Nerves fire rhythmically to power body movement

By Chelsea Whyte on June 4, 2012 7:45 PM EDT

Neuron
This neuron plays in time with others to make body movement possible. (Photo: Nicolas P. Rougier)

In a discovery that turns current thinking on its head, researchers at Stanford have discovered the mechanism that makes our body movement tick.

Motor neurons have long resisted understanding, but now a team of engineers and neuroscientists propose a new theory of brain activity that powers arm movements. Their findings are published in the journal Nature and are a significant departure from what we already understand about the functions of neurons.

Unlike visual neurons, which take in outside information and encode things like color, light and form in a predictable pattern, motor neurons are harder to pin down. The neurons that govern movement don't have a one-to-one relationship between a neuron's behavior and a muscle's movement or velocity.

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"Visual neurons encode things in the world. They are a map, a representation," said lead author Mark Churchland of Columbia University. "It's not a leap to imagine that neurons in the motor cortex should behave like neurons in the visual cortex, relating in a faithful way to external parameters, but things aren't so concrete for movement."

Scientists have disagreed about exactly which motions are being controlled by specific neurons, and determinations of exactly which pieces of information each neuron is sending - direction, distance, speed - can't be made with any confidence.

"Many experiments have sought such lawfulness and yet none have found it. Our findings indicate an alternative principle is at play," said co-lead author John Cunningham of Cambridge University, now at Washington University in St. Louis. 

By monitoring electrical brain activity in the motor cortex, the team found that it's not just one neuron or a few firing up that brings on movement. Instead, they found that each neuron pulses on and off, and they do it in concert with the entire neural population of the motor cortex. The electrical signal that drives movement is really a beautifully orchestrated composition of all of these beats pulsing together.

"Each neuron behaves like a player in a band. When the rhythms of all the players are summed over the whole band, a cascade of fluid and accurate motion results," said Churchland.

To test their hypothesis, the researchers studied the brain activity of monkeys reaching out for targets. These experiments showed that the underlying neural rhythm explains both the brain and muscle activity, with the pattern of shoulder-muscle behavior described by the sum of two underlying patterns. These patterns showed up even when the motion being tested was decidedly lacking any visible rhythm.

"Say you're throwing a ball. Beneath it all is a pattern. Maybe your shoulder muscle contracts, relaxes slightly, contracts again, and then relaxes completely, all in short order," said Churchland. "That activity may not be exactly rhythmic, but it can be created by adding together two or three other rhythms. Our data argue that this may be how the brain solves the problem of creating the pattern of movement."

"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," Cunningham told Newswise.

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