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Monday, February 28, 2011

The Other Brain

An interesting new book from R. Douglas Fields: The Other Brain.

"Glia" is a catch-all term for every cell in the nervous system that's not a neuron. We have lots and lots of them: on some estimates, 85% of the cells in the brain are glia. But to most neuroscientists at the moment, they're about as interesting as dirt is to archaeologists. They're the boring stuff that gets in the way. The name is Greek for "glue", which says a lot.

It's telling that most neuroscientists (myself included I confess) use the term "brain cells" to mean neurons, even though they're a minority. Hence the book's title: Douglas Fields argues that glia constitute a whole world, another brain - although of course, it's not seperate from the neuronal brain, and neuron-glia interactions are the really interesting thing and the central theme of the book.

Glia have historically been regarded as mere "housekeepers", keeping the brain neat and tidy by cleaning up the byproducts of neural activity. Douglas Fields explains that there's actually a lot more to glia than that, but that even if they were just housekeepers, the housekeeping they do is extremely important.

Astrocytes, one kind of glial cell, are key to the regulation of glutamate levels in the brain. Glutamate is by far the most common neurotransmitter yet it's also the most dangerous: glutamate can kill neurons if they receive too much of it (excitotoxicity). I previously wrote about some bad clams which can cause permanent brain damage if who eat them; the toxin responsible mimics the action of glutamate.

By quickly clearing up glutamate as it's released from neurons, astrocytes perform a vital function which saves the brain from self-destruction. Yet recent evidence has shown that they don't just mop up neurotransmitters, they also respond to them, and even release them. People are nowadays talking about the "tripartite synapse" - presynaptic neuron, postsynaptic neuron, and glia.

Glia even have their own communication network quite seperate from the neuronal one. Whereas neurons use electrical currents to convey signals, and chemicals to talk to other cells, astrocytes are interconnected via direct gap-junctions - literally, little holes bridging the membranes between neighbors.

Waves of calcium can travel through these junctions across long distances. The function of this glial network is almost entirely mysterious at present, but it's surely important, or it wouldn't have evolved. (A few types of human neurons do the same thing; in some animals it's more common.)

The subtitle is overblown, as subtitles often are ("From Dementia to Schizophrenia, How New Discoveries About the Brain are Revolutionizing Medicine and Science"); the book also repeats itself in a number of places, especially when it's castigating neuroscientists for overlooking glia for so long (a fair point, but it gets old.) Overall though it's very readable and it's got some nice anecdotes as well as the science.

The Other Brain makes an excellent case that neuroscience can't remain neuron-science if it hopes to answer the big questions. It's certainly opened my eyes to the importance of glia and given me ideas for my own research. As such it's one of those rare popular science books that will prove interesting to professionals and others too.

Link: Also reviewed here.

Disclaimer: I got a free review copy.

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