The dark matter of the brain

Discover Magazine has an excellent Carl Zimmer article on glial cells. They make up the majority of the brain’s volume but they get relatively little attention from the neuroscience community who would rather focus on the seemingly more lively neurons.

There’s a traditional format for these stories, that says that we used to think that glial cells were just ‘scaffolding’ for the brain that gave protected padding for the neurons, but now we are on the verge of a breakthrough in understanding what they do.

Here’s one from New Scientist in 1994, and a pdf of another from Scientific American in 2004.

One difficulty has been integrating the action of glial cells into the popular cognitive model of the brain that suggests that it works as an information processing device.

While there have been various discoveries about the biological function of glia, this is the first article I’ve read which gives a clear idea of how one type of glial cell, the astrocyte, might be involved in information processing.

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.”

Obviously this is based on the idea that we need to fit new biological findings into the computational model, rather than fitting our model of the mind into the biology, but that’s a whole different battle.

Link to Discover article ‘The Dark Matter of the Human Brain’.