Scientists Reveal the Body’s Secret Balancing Act

by Tanya Thomas on Oct 10 2008 7:10 PM

The mysteries of the body continue to plague scientists even as they unravel another of its little secrets- how the human body maintains balance during walking and running. Scientists at the Salk Institute for Biological Studies attribute this phenomenon to a class of spinal cord nerve cells, the V3 neurons, which ensures co-ordination between the left and right sides of the body.

The V3 neurons also ensure that the stepping rhythm is robust and well-organized, said the researchers.

"V3 neurons provide an additional level of control, which makes sure that when you walk and run, the intensity of the activity is matched on both sides of the body. If that were not the case, we would be unable to walk or run along a straight line," said research leader Dr. Martyn Goulding, a professor in the Molecular Neurobiology Laboratory.

Publishing their findings in the journal Neuron, they said that their findings marked an important milestone in understanding the neural circuitry that coordinates walking movements, one of the main obstacles in developing new treatments for spinal cord injuries.

"In the case of cervical spinal cord injuries, the spinal network that drives your limbs and allows you to walk is still there but no longer receives appropriate activating inputs from the brain. The fact that the V3 neurons are important for generating a robust locomotor rhythm makes them good candidates for efforts aimed at therapeutic intervention after spinal cord injury," said Goulding.

During the study, some mice were genetically engineered to shut off their V3 neurons, and electrophysiological experiments were performed on spinal cords isolated from such mice.

Dr. Ying Zhang, the first author, said that the study showed that without functioning V3 neurons, the length of individual motor neuron bursts began to fluctuate wildly.

"Instead of a stable, alternating pattern, we found irregular oscillations between the left and the right side," she said.

Since the activity of the motor neurons determines how much the muscle contracts and for how long, the researchers wanted to know how this irregular activity pattern of motor neurons influences the gait of mice strolling down a walkway.

They temporarily shut off V3 neurons in adult mice, and sent them on their way along a narrow Plexiglas walkway.

While the mice still alternated steps with their left and right hind limbs, the length of each step varied markedly, making it difficult for them to walk with a smooth cadence.