It seems that the OP is asking what happens if the brain stops controling anything.
And I think the answer would be what would happen to the body of brain dead people, if they were just kept ventilated (for instance not given any drug to regulate blood pressure or anything of that sort).
And it seems to me this was discussed in great detail in a GD thread about some American girl who had been declared dead while her parents insisted on her being kept alive last year. But since I don’t remember her name, I can’t find the thread.