Medical:
Latin Commotio cerebri. A short
paralysis of the
brain as a result of a
blow to the head.
The causes to unconsciousness following concussions aren't entirely known, but it is believed that especially the reticular system is sensitive to sudden movements. Essentially what happens is that the rapid movement of the head makes the brain hit the inside of the skull. Both the point of impact and the opposite side of the brain is usually affected, resulting in bruises called coup and contrecoup injuries, respectively.
Short-term effects are headaches and tiredness, apart from unconsciousness. Long-term effects such as permanent brain damage causing both physical and psychological defects, may also come as a result. It is not uncommon to treat severe concussions with medicines to prevent edema.
Recent research has also suggested that by artificially increasing the concentration of cerebrospinal fluid can have positive effect. Mannitol infusion evens out the electrical signals in the brain in a positive way, so that "spikes" in the electrical activity causing seizures do not appear. It also reduces the risk of swelling causing the brain the pressure against the skull, creating permanent brain damage. Usually, all effects of a concussion are reversible, and the brain soon returns to normal activity.
Source: Scientific American, ne.se