Hard Knocks: The Science of Concussions

  • Published3 Oct 2012
  • Reviewed3 Oct 2012
  • Author Jennifer Carr
  • Source BrainFacts/SfN

Scientists have long known that blows to the head can be dangerous. Reports of increased memory loss and depression by retired athletes years after their retirement raise new questions about the long-term effects of concussions, traumatic brain injuries caused by these violent hits.

On sports fields and arenas across the United States, an estimated 300,000 concussions will take place this year. Some concussions you can spot from the stands (the football player lying motionless on the field); others will be hidden (the soccer star jogging slowly to the bench). Brain scientists want to understand the differences among the types and intensities of impacts on athletes. This knowledge is critical to help players recover faster and protect themselves from permanent harm.

A concussion is caused by a sudden blow to the head, leading to a temporary disruption in brain activity. Symptoms in the minutes and hours following a concussion can range from a temporary loss of consciousness and general disorientation to dizziness, nausea, and vomiting. With rest, these symptoms generally subside within several days. But for some, persistent headaches, sleep disturbances, memory problems, and difficulties concentrating will continue for weeks or longer.

So what causes these symptoms, and how do athletes protect against lasting damage? Studies show repeat concussions lead to severe long-term symptoms and longer recovery times. This suggests recognizing the signs of a concussion early and giving the brain time to fully recover before returning to play is key to long-term brain health.

Energy dip slows pace of brain

Most immediate damage from concussions is hard to detect using standard imaging procedures, such as CT scans or MRIs. As a result, scientists have spent the better part of a decade studying animals, predominately rodents, for clues about the changes that take place in the brain.

When you slam your head, the force rapidly pushes the brain against the interior of the skull. This sudden movement causes brain cells to stretch and tear, altering the electrical and chemical balance critical to cell function and communication. These cells then spring into action, working harder to return to the correct chemical state. This puts the cell in a “highly stressful” state, leaving it unable to function properly or even survive.

“At the end of the day, a concussion is caused by an energy crisis to brain cells,” says Michael Collins, a clinical psychologist at the University of Pittsburgh Medical Center, who specializes in sports-related concussions. “The brain must work much harder to perform tasks.”

Rapid stretching from concussions also causes axons — the long fibers that brain cells use to communicate with one another — to swell, compromising the ability of the cells to send signals. Too much swelling and the connections between cells can be permanently lost.

Stopping the concussion cycle

Researchers believe that for a single concussion, most of the changes to brain cells are temporary. While evidence is limited, it is thought both physical and mental rest helps most injured brain cells restore their normal internal chemistry and recover. However, experimental studies suggest that some of the damage is permanent, potentially reflected by persisting cognitive dysfunction in some cases of single concussion. Moreover, experiencing multiple concussions before the brain has had time to fully heal can lead to more significant and potentially long-term changes in the brain.

“At the moment, there is no one who can tell you definitively how long to wait after a concussion before putting your head back at risk,” says Doug Smith, a neuroscientist at the University of Pennsylvania who studies the damage to axons following concussions.

Researchers are hopeful new noninvasive techniques will one day improve physicians’ ability to recognize people who need longer to recover from concussions, and provide clues about the relationship between concussions and the deterioration of the brain later in life.

Late life risks

In recent years, scientists have found signs of a neurodegenerative disease called chronic traumatic encephalopathy (CTE) in the brains of deceased athletes who either experienced multiple concussions or played positions where they were exposed to frequent impacts. Symptoms of the disease include increased irritability and impulsivity early on and dementia in later stages.   

Confirmation a person has CTE requires analysis of the brain after death. According to Ann McKee, a neuropathologist at Bedford VA Medical Center who performs this type of analysis, brains of patients with CTE have unique patterns of abnormal protein tangles and cell loss.

McKee has spent years analyzing the tissue of former athletes and war veterans who survive bomb blasts, whose families noticed behavioral changes in their loved ones suggestive of CTE and donated their brains to science. Of the 85 brains her group has analyzed to date, 68 had CTE, McKee says.

McKee’s group and others now study animals for clues about how CTE develops and ways to diagnose it in living patients, so they can create new therapies to treat the disease.

"One of the big questions at this point is: How are concussions are related to CTE?” McKee says. “It seems that repetitive injury superimposed on unrecovered brains is key, but how they are connected and at what point the injuries turn into a vicious cycle, we just don't know.” 



Barkhoudarian G, Hovda DA, Giza CC. The molecular pathophysiology of concussive brain injury. Clin Sports Med. 2011 Jan;30(1):33-48, vii-iii.

Baugh CM, Stamm JM, Riley DO, et. al. Chronic traumatic encephalopathy: neurodegeneration following repetitive concussive and subconcussive brain trauma. Brain Imaging Behav. 2012 Jun;6(2):244-54.

Goldstein LE, Fisher AM, Tagge CA, et. al. Chronic traumatic encephalopathy in blast-exposed military veterans and a blast neurotrauma mouse model. Sci Transl Med. 2012 May 16;4(134):134ra60.

Guskiewicz KM, McCrea M, Marshall SW, et. al. Cumulative effects associated with recurrent concussion in collegiate football players: the NCAA Concussion Study. JAMA. 2003 Nov 19;290(19):2549-55.

Longhi L, Saatman KE, Fujimoto S, et. al. Temporal window of vulnerability to repetitive experimental concussive brain injury. Neurosurgery. 2005 Feb;56(2):364-74

Meehan WP 3rd, Zhang J, Mannix R, et. al. Increasing Recovery Time Between Injuries Improves Cognitive Outcome After Repetitive Mild Concussive Brain Injuries in Mice. Neurosurgery. 2012 Jun 27. 

Stern RA, Riley DO, Daneshvar DH, et. al. Long-term consequences of repetitive brain trauma: chronic traumatic encephalopathy. PM R. 2011 Oct;3(10 Suppl 2):S460-7. 

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