In simple terms it is a brain injury sustained by a blow either to the head or to the body resulting in traumatic unnatural forces being applied to the brain.
It is not a “bruise” to the brain, rather a trauma. That is why you may see it as mTBI (mild traumatic brain injury) in some texts. The concussion and mTBI are relatively interchangeable.
Webster defines a concussion as;
About.com defines, more specifically, sports concussion as;
Concussions are traumatic head injuries that occur from both mild and severe blows to the head. Some head injuries may appear to be mild but research is finding that concussions can have serious, long-term effects, especially repeat head injuries or cumulative concussions.A concussion is typically caused by a severe head trauma during which the brain moves violently within the skull. The brain cells all fire at once, much like a seizure. Some studies show that patients who suffer a concussion appear to have the brain activity of people in a coma.
A concussion may result from a fall in which the head strikes against an object or a moving object strikes the head. A suddenly induced turning movement such as a blow that twists the head (like a punch to the side of the face) is more likely to produce unconsciousness. However, significant jarring in any direction can produce unconsciousness.
In 2004, data has collected from the head impact telemetry system used in the NFL concussion studies found that 58 of 623 (9.3 percent) of professional football players who suffered a concussion also had a loss of consciousness.
What is Happening to Your Brain…
In the immediate aftermath of a concussion, there may be some structural damage at the microscopic level with some cell death. However, the primary problem is a chemical one in the cells that have survived the incident.
John Doherty goes on to explain via the research of David Hovda, PhD at UCLA.
When there is a sufficient blow to the brain, the membranes of the affected nerve cells in the brain are stretched or twisted, allowing potassium to exit those cells, which triggers those cells to depolarize, thus the phenomenon of seeing stars if the affected area is involved with sight or ringing in the ears if the affected area is involved with hearing. The exit of potassium (K+) peaks approximately two minutes after the incident but continues for another 3-4 minutes.
Then, until the chemical balance is somewhat restored,
those neurons (nerve cells) are unable to fire again. Furthermore, in a protective reflex of sorts, surrounding cells begin to shut down, a process Hovda calls “spreading depression.”
If enough cells become depressed, confusion, amnesia, and even loss of consciousness result.
Meanwhile, in an attempt to recover, the brain starts using up massive amounts of blood sugar and will continue to do so for as long as 30 minutes. This overuse of this glucose results in the production of lactic acid which, in excess amounts, inhibits brain function.
A demand for glucose by the brain, such as when one is studying, logically causes an increase in blood flow to the brain. However for reasons not entirely clear, within two minutes of a concussion, the body decreases blood flow to the brain by up to 50 percent. This process continues unabated for 3-4 days but fully normal blood flow may not resume until an average of 10 days have passed. Consequently, at precisely the time the brain needs extra fuel to repair itself, it ends up getting less.
Meanwhile, the exit of potassium allows calcium to enter the neuron. And while the exit of potassium ceases in minutes, it takes the cell 2-4 days to rid itself of the energy-inhibiting calcium.
While the immediate chemical reaction of the brain to the concussion is brief and generally completed in 30 minutes, it takes days for the individual cell and the brain as a whole to restore that chemical balance which was lost so quickly. Until that balance is restored, the brain doesn’t work as well and is particularly vulnerable to re-injury.
That is what is happening down to the cellular level. This is where the research is getting deeper, how to control or abate the reaction of the brain. Is it necessary for the blood flow to be compromised for that long? Can we safely alleviate that? Are there pharmaceuticals that can help? What is the “general” threshold of the mechanism.
The Concussion Blog 