation, and Crosscut Web sites.