Are neocortical synapses bridges that are formed by a bridging material, or are they two connection points that are drawn together (kind of like magnets)?
Does that bridging, or repositioning motion happen from both sides, or does one side do most of the work?
Axons grow, usually directionally, meaning there is some reason they go the way they go. Dendrites, from what I understand, are more just branching out into space somewhat randomly, although some will route to other layers first? (not really sure, but in HTM this happens)
As axons and dendrites approach each other, potential synapses appear. They become more “permanent” as they are reinforced by activity across the synapse. I’m not sure whether the axon or dendrite initiates the initial connection, but I’m not sure it matters.
I hope I’m not out-of-topic, but axonal and/or dendritic segments actively getting close to each other could also be part of the answer you’re looking for, here, I guess.
That paper explores that question
A Stepanyants, G Tamás and DB Chklovskii,
Neuron, Jul 22 2004
Brain function relies on specificity of synaptic connectivity patterns among different classes of neurons. Yet, the substrates of specificity in complex neuropil remain largely unknown. We search for imprints of specificity in the layout of axonal and dendritic arbors from the rat neocortex. An analysis of 3D reconstructions of pairs consisting of pyramidal cells (PCs) and GABAergic interneurons (GIs) revealed that the layout of GI axons is specific. This specificity is manifested in a relatively high tortuosity, small branch length of these axons, and correlations of their trajectories with the positions of postsynaptic neuron dendrites. Axons of PCs show no such specificity, usually taking a relatively straight course through neuropil. However, wiring patterns among PCs hold a large potential for circuit remodeling and specificity through growth and retraction of dendritic spines. Our results define distinct class-specific rules in establishing synaptic connectivity, which could be crucial in formulating a canonical cortical circuit.
Only me inferring a rough outline from their paper :
IN : very actively trying to wire
PC axons : clueless fire&forget
PC dendrites : I’ll hypothesize quite active
That’s interesting, so synapses form from growth, like a plant growing toward the sun. Except, in this case, maybe the sun is also growing toward the plant.