During a sudden deceleration, as shown in the beginning of the animation sequences,
the brain violently impacts against the inner surface of the skull
at two times--first when the head impacts with the airbag
or otherwise stops its forward motion and begins its immediate, rapid, violent reversal
of motion, and second when the
head impacts against the headrest and repeats the immediate, rapid, violent reversal
By observing the collision from a view that follows the motion of the head,
one can appreciate the deformation of the brain as it violently impacts
against the front and back of the skull.
When the impacts of the brain against the skull are viewed in slow motion,
the shock waves that travel through the brain during the impacts
can be observed.
Close inspection of an area of the outer surface of the brain
and inner surface of the skull during the initial impact
shows the soft, fragile brain scraping against the hard, jagged
inner surfaces of the skull to create shearing forces.
As the gray matter, comprised of cell bodies, and the white matter,
comprised of axons, are of two different densities,
the shearing forces create a plane of cleavage where many axonal injuries occur.
The axons may be completely torn, partially torn, or separated from their connections
with other cells. Thousands or even millions of scattered axons may be torn,
but unless some of the larger and more resilient arteries are also torn,
no bleeding occurs.
Traditional imaging studies such as CT or MRI are not nearly sensitive enough
to detect individual axonal injuries or even relatively large groups of axonal injuries.
CT and MRI are designed to detect areas of bleeding.
Unless a blood vessel or multiple vessels are torn, creating a relatively large bleed,
these studies fail to demonstrate any findings that would indicate the presence
of multiple, widespread, and microscopic axonal injuries that can result in
devastating neuropsychological deficits.
In situations when the forces involved are severe enough to result in injury
to the blood vessels, the injuries to the axons are even more severe.
An injury to one or more blood vessels results in the release of red blood cells
into the surrounding brain tissue.
CT and MRI are designed to detect blood or, after a period of time, the remnants of blood,
Large quantities of red blood cells must hemorrhage from a blood vessel
or blood vessels to be detectable on CT or MRI.
If even one small hemorrhage occurs that is detectable,
it is an indicator that there are likely vast numbers of associated
axonal injuries that are not depicted in the scan.