This is the kind of discovery that leads scientists to say, "What's happening is not a simple, monolithic process."
And when they say that, you know they're starting to pay attention.
This fascinating discovery is a matter of function, not form. I gripe about the simplistic, even boneheaded science that seeks to uncover neurology by approaching it with a backhoe. This is one example of the more productive & telling approach, using the scientific equivalent of brushes and trowels. Good stuff, intelligent and apt.
The radical new idea is this: the little blobs of neurotransmitters that sit between the nerve endings are not identical little blobs, but are in two separate sets of blobs.
So what do they do?
The smaller group of blobs is the "recycling" set, called that because they get used and restocked each time the nerve fires. The larger group of blobs hangs back out of the way, and occasionally a blob takes off but not because the nerve just fired. Due to their relaxed attitude, these are called the "resting" set.
In the article, the blobs are referred to as sacs (or "vesicles," in med-speak.) They're sacs in precisely the same way that the drop of water rolling across the hood of your car is a sac. The blobs have a sort of "skin" created by molecules gathering more densely at the surface, but nothing more.
It's an important layer of molecules, though, since the detectable difference between the "resting" blobs/sacs and "recycling" sacs is the proteins that gather on their surfaces.
Now, to discover what those proteins mean ... That's another study. I look forward to it.