# Connected (DD-635) > 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. - Department: Science - First published: 2025-08-12 - Last updated: 2025-08-12 - Permalink: https://detailenjoyer.com/case/connected/ - Author: Marcus Easterling (DetailEnjoyer), The Detail Department ## Quick facts - **DD-635-F01**: The human brain has about 86 billion neurons. _(Write-up · Exhibit A (Where 86 billion comes from))_ - **DD-635-F02**: About 69 billion neurons are in the cerebellum. _(Write-up · Exhibit C (Mostly in the cerebellum))_ ## Write-up 86 billion neurons, trillions of links. Synapses wire the brain together. 1. **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. **Brain soup**: The team dissolved brain tissue into a uniform suspension of nuclei and counted samples. 3. **Mostly in the cerebellum**: About 69 billion neurons are in the cerebellum and about 16 billion in the cerebral cortex. 4. **Counting synapses**: The neocortex alone holds an estimated 150 trillion synapses. 5. **Rough numbers**: Synapses per neuron vary hugely by cell type, so whole-brain totals are estimates, not counts. ## Related case files - [Happiness (DD-743)](https://detailenjoyer.com/case/happiness/): Brain chemistry in action - [SSRIs (DD-749)](https://detailenjoyer.com/case/ssris/): How drugs act on synapses - [Synaptic Transmission (DD-275)](https://detailenjoyer.com/case/synaptic-transmission/): I linked more on how the brain is wired ## Reel vs. record - **Note**: 'Approximately 86 billion': correct, but it is a 2009 estimate (86.1 ± 8.1 billion) from the brains of four adult men, not an exact count. - **Note**: 'Each connecting... 1,000 to 10,000 synapses per neuron': synapse counts vary enormously by cell type, so this is a rough rule of thumb, not a range every neuron falls within. - **Note**: '100 to 1,000 trillion': a reasonable range of estimates. The neocortex alone has been estimated at about 150 trillion synapses (2003). - **Missing**: most neurons, about 69 billion, are in the cerebellum, not the cerebral cortex (about 16 billion). - **Note**: 'Vast processing power': synapse counts alone don't measure what a brain can do. How the connections are organized and how strong they are matters too. ## Video: Connected: 86 Billion Neurons, Trillions of Links - Duration: 23.9 s · Published: 2025-08-12 - File: https://detailenjoyer.com/assets/videos/DNRYax-S9le.mp4 ### Transcript 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! ## Questions & answers **Q: About how many neurons are in the human brain?** A: 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))_ **Q: What figure did the 86 billion estimate replace?** A: 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))_ **Q: Which part of the brain holds the most neurons?** A: 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))_ **Q: How many synapses are estimated in the neocortex alone?** A: About 0.15 quadrillion. _(Source: Write-up · Exhibit D (Counting synapses) · Pakkenberg et al. (2003): Aging and the human neocortex, Experimental Gerontology (PubMed))_ **Q: Where do neurons pass signals to each other?** A: 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))_ ## Sources - [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)](https://pubmed.ncbi.nlm.nih.gov/19226510/) - [Herculano-Houzel (2009): The human brain in numbers: a linearly scaled-up primate brain, Frontiers in Human Neuroscience (PMC)](https://pmc.ncbi.nlm.nih.gov/articles/PMC2776484/) - [Pakkenberg et al. (2003): Aging and the human neocortex, Experimental Gerontology (PubMed)](https://pubmed.ncbi.nlm.nih.gov/12543266/) ## 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/