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The overall number of synapses in the human brain is roughly 1,000 trillion, i.e. 10,000 synapses per neuron.

My question here is three-fold:

  1. I assume that each structural type of neuron (basket, pyramidal, ...) has a somehow characteristic average number of in-coming and out-going synaptic connections, i.e. not every type of neuron will have about 10,000 in-coming and 10,000 out-going synaptic connections. I am looking for a thorough overview of these characteristic numbers by structural type.

  2. I assume that for some functional groups of neurons (see The human brain in numbers I: neurons) there are characteristic average numbers of in-coming and out-going synaptic connections per neuron. I am looking for a thorough overview of these characteristic numbers by functional group.

  3. Finally, for some functional groups there are presumably characteristic ratios of inner and outer synaptic connections: "x % of the in-coming synaptic connections of a neuron in group X come from other neurons in this group, the rest comes from the outside. y % of the out-going synaptic connections of a neuron in group X go to other neurons in this group, the rest goes to the outside". I am looking for a thorough overview of these characteristic ratios by functional group.

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    $\begingroup$ Woah, that's just too broad a question! Divide it into (at least) three questions, lest even a bounty of +500 points is not very likely to get you a complete answer for such a big question... $\endgroup$ – another 'Homo sapien' Jul 23 '17 at 10:32
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    $\begingroup$ Yes, incredibly incredibly too broad. You might look into projects like the Blue Brain Project in Europe and the BRAIN initiative in the US. $\endgroup$ – Bryan Krause Jul 23 '17 at 17:29
  • $\begingroup$ I'd like to, but these are real huge projects with vast web sites. And I am not sure if I would find thorough and concise overviews of the kind I'm looking for. So if anyone knows... $\endgroup$ – Hans-Peter Stricker Jul 23 '17 at 18:45
  • $\begingroup$ @hansstricker honestly, if you ain't sure you'd get all that you want even on such vast websites, how do you expect somebody here to pack all of it in a little answer? $\endgroup$ – another 'Homo sapien' Jul 24 '17 at 11:41
  • $\begingroup$ The little answer might be a link that would be hard to find. $\endgroup$ – Hans-Peter Stricker Jul 24 '17 at 12:09