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Knowing there are 100,000,000,000 neurons in an average human brain, and 7,000 dendrites in each neuron, and neurons are connected to each other by dendrites and axon terminals, how many synapses are there in an average human brain? (I don't know how many axon terminals per neuron there are on average.)

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well this what I learned using this beatiful schema :) en.wikipedia.org/wiki/Neuron#mediaviewer/… –  mounaim Aug 12 at 12:02

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The human neocortex contains approximately 1.5x10^14 synapses, connecting its 19–23 billion neurons.

Source: Pakkenberg, B., Pelvig, D., Marner, L., Bundgaard, M. J., Gundersen, H. J. G., Nyengaard, J. R., & Regeur, L. (2003). Aging and the human neocortex. Experimental Gerontology, 38(1-2), 95-99. doi:10.1016/S0531-5565(02)00151-1

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For the less mathematically inclined, $1.5 \times 10^{14} = 100,000,000,000,000$, or 150 trillion. –  Eoin Aug 11 at 11:04
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150 000 000 000 000 instead :) –  mounaim Aug 12 at 11:56
    
@Dylan the average number of human brain neurons is 100 billion :) not 19-23. This is what I'm reading everywhere human-memory.net/brain_neurons.html for exemple –  mounaim Aug 12 at 12:08
    
The numbers in Pakkenberg et al apply to human neocortex, presumably ignoring subcortical structures (plus cerebellum, for instance), which would inflate the numbers somewhat. Also, if you have a solid reference from a research article using stereology, please feel free to include it. The numbers bandied around are usually just orders of magnitude (i.e. accurate to within a multiple of 10). –  Dylan Richard Muir Aug 12 at 13:35
    
This 2009 paper estimates the human brain as having 85 billion neurons. Interesting TED talk about it, too. –  Chelonian Aug 30 at 5:15

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