The sentence looks good grammatically if you add "the" before "brain"; it should read "...about the functioning of the brain...". However, I find the sentence slightly unnatural. I think the passive construction ("is known") throws me off. I would rather write something like "This one tweet shows how much you know about the functioning of the brain."
On a final note, the sentence as written above does not provide much context to know the connotation. I don't know if you mean to imply that this person knows much or little about the functioning of the brain.