About 100 billion neurons make up the brain.
Older estimates have long suggested that 100 billion neurons in the human brain was the magic number, but some more recent research suggests that the brain actually contains fewer neurons than previously believed.
The human brain is made up of a complex network of neurons. These neurons serve as the building blocks of the nervous system, transmitting information to and from the brain and throughout the body. You probably expect that a large number of neurons are required for such a complex process, but just how many neurons are there in the human brain?
Neurons in the Human Brain
According to many estimates, the human brain contains around 100 billion neurons (give or take a few billion). This estimate has often been reported for many years in neuroscience and psychology textbooks and for many years was simply accepted as a relatively close approximation.
Recently, however, Brazilian researcher Dr. Suzana Herculano-Houzel discovered that these estimates might not be entirely accurate. While the number is widely cited, she found that no one seemed to know where or when this number originated. She then decided to investigate in order to determine if the number is accurate.
Estimating the number of neurons in the brain seems fairly simple on the surface. Simply take a sample of the brain, count the number of neurons in that sample and then extrapolate that information to account for the remaining brain volume.
While this seems like a fairly straightforward approach, neuron density differs in different regions of the brain. Counting neurons in a high-density part of the brain might lead to a high estimate while counting those in a lower density region might lead to an excessively low estimate.
To overcome this problem, the researchers utilized a method that involved dissolving the cell membranes in order to create a sort of "brain soup" so that they can then count the number of cell nuclei in a sample. The nuclei of the cells were also stained to differentiate between neurons and glia, allowing researchers to then count the cell nuclei that belong to neurons.
"It took me a couple of months to make peace with this idea that I was going to take somebody's brain or an animal's brain and turn it into soup," Herculano-Houzel explained to Nature. "But the thing is we have been learning so much by this method we've been getting numbers that people had not been able to get … It's really just one more method that's not any worse than just chopping your brain into little pieces.