In a study that could eventually restore movement to humans’ paralyzed limbs, researchers at California’s Stanford University have used light to induce muscle contractions in mice. A gene derived from algae was inserted into the mice, encoding a light-sensitive protein which adhered to their nerve cell surfaces. Scientists then placed an “optical cuff” lined with tiny, inwards-facing LEDs around the mice’s sciatic nerves. By penetrating those nerves with brief, high-intensity bursts of blue light, they were able to produce muscle contractions similar to those that would occur naturally. The technology is called “optogenetics.”
Previously, electrical versions of the optical cuff have been used to cause muscle movement. These have even been tested on paralyzed human subjects, allowing them to walk for a few minutes. Unfortunately, the electrical cuffs caused the muscle-activating nerves to fire in the wrong order.
Because it normally takes less stimulation to fire slow-twitch muscles, they are usually activated first, with the energy-burning fast-twitch muscles taking over only where needed (fast-twitch muscle fibers are used for quick, powerful movements, while slow-twitch muscles are used for slower, more delicate movements, and for fine-tuning fast-twitch-type movements). When the electrical cuff was tested, however, it activated the fast-twitch muscles first. This resulted in a very jerky walking gait, that tired the test subjects out quickly.
Using the optical cuff, the slow-twitch-activating nerves were fired first, the way it’s meant to be. The researchers also tried electrical cuffs on the mice, so they could see the difference. “With optical stimulation, the muscles retained about one-third of their initial maximum force after 20 minutes, and remained at that plateau for quite a while afterward,” said the study’s lead author, Michael Llewellyn. “Electrical stimulation completely exhausted the same muscles within four minutes.”
Although the optical cuffs are currently a research tool, it is hoped that they could one day be surgically implanted in humans along motor nerve bundles, and controlled by computer algorithms. Before that could happen, however, the protein-encoding genes would also need to be safely introduced into the recipients’ bodies. The Stanford scientists are also experimenting with another protein that inhibits nerve function when subjected to light, in hopes that it could someday be used to control involuntary muscle movements, such as those caused by cerebral palsy.
The research was published this week in the journal Nature Medicine.