It's not football season, but the game is still making headlines: In a new study published in the Journal of the American Medical Association, neuropathologist Ann McKee and her colleagues examined the brains of 111 N.F.L. players and found 110 of them to have the degenerative disease chronic traumatic encephalopathy (CTE).

The condition has been linked to repeated blows to the head—and every year in the U.S., professional and novice athletes alike receive between 2.5 and 4 million concussions. This raises the question: What happens to the human brain when we get a concussion or suffer a hard blow to the head, and how dangerous are these hits to our long-term health?

Expert Clifford Robbins explains in the TED-Ed video below: