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.[38][39] 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.[5]
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.[5][8][38] Excitatory neurotransmitters such as glutamate are released in excessive amounts as the result of the injury[25] and lead to an imbalance of ions such as potassium and calcium across the cell membranes of neurons like that seen in excitotoxicity.[38] 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.[38] 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).[38] At the same time, cerebral blood flow is relatively reduced,[40] leading to an energy crisis for cells.[38]
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.[40] 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.[40] Animal studies have also found that a second concussion occurring during this window of time has severe effects on metabolism within the brain.[41]
Concussion involves diffuse brain injury[42] (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.[4][5] Axonal damage has been found in the brains of patients who suffered concussion and then died from other causes.[33] 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.