Some of the common words we use are frozen mistakes. The term influenza comes from the Italian word meaning “influence”—an allusion to the influence the stars were once believed to have on our health. European explorers searching for an alternate route to India ended up in the New World and uncomprehendingly dubbed its inhabitants indios, or Indians. Neuroscientists have a frozen mistake of their own, and it is a spectacular blunder. In the mid-1800s researchers discovered cells in the brain that are not like neurons (the presumed active players of the brain) and called them glia, the Greek word for “glue.” Even though the brain contains about a trillion glia—10 times as many as there are neurons—the assumption was that those cells were nothing more than a passive support system. Today we know the name could not be more wrong.

Glia, in fact, are busy multitaskers, guiding the brain’s development and sustaining it throughout our lives. Glia also listen carefully to their neighbors, and they speak in a chemical language of their own. Scientists do not yet understand that language, but experiments suggest that it is part of the neurological conversation that takes place as we learn and form new memories.

If you had to blame one thing for the mistaken impression about glia, it would have to be electricity. The 18th-century physiologist Luigi Galvani discovered that if he touched a piece of electrified metal to an exposed nerve in a frog’s leg, the leg twitched. He and others went on to show that a slight pulse of electricity moving through the metal to the nerve was responsible. For two millennia physicians and philosophers had tried to find the “animal spirits” that moved the body, and Galvani discovered that impetus: It was the stuff of lightning.


Over the next two centuries scientists got a clearer understanding of how those signals work. When a branch at one end of a nerve cell, or neuron, is stimulated, an electric pulse races toward the main body of the cell. Other branches might send separate pulses at the same time. The main body of the neuron conveys those pulses to an outgoing arm, or axon, which splits into numerous branches, each of which nearly touches other neurons. The slight gap between two nerve cells is called a synaptic cleft. The signal-sending neuron pumps chemicals into the space, and the signal-receiving neuron takes up some of them, triggering a new electric pulse.

All neurons have certain characteristic attributes: axons, synapses, and the ability to produce electric signals. As scientists peered at bits of brain under their microscopes, though, they encountered other cells that did not fit the profile. When impaled with electrodes, these cells did not produce a crackle of electric pulses. If electricity was the language of thought, then these cells were mute. German pathologist Rudolf Virchow coined the name glia in 1856, and for well over a century the cells were treated as passive inhabitants of the brain.

At least a few scientists realized that this might be a hasty assumption. The pioneering neuroscientist Santiago Ramón y Cajal earned a Nobel Prize in 1906 for what came to be known as the neuron doctrine—the theory that neurons are the fundamental units of the brain. Ramón y Cajal didn’t think glia were necessarily just glue, however. Instead, he thought they were a mystery—a mystery, he wrote, that “may remain unsolved for many years to come until physiologists find direct methods to attack it.”

Today the mystery of glia is partially solved. Biologists know they come in several forms. One kind, called radial glia, serve as a scaffolding in the embryonic brain. Neurons climb along these polelike cells to reach their final location. Another kind of glia, called microglia, are the brain’s immune system. They clamber through the neurological forest in search of debris from dead or injured cells. A third class of glia, known as Schwann cells and oligodendrocytes, form insulating sleeves around neurons to keep their electric signals from diffusing.