What happens when you have a concussion? - Clifford Robbins

Each year in the United States, players of sports and recreational activities receive between 2. 5 and 4 million concussions. How dangerous are all those concussions?

The answer is complicated, and lies in how the brain responds when something strikes it. The brain is made of soft fatty tissue, with a consistency something like jello. Inside its protective membranes and the skull's hard casing, this delicate organ is usually well-shielded.

But a sudden jolt can make the brain shift and bump against the skull's hard interior, and unlike jello, the brain's tissue isn't uniform. It's made of a vast network of 90 billion neurons, which relay signals through their long axons to communicate throughout the brain and control our bodies. This spindly structure makes them very fragile so that when impacted, neurons will stretch and even tear.

That not only disrupts their ability to communicate but as destroyed axons begin to degenerate, they also release toxins causing the death of other neurons, too. This combination of events causes a concussion. The damage can manifest in many different ways including blackout, headache, blurry vision, balance problems, altered mood and behavior, problems with memory, thinking, and sleeping, and the onset of anxiety and depression.

Every brain is different, which explains why people's experiences of concussions vary so widely. Luckily, the majority of concussions fully heal and symptoms disappear within a matter of days or weeks. Lots of rest and a gradual return to activity allows the brain to heal itself.

On the subject of rest, many people have heard that you're not supposed to sleep shortly after receiving a concussion because you might slip into a coma. That's a myth. So long as doctors aren't concerned there may also be a more severe brain injury, like a brain bleed, there's no documented problem with going to sleep after a concussion.

Sometimes, victims of concussion can experience something called post-concussion syndrome, or PCS. People with PCS may experience constant headaches, learning difficulties, and behavioral symptoms that even affect their personal relationships for months or years after the injury. Trying to play through a concussion, even for only a few minutes, or returning to sports too soon after a concussion, makes it more likely to develop PCS.

In some cases, a concussion can be hard to diagnose because the symptoms unfold slowly over time. That's often true of subconcussive impacts which result from lower impact jolts to the head than those that cause concussions. This category of injury doesn't cause noticable symptoms right away, but can lead to severe degenerative brain diseases over time if it happens repeatedly.

Take soccer players, who are known for repeatedly heading soccer balls. Using a technique called Diffusion Tensor Imaging, we're beginning to find out what effect that has on the brain. This method allows scientists to find large axon bundles and see how milder blows might alter them structurally.

In 2013, researchers using this technique discovered that athletes who had headed the ball most, about 1,800 times a year, had damaged the structural integrity of their axon bundles. The damage was similar to how a rope will fail when the individual fibers start to fray. Those players also performed worse on short-term memory tests, so even though no one suffered full-blown concussions, these subconcussive hits added up to measurable damage over time.

In fact, researchers know that an overload of subconcussive hits is linked to a degenerative brain disease known as Chronic Traumatic Encephalopathy, or CTE. People with CTE suffer from changes in their mood and behavior that begin appearing in their 30s or 40s followed by problems with thinking and memory that can, in some cases, even result in dementia. The culprit is a protein called tau.

Usually, tau proteins support tiny tubes inside our axons called microtubules. It's thought that repeated subconcussive hits damage the microtubules, causing the tau proteins to dislodge and clump together. The clumps disrupt transport and communication along the neuron and drive the breakdown of connections within the brain.

Once the tau proteins start clumping together, they cause more clumps to form and continue to spread throughout the brain, even after head impacts have stopped. The data show that at least among football players, between 50 and 80% of concussions go unreported and untreated. Sometimes that's because it's hard to tell a concussion has occurred in the first place.

But it's also often due to pressure or a desire to keep going despite the fact that something's wrong. This doesn't just undermine recovery. It's also dangerous.

Our brains aren't invincible. They still need us to shield them from harm and help them undo damage once it's been done.