1. Is the Brain Made of Only Neurons?
No. In a study of adult male human brains, researchers counted about 86 billion neurons and about 85 billion nonneuronal cells (cells that are not neurons) [1]. That is almost a one-to-one match. That group is not all glia, because it also includes cells such as those lining blood vessels. Still, a large share of it is glial cells [6].
Older books often said something very different: that glia outnumber neurons ten to one. According to a review of 150 years of cell counting, the human brain was believed for half a century to hold about 100 billion neurons and about one trillion glial cells [2]. A newer counting method, the isotropic fractionator (it dissolves tissue so the cell nuclei can be counted), pointed to a glia-to-neuron ratio much closer to one to one [2]. The ratio also differs from one part of the brain to another, so "how many glia" depends on where and how you count [6].
If glia are that common, what are they doing? First, a look at why they got such a dismissive name.
2. Why Were Glial Cells Named After Glue?
According to an encyclopedia entry, the word "glia" comes from a Greek word meaning "glue." Early observers thought these cells held the nervous system together [6]. The German pathologist Rudolf Virchow is credited with first describing glia in 1856, while he was looking for a "connective tissue" in the brain [6]. (This history comes from an encyclopedia entry only, so treat the date as a good lead to check, not as settled.)
For a long time, attention went to neurons, which send electrical signals. Glia looked like the supporting cast. The name stuck even after scientists learned that glia are several different kinds of cell with different jobs. Here are three of the best-known kinds.
3. Which Glial Cells Insulate Nerve Fibers?
A neuron sends signals along a long, thin fiber called an axon. In the brain and spinal cord, cells called oligodendrocytes wrap parts of axons in a fatty layer called myelin [6]. Think of the plastic coating on an electric wire. The coating insulates the fiber and affects how signals travel along it [6].
So some of the cells once called "glue" are actually part of how fast signals move through the nervous system.
4. What Do Star-Shaped Astrocytes Do?
Under a microscope, astrocytes look like stars with many branching arms. They sit between neurons and blood vessels, and, according to the encyclopedia entry, they help form the blood-brain barrier. They also help keep the neurons' surroundings stable, for example by clearing extra potassium and recycling chemical messengers [6].
One striking result came from mice in 2012. Cerebrospinal fluid, the clear fluid around the brain, was seen flowing into brain tissue along the spaces around arteries and draining out along veins. In mice that lacked a water channel called aquaporin-4 in their astrocytes, this flow slowed and the clearing of waste from between cells dropped by about 70 percent. A protein linked to Alzheimer's disease, amyloid beta, was carried along the same route [3]. These are mouse results, and how much of this happens in people is still being studied.
5. How Do Microglia Clean Up and Trim the Brain?
Microglia are the brain's own immune cells. They engulf debris and help defend against harm, and they multiply when the brain is injured [6].
The surprise is that they also work in healthy, growing brains. In 2011, researchers reported that in young mice, microglia take up pieces of synapses, the contact points between neurons, and that this is involved in "pruning," the way a young brain removes extra connections [4]. In 2012, a second team studied the pathway from the eye to the thalamus, a relay station deep in the brain. In developing mice, microglia took in the incoming connections depending on how active they were, using a protein system called complement (C3 and its receptor CR3). When that system was disrupted, the mice were left with lasting defects in their connections [5].
A gardener trimming a hedge is a rough picture of this, though microglia are of course just cells following chemical signals. Both studies were in mice, so the question of exactly how this works in the human brain remains open.
6. What Do Scientists Still Disagree About?
Quite a lot. Many of the best results come from mice, and researchers are still working out how closely human brains match them [3][5]. How far astrocytes take part in handling information, rather than only supporting neurons, is also debated among researchers (this article draws on the abstracts listed below and does not settle that argument).
What is no longer in doubt is that "glial cells are just glue" is too simple. The brain works as a team of several kinds of cells, with neurons sending signals and glia insulating, tending, clearing, and trimming.
7. How Can You Check a Number Like "Ten to One" Yourself?
If you find a book or website saying glia outnumber neurons ten to one, look at when it was written and whether it names a study. An old number may still be repeated long after it was corrected. The review in source [2] explains how the figure arose and why it lasted; its abstract is free to read.
When you read about a brain-cell discovery, also ask which animal it was tested in. "In mice" and "in people" are different claims, even when the headline sounds the same.