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An image from Terrence Sejnowski's lab is around since at least 2015 when it appeared in an article about hippocampal spine head sizes and only recently (May 2024) in an article about synaptic information storage capacity:

neuron image from Sejnowski lab

Visualization of a pair of spines (an SDSA pair) (with gray necks) from the same dendrite (yellow) and the postsynaptic density (PSD) associated area (red, indicated by white arrows) formed by the same axon (black stippling) with presynaptic vesicles (white spheres)

What I wonder about in this image is the appearance of the axon. It is neither a regular tube with a somehow fixed diameter nor an obvious terminal. It has several junctions/synapses along its length, while I believed an axon forms synapses only at its ends. Can someone please clarify? Which part of an axon do I see, and how realistic is it supposed to be?

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Your belief is wrong; axons form synapses along their lengths. I'm most familiar with these being referred to as en passant boutons. See for example this image and description from: https://synapseweb.clm.utexas.edu/axons

enter image description here

To clarify, though, this is something that would happen in the terminal field of an axon; in other places an axon is entirely an axon of passage making no synapses at all. The specific anatomy is going to depend on the exact type of neuron and it's origin and target(s).

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  • $\begingroup$ Thanks, Bryan. Can it be said that along the axon of passage its diameter is roughly constant, and only in the terminal field its diameter is as irregular as the images display? $\endgroup$ Commented May 27 at 12:48
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    $\begingroup$ The swellings/boutons are specific structures just like post-synapses, they contain vesicles and all the presynaptic release machinery. All that will only be found where actual synapses are made. I'd presume the axon would be mostly constant diameter elsewhere and certainly anatomists use the presence of swellings to indicate synaptic contacts, but I don't know what the magnitude of diameter variance is outside of synapses. $\endgroup$
    – Bryan Krause
    Commented May 27 at 12:56
  • $\begingroup$ Another point, although I don't think it applies to this particular axon, is that many axons are myelinated and thus vary in apparent diameter along their length. $\endgroup$ Commented May 27 at 15:54
  • $\begingroup$ I have only seen synaptic boutons in unmyelinated parts of an axon. In the neocortex and the hippocampus, en passant boutons are very common. As is seen in both images, their inter-bouton distance is roughly 1-2 μm. The axons in both images appear typical. $\endgroup$
    – vkehayas
    Commented May 28 at 9:48
  • $\begingroup$ Interestingly enough - and probably not by chance - is Kristen Harris both co-author of Terry Sejnowski (and the two papers I have linked) and author of SynapseWeb (that Bryan has linked). By private communication she and Tom Bartol confirmed the correctness of Bryan's answer. $\endgroup$ Commented May 28 at 16:24

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