User blog comment:Xilinoc/Ch. 579 - THE UNDEAD 6/@comment-2211792-20140508131732/@comment-1268838-20140508202227

Spiritual power is intimately tied to the soul (the spirit body). There are pressure points and ducts through which it flows and is stored. The series has not gone into much detail about them, but we know they are there. That power can swell and explode, under the wrong circumstances. Ideally, that spirit body grows and strengthens along with the body. Otherwise, that power would tear them into pieces; how else could captain-level Shinigami, whose power can literally crush everything around them, contain it with being crushed themselves?

It doesn't matter that his body was "fake," because anything he imagines is real. But I think it is safe to presume that his brain took that damage as well.