The New Yorker has published a comprehensive article on the effects of high-impact sports, particularly football, on the human brain. Penned by Malcolm Gladwell.
football injury
Upon closer analysis of brain plaques, particularly a trauma-induced scarring called tau, researchers have found considerable damage in the samples viewed. Only a post-mortem autopsy and proper staining can reveal the damage so the sample size is very small but the evidence is overwhelming.

Here are some notable quotes:

Chronic traumatic encephalopathy (C.T.E.), which is a progressive neurological disorder found in people who have suffered some kind of brain trauma. C.T.E. has many of the same manifestations as Alzheimer’s: it begins with behavioral and personality changes, followed by disinhibition and irritability, before moving on to dementia.

…a man who had been a linebacker for sixteen years, you could see, without the aid of magnification, that there was trouble: there was a shiny tan layer of scar tissue, right on the surface of the frontal lobe, where the brain had repeatedly slammed into the skull. It was the kind of scar you’d get only if you used your head as a battering ram.

Gladwell ties in Michael Vick and the aggressive nature of dogfighting and football. It’s a strange comparison that in the end reveals much about the will and fight in the football players to go beyond their pain and sacrifice themselves for the good of the team, or in this case the dog being loyal to its owner.

…data suggest that, in an average football season, a lineman could get struck in the head a thousand times, which means that a ten-year N.F.L. veteran, when you bring in his college and high-school playing days, could well have been hit in the head eighteen thousand times: that’s thousands of jarring blows that shake the brain from front to back and side to side, stretching and weakening and tearing the connections among nerve cells, and making the brain increasingly vulnerable to long-term damage.

A professor at the University of Chicago called it a “boy-killing, man-mutilating, money-making, education-prostituting, gladiatorial sport.” I can now agree with this statement wholeheartedly.