Light-activated muscles beat fatigue – Neuroscience News

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Resume: Researchers have developed a new approach to muscle control using light instead of electricity. This optogenetic technique provides more precise muscle control and significantly reduces fatigue in mice. Although not currently feasible in humans, this approach could revolutionize prosthetics and help people with reduced limb function.

Key Facts:

  • Optogenetic muscle stimulation provides more precise control than electrical stimulation.
  • This method significantly reduces muscle fatigue compared to traditional approaches.
  • Researchers are working on ways to safely deliver light-sensitive proteins to human tissue.

Source: MIT

For people with paralysis or amputation, neuroprosthetic systems that artificially stimulate muscle contraction with electrical current can help them regain limb function. However, despite years of research, this type of prosthesis is not widely used because it leads to rapid muscle fatigue and poor control.

MIT researchers have developed a new approach that they hope could one day provide better muscle control with less fatigue. Instead of using electricity to stimulate the muscles, they used light. In a study in mice, researchers showed that this optogenetic technique provides more precise muscle control, along with a dramatic decrease in fatigue.

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One hurdle the researchers now want to overcome is how to safely deliver light-sensitive proteins into human tissue. Credit: Neuroscience News

“It turns out that by using light, via optogenetics, you can control your muscles in a more natural way. In terms of clinical application, this type of interface could have very broad utility,” said Hugh Herr, professor of media arts and sciences, co-director of the K. Lisa Yang Center for Bionics at MIT, and associate member of MIT’s McGovern Institute for Brain Research.

Optogenetics is a method based on genetically engineering cells to express light-sensitive proteins, allowing researchers to control the activity of those cells by exposing them to light. This approach is not currently feasible in humans, but Herr, MIT graduate student Guillermo Herrera-Arcos and their colleagues at the K. Lisa Yang Center for Bionics are now working on ways to safely and effectively deliver light-sensitive proteins into human tissue.

Herr is the lead author of the study, which appears today in Science Robotics. Herrera-Arcos is the lead author of the paper.

Optogenetic control

For decades, researchers have been exploring the use of functional electrical stimulation (FES) to control muscles in the body. This method involves implanting electrodes that stimulate nerve fibers, causing a muscle to contract. However, this stimulation tends to activate the entire muscle at once, which is not the way the human body naturally controls muscle contraction.

“Humans have this incredible fidelity of control that is achieved through a natural recruitment of the muscle, recruiting small motor units, then medium, then large motor units, in that order, as the signal strength increases,” says Herr. “In FES, when you artificially inflate the muscle with electricity, the largest units are recruited first. So if you increase the signal, you get no power at first, and then suddenly you get too much power.”

This high force not only makes it more difficult to achieve fine muscle control, but it also wears out the muscle quickly, within five to ten minutes.

The MIT team wanted to see if they could replace that entire interface with something else. Instead of electrodes, they decided to control muscle contraction using optical molecular machines via optogenetics.

Using mice as an animal model, the researchers compared the amount of muscle force they could generate using the traditional FES approach with forces generated by their optogenetic method. For the optogenetic studies, they used mice that had already been genetically engineered to express a light-sensitive protein called channelrhodopsin-2. They implanted a small light source near the tibial nerve, which controls the muscles of the lower leg.

The researchers measured muscle strength while gradually increasing the amount of light stimulation, and found that optogenetic control, unlike FES stimulation, produced a steady, gradual increase in muscle contraction.

“If we change the optical stimulation we deliver to the nerve, we can proportionally, in an almost linear fashion, control the force of the muscle. This is similar to how signals from our brain control our muscles. This makes it easier to control the muscle compared to electrical stimulation,” says Herrera-Arcos.

Fatigue resistance

Using data from those experiments, the researchers created a mathematical model of optogenetic muscle control. This model relates the amount of light entering the system to the muscle’s output (how much force is generated).

With this mathematical model, the researchers were able to design a closed-loop controller. In this type of system, the controller delivers a stimulating signal and after the muscle contracts, a sensor can detect how much force the muscle is exerting. This information is sent back to the controller, which calculates whether and how much the light stimulation needs to be adjusted to achieve the desired force.

Using this type of control, the researchers found that muscles could be stimulated for more than an hour before becoming fatigued, while muscles fatigued after just 15 minutes using FES stimulation.

One hurdle the researchers now want to overcome is how to safely deliver light-sensitive proteins into human tissue. Several years ago, Herr’s laboratory reported that these proteins can trigger an immune response in rats that inactivates the proteins and can also lead to muscle atrophy and cell death.

“A major goal of the K. Lisa Yang Center for Bionics is to solve that problem,” says Herr. “Work is underway on several fronts to design new light-sensitive proteins and strategies to deliver them without triggering an immune response.”

As additional steps toward reaching human patients, Herr’s lab is also working on new sensors that can be used to measure muscle strength and length, as well as new ways to implant the light source. If successful, the researchers hope their strategy could benefit people who have suffered strokes, limb amputations and spinal cord injuries, as well as others who have a reduced ability to control their limbs.

“This could lead to a minimally invasive strategy that would change the game in terms of clinical care for people suffering from limb pathology,” says Herr.

Financing: The research was funded by MIT’s K. Lisa Yang Center for Bionics.

About this optogenetics and neuroscience research news

Author: Melanie Grados
Source: MIT
Contact: Melanie Grados – MIT
Image: The image is credited to Neuroscience News

Original research: Closed access.
“Closed-loop optogenetic neuromodulation enables high-fidelity fatigue-resistant muscle control” by Hugh Herr et al. Science Robotics


Abstract

Closed-loop optogenetic neuromodulation enables high-fidelity fatigue-resistant muscle control

Closed-loop neuroprostheses show promise in restoring movement in individuals with neurological disorders.

However, conventional activation strategies based on functional electrical stimulation (FES) fail to accurately modulate muscle force and exhibit rapid fatigue due to their unphysiological recruitment mechanism.

Here we present a closed-loop control framework that utilizes physiological force modulation under functional optogenetic stimulation (FOS) to enable high-fidelity muscle control for extended periods of time (>60 minutes) in vivo.

We first uncovered the force modulation characteristic of FOS, which showed greater physiological recruitment and significantly higher modulation ranges (>320%) compared to FES.

Second, we have developed a neuromuscular model that accurately describes the highly nonlinear dynamics of optogenetically stimulated muscles.

Third, based on the optogenetic model, we demonstrated real-time muscle force control with improved performance and fatigue resistance compared to FES.

This work lays the foundation for fatigue-resistant neuroprosthetics and optogenetically controlled biohybrid robots with high-fidelity force modulation.

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