The first optogenetic stimulation of mammalian skeletal muscle is reported this week in Nature Communications. The study describes the light-mediated control of contractions in surgically removed vocal cords from mice, which were genetically engineered to express the light responsive channel, Channelrhodopsin2 (ChR2).
Optogenetics allows the precision stimulation of specific cell types (usually neurons) that have been genetically sensitised to light. This technology has been used before to indirectly stimulate skeletal muscle via the secondary motor cortex or peripheral motor neurons. Direct optogenetic stimulation of skeletal muscle cells was previously shown only in nematodes and in individual muscle fibres in the lab. These experiments did not provide insight into the feasibility of this technique in intact skeletal muscle as generating enough force to cause contractions requires the stimulation of multiple muscle fibres at once and it was unclear if enough light could pass through muscle to allow this to take place.
Now, Philipp Sasse and colleagues genetically engineered mice to express ChR2 and show that stimulation with light pulses as short as 2 milliseconds can induce contractions in laryngeal muscles that had been surgically removed from mice. They also found that by modulating the duration and intensities of light pulses they could fine-tune the force generated.
The authors suggest that direct optogenetic stimulation of muscle may be superior to electrical stimulation as it can overcome discomfort due to the activation of sensory nerves and unwanted stimulation of nearby muscles by electrical pulses. They also speculate that, given similarities between larynges from mice and humans, this approach may someday be adapted to be a therapeutic option for patients with laryngeal paralysis where neuronal stimulation is not an option, though considerable further testing is needed to determine whether this is the case.