A new model for understanding how nerve cells in the brain control movement may help unlock the secrets of the motor cortex, a critical region that has long resisted scientists' efforts to understand it, researchers report June 3 in Nature.
Scientists at Washington University in St. Louis, Stanford University and Columbia University have shown that the motor cortex's effects on movement can be much more easily understood by looking at groups of motor cortex neurons instead of individual nerve cells. In the study, scientists identified rhythmic brain cell firing patterns coordinated across populations of neurons in the motor cortex. They linked those patterns to different kinds of shoulder muscle movements.
"Populations of neurons in the motor cortex oscillate in beautiful, coordinated ways," says co-first author John Cunningham, PhD, assistant professor of biomedical engineering at Washington University in St. Louis. "These patterns advance our understanding of the brain's control of movement, which is critical for understanding disorders that affect movement and for creating therapies that can restore movement."
Until now, scientists had based their studies of the motor cortex on decades-old insights into the workings of the visual cortex. In this region, orientation, brightness and other characteristics of objects in the visual field are encoded by individual nerve cells.
However, researchers could not detect a similar direct encoding of components of movement in individual nerve cells of the motor cortex.
"We just couldn't look at an arm movement and use that to reliably predict what individual neurons in the motor cortex had been doing to produce that movement," Cunningham says.
For the new study, conducted at Stanford University, scientists monitored motor cortex activity as primates reached for a target in different ways. They showed that the motor cortex generated patterns of rhythmic nerve cell impulses.
"Finding these brain rhythms surprised us a bit, as the reaches themselves were not rhythmic," says co-first author Mark Churchland, PhD, who was a postdoctoral researcher at Stanford at the time of the study and is now assistant professor of neuroscience at Columbia University. "In fact, they were decidedly arrhythmic, and yet underlying it all were these unmistakable patterns."
Cunningham compares the resulting picture of motor cortex function to an automobile engine. The engine's parts are difficult to understand in isolation but work toward a common goal, the generation of motion.
"If you saw a piston or a spark plug by itself, would you be able to explain how it makes a car move?" Cunningham asks. "Motor-cortex neurons are like that, too -- they are understandable only in the context of the whole."
Researchers are applying their new approach to understanding other puzzling aspects of motor cortex function.
"With this model, the seemingly complex system that is the motor cortex can now be at least partially understood in more straightforward terms," says senior author Krishna Shenoy, PhD, associate professor of electrical engineering at Stanford.
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