User blog comment:Xilinoc/Ch. 636 - Sensitive Monster/@comment-10497166-20150723101510/@comment-25121259-20150723135106

From what I have seen/heard, the popular theory is that it is Souken's head connected to a bunch of Quincy nerves...

From what I understood, Mayuri had removed the Myelin Sheath, thus exposing the nerves to the outside environment, making them extremely sensitive (reference to the chapter's title) and hurtful ?