What forms the spine of a story? Why, the three structures of course! They are isolated out and focused upon at the beginning of the class, in the establishing meaning phase, and they are practiced and repeated amidst a smattering of words in PQA, and then they reappear from time to time in the general word fabric of the story, just like water from underground streams, because the process is all unconscious.
So, if the structures keep appearing and reappearing over and over in all phases of the story creation process, they have to be called the spine of the story. A TPRS class without those structures would, like a scarecrow, flop around, brainless. Thank goodness for the three structures!