For what i understand is that the brain controls all the functions of the body..It controls ur body organs to breath..To touch,taste etc.. A coma i think is different from being brain dead.. A person in a coma is in a state of deep and usually prolonged unconsciousness..So theres a chance for the person to be aware and concious again.. hence a brain dead person has lost his brain functions IRREVERSIBLY.. And i dont think i ever heard of a brain transplant..Correct me if im wrong...