Summary: Researchers at MIT have developed an optogenetic method that uses light instead of electrical current to control muscle contractions. In mice, this technique produced smoother, more natural muscle force modulation and dramatically reduced muscle fatigue. While translating the method to humans remains a challenge, the approach offers promising avenues for future neuroprosthetic devices and therapies for people with impaired limb function.
Key Facts:
- Optogenetic stimulation can modulate muscle force more precisely than conventional electrical stimulation.
- This approach substantially increases resistance to fatigue compared with functional electrical stimulation (FES).
- Current efforts focus on safely delivering light-sensitive proteins to human tissue and developing implantable hardware for clinical use.
Source: MIT
Neuroprosthetic systems that use electrical currents to trigger muscle contraction can restore some limb function for people with paralysis or amputation, but conventional functional electrical stimulation (FES) faces serious limitations. FES tends to activate large motor units first, producing abrupt, excessive force and causing rapid muscle fatigue. These factors make fine motor control difficult and limit practical clinical use.
MIT researchers led by Hugh Herr, co-director of the K. Lisa Yang Center for Bionics, tested an alternative interface that replaces electrical pulses with optical control using optogenetics. Optogenetics involves genetically engineering cells to produce light-sensitive proteins. When exposed to light, these proteins open ion channels and modulate cell activity. In the study, mice engineered to express the protein channelrhodopsin-2 were used to evaluate optical stimulation of peripheral nerves that control leg muscles.
Physiological recruitment and finer control
Unlike FES, which often recruits large motor units first and produces a sudden jump in force, optogenetic stimulation produced a gradual, proportional increase in muscle contraction as light intensity rose. This modulation closely resembles natural motor unit recruitment, where small units are activated first, followed by progressively larger units as neural drive increases. As a result, researchers achieved smoother, more predictable control of muscle force over a range of outputs.
Lead author Guillermo Herrera-Arcos explains that optical stimulation allowed near-linear control of force: by adjusting the light delivered to the nerve, the team could reliably scale muscle output in a way that mimics normal neural control. This fidelity makes precise movements easier and reduces the abrupt overactivation associated with electrical stimulation.
Closed-loop system and fatigue resistance
To capitalize on the optogenetic force characteristics, the team developed a mathematical model relating optical input to muscle force. That model served as the foundation for a closed-loop controller: a system that delivers light stimulation, measures the resulting muscle force with sensors, and then adjusts the light input in real time to achieve a target force.
In trials using that closed-loop approach, optogenetically stimulated muscles maintained function for more than an hour before fatiguing, compared with roughly 15 minutes under FES. The combination of physiological recruitment, accurate modeling, and feedback control yielded both high-fidelity force modulation and markedly improved fatigue resistance.
Translational challenges and future directions
A major obstacle to clinical translation is safe and durable delivery of light-sensitive proteins into human muscle and nerve tissue. Previous work in rodents showed that some viral delivery strategies can provoke immune responses that inactivate the proteins and risk muscle damage. To address this, the K. Lisa Yang Center for Bionics is pursuing multiple strategies: designing new proteins less likely to trigger immunity, improving delivery methods, and developing implantable light sources and sensors that are minimally invasive.
In parallel, Herr’s lab is developing sensors to measure muscle force and length in vivo and refining implantation techniques for the light emitters. If those technical and safety hurdles can be overcome, the researchers believe optogenetic neuroprostheses could benefit people recovering from stroke, those with spinal cord injury, and amputees, offering finer control and longer-lasting muscle performance than current electrical systems.
“This approach could enable a minimally invasive strategy that substantially improves clinical care for people with limb dysfunction,” Herr says.
Funding: 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: 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 have the potential to restore movement for people with neurological impairments. Conventional activation strategies based on functional electrical stimulation (FES) struggle to modulate muscle force accurately and suffer rapid fatigue because they recruit motor units in an unphysiological order.
Here, the authors present a closed-loop framework that leverages physiological force modulation achieved with functional optogenetic stimulation (FOS) to enable sustained, high-fidelity muscle control in vivo for periods longer than 60 minutes. First, they characterize the force modulation properties of FOS and show that it produces more physiological recruitment and a much larger modulation range compared with FES. Second, they develop a neuromuscular model that captures the nonlinear dynamics of optogenetically stimulated muscle. Third, using that model, they demonstrate real-time closed-loop control of muscle force with improved performance and fatigue resistance relative to FES. This work establishes a foundation for fatigue-resistant neuroprostheses and for optogenetically controlled biohybrid robots with precise force modulation.