Brain neurons how many

For half a century, neuroscientists thought the human brain contained billion nerve cells. But when neuroscientist Suzana Herculano-Houzel devised a new way to count brain cells, she came up with a different number — 86 billion.

Read More. See how discoveries in the lab have improved human health. For Educators Log in. How Many Neurons Are in the Brain? Transcript Narration: It was once widely believed that the human brain had over billion neurons, and 10 times as many glia cells.

For decades, this number was repeated over and over. Then, one scientist sat down to actually test this number. I'm an associate professor at Vanderbilt University in Nashville. Narration: Were you originally taught that the brain had over billion neurons? Herculano-Houzel: Oh yeah, the billion neurons in the human brain, that's the number that appears in major text books, on the internet. You can write it in the opening paragraph of your review papers without ever bothering to give a reference.

It's as good a truth as genes are made of DNA. Who gives a reference for that anymore, right? You open textbooks and it's right there, so you just take it for the truth. Narration: Why did everyone think there were that many neurons in the brain? Herculano-Houzel: The way that people could count neurons or any other type of cells in brains was, and still is, stereology which essentially amounts to taking whole brains, slicing them up perfectly, and then sub-sampling a few sections under the microscope.

You look up how many cells you find in a given volume or within a given little dissector, just a probe that you place on the sections. That works beautifully provided that you know how to do it appropriately and that you're looking at a structure that's very homogeneous in the distribution of cells. It works perfectly for small parts of the brain; you want to count how many cells you have in the thalamus and motor nuclei, that's fine. If you want to apply that to the whole brain, you run into the problem of how different the distribution of neurons is from one millimeter to the next.

It's like taking a poll without knowing what you're doing, how people are distributed, or how they're concentrated You'd get a result but that doesn't necessarily mean that it's a good result, that it really represented the truth, let's say what you're going for.

It made it very difficult for people to actually get estimates of how many neurons composed whole brains of different species, much less a human brain, which is really large I think it was a mixture of different factors that got this magic number propagated so long and along with it, that story that we have 10 times as many glial cells as neurons in the human brain, which is just so not true.

Even though Barres is confident in his own unpublished calculations—and intends to write that glia far outnumber neurons in the newest edition of the Principles of Neural Science —he argues that no one has conducted the kind of rigorous study that would definitively answer the question of the glia to neuron ratio once and for all.

Barres envisions a study in which researchers stain whole human brains with just about every known marker for both neurons and glia—making sure to capture as many of the different cell types as possible—before slicing up the brains and meticulously counting the cells in each section. He says all the necessary tools are available. It is only a matter of funding the project and finding the time for all that counting. Let's say scientists figure out exactly how many glia and neurons the brain contains and everyone agrees on the numbers—what will that accomplish?

Why does it matter? Some scientists think that the glia to neuron ratio is just about one of the least important questions you can ask about the brain. Instead, they argue, scientists should focus on how brain cells behave. Other scientists point out that aging, as well as many neurological diseases, involve the loss of brain cells.

Understanding exactly which brain cells die and which survive could spur the development of new treatments. Some biologists and neuroscientists are also very interested in whether the glia to neuron ratio has changed over the course of evolution and whether, for example, animals with large brains—or brains that are large for their body size—have unusually high or low numbers of glia.

In a study , scientists sliced up five minke whale brains, counted the cells with the help of computers and found However, the study did not include the cerebellum, which contains most of the mammalian brain's neurons according to Herculano-Houzel's work.

Many researchers have argued that glia deserve more attention in part because they are so numerous. But prevalence is not equivalent to significance. Scientists no longer need to depend on the alleged ratio to justify glia research.

Glial cells are fascinating and important because of their structural diversity, functional versatility and the fact that they can change the behavior of firing neurons even though they cannot discharge electrical impulses of their own.

They guide early brain development and keep their fellow brain cells healthy throughout life. Glia are not mere structural filler, but—as the origin of their name implies Greek for glue —they help keep things together. Regardless of the true glia to neuron ratio, scientists have already shown that glia are, functionally, the brain's other half.

Azevedo, Frederico A. Carvalho, Lea T. Ferretti, Renata E. Dobbing, J and Sands, J. Quantitative growth and development of human brain. Arch Dis Child. Eriksen N, Pakkenberg B. Total neocortical cell number in the mysticete brain. Anat Rec Hoboken. Herculano-Houzel, Suzana and Roberto Lent. Hilgetag, Claus and Helen Barbas.

Principles of Neural Science, 4th ed. McGraw-Hill, New York. Pakkenberg, B. Journal of Microscopy, 1— The Neurocritic. Fact or Fiction? There are ten times more glia than neurons in the brain.

Sep 27, Ferris Jabr is a contributing writer for Scientific American. Already a subscriber? Sign in. F or a long time, neuroscientists would say that there are about billion neurons in the human brain. Interestingly, no one has ever published a peer-reviewed scientific paper supporting that count. Rather it's been informally interpolated from other measurements. A recent study from published by Azevedo and colleagues took a crack at a more precise estimate. Their answer? Approximately 86 billion neurons in the human brain.

The latest estimates for the number of stars in the Milky Way is somewhere between and billion. So close, but the human brain certainly doesn't quite stack up! But why do scientists think there are 86 billion neurons? How did they get that number? Well the easiest way to estimate the number of neurons in the brain is to count how many are in one part of the brain and then extrapolate out for the rest of the brain's volume.

Interestingly, this method can also be used to estimate how many stars are in the Milky Way! But the method has a few issues:. The brain's neuronal density isn't uniform. For example, the cerebellum the artificially purple-colored structure in the bottom back in the image to the left source: wikipedia contains about half of all the neurons in the central nervous system, but it is well below half the volume.

It's hard to get an estimate even for one brain region, because the neurons are so dense and intertwined and mostly clear! One method is to use a staining technique to make neurons visible enough to count them. A classic method is the "Golgi stain" named after Nobel prize winner Camillo Golgi.

This method stains only a few percent of neurons no one's quite sure why. So in the stain below source: Scholarpedia , even though only one neuron is visible, there may be hundreds more in that space that you can't see because they didn't stain.

Using this method, you can estimate what proportion of neurons gets stained, count the number in some patch of brain, then extrapolate. But you're introducing two variables for your guess here! Not very accurate. The new method that gives us the 86 billion figure is The method involves dissolving the cell membranes of cells within the brain and creating a homogeneous mixture of the whole soup.

The nuclei of these cells are stained using different markers to differentiate neurons from glia, allowing you to count the number of cell nuclei belonging to neurons as opposed to other cells in the brain such as glia and then scale up to get the overall number. The great advantage of this method is that unlike counting the number of neurons in one part of the brain and then extrapolating from that, it gets over the problem that different brain regions may have more or less densely packed neurons.

There you go! This is the latest plausible estimate. But you notice that to do this, the researchers are still using the extrapolation method.