When the feeling of "heart broken" come across to my mind, the very first description about this feeling is "pain from inside", it is obviously does not referring the physical pain, but its an "imaginary pain".
Well, to be precise and try not to lead this "heart broken" as an imagination, "heart broken" is an "abstracted pain", meaning, it actually doesn't exist as a signal of pain to your neuron system (i.e. your brain), but its a signal of pain to your consciousness (roughly speaking, your soul/spirit).
Of course, when we talk about brain and consciousness, we hardly differentiate both of them, as we don't know much about consciousness, the furthest we know is consciousness can affect/control our hormone, and hormone can affect the physical of one.
As the emotion of one, are almost cause of hormone, but in the other hand, consciousness of one has partially controlled of hormone, so emotion actually is partially cause of physical and partially cause of consciousness. So the "heart broken" feeling, might because of the consciousness received some signal, then giving a signal to our hormone, and from hormone generates an "emotion", building up an "abstract pain", run around your physical back to your consciousness, which giving us an "inside out pain", when the pain run from "inside" to "outside", and from "outside" back to "inside".
And, that's why the feeling of "heart broken" is so terrible! (That's Tony Leung)