The Brain: The Dark Matter of the Human Brain

By | August 24, 2009

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. …

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.

But the more neuroscientists examine glia, the more versatile these cells turn out to be. Microglia do not just keep the brain clean; they also prune away extra branches on neurons to help fine-tune their developing connections. Oligodendrocytes and Schwann cells don’t just insulate cells; they also foster new synapses between neurons. And once radial glia are finished helping neurons move around the developing brain, they don’t die. They turn into another kind of glia, called astrocytes.

Astrocytes—named for their starlike rays, which reach out in all directions—are the most abundant of all glial cells and therefore the most abundant of all the cells in the brain. They are also the most mysterious. A single astrocyte can wrap its rays around more than a million synapses. Astrocytes also fuse to each other, building channels through which molecules can shuttle from cell to cell.

All those connections put astrocytes in a great position to influence the goings-on in the brain. They also have receptors that can snag a variety of neurotransmitters, which means that they may be able to eavesdrop on the biochemical chatter going on around them. Yet for a long time, neuroscientists could not find any sign that astrocytes actually responded to signals from the outside. Finally, in 1990, neuroscientist Ann Cornell-Bell at Yale discovered what seemed to be a solution to the mystery. It turned out that astrocytes, like neurons, can react to neurotransmitters—but instead of electricity, the cells produce waves of charged calcium atoms. …

For some brain scientists, these discoveries are puzzle pieces that are slowly fitting together into an exciting new picture of the brain. Piece one: Astrocytes can sense incoming signals. Piece two: They can respond with calcium waves. Piece three: They can produce outputs—neurotransmitters and perhaps even calcium waves that spread to other astrocytes. In other words, they have at least some of the requirements for processing information the way neurons do. Alfonso Araque, a neuroscientist at the Cajal Institute in Spain, and his colleagues make a case for a fourth piece. They find that two different stimulus signals can produce two different patterns of calcium waves (that is, two different responses) in an astrocyte. When they gave astrocytes both signals at once, the waves they produced in the cells was not just the sum of the two patterns. Instead, the astrocytes produced an entirely new pattern in response. That’s what neurons—and computers, for that matter—do.

If astrocytes really do process information, that would be a major addition to the brain’s computing power. After all, there are many more astrocytes in the brain than there are neurons. Perhaps, some scientists have speculated, astrocytes carry out their own computing. Instead of the digital code of voltage spikes that neurons use, astrocytes may act more like an analog network, encoding information in slowly rising and falling waves of calcium. In his new book, The Root of Thought, neuroscientist Andrew Koob suggests that conversations among astrocytes may be responsible for “our creative and imaginative existence as human beings.”

via The Brain: The Dark Matter of the Human Brain | Memory, Emotions, & Decisions | DISCOVER Magazine.

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