Until recently, neuroscientists thought cells called glia were the nervous system’s supporting players, helping keep brain cell communication in working order. Researchers focused more attention on the brain’s 100 billion nerve cells called neurons. Recent studies, however, suggest glia play a vital role in brain cell communication, and perhaps in the development of human intelligence.
Cells known as glia (Greek for “glue”) were long believed to provide nothing more than support to nerve cells. Research is showing, however, that glia are active participants in brain function.
After legendary genius Albert Einstein died in 1955, his brain was removed from his body and placed in a jar of formaldehyde. For the next 30 years, scientists examined small slices of his brain, hoping to uncover clues to the great man’s genius. Most people expected that Einstein’s brain would be larger than average. But it was not. Nor was there anything unusual about the number or size of its neurons, the brain cells responsible for everything from breathing to thinking.
Then, in the late 1980s, a scientist discovered something that was different about Einstein’s brain. It had more brain cells called glia, especially in the association cortex, an area of the brain involved with imagination and complex thinking. At first, scientists found this discovery surprising and confusing. They had long believed glial cells served solely as supporting actors to neurons. Glia’s only known duties at the time — tasks like carrying nutrients to neurons and cleaning up dead nerve cells and other debris — were, after all, seemingly less important.
Recent research, however, has redirected the spotlight onto glia. Glia are now known to be active players in the formation and function of synapses, the tiny gaps between neurons that allow them to communicate with each other. Ongoing research in this field is leading to:
- A better understanding of how brain cells communicate and process information.
- Insight into brain development.
- New approaches to treating neurological disorders, including chronic pain.
One reason why scientists underestimated glia for so long was because they saw no evidence that these cells communicated with each other. Neurons “speak” across synapses by generating action potentials, electrical impulses that trigger chemical communication between neurons and prompt more impulses in other neurons. But glial cells lack the ability to generate action potentials. Recent advances in imaging technology helped scientists discover that glia were actually communicating, although by chemical and not electrical means.
Research soon revealed that glial cells were “talking” not only among themselves, but also to neurons. Neuroscientists found that glia have receptors (receiving docks) for many of the same chemical messages used by neurons. These receptors enable them to eavesdrop on the neurons and respond in ways that help strengthen their messages.
Studies have shown that without glial cells, neurons and their synapses fail to function properly. For example, neurons removed from rodents were found to form very few synapses and to produce very little synaptic activity until they were surrounded by glial cells known as astrocytes. Once the astrocytes were introduced, the number of synapses jumped, and synaptic activity increased by 10 times.
Additional research supports the idea that glia are important in the formation of synapses. Researchers have discovered, for example, that astrocytes secrete a chemical called thrombospondin that encourages synapse formation in neurons.
Glia also contribute to the normal destruction of synapses that happens during brain development. Like trimming an overgrown tree, the developing brain cuts back unnecessary connections to simplify its circuits. Recent studies suggest that glia may encourage this process with the help of the immune system. The abnormal “pruning” of healthy synapses may be a factor in neurodegenerative disorders like Alzheimer’s disease, making it all the more important to understand how glia contribute to this process.
In addition to helping build and destroy synapses, glia may be involved in brain functions like learning in a more direct way. Some varieties of glia wrap around axons, the “wires” that connect neurons, forming insulation called myelin. When animals are raised in learning-rich environments, myelination increases, suggesting that glial cells may actively contribute to learning.
Understanding the how and why of glia communication is helping scientists rethink how the brain operates and how to treat it when it malfunctions. Glia have been associated with such varying neurological disorders as dyslexia, autism, stuttering, tone deafness, chronic pain, epilepsy, sleep disorders, and even pathological lying.
As research into glia continues, these formerly less understood brain cells — perhaps a source of Einstein’s genius — will continue creating headlines.