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What is the roughly the distribution of the various synaptic connections in the brain.

Consider the following types:

  • axoaxonic synapse between the axon of one neuron and the axon of another neuron.
  • axodendritic synapse between the axon of one neuron and the dendrites of another.
  • axodendrosomatic synapse between a neuron's axon and the dendrites and body of another.
  • axosomatic synapse between the axon of one neuron and the body of another.
  • dendrodendritic synapse from a dendrite of one cell to a dendrite of another.

I know that the axodendritic synapse is the most prevalent, but what is order of the others? Are there any sources for this (couldn't find this anywhere). Specifically, rough percentages of the distribution (amount) of these synaptic connections in the nervous system would be very helpful.

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  • $\begingroup$ The adult human brain contains ~86 billion neurons with trillions of synapses estimated at 0.15 quadrillion. I am unsure if this question can be answered. $\endgroup$
    – AliceD ♦
    Nov 1, 2015 at 11:29
  • $\begingroup$ @AliceD I understand that there are many synapses, but don't we know anything about which synaptic connections are more common than others? $\endgroup$
    – Jean-Paul
    Nov 1, 2015 at 11:52
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    $\begingroup$ I expect there may be studies that have addressed some of the various connections in particular brain structures, but to find data that provide an order of appearance of all of your mentioned connections in the whole brain, I am doubtful of. It's an intriguing question, though. It's quite of a textbook question and I doubt a literature search will be helpful. $\endgroup$
    – AliceD ♦
    Nov 1, 2015 at 12:02
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    $\begingroup$ I don't have time for a full answer, but here is a study on the hippocampal CA1 pyramidal neuron that makes an exhaustive catalog of all synapse types and locations: Megías, M., Z. Emri, T. F. Freund, and A. I. Gulyás. “Total Number and Distribution of Inhibitory and Excitatory Synapses on Hippocampal CA1 Pyramidal Cells.” Neuroscience 102, no. 3 (2001): 527–40. I know such papers exist for a few other cell types. This paper might provide a lead for finding those too. $\endgroup$
    – yamad
    Nov 2, 2015 at 3:30
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    $\begingroup$ Are you still interested in this kind of questions? Have a look at mine: biology.stackexchange.com/questions/53587/…, biology.stackexchange.com/questions/63950/… $\endgroup$
    – Hans-Peter Stricker
    Aug 11, 2017 at 9:45

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