Leg nerves activated by light offer new path to restoring mobility

New optogenetic technique could help restore limb movement, treat muscle tremor.


MIT researchers recently have shown that nerves made to express proteins that can be activated by light can produce limb movements utilizing cues produced by the motion of the limb itself. These limb movements can be adjusted in real-time seamlessly.

However, the technique leads to smoother and less fatiguing movement than analogous to electrical systems.

Electrical stimulation of nerves is actually utilized clinically to treat breathing, bowel, bladder, and sexual dysfunction in spinal cord injury patients. Also, it is utilized to improve conditioning in people with muscular degenerative diseases.

Similarly, electrical stimulation capable enough to control paralyzed limbs and prosthetics. In all cases, electrical pulses delivered to nerve fibers called axons trigger movement in muscles activated by the fibers.

Scientists have been in search to find another method of nerve stimulation, since this technique fatigues muscles, it can be painful and not so precise to target.

Optogenetic stimulation depends on nerves that have been genetically engineered to express light-sensitive algae proteins called opsins. These proteins monitor electrical signals such as nerve impulses.

The researchers made this experiment on mice and rats. They implanted these opsins within the leg or attached over the skin to monitor the up and down movement of the rodents’. This is first ever time a “closed-loop” optogenetic system has been used to power a limb.

Shriya Srinivasan, a Ph.D. student in medical engineering and medical physics at the MIT Media Lab explained, “When you’re walking slowly, you’re only activating those small fibers, but when you run a sprint, you’re activating the big fibers.”

“Electrical stimulation activates the big fibers first, so it’s like you’re walking but you’re using all the energy it requires to do a sprint. It’s quickly fatiguing because you’re using way more horsepower than you need.”

Further, she added, “When we kept doing these experiments, especially for extended periods of time, we saw this really interesting behavior.”

“We’re used to seeing systems perform really well, and then fatigue over time. But here we saw it perform really well, and then it fatigued, but if we kept going for longer the system recovered and started performing well again.”

Scientists concluded that this is related to how opsin activity cycles in the nerves, in a way that allows the full system to regenerate.

Srinivasan, said, “While this method was tested on animals, with further research and future trials in humans this optogenetic technique could be used someday to restore movement in patients with paralysis or to treat unwanted movements such as muscle tremor in Parkinson’s patients.”

Initially, the technology is applied to restore motion to paralyzed limbs or to power prosthetics. But, the scientists participated in the study suggest, an optogenetic system has the strength to restore limb sensation, turn off unwanted pain signals or treat spastic or rigid muscle movements in neurological diseases such as amyotrophic lateral sclerosis or ALS.

Srinivasan added, “Most people are using optogenetics as sort of a tool to learn about neural circuits, but very few are looking at it as a clinically translatable therapeutic tool as we are.”

Hugh Herr, who led the research team and heads the Media Lab’s Biomechatronics group, said, “Artificial electrical stimulation of muscle often results in fatigue and poor controllability. In this study, we showed mitigation of these common problems with optogenetic muscle control.”

“This has great promise for the development of solutions for patients suffering from debilitating conditions like muscle paralysis.”

To implement this method into humans, researchers require an experiment with the best ways to deliver light to nerves deep within the body and find ways to express opsins in human nerves safely and efficiently.

The research published in the December 13 issue of Nature Communications.


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