With regards to the synapses between axon terminals and dendrites, what is the relation between a given neuron's axon terminals and its neighbouring neurons' dendrites?

  1. Does each axon synapse on only a single dendritic spine from another neuron, whereby all other connections that axon makes are with distinct neurons, or
  2. Can multiple axon terminals from a given neuron end up connecting to a single neuron via multiple dendritic spines?
  • $\begingroup$ @BryanKrause am seeing you are both a neuroscientist, and programmer. Mind if we connect separately about neurons and AI? :-) $\endgroup$
    – Peter
    Mar 29 '21 at 22:05
  • $\begingroup$ Cleaned up the comments and wrote a formal answer. I generally check the Biology.SE chat: chat.stackexchange.com/rooms/1997/the-biosphere every day or two, and will respond to pings there. I'm okay answering some questions there outside the StackExchange Q&A format, but won't necessarily commit to having any long conversations. If there are some specific things you'd like to ask about or you're looking for some direction I'm happy to help with what I can. $\endgroup$
    – Bryan Krause
    Mar 29 '21 at 22:31

Synapses are pretty much one-to-one

Here's some EM pictures of synapses, from Wikimedia commons:


You should gather from these images that these are super organized structures. There's a dense, dense gathering of cellular machinery at the synapse that creates a dark electron-dense region shown by the arrows. There's no room for more than one cell to be directly involved in this area, it's one membrane closely coupled with another membrane. It's as if you put the palms of your two hands together.

Some neuromodulators (and sometimes traditional neurotransmitters) are not released synaptically, though, and are just dumped into the extracellular space from where they diffuse in to the cleft or bind outside the synaptic region.

There can be multiple connections between two given cells

Though a synapse is a one-to-one connection, axons make multiple synaptic contacts as do dendrites. Though this can mean that cells receive and send input to 1000s of different cells, it's also very common for there to be multiple individual synapses between two specific cells.

For some examples, Tamas et al 1997 looked at synapses made from inhibitory interneurons to excitatory cells in visual cortex:

All presynaptic cells established multiple synaptic junctions on their postsynaptic target cells. A basket cell innervated a pyramidal cell via fifteen release sites; the numbers of synapses formed by three dendrite-targeting cells on pyramidal cells were seventeen and eight respectively, and three on a spiny stellate cell; the interaction between a double bouquet cell and a postsynaptic pyramidal cell was mediated by ten synaptic junctions.

Note in this paragraph they are literally talking about 5 specific pairs of cells, where they trace both cells in EM. This is an exhausting process. The presynaptic cells were one basket cell, three dendrite-targeting cells, and one double bouquet cell. The post-synaptic cells were all pyramidal cells except one was a spiny stellate cell. In the five pairs they found 15, 17, 8, 3, and 10 connections. Of course they could have missed some, too.

Tamas, G., Buhl, E. H., & Somogyi, P. (1997). Fast IPSPs elicited via multiple synaptic release sites by different types of GABAergic neurone in the cat visual cortex. The Journal of physiology, 500(3), 715-738.


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