'Optical cuffs' been used to stimulate muscle movements in
mice (Photo: Rama)
n 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.
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