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Connected

Short answer: The '86 billion neurons' figure comes from a 2009 study by Frederico Azevedo, Suzana Herculano-Houzel and colleagues. They dissolved the brains of four adult men into a uniform 'soup' of cell nuclei and counted samples.

They found about 86.1 billion neurons (give or take 8.1 billion), replacing the round 100 billion long quoted in textbooks. Most of those neurons aren't in the wrinkled cortex. About 69 billion are in the cerebellum, and about 16 billion are in the cerebral cortex. Neurons pass signals to each other at synapses. Synapse counts per neuron vary enormously by cell type, so '1,000 to 10,000 per neuron' is only a rough rule of thumb. Whole-brain totals are estimates too. A 2003 stereology study by Bente Pakkenberg and colleagues put the neocortex alone at about 0.15 quadrillion (150 trillion) synapses, which fits within the Reel's range of 100 to 1,000 trillion. No one has counted every synapse in a human brain, so the totals are estimates. The footage is golden, branching animation of neurons.

VideoWrite-up3 sources

In collections:The Brain

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What should you know about Connected?

Quick facts. Each one stands alone and has a stable ID you can cite.

  1. The human brain has about 86 billion neurons.

    DD-635-F01 · Write-up · Exhibit A (Where 86 billion comes from)

  2. About 69 billion neurons are in the cerebellum.

    DD-635-F02 · Write-up · Exhibit C (Mostly in the cerebellum)

Video transcript: Connected: 86 Billion Neurons, Trillions of Links

DID YOU KNOW? The human brain contains approximately 86 billion neurons, each connecting to other neurons through synapses, with estimates ranging from 1,000 to 10,000 synapses per neuron. This results in roughly 100 to 1,000 trillion synaptic connections, forming an extraordinarily complex neural network. These connections enable the brain's vast processing power, supporting everything from basic reflexes to complex cognition. NOW YOU KNOW!

Burned-in captions, read frame by frame (see transcript.md) · 23.9s Reel ·

Video coming soonConnected · Instagram Reel
Connected: 86 Billion Neurons, Trillions of Links Instagram Reel · · 23.9s

The write-up

86 billion neurons, trillions of links. Synapses wire the brain together.

  1. Exhibit A

    Where 86 billion comes from

    A 2009 study counted about 86.1 billion neurons in adult male brains, replacing the old 100 billion figure.

  2. Exhibit B

    Brain soup

    The team dissolved brain tissue into a uniform suspension of nuclei and counted samples.

  3. Exhibit C

    Mostly in the cerebellum

    About 69 billion neurons are in the cerebellum and about 16 billion in the cerebral cortex.

  4. Exhibit D

    Counting synapses

    The neocortex alone holds an estimated 150 trillion synapses.

  5. Exhibit E

    Rough numbers

    Synapses per neuron vary hugely by cell type, so whole-brain totals are estimates, not counts.

Questions people ask about Connected

About how many neurons are in the human brain?

About 86 billion, per a 2009 study.

Source: Write-up · Exhibit A (Where 86 billion comes from) · Azevedo et al. (2009): Equal numbers of neuronal and nonneuronal cells make the human brain an isometrically scaled-up primate brain, J. Comp. Neurol. (PubMed)

What figure did the 86 billion estimate replace?

The textbook 100 billion.

Source: Write-up · Exhibit A (Where 86 billion comes from) · Azevedo et al. (2009): Equal numbers of neuronal and nonneuronal cells make the human brain an isometrically scaled-up primate brain, J. Comp. Neurol. (PubMed)

Which part of the brain holds the most neurons?

About 69 billion.

Source: Write-up · Exhibit C (Mostly in the cerebellum) · Herculano-Houzel (2009): The human brain in numbers: a linearly scaled-up primate brain, Frontiers in Human Neuroscience (PMC)

How many synapses are estimated in the neocortex alone?

About 0.15 quadrillion.

Source: Write-up · Exhibit D (Counting synapses) · Pakkenberg et al. (2003): Aging and the human neocortex, Experimental Gerontology (PubMed)

Where do neurons pass signals to each other?

Synapses are the junctions between neurons.

Source: Write-up · Exhibit D (Counting synapses) · Pakkenberg et al. (2003): Aging and the human neocortex, Experimental Gerontology (PubMed)

Test Your Knowledge

All quizzes →

5 questions, all from this case file. Answers are revealed instantly.

  1. About how many neurons are in the human brain?

    1. 86 billion
    2. 8.6 million
    3. 860 billion
    4. 86 trillion
    Show the answer

    Answer: A. 86 billion About 86 billion, per a 2009 study. Source: Write-up · Exhibit A (Where 86 billion comes from) · Azevedo et al. (2009): Equal numbers of neuronal and nonneuronal cells make the human brain an isometrically scaled-up primate brain, J. Comp. Neurol. (PubMed)

  2. What figure did the 86 billion estimate replace?

    1. 100 billion
    2. 10 billion
    3. 1 trillion
    4. 50 billion
    Show the answer

    Answer: A. 100 billion The textbook 100 billion. Source: Write-up · Exhibit A (Where 86 billion comes from) · Azevedo et al. (2009): Equal numbers of neuronal and nonneuronal cells make the human brain an isometrically scaled-up primate brain, J. Comp. Neurol. (PubMed)

  3. Which part of the brain holds the most neurons?

    1. The cerebellum
    2. The cerebral cortex
    3. The brainstem
    4. The spinal cord
    Show the answer

    Answer: A. The cerebellum About 69 billion. Source: Write-up · Exhibit C (Mostly in the cerebellum) · Herculano-Houzel (2009): The human brain in numbers: a linearly scaled-up primate brain, Frontiers in Human Neuroscience (PMC)

  4. How many synapses are estimated in the neocortex alone?

    1. About 150 trillion
    2. About 150 million
    3. About 1,500
    4. About 150 billion
    Show the answer

    Answer: A. About 150 trillion About 0.15 quadrillion. Source: Write-up · Exhibit D (Counting synapses) · Pakkenberg et al. (2003): Aging and the human neocortex, Experimental Gerontology (PubMed)

  5. Where do neurons pass signals to each other?

    1. At synapses
    2. At ribosomes
    3. At mitochondria
    4. At alveoli
    Show the answer

    Answer: A. At synapses Synapses are the junctions between neurons. Source: Write-up · Exhibit D (Counting synapses) · Pakkenberg et al. (2003): Aging and the human neocortex, Experimental Gerontology (PubMed)

Cite this case file

Easterling, Marcus. "Connected." The Detail Department (DetailEnjoyer), case file DD-635, last updated Aug 12, 2025. https://detailenjoyer.com/case/connected/

Permalink: https://detailenjoyer.com/case/connected/ · Last updated

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