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Researchers are discovering how light can manipulate the nervous system

A FEW years ago researchers found a way to create a remotely controlled on-off switch in a Neuron by inserting a light-sensitive gene into the nerve cell. Now the same technique has been used experimentally in laboratory rats in a study that could help with spinal-cord injuries.

When the spinal cord is severed instructions being sent from the brain are interrupted. This means not just the loss of the ability to move limbs, but also Impairment of the up and down movement of the diaphragm too. This leaves patients unable to breathe on their own and often causes death.

Jerry Silver, a neuroscientist at Case Western Reserve University in Cleveland, Ohio, wanted to know if he could use light-sensitive genes to stimulate activity in neurons that control the diaphragm. Dr Silver and his colleagues partly severed the spinal cords of laboratory rats at the second vertebra. This is where the neck swivels, and where many humans suffer spinal-cord injuries. It is also where neurons that control breathing are located.

The partial injury (there are no ventilators for rats, so the researchers could not completely sever the cord without killing them) allowed only one side of the diaphragm to work and the rats had trouble breathing. A virus carrying genes for a light-sensitive protein called Channelrhodopsin-2 was then injected into neurons located just below the injury, at an area also involved with the diaphragm.

Four days later, the animals’ spines were opened. The application of continuous light on them had no effect. But as cells seem to respond most to patterns, the researchers experimented with various pulses of light. Several hours after a session which involved one-second pulses for five minutes followed by five minutes of rest, the rats began breathing normally for a day and a half.

Tests confirmed that their blood was well oxygenated and, perhaps more remarkably, that the two halves of their diaphragms were working in step with each other. This, Dr Silver is convinced, is because the activity in the light-sensitive neurons somehow activated a latent nerve pathway that spans both sides of the spinal cord, allowing them to synchronise.

Dr Silver is hopeful that the work on rats, which is published in the Journal of Neuroscience, will have relevance to humans—and not just those with spinal-cord injuries. The technique could eventually help people suffering from amyotrophic lateral sclerosis and Multiple Sclerosis. It might also lead to useful “off” switches as well as “on” ones, which could allow certain conditions, such as chronic pain, to be turned off.

How to get light to the neurons once they have been treated without cutting people up is a problem that Dr Silver expects soon to be resolved. Methods are already being developed to implant a light source inside the body which could be activated by remote control. Another method would be to pipe the light in through tiny cables. The fibre-optic age may be about to strike neurosurgery.

From The Economist print edition

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