In a first, MIT neuroscientists have been able to control muscle movement by shining light on spinal cord neurons in mice.
Researchers have shown they can control muscle movement by applying optogenetics - a technique that allows scientists to control neurons' electrical impulses with light - to the spinal cords of animals that are awake and alert.
Led by Massachusetts Institute of Technology (MIT) Professor Emilio Bizzi, the researchers studied mice in which a light-sensitive protein that promotes neural activity was inserted into a subset of spinal neurons.
When the researchers shone blue light on the animals' spinal cords, their hind legs were completely but reversibly immobilised.
The findings, described in the journal PLoS One, offer a new approach to studying the complex spinal circuits that coordinate movement and sensory processing, researchers said.
In the study, Bizzi and Vittorio Caggiano, a postdoc at MIT's McGovern Institute for Brain Research, used optogenetics to explore the function of inhibitory interneurons, which form circuits with many other neurons in the spinal cord.
Inhibitory neurons in the spinal cord suppress muscle contractions, which is critical for maintaining balance and for coordinating movement.
For example, when you raise an apple to your mouth, the biceps contract while the triceps relax. Inhibitory neurons are also thought to be involved in the state of muscle inhibition that occurs during the rapid eye movement (REM) stage of sleep.
To study the function of inhibitory neurons in more detail, the researchers used mice developed by Guoping Feng, the Poitras Professor of Neuroscience at MIT, in which all inhibitory spinal neurons were engineered to express an opsin called channelrhodopsin 2.
This opsin stimulates neural activity when exposed to blue light. They then shone light at different points along the spine to observe the effects of neuron activation.
When inhibitory neurons in a small section of the thoracic spine were activated in freely moving mice, all hind-leg movement ceased.
This suggests that inhibitory neurons in the thoracic spine relay the inhibition all the way to the end of the spine, Caggiano said.
The researchers also found that activating inhibitory neurons had no effect on the transmission of sensory information from the limbs to the brain, or on normal reflexes.
"The spinal location where we found this complete suppression was completely new," Caggiano said.
"It has not been shown by any other scientists that there is this front-to-back suppression that affects only motor behaviour without affecting sensory behaviour," Caggiano said.
