Repeated hits to the head — even mild ones — can take their toll. But a new study shows that to let the brain recover, taking a few days to rest may be essential.
A typical high school or college football player experiences about eight hits to the head each week of the football season. And those hits can add up. None may seem serious by itself. They may not even give rise to symptoms of concussion — unconsciousness, headache, dizziness or forgetfulness. But continued hits to the head rob a player’s brain of the time it needs to heal, new data show.
One problem with mild brain injuries, even mild concussions, is that they can be difficult to diagnose. They’re not fatal. And researchers can’t peer directly into the brains of people who have experienced a hit to the head. What’s more, any damage that may be done doesn’t show up when living brains are scanned, notes Mark Burns, who led the study. He’s a neuroscientist at Georgetown University in Washington, D.C. A neuroscientist studies the brain and nerve cells.
Working with an animal model, such as mice, allows scientists to learn what happens inside the brain. So Burns and his team studied concussions in mice. They developed a method that concussed the mice — without severely damaging their brains. Some mice received only a single concussion. Others experienced many. Giving mice many concussions mimicked all the hits a football player might experience during a single season, Burns says.
Throughout the study, the researchers tested the mice for balance, anxiety and the ability to find and remember their way through a maze. Burns and his coworkers also examined the animals’ brains. Some were checked soon after a concussion, others after a delay. The team looked for signs of brain damage, including dead cells, inflammation (swelling) and broken connections between cells.
Nerve cells, or neurons, in the brain have long structures that send and receive signals from other cells. The place where two cells meet is called a synapse. Cells send chemical and electrical signals between brain cells at synapses. The axon, or signal-sending part of one neuron, releases chemicals at the synapse. Those chemical messengers move to the dendrite of the next cell. Dendrites are the parts of neurons that receive signals. This allows cells to communicate. If the dendrites shrink, they can’t reach the axons of neighboring cells or receive signals.
A one-time hit
Mice that had a single head injury did not suffer long-term problems. Immediately after a concussion, these mice behaved normally. And when the team looked at some animals’ brains right after this concussion event, they saw no serious injury. No brain cells had died. No axons were damaged. Dendrites remained in place. And the brain tissue showed no sign of inflammation.
A day after a concussion, though, some dendrites had shrunk away from their synapses and stopped communicating with other cells. Calcium also had built up in the cells, says Burns. Calcium ions play an important role in signaling between nerve cells. The ions move into the neuron when the cell fires, or signals. When calcium built up in the neurons of concussed mice, the cells stopped firing until the backlog could be cleared. In fact, after three days’ rest, these mice were back to normal.
“It’s great news,” Burns says. “Even through the brain has lost some function, it reestablishes connections when given time to recover.”
Mice that took one hit a day over six weeks, though, fared worse. These mice took hits comparable to a person playing a season of football. They had trouble with balance and tended to hide more, a sign of anxiety. Those changes lasted up to a year after the last concussion, Burns says.
The mice that took daily hits also showed long-term brain inflammation.
That inflammation worsened for months after the last concussion. Inflammation in the brain harms a nerve cell’s ability to communicate, Burns explains. And that can lead to all types of harm, such as trouble with memory and balance.
The researchers then looked at what happened when repeated head hits were spread out over time. This group of mice experienced a concussion once a week for 30 weeks. They had the same total number of hits to the head as the one-hit-a-day group. But these mice had time to recover between each concussion. Like the mice that had a single concussion, these mice showed no problems with their balance, anxiety, learning or memory. That suggests their brains were able to repair the damage in between hits.
“The brain can cope with mild brain injuries — as long as it’s given time to recover,” Burns now concludes. These results suggest athletes should be sure to report any symptoms of concussion, he says. “It’s okay to sit out. Not resting can lead to long-lasting damage.”
The findings were published online February 5 in the American Journal of Pathology.
The study gives important clues into how repeated concussions affect the brain, says Andre Obenaus. He’s a brain researcher at Loma Linda University in California and was not involved with the study. He questions the one-hit-a-day approach as a useful model of concussions in high-school athletes, however. That’s because new and extensive “return to play” guidelines already in use should mean these athletes have time to recover before risking a second hit to the head, he says.
(for more about Power Words, click here)
animal model A nonhuman animal used to stand in for people in research testing. Which animal a lab uses will depend on how closely parts of its body or chemical-signaling systems match those in people.
anxiety A nervous disorder causing excessive uneasiness and apprehension. People with anxiety may even develop panic attacks.
axon The long, tail-like extension of a neuron that conducts electrical signals away from the cell.
brain scan The use of an imaging technology, typically using X rays or a magnetic resonance imaging (or MRI) machine, to view structures inside the brain. With MRI technology — especially the type known as functional MRI (or fMRI) — the activity of different brain regions can be viewed during an event, such as viewing pictures, computing sums or listening to music.
calcium A chemical element which is common in minerals of the Earth’s crust. It is also found in bone mineral and teeth, and can play a role in the movement of certain substances into and out of cells.
cell The smallest structural and functional unit of an organism. Typically too small to see with the naked eye, it consists of watery fluid surrounded by a membrane or wall. Animals are made of anywhere from thousands to trillions of cells, depending on their size.
chemical A substance formed from two or more atoms that unite (become bonded together) in a fixed proportion and structure. For example, water is a chemical made of two hydrogen atoms bonded to one oxygen atom. Its chemical symbol is H2O. Chemical can also be an adjective that describes properties of materials that are the result of various reactions between different compounds.
concussion Temporary unconsciousness, or headache, dizziness or forgetfulness due to a severe blow to the head.
dendrite Projection from a neuron that is involved in receiving chemical signals from neighboring cells.
inflammation The body’s response to cellular injury and obesity; it often involves swelling, redness, heat and pain. It is also an underlying feature responsible for the development and aggravation of many diseases, especially heart disease and diabetes.
ion An atom or molecule with an electric charge due to the loss or gain of one or more electrons.
model A simulation of a real-world event (usually using a computer) that has been developed to predict one or more likely outcomes.
neuron or nerve cell Any of the impulse-conducting cells that make up the brain, spinal column and nervous system. These specialized cells transmit information to other neurons in the form of electrical signals.
neuroscience Science that deals with the structure or function of the brain and other parts of the nervous system. Researchers in this field are known as neuroscientists.
pathology The study of diseases and/or conditions that or did lead to death. People who work in this field are called pathologists. They look at what causes a disease, how symptoms develop and may examine all stages of an illness. In some cases, they will look at tissues removed from people who are sick, in surgery or have suspicious-looking tissues.
synapse The junction between neurons that transmits chemical and electrical signals.
traumatic brain injury Damage to the brain that results from an external shock, such as an explosion, or a direct impact (as can occur in a car accident). Also called TBI, the damage can lead to either temporary or permanent impairment of thinking, memory and movements of the body.