Taken from wikipedia. Link: http://en.wikipedia.org/wiki/Concussion
Concussion can alter the brain's physiology for a period of weeks, setting into motion a variety of pathological events. In a large majority of affected brain cells, the metabolic processes that follow concussion are reversed within hours or days; however a small number of cells may die after the injury.
Concussion is thought to unleash a metabolic cascade of events including impaired neurotransmission, loss of regulation of ions, deregulation of energy use and cellular metabolism, and a reduction in cerebral blood flow. Excitatory neurotransmitters such as glutamate are released in excessive amounts as the result of the injury and lead to an imbalance of ions such as potassium and calcium across the cell membranes of neurons like that seen in excitotoxicity. The ionic imbalance leads to neuronal depolarization, which in turn causes the sodium-potassium pump to work more than it normally does in an attempt to restore resting potential. For this, the pump requires greater-than-usual amounts of ATP, which causes cells to enter a state of hypermetabolism (to use more glucose than usual). At the same time, cerebral blood flow is relatively reduced, leading to an energy crisis for cells.
For a period of minutes to days after a concussion, the brain is especially vulnerable to changes in intracranial pressure, blood flow, and particularly anoxia. According to studies performed on animals, large numbers of neurons can die during this period in response to slight, normally innocuous changes in blood flow. Animal studies have also found that a second concussion occurring during this window of time has severe effects on metabolism within the brain.
Concussion involves diffuse brain injury (as opposed to focal brain injury), meaning that the dysfunction occurs over a widespread area of the brain rather than in a particular spot. Concussion is thought to be a milder type of diffuse axonal injury because axons may be injured to a minor extent due to stretching. Axonal damage has been found in the brains of patients who suffered concussion and then died from other causes. A stretched or damaged axon may be further damaged or killed by fluctuations of ions across the axonal membrane in the hours and days following the injury.