The human brain is protected by the skull and floats in a heavy, gelatinlike substance called cerebral spinal fluid.

Harsh, quick hits such as those sustained by boxers, football players and other athletes can stretch nerve cells and trigger small hemorrhages responsible for creating a tangled mess of brain proteins called tao. Years or decades after hits to the head, tao can hamper the brain's ability to communicate with the body and cause cell death.

Unlike broken limbs, dead brain cells cannot be restored.

The presence of tao is central to chronic traumatic encephalopathy, or CTE, which can be diagnosed only after death. Since 2008, Boston University's Center for the Study of Traumatic Encephalopathy found significant tao and CTE in the brain tissue of 12 deceased NFL players, including former Tampa Bay Buccaneers offensive lineman Tom McHale.

McHale's widow, Tampa resident Lisa McHale, said her husband never thought playing in the NFL would damage his brain. Instead, they attributed his spiral into mental illness to an addiction to medication prescribed to treat muscular-skeletal injuries. Tom McHale played nine seasons in the NFL, six with the Bucs from 1987 to 1992. He died of an overdose in 2008, not knowing the hits killed brain cells.

“I think every player has always known, ‘I could have bad knees when I retire. There's a possibility I could have fairly severe physical handicaps on the extreme side,'” Lisa McHale said. “I never conceived of this in a million years, that it could destroy the person that Tom was. To me, that's not OK. That's different. And I think we need to find a way to prevent it.”

Midway through the 2009 season, the NFL adopted stricter guidelines for assessing players with concussions, including clearance from an independent neurologist before returning to the field. Also, the league began baseline brain-activity testing for all players drafted in 2010.

The NFL also dismantled its longtime committee on concussions, which was adamant in its criticism of research conducted at the University of North Carolina and Boston's CTE center. Leaders of the Mild Traumatic Brain Injury committee resigned last fall. A new group, the NFL Head, Neck and Spine Medical committee, was announced in February.

-- Mary Shedden