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I'm not asking why the synaptic cleft exists, i.e. what function it holds, rather how.
So I know that the neurotransmitter diffuses across it, it is 20-40 nm wide and contains basal lamina (in NMJs at least), but I cannot find any allusion as to what causes the gap; how the two membranes don't just touch, and how so consistent a distance is created across neurons.
Google has so far failed me on this one, though perhaps I am not searching well enough, any help would be greatly appreciated.

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There are membrane proteins that act as structural components of the gap (i.e. the synapses aren't just floating there, they are anchored to each other via membrane proteins).

https://en.wikipedia.org/wiki/Neuroligin https://en.wikipedia.org/wiki/Neurexin

The most common examples are neurexin (expressed on the pre-synaptic terminal) and neuroligin (expressed on the post-synaptic terminal). Not surprisingly, these proteins are involved in synapse formation in the developping brain, as well as the adult brain (in plastic processes where new synapses are formed). To a first approximation, as synapses are growing, their membrane proteins bind their partners and become anchored. This also leads to intracellular signalling and further maturation of the synapse. That's how synapses know where to attach.enter image description here

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  • $\begingroup$ Thank you very much this is a great answer and just what I was looking for :) $\endgroup$ – joe_young Apr 4 '16 at 8:36

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