Leg nerves activated by light offer new path to restoring mobility

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

For the first time, MIT researchers have shown that nerves made to express proteins that can be activated by light can produce limb movements that can be adjusted in real-time, using cues generated by the motion of the limb itself. The technique leads to movement that is smoother and less fatiguing than similar electrical systems that are sometimes used to stimulate nerves in spinal cord injury patients and others.

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.

The first applications of the technology might be to restore motion to paralyzed limbs or to power prosthetics, but an optogenetic system has the potential 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.

The MIT team is one of very few research groups using optogenetics to control nerves outside the brain.

Electrical stimulation of nerves is used clinically to treat breathing, bowel, bladder, and sexual dysfunction in spinal cord injury patients, as well as to improve muscle conditioning in people with muscular degenerative diseases. Electrical stimulation can also control paralyzed limbs and prosthetics. In all cases, electrical pulses delivered to nerve fibers called axons trigger movement in muscles activated by the fibers. This type of electrical stimulation quickly fatigues muscles, can be painful, and is hard to target precisely, however, scientists look for alternative methods of nerve stimulation.

Optogenetic stimulation relies on nerves that have been genetically engineered to express light-sensitive algae proteins called opsins. These proteins control electrical signals such as nerve impulses — essentially, turning them on and off — when they are exposed to certain wavelengths of light.

Using mice and rats engineered to express these opsins in two key nerves of the leg, the researchers were able to control the up and down movement of the rodents’ ankle joint by switching on an LED that was either attached over the skin or implanted within the leg.

This is the first time that a “closed-loop” optogenetic system has been used to power a limb.

Closed-loop systems change their stimulation in response to signals from the nerves they are activating, as opposed to “open-loop” systems that don’t respond to feedback from the body.

In the case of the rodents, different cues including the angle of the ankle joint and changes in the length of the muscle fibers were the feedback used to control the ankle’s motion. Optogenetic stimulation also led to less fatigue during cyclic motion than electrical stimulation. With less fatigue involved, the optogenetic system might be a good future fit for long-term motor operations such as robotic exoskeletons that allow some people with paralysis to walk, or as long-term rehabilitation tools for people with degenerative muscle diseases.

News Source: MIT