How many neurons are in the human brain?
Where the 86 billion estimate comes from and how far it holds
Matic Broz, PhD9 min
An adult human brain contains about 86 billion neurons. The figure comes from a 2009 study that dissolved four donated brains into a suspension of cell nuclei and counted the neurons among them. Most of those neurons are not where people expect: about 69 billion sit in the cerebellum, the smaller structure tucked beneath the back of the brain, and only about 16 billion in the cerebral cortex.
Counting glial and other support cells as well, the brain holds about 170 billion cells. The older answer of 100 billion neurons was never a measurement, and 86 billion is an average rather than a fixed number: the nine adult brains counted with the same method held between about 62 billion and 95 billion neurons.
How many neurons are in an adult human brain?
The best direct estimate is 86.1 billion neurons, the mean that Azevedo and colleagues reported for four men aged 50 to 71 who died without neurological disease. Individual brains ranged from 78.8 billion to 95.4 billion.[1][2]
A second study using the same method examined five women aged 71 to 84 and found a mean of 67.3 billion, with individual brains from 62.1 billion to 72.1 billion.[3][2] The women were older and their brains lighter, averaging 1,291 grams against 1,509 grams for the men, so the two samples are too small and too different to show that one sex generally has more neurons.[2]
Four brains make a thin basis for a population average. Goriely calculated that the 2009 data support a male mean somewhere between about 73 billion and 99 billion at 95% confidence, and between 61 billion and 73 billion for the five women. In reply, von Bartheld agreed that the exact average is unknown and noted that estimates from two independent counting methods fall between about 60 billion and 95 billion.[2][4] “About 86 billion” is fair shorthand as long as it is read as an approximation.
Where did the 100 billion figure come from?
For decades, textbooks and reviews stated that the human brain has 100 billion neurons and ten times as many glial cells, usually without a reference. Earlier estimates had ranged from 3 billion in 1895 to 1 trillion in 1981 before the round number settled in the literature around 1991.[2][5]
When Herculano-Houzel's group asked senior neuroscientists in 2007, most believed the count was already known, yet none could name an original study for it. The closest thing to a source was a 1988 compilation by Williams and Herrup, who added up partial counts from different regions and arrived at about 85 billion.[6]
The 2009 count then placed 100 billion outside the range of every brain measured.[6] The round number persists because it is easy to remember, not because a whole-brain count ever produced it.
How do scientists count neurons?
No one counts 86 billion neurons one at a time. The 2009 study used the isotropic fractionator, informally called brain soup. Each region is dissected and its tissue dissolved into an evenly mixed suspension of free cell nuclei, so a small sample counted under the microscope scales up to the whole region. Staining for NeuN, a protein in the nuclei of most neurons, then separates neuronal from non-neuronal nuclei.[1]
The older approach, stereology, counts cells in thin sections sampled systematically through fixed tissue and multiplies their density by the volume of the region. It works best for well-defined regions with clear borders, and it produced most of the published counts for the neocortex, including those for newborns and older adults discussed below.[5]
Both methods have blind spots. NeuN, the product of the RBFOX3 gene, is not expressed in every neuron type, and its levels rise as neurons mature; its sequence and annotations can be retrieved with ProteinIQ's UniProt download tool.[2]
In a brain with thousands of molecularly distinct cell subtypes, a single marker is always an approximation. Dissection boundaries, tissue processing, and age at death add further variation, which is why each published total is an estimate rather than an inventory.[2][5]
Which brain region has the most neurons?
The cerebellum holds by far the most: about 69 billion neurons, or 80% of the total, in roughly a tenth of the brain's mass. The cerebral cortex, the folded outer layer that makes up 82% of brain mass, holds about 16.3 billion, or 19%. The brainstem, thalamus, basal ganglia, and other deep structures together hold about 690 million, less than 1%.[1][6]
The imbalance comes from the cerebellum's granule cells, tiny neurons packed at enormous density, which make up the overwhelming majority of its neurons.[5] A larger region does not necessarily hold more neurons; what matters is how small and tightly packed they are.
| Region | Share of brain mass | Neurons | Non-neuronal cells | Non-neuronal cells per neuron |
|---|---|---|---|---|
| Cerebral cortex | 82% | 16.3 billion | 60.8 billion | 3.7 |
| Cerebellum | 10% | 69.0 billion | 16.0 billion | 0.23 |
| Rest of the brain | 8% | 0.69 billion | 7.7 billion | 11 |
| Whole brain | 100% | 86.1 billion | 84.6 billion | 0.98 |
The cortical count is the one that sets humans apart. An African elephant brain, about three times heavier than ours, holds about 257 billion neurons, but 97.5% of them are in its cerebellum. Its cerebral cortex, twice the mass of the human cortex, holds only 5.6 billion neurons, about a third of the human number.[7] Herculano-Houzel and colleagues read this as evidence that humans stand out in cortical neurons, not in brain size or total neuron count.
Are there more neurons or glial cells in the human brain?
Neurons probably outnumber glia. The 2009 study counted 84.6 billion non-neuronal cells beside its 86.1 billion neurons, but that group includes an estimated 20 billion to 25 billion endothelial cells lining blood vessels as well as glia, the cells that insulate axons, regulate the chemical environment, and clear debris. Glia alone probably number closer to 40 billion to 50 billion.[1][5]
The textbook ratio of ten glial cells per neuron, which implied about a trillion glia, never had a primary source. One likely origin is a single researcher's work on brainstem nuclei where glia do outnumber neurons 30 to 50 times, a local ratio that was later repeated as a rule for the whole brain.[5]
Table 1 shows why no single ratio describes the brain. The cortex has about 3.7 non-neuronal cells per neuron, the cerebellum about one for every four neurons, and the deep structures about 11 per neuron.[1]
How many synapses are in the human brain?
Synapses, the junctions where one neuron passes a signal to the next, are counted in the trillions. The best direct counts cover the neocortex, the six-layered tissue that forms most of the cerebral cortex. Tang and colleagues counted synapses by electron microscopy in five young men's brains and estimated an average of 164 trillion; a later summary from the same Danish group gave about 150 trillion.[8][9]
Neither study counted synapses in the cerebellum or the deep structures. Whole-brain totals quoted online, often 100 trillion to 1 quadrillion, are therefore extrapolations rather than measurements.
How does the number of neurons change with age?
Most neurons are made before birth. There is no reliable whole-brain count for a newborn, but stereological studies of the neocortex find roughly the adult number already in place: a mean of 19.8 billion neurons in 10 full-term newborns, and 20.7 billion (range 18.0 billion to 24.8 billion) in 10 children from birth to age 3.[10][11]
Neuron numbers did not rise across those first three years, while astrocytes and oligodendrocytes, the glia that wrap axons in insulating myelin, increased steadily.[11] The often repeated claim that a baby is born with 100 billion neurons does not come from a whole-brain count.
In adulthood the loss is modest. A study of 94 Danish brains from people aged 20 to 90 found that about 10% of neocortical neurons are lost over that span in both sexes. The same study found averages of 19 billion neocortical neurons in women and 23 billion in men.[12]
For decades, textbooks taught that the brain loses large numbers of neurons every day after age 30. Those claims rested on cell densities that ignored how brains of different ages shrink differently when preserved; counts that corrected for this found little loss in most regions.[5] What declines more is the wiring: the total length of myelinated nerve fibers, 150,000 to 180,000 kilometers in young adults, falls substantially with age.[9]
Large losses of neurons point to disease rather than age alone. Andrade-Moraes and colleagues found a great reduction of neurons in the hippocampus and cerebral cortex of people with severe Alzheimer's dementia, but not in people whose brains showed the same plaques and tangles without symptoms.[3]
What the numbers add up to
Taken together, the counts describe a brain of about 170 billion cells, half of them neurons. Four in five of those neurons crowd into the cerebellum, yet the 16 billion in the cortex are what most clearly set it apart from larger brains such as the elephant's. Even so, the brain's cells are a small share of the roughly 36 trillion cells in a reference adult body, most of which are blood cells, and whole-brain counts leave out the neurons of the spinal cord and peripheral nerves.
