The human brain has roughly the consistency of barely congealed jelly, encased in a bony helmet with just a little room to move.
That snug fit likely evolved to protect the brain against most of the bumps and knocks humans would suffer in the course of hunting and gathering. But it becomes a problem when you add far more energy to the mix -- when a head moving forward at high speed (say, in a car) is stopped suddenly or when a head at rest is struck by a fast-moving object or a wave of concussive energy rippling outward from a bomb's blast.
In those moments of rapid acceleration or deceleration, the gelatinous brain has just enough room to crash into the wall of its protective shell.
As it hits that wall, the brain's delicate folds of interlocking tissue are compressed in some places, stretched and, scientists surmise, torn in others. Often, after impact, the whole gelatinous pile bounces back against the other wall, compressing, stretching and tearing in the other direction.
Depending on the energy and sometimes the direction of that motion, small rips and cavities appear in places where interlocking colonies of cells used to thrive. Other brain cells that have been squashed respond to the insult by releasing chemicals that stimulate neighboring cells, which in turn, release more excitatory chemicals. The result is waves of chaotic activity that disrupt communications among cells for days or weeks, resulting in symptoms such as dizziness, confusion, slowed response rates and attention deficits.
This storm of activity boosts the brain's demand for fuel. But during this period, oxygen- and fuel-carrying blood is often being lost to bleeding or diverted to other organs. Just when the brain most needs extra fuel and oxygen to restore normal function, its prime source of energy ebbs.
Some cells may die. Others will perform poorly for long periods of time until they heal. And while they heal, these cells are extremely vulnerable to a second injury. That's why an athlete who suffers a concussion must be removed from a game.
On the sidelines, at the bomb site, in the quiet aftermath of a motor vehicle crash, the owner of the concussed brain may have lost consciousness, or feel dizzy, disoriented and confused. Her memory for what just happened -- or for more distant events -- may be dim. His ability to pay attention, and to anticipate predictable patterns of movement, may be affected.
Physicians call this cluster of symptoms post-concussion syndrome.
For most, the resilient brain, given the chance to rest, will rebound, and normal function will return. Depending on the severity of the injury, it can take two weeks or it can take several months before symptoms such as headaches, dizzy spells and irritability subside and memory and concentration return to normal.
Until they do, many neurologists encourage people experiencing post-concussion syndrome to lay off the video games, classwork or other activities demanding keen concentration, and to avoid activities that could expose them to a potential second concussion. For those who go back to demanding and dangerous activities too soon, not only can healing be delayed; faulty judgment and slowed reactions make a subsequent injury more likely. The result can be "second-impact" syndrome, with greatly worsened and more persistent symptoms. In some cases, it can be fatal.
For about one in five of those with concussion, some symptoms may linger for longer than a few months, and for an unlucky few, may persist. Scientists are only now beginning to discover that for some with as few as a single concussion, the injury can set in motion changes in the brain that could contribute to the development of neurodegenerative disease later in life.