If I may ramble about something (prob obvious, sorry) using my half-forgotten biology education…
Quirks are extensions of the body, they’re biological functions of the body. So like any other voluntary action, like moving an arm, using a quirk requires the brain to be responsible for its function - the unconscious maintenance and regulation of the so call quirk factors, as well as the conscious motor control.
Brain activity, basically. Besides being a physical thing, quirks are also likely mental - emotion, mood, brain health, etc. all affect quirk function.
(I wonder if Aizawa’s quirk is psychic, if he’s more blocking the brain from sending the signal to use the quirk, preventing it’s activation; that seems likely.)
Having multiple quirks means that the brain’s got to control so much more at once, so no wonder people giving multiple quirks overloads them, rendering unable to even speak. So Noumu’s created to give up any brain function, any thinking, in order to use multiple quirks. But I think this also means the brain is still working really hard, which means heighten blood flow to the brain, and must generate lots of heat, which means the brain also needs a way to cool down quickly.
That’s why Noumu’s brains are exposed. For radiative cooling.
The downside is that the brain is exposed, with nothing to protect it; but that’s only if one can get near enough to a Noumu to injure/destroy it’s brain. So, at least that problem half-solved?
