What are these bulb structures?

Questions
#1

I’ve sometimes seen these very small (maybe 5 pixel wide) bulbs that seem to have pushed their way into a fiber. You’ll see the fiber get a black dot in it, which expands to a small circle, and then the fiber sort of peels away from it. Synapse? I thought those looked like big showerheads nuzzled up against fibers.





#2
A dark spot growing into a circle is a very general pattern that would appear when you cut through a rounded top of things. Let's say you have a hollowed sphere with shell of some thickness. and Imagine you cut off the top of it little by little. Then the cross sections will be small circles until you get to the hollow, where the cross section now will be circle for the hollow part + ring strip for the shell part. Of course, the shell is the dark cell membrane and the hollow is inside of neurons, in our case. 

So in principle any part of the neuron can show this pattern. Four slices of images are not enough to tell what it is. It would be better to look at the 3D model for the purpose. If what you are asking is small bulbs that budded out of main branches, then they could be synaptic boutons or dendritic spines, but not always. 
#3

Looking at the 3D of the neuron, there appears to be an indentation where the bulb fits in, almost like a ball-and-socket joint. I guess the problem is that I’m looking at the socket, not the ball!