The Brain's Silent Majority

The Brain's Silent Majority: How the Other 90 Percent of Your Brain Works  link
By Bruce Goldman
"Fall" 2009

What a great read! The article's focus is on glial cells (latin for glue) and our long overdue understanding of what their functions are. It's funny because the main focus of pretty much any neuroscientist out there is on the neurons, which of course ARE very important and talented, but thanks to scientist Ben Barres we also now know a lot more about what the majority of the cells in our brain do.

There are three types of glial cells; astrocytes, microglia, and oligodendrocytes. Each one having their own individual job functions in the brain.

Astrocytes, making up a cool 50% of cells in the human brain, are the least understood glial cell of the three, but also apparently has the most interesting job skills. First and foremost it appears they have a lot to do with where and when synapses form (an experiment was done where neurons were in a lab all by themselves, and they seemed finally and even functioned fine by sending along electrical signals. But when they took a closer look they noticed the synapses were few and far between and even then they didnt function as well as they normally do. After adding astrocytes there was an immediate improvement, where a lot more synapses were formed and their messages conveyed a lot faster!) Astrocytes also take care of housekeeping functions such as feeding the neurons (supplying nutrients, energy-rich molecules and neurotransmitter precursors) or mopping up after them (for example, speedily slurping up spent neurotransmitter molecules from the synapses so that the next signal will be a clean one). Astrocytes also secrete a protein called thrombospondin (trying saying that three times fast!) during brain development and when brain maturation is complete thrombospondin expression shuts down everywhere in the brain except the hippocampus (which you should know by now is the center where new memories are formed and where neurogensis still occurs). But, interestingly enough, when the brain is injured astrocytes turn back on thrombospondin expression. Barres asks: Could those astrocytes be playing a part in inducing and repairing synapses in the injured brain? They of course are not sure one way or another, but it sure looks that way doesnt it?

It is also worth noting thrombospondin is one of only two genes that are far more highly expressed in human brains than in those of other primates. The connection is currently unknown.

Oligodendocytes, which account for roughly 40% of the cells in the human brain, extrude a fatty product called myelin, which insulates neuronal surfaces and speeds signals along axons. Myelin is largely responsible for giving heavily myelinated regions of the brain (the "white matter") their lighter color.

Microglia, which make up to about 10% of the cells in a human brain, serve as an immune function in our brains, which are somewhat impervious to immune cells attempting entry from across the blood-brain barrier. Microglia are thought to migrate into the brain in early development, before the barrier is in place. I find this really interesting, I suppose I never thought of our brains to have a "barrier" to otherwise potentially helpful cells in the rest of our body. Its good to know it has its own security team.

So there you have it folks. The neurons may be celebrities, but they surely didnt get there all by themselves!