Cambridge, MA. — September 20, 2005 — Neuroscientists at The McGovern Institute for Brain Research at MIT are one step closer to understanding how the central nervous system (CNS) solves a gigantic problem-the production of voluntary movements. The simplest movement requires choosing which combination of motor neurons will innervate which of thousands of muscles fibers, with just the right amount of force and at the proper timing.

The vast number of degrees of freedom-the superabundance of independent variables to choose among-overwhelms the computational power of any computer, thwarting attempts to engineer a multi-joined prosthesis that could perform basic movements for an amputee. Yet the CNS solves this problem seemingly effortlessly, and engineers designing robots and prosthetics hope to mimic the way that biological systems approach this challenge.

For many years, scientists wondered whether vertebrates tackle this problem in a top-down way, with the brain micromanaging the process, or by establishing mini-command centers in the spinal cord that relieve the brain of this onerous oversight. Emilio Bizzi, an MIT Institute Professor and Principle Investigator in the McGovern Institute, has proposed the latter, that the CNS handles the daunting number of variables involved in a single movement by grouping sets of muscles, and their innervating neurons, into a small number of integrated units called muscle synergies.

In recent studies in frogs, Bizzi and his collaborators found solid evidence for muscle synergies. They showed that grouping muscles in a small set of muscle synergies simplifies the CNS's control issues.

But do muscle synergies in the spinal cord operate independently of sensory input, or does the frog react to sensory information from the environment? Resolving that question became the focus of a new study in the Journal of Neuroscience in July 2005.

Vincent Chi-Kwan Cheung, Bizzi's graduate student and the first author of the paper, recorded the electrical activity of hind leg muscles both before and after severing the nerve roots feeding sensory information into the spinal cord from the muscles. He left in tact the nerve roots carrying the commands to the muscles.

Cheung found that, for the most part, shutting off sensory input from the muscles did not perturb the synergies involved in natural jumping and swimming movements.

Bizzi explains the value of having both fixed motor synergies and some feedback from the environment. "If you're walking on an a mountain trail, you need to be able to make many small adjustments as you walk, and having a little sensory feedback helps you match your movements to specific conditions."

In practical terms, the near autonomy of the muscle synergies makes it possible to control a large number of muscles with just a few signals generated in the areas of the CNS involved in programming voluntary movements. According to Cheung. "That simplifies the future design of neuroprosthetics." Importantly, using a rigorous mathematical analysis, the researchers also found that a computer model representing specific combinations of muscle synergies could predict the movements produced by the animal.

This research was supported by the National Institute of Neurological Disorders and Stroke (NINDS).

About the McGovern Institute at MIT

The McGovern Institute at MIT is a research and teaching institute committed to advancing human understanding and communications. Led by a team of world-renowned, multi-disciplinary scientists, The McGovern Institute was established in February 2000 by Lore Harp McGovern and Patrick McGovern to meet one of the great challenges of modern science - the development of a deep understanding of thought and emotion in terms of their realization in the human brain. Additional information is available at: http://web.mit.edu/mcgovern

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