As a linebacker for the UW–Madison Badgers, Chris Borland made a name for himself as a hard-hitting tackler. His senior year, he was selected as a first-team All-American as well as the top linebacker and defensive player in the Big Ten Conference.

A third-round draft pick, Borland seemed destined for a headline career in the National Football League. But during a full-contact practice at the San Francisco 49ers summer training camp in August 2014, Borland got his “bell rung” by a 290-pound fullback during a routine exercise. Though Borland felt dazed, he played through—as he’d done dozens of times before.

Like many football players, Borland had endured his share of hard hits, including two diagnosed concussions. This particular hit, however, got him thinking seriously about the future, and about the negative effects that repeated collisions could have on his long-term physical and cognitive health. Even so, he went on to play a dynamite rookie year.

Then, after the season was over, Borland quit.

The announcement shocked the sports world. Borland was 24 years old and healthy, yet chose to walk away from a $2.3 million, four-year contract.

“I just honestly want to do what’s best for my health,” Borland explained on ESPN’s Outside the Lines. “From what I’ve researched and what I’ve experienced, I don’t think it’s worth the risk.”

With their repeated hits, football players—along with boxers—are at increased risk of developing chronic traumatic encephalopathy (CTE), a degenerative brain disease marked by memory loss, depression, suicidal thoughts, aggression and dementia. Of 91 brains donated to science by former NFL players, 87 have tested positive for CTE. It’s seen as a likely contributing factor to nine suicides by current and retired football players over the past decade.

Borland didn’t want to share that fate.

“To me, Chris Borland is a hero. He walked away before he made the big bucks and he was very explicit about why he quit—that it was not worth it to him,” says CALS genetics professor Barry Ganetzky, whose findings about the central nervous system in fruit flies are shedding light on what hard hits do to humans.

Ganetzky isn’t a sports guy, but he started paying attention to football-related brain injuries after the 2012 suicide of New England Patriots linebacker Junior Seau, intrigued by the biological processes driving this tragic phenomenon.

“I started wondering, what’s the link between a blow to the head and neurodegeneration 10 or 20 years down the line? When I started digging into the scientific literature, it became clear that we know very little,” says Ganetzky, who held the Steenbock Chair for Biological Sciences for 20 years. “And my usual response is, well, if we don’t understand something about the brain, then we should be studying it in flies.”

Fruit flies, officially known as Drosophila melanogaster, are a widely studied model organism, with a vast arsenal of genetic and molecular tools available to support that work. Flies reproduce rapidly and are easy to work with, enabling swift research progress. They are well suited for brain research because they have nerve cells, neural circuitry and a hard skull-like cuticle remarkably similar to our own, allowing scientists to conduct probing experiments that would be difficult in rodent models—and impossible in human subjects.

Fly models already exist to study Alzheimer’s, Parkinson’s and a number of other neurological diseases. Why not concussion? But there wasn’t a model available.

Then Ganetzky remembered work he’d done decades earlier.

“It occurred to me that I knew how to make flies have a concussion, and I had done it 40 years ago as a post-doc,” says Ganetzky. “I thought, ‘That’s it!’”

It was a simple thing: As a post-doctoral researcher at the California Institute of Technology, Ganetzky decided to see if any of his flies happened to be bang-sensitive mutants, flies that display seizures and paralysis after given a high-powered swirl on a vortex machine. But he didn’t have a vortex nearby, so he decided to just bang the vials against his hand.

“After a couple of sharp whacks, some of the flies were hanging out at the bottom of the vial, stunned. Others were on their backs, obviously knocked out. And after a few minutes, they all got up and started walking around again,” recalls Ganetzky.

He immediately knew the flies weren’t bang-sensitive—it’s an extremely rare mutation—but Seau’s death helped Ganetzky realize they had displayed symptoms “very similar in many respects to the empirical definition of a concussion.”

After developing and validating the new fly model, Ganetzky and UW genetics professor David Wassarman have been able to charge forward with brain injury research. The model has already been used to reveal key genes involved in the body’s response to brain injury. It’s also poised to help unlock medical applications, including a genetic test for high-risk individuals and an assortment of promising drugs and treatments.

In addition to helping athletes in contact sports, these advances will benefit the millions of Americans each year who experience traumatic brain injury due to falls, car accidents and violent assaults.

“At the most fundamental level, we just want to understand how traumatic brain injury works,” explains Ganetzky. “However, this is a major medical problem for which there are not many good—or any good—treatments or therapies or preventives, and so that is part of our motivation. If we can learn the genes and the molecules and the pathways, can we come up with interventions?”

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