Dear Dr. Phil,

How many brain cells do humans have?

Thanks, Brian, age 7

Hi Brian,

Great question, but not an easy one to answer.

When I was in school, it was commonly said that the “human brain and the Milky Way Galaxy have about 100 billion parts.” This comparison was used because it is very hard to explain such large numbers. Over the years, science has learned that the 100 billion number for brain cells and stars is wrong.

Researchers in the Journal of Comparative Neurology reported in 2009 that 86.1 billion neurons are in the average human male brain. (Give or take 8.1 billion if you want to be very accurate.) But there is more, the brain also has some 84.6 billion non neuronal brain cells. These are the support cells of the brain, mostly made up of glial cells. Glial, from the Greek meaning “glue,” hold the brain together - kind of like warm gray and white Jello.

According to Sky and Telescope, our Milky Way galaxy has “around 300 billion stars” give or take 100 billion stars. Thus, our galaxy has way more stars than we have neurons.

How do you count brain cells or stars?

If you wanted to count all the cells in the brain it would take a really long time. Instead, scientists would take a representative tiny section of the brain, count the cells in that section and then extrapolate (multiply out) that number to get a ballpark figure for the whole brain.

The problem with this method is that some parts of the brain have lots more cells packed closer together, than other parts of the brain. If that isn’t difficult enough, brain cells are wrapped up and around each other, weaving all over the place. To top this all off, the interweaving neurons are mostly see through. Making it very hard to count them.

This method of estimation is also used in counting the stars in the Milky Way. That is why you tend to get a range as an answer. Such as 200-400 billion stars in our galaxy.

Science to the rescue

Is this opening addition okay? To conquer these challenges, scientists have developed a new ingenious method for estimating brain cells. It was used to estimate the neuron count of 86.1 billion.

!!!!! Gross alert. This next section is kind of yucky.

What the scientists did was take a portion of the brain, put it in a dissolving mixture that broke down all the cell membranes, and making a brain soup of sorts. This brain soup was carefully mixed together to get all the cell bits evenly mixed up. Then a small portion of this soup was removed for the counting process. Stains were added to this. These stains only change the color of nerve cells. By counting the colored neurons in the portion, the scientists were able to estimate the number of nerves in the total brain soup. This is how we got the number 86.1 billion. Because scientists like to be as accurate as possible, here is how they reported their findings.

Here we determine these numbers by using the isotropic fractionator and compare them with the expected values for a human-sized primate. We find that the adult male human brain contains on average 86.1 +/- 8.1 billion NeuN-positive cells ("neurons") and 84.6 +/- 9.8 billion NeuN-negative ("nonneuronal") cells.

Your spinal cord learns too

It is well known that when you are learning a task, like playing music on the piano, your brain is learning how to play the song. Your brain controls your movements and it stores this information so you can do that movement again.

There has been some question about whether your brain does all of the learning. Could it be that when you learn a complex task, like playing music, that your spinal nerves also learn? It has been hard to test this question. How do you look at the brain and the spinal nerves while motor learning is taking place? The answer: by using an fMRI

New research on learning

Richard Robertson reports in this month's PloS/Biology that the spine also “learns”:

Those difficulties have been overcome in a new study from Shahabeddin Vahdat, Ovidui Lungu, Julien Doyon, and colleagues, who use simultaneous functional magnetic resonance imaging (fMRI) of the brain and spinal cord to show that motor output areas of the cord that control the finger muscles display learning-related changes in blood flow independent of those in the brain while learning a complex finger movement.

Read the study: Simultaneous Brain-Cervical Cord fMRI Reveals Intrinsic Spinal Cord Plasticity during Motor Sequence Learning.

In the future we may be wishing for a big brain and a big spinal cord to help us learn complex tasks like hitting a ball, dancing the cha cha, or diving off the high board.