A researcher at the National Institutes of Health (NIH) has captured video images of a previously unknown form of communication between brain cells that might hold clues to the way learning shapes the brain. The videos are offered as a resource for educators teaching high school, undergraduate and graduate students.
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The newly recorded signals are emitted along the length of axons, the long nerve fibers that stretch between neurons. Research has documented signal transmission across the synapse. When neurons communicate, the new videos show, electrical signals emitted along their length stimulate nearby glial cells. These cells begin making myelin, a substance that coats the axons and allows electrical charges to travel with greater speed through brain networks. The process of myelination underlies learning and is crucial for the development of new skills, other studies have shown.
Additional research might help clarify how signaling between neurons and glial cells triggers myelination and influences skill development, according to R. Douglas Fields, PhD, of the Nervous Systems Development and Plasticity Section at the National Institute of Child Health and Human Development. "For the last 100 years researchers have studied how information traverses the brain, crossing synapses and traveling from one nerve cell to the next," said Dr. Fields. "We can now see another type of communication, in which cells along a neuron's length can sense the chemical signals the neuron releases."
To detect these signals Dr. Fields observed neurons under a microscope. Working with the corresponding neuron of mice, he stimulated the neuron with electrical charges and recorded his findings in three videos.
The videos are available on the website of Science Signaling, a resource of the American Association for the Advancement of Science. The teaching resource on the site features short video clips that document these previously unknown non-synaptic signals (
http://stke.sciencemag.org/cgi/content/full/sigtrans;4/155/tr1).
Movie 1 shows an increase in the transmission of light through the axons after they are stimulated to fire impulses. The researchers shone a light through the axon and measured the amount that passed through. The increased transmission of light is caused by the microscopic swelling of the axons, which reduces the scattering of light passing through them.
As an electrical charge travels the length of the axon, the wall of the axon becomes porous, and the axon swells with water. To relieve the pressure and keep from bursting, the axon releases water and a chemical called adenosine triphosphate (ATP). This ATP signal is intercepted by nearby glial cells. Movie 2, taken over the course of a minute, shows the axon swell as charges pass along its length. As the axon swells, more light from the microscope passes through. Increases in light intensity appear as warmer colors in the video.
Movie 3 shows red dots that mark ATP as it is released from the round neuron in the center of the image. The dots are visible because ATP combined with a special enzyme deposited by Dr. Fields near the axons. When ATP binds to the enzyme, a photon of light is released, which appears as red spots in the time-lapse video, which was filmed over the span of 10 minutes.
"It's humbling, really," he said. "We had taken glial cells for granted for so many years, and we've since come to see that they're not just supporters of neurons, but integral partners that enable learning and the acquisition of new skills to take place."
