Using light-sensitive proteins from a single-celled alga and a tiny LED "cuff" placed on a nerve, researchers have triggered the leg muscles of mice to contract in response to millisecond pulses of light.
The study, published in the journal Nature Medicine, marks the first use of the nascent technology known as optogenetics to control muscle movements. Developed by study coauthor Karl Deisseroth, an associate professor of bioengineering and of psychiatry and behavioral science at Stanford University, optogenetics makes it possible to stimulate neurons with light by inserting the gene for a protein called channelrhodopsin-2, from a green alga. When a modified neuron is exposed to blue light, the protein initiates electrical activity inside the cell that then spreads from neuron to neuron. By controlling which neurons make the protein, as well as which cells are exposed to light, scientists can control neural activity in living animals with unprecedented precision. The paper's other senior author, Scott Delp, a professor of bioengineering, mechanical engineering, and orthopedic surgery at Stanford, says that the optical control method provides "fantastic advantages over electrical stimulation" for his study of muscles and the biomechanics of human movement.
Members of Deisseroth's lab had engineered mice to produce channelrhodopsin-2 in both the central and the peripheral nervous systems. Michael Llewellyn, a former graduate student in Delp's lab, developed a tiny, implantable LED cuff to apply light to the nerve evenly. He placed the cuff on the sciatic nerves of anesthetized mice and triggered millisecond pulses of light. This caused the leg muscles of the mice to contract. When Llewellyn compared the muscle contractions stimulated by light to those generated using a similar electrical cuff, he found that the light-triggered contractions were much more similar to normal muscle activity.