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How Many Neurons Are in the Brain?

By Kendra Cherry - Reviewed by a board-certified physician.
Updated March 30, 2017

How many neurons are in the human brain?
Older estimates have long suggested that 100 billion 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?

How Many Neurons are 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."

            How many neurons did the researchers find in the brains they analyzed?

            "We found that on average the human brain has 86 billion neurons. And not one that we looked at so far has the 100 billion. Even though it may sound like a small difference the 14 billion neurons amount to pretty much the number of neurons that a baboon brain has or almost half the number of neurons in the gorilla brain. So that's a pretty large difference actually," explained Herculano-Houzel.

            So, according to this new research, the human brain likely has somewhere around 86 billion neurons.

            Neurons in Other Animals
            According to Herculano-Houzel, human brains are remarkably similar to primate brains with one important distinction: we have far more brain cells that require a tremendous amount of energy to fuel and maintain.

            Experts suggest that an estimated 25 percent of all our energy expenditure goes toward fueling all of these cells.

            The sheer number of neurons present in the human brain becomes more apparent when compared to other species. So how many neurons are in the brains of other animals?
            Fruit Fly: 100 thousand neurons
            Cockroach: One million neurons
            Mouse: 75 million neurons
            Cat: One billion neurons
            Chimpanzee: 7 billion neurons
            Elephant: 23 billion neurons

While the human brain might not have the mythic 100 billion neurons as long suspected, 86 billion is still nothing to sneeze at.
Learn more about neurons and the human brain:
            What Is a Neuron?
            The Structure of a Neuron
            The Anatomy of the Human Brain
            How Big Is the Brain?
            Do We Only Use 10-Percent of Our Brain?

Herculano-Houzel, S. The human brain in numbers: A linearly scaled-up primate brain. Frontiers in Human Neuroscience. 2009; 3(31): doi:10.3389/neuro.09.031.2009.
Orca, S. The race to reverse engineer the human brain. H + Magazine.2009.
Randerson, J. How many neurons make a human brain? Billions fewer than we thought. The Guardian. 2012.
Williams, Robert W. Mapping Genes that Modulate Mouse Brain Development: A Quantitative Genetic Approach. In: Mouse brain development (Goffinet AF, Rakic P, eds), Springer Verlag, New York, 2149; 2000.
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