The brain is an organ present in all but the simplest of creatures which exerts control over all other organs in the body.
In a certain room of the Department of Mysteries, there exists a large tank containing a number of brains or brain-like objects. These brains exude thoughts as thin tendrils along their "bodies", and appear to have a certain amount of sentience on their own. They are used by Unspeakables to study human thought.
During the Battle of the Department of Mysteries, Ronald Weasley, under influence of a Death Eater's spell that made him disoriented (possibly the Confundus Charm), used a Summoning Charm on one of these brains. The brain immediately attacked when it touched Ron, wrapping its tentacle-like thoughts around his body and attempting to suffocate him. Though he was eventually able to get free and was treated to by Poppy Pomfrey, he suffered a great deal of damage and had scars from the event that would presumably last the rest of his life.