Light-activated muscles beat fatigue – Neuroscience News

Summary: Researchers have developed a new approach to muscle control using light instead of electricity. This optogenetic technique enables 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 impaired limb function.

Key facts:

  • Optogenetic muscle stimulation offers 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 will someday offer better muscle control with less fatigue. Instead of using electricity to stimulate the muscles, they used light. In a mouse study, researchers showed that this optogenetic technique offered more precise muscle control, along with a dramatic reduction in fatigue.

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

“It turns out that by using light, through optogenetics, one can control muscles more naturally. From a clinical application perspective, this type of interface could have very broad utility,” says Hugh Herr, professor of media arts and sciences, co-director of the K. Lisa Yang Center for Bionics at MIT and an associate member of MIT’s Institute for Brain Research McGovern.

Optogenetics is a method based on genetic engineering of cells to express light-sensitive proteins, which allows researchers to control the activity of these 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 Scientific 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 the muscle to contract. However, this stimulation tends to activate the entire muscle at once, which is not how the human body naturally controls muscle contraction.

“Humans have this incredible precision of control that is achieved through natural muscle recruitment, where you recruit small motor units, then medium, then large motor units, in that order as the strength of the signal increases,” says Herr. “With FES, when you artificially blast the muscle with electricity, the largest units are recruited first. So when you boost the signal, you get no power at first and then all of a sudden you get too much power.

This great force not only makes it more difficult to achieve fine muscle control, but also wears out the muscle quickly, within five or 10 minutes.

The MIT team wanted to see if they could replace that entire interface with something different. Instead of electrodes, they decided to try to control muscle contraction using optical molecular machines through 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 modified 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, unlike FES stimulation, optogenetic control resulted in a steady, gradual increase in muscle contraction.

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

Fatigue resistance

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

This mathematical model allowed the researchers to design a closed-loop controller. In this type of system, the controller delivers a stimulus 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 should be adjusted to reach the desired strength.

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

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

“A key goal of the K. Lisa Yang Center for Bionics is to solve this problem,” says Herr. “Multiple efforts are underway to design new light-sensing proteins and strategies to deliver them without inducing 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 survived strokes, limb amputations and spinal cord injuries, as well as others who have an impaired ability to control their limbs.

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

Financing: The research was funded by the K. Lisa Yang Center for Bionics at MIT.

About this optogenetics and neuroscience research news

Author: Melanie Grados
source: MIT
Contact: Melanie Grados – MIT
Image: Image credit: Neuroscience News

Original Research: Closed access.
“Closed-loop optogenetic neuromodulation enables high-quality, fatigue-resistant muscle control” by Hugh Herr et al. Scientific robotics


Summary

Closed-loop optogenetic neuromodulation enables highly precise, fatigue-resistant muscle control

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

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

Here, we present a closed-loop control framework that uses physiological force modulation under functional optogenetic stimulation (FOS) to enable high-precision muscle control over extended periods of time (>60 min) in vivo.

We first revealed the force modulation characteristic of FOS, showing more physiological recruitment and significantly higher modulation ranges (>320%) compared to FES.

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

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

This work lays the groundwork for fatigue-resistant neuroprostheses and optogenetically controlled biohybrid robots with high-quality force modulation.

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