Just when you thought the brain was complicated enough as it is, it turns out we barely even know what most of the brain cells is doing. In The Other Brain: From Dementia to Schizophrenia, How New Discoveries about the Brain Are Revolutionizing Medicine and Science, by R. Douglas Fields, the “other brain” refers to the 85% of the brain cells that aren’t neurons: the glial cells.
When you think of a brain cell, you mostly think of neurons, which receive input from the neurons around them through the synapses, and transmit one output channel into the long axon. When early researchers were trying tease out the tangled network of neurons, other cells just seemed to be glued them together. So they called them glial cells. The obscurity of glial cells is compounded by the fact that since neurons communicate by electric impulses, their activity is relatively easy to detect. It was only recently discovered that glia communicate by chemical means, such as waves of calcium ions. Not exactly something you can stick a probe into. Early in this interview, the author describes how they found this out by making the cells light up. Very cool.
The book introduces four classes of glia: astrocytes, microglia, and oligodendrocytes in the brain and spinal cord; and Schwann cells in the rest of the body. Astrocytes were first seen only as their starry skeletons, which is what took up the dye used. They supply nutrients. After a synaptic transmission, they collect neurotransmitters and respond with waves of calcium ions among the astrocytes. Microglia are the equivalent of white blood cells, protecting the brain from injury and disease organisms. Oligodendrocytes cover the axons of neurons like octopi with tendrils wrapped around, and coat the axons with myelin, the fatty insulation that allows neurons and nerves to transmit electrical impulse over distance. In the rest of the body, some Schwann cells cover the nerves with myelin, other cover clusters of smaller nerves, and others seal off the ends of nerves where they meet their target, such as muscles.
Most of the book covers current research and speculation about the role of glia in various conditions and diseases. For example, the chapter on spinal cord injury explains why it’s so hard to recover. When a nerve is injured, it dies. When the new one grows back, chemical signals from the Schwann cells guide regrowth down the channel of the old nerve. Inside the spinal cord and brain, microglia attack the blood crossing the broken barrier. Unfortunately oligodendrocytes are killed in the crossfire, leaving the axons of neurons bare, which can lead to further neural deaths. In the days following a spinal injury, the damage threatens to spread through the spinal cord. But astrocytes wall it off. Microglia consume the debris. Within a week the injury bound up in a scab around a cyst filled with fluid. This barrier means paralysis is permanent. Until, of course, we figure out how to get around that.
What this book does best is convey how embryonic this whole field is. I found some parts of it confusing, including basic ideas like what the four classes of glia are. (It doesn’t help that there are three kinds of Schwann cells and it really doesn’t help that they are lumped together as a class.) I wanted more pictures, and I wanted more organization. On the whole, though, it’s a fascinating topic sure to yield more.