From three answers to the same question at Quora I've learned of three forces that keep synapses together:
They are held together by cell adhesion molecules (e.g. neurexin and neuroligin).
They are surrounded by support cells (glia?) that hold them together.
They are packed densely together (and just couldn't move, even without specialized support cells).
I'm sure all of these ways of holding synapses together play a role.
But are there others?
In any case it may make sense to ask
which of these "forces" is the strongest, and how strong is it?
For example: If axonal tension is supposed to be a driving force of gyrification, the "weakest link in the chain" is probably the synapse. So the forces holding the synapse together must be strong enough to counteract the forces that drives the neurons in different areas of the cortex apart (whatever these may be). For "axonal tension" or "axonal tethering", see also here.