By Ashley Fantz and Steve Almasy, Mar. 18, 2015, CNN

It was a ding, just a "minor concussion," that got Chris Borland thinking that playing football might not be worth the millions that come with being an NFL player.

Borland was a rookie linebacker in his first training camp in August when he got his "bell rung," he told ESPN's "Outside The Lines."

What did he do? He kept playing that day, and through an excellent rookie season. But the thought of the permanent damage that might be happening to him kept nagging at him.

He talked to his teammates and family, read about concussions and their effects, each day coming closer to the decision that he finally announced this week.

At age 24, not even yet in his prime as an NFL player, Borland told his team he was retiring because he was worried about the long-term effects of head trauma.