Borland says he's only had two diagnosed concussions, but doesn't want to take any chances. He'll pass up a $540,000 paycheck this season, on top of future earnings. His full, four-year contract was worth almost $3 million.

"I just thought to myself, 'What am I doing? Is this how I'm going to live my adult life, banging my head, especially with what I've learned and know about the dangers?'" he told ESPN.

In what might be a sign of the powerful pull professional sports still exerts, even in a chastened age, Borland says he wants to return to school and potentially study sports management. (One thing that may make it easier for Borland to step aside and surrender those future earnings is the fact that he attended one of the nation's best public universities and graduated with a bachelor's degree in history. Not all players have that to fall back on.)

Borland's team seemed taken aback, but generally supportive. In a statement, the 49ers general manager Trent Baalke said:

From speaking with Chris, it was evident that he had put a great deal of thought into this decision. He was a consummate professional from day one and a very well respected member of our team and community. Chris is a determined young man that overcame long odds in his journey to the NFL and we are confident he will use the same approach to become very successful in his future endeavors.

More interesting is how current and former NFL players responded to the announcement. Generally, they seem to be understanding, if shocked. The Broncos wide receiver Brandon Marshall took heat for this tweet:

So P. Willis and Chris Borland? They know something that we don’t? — Brandon Marshall (@BMarshh54) March 17, 2015

But many reactions are more like this one from the Rams defensive end Chris Long:

WOW. I loved Chris Borland's game but I can't fault him for calling it quits. His concerns are real. Still it takes a man to do the logical. — Chris Long (@JOEL9ONE) March 17, 2015

(Later, Long tweeted that the small number of people reacting negatively "need to be reminded that they don't own pro athletes.")

Here's retired star Donté Stallworth:

49ers young star Chris Borland's retirement after one season is the beginning of many more to come. Best wishes, kid! http://t.co/SxTwvrCPL6 — Donté Stallworth (@DonteStallworth) March 17, 2015

The Colts long snapper Matt Overton:

Guys deciding to walk away from the game at a young age is a great reminder to us all that life has a bigger picture. I wish them the best! — Matt Overton (@MattOverton_LS) March 17, 2015

The 49ers linebacker Chase Thomas:

shocked to hear the news about my dude Borland, but i totally understand his decision to retire #muchrespect — Chase Thomas (@CTcard44) March 17, 2015

The Eagles nose tackle Beau Allen, a teammate of Borland's at Wisconsin:

Nothing but love and respect for you @ChrisDec26. At the end of the day there are bigger things than the game of football #priorities — Beau Allen (@Beau_Allen) March 17, 2015

A free agent tight end, Tom Crabtree, struck a somber note:

It scares the shit out of me when i can't recall a name or forget where i put my keys as we all do. But thoughts of CTE always creep in. — Tom Crabtree (@itsCrab) March 17, 2015

Slower to respond was the NFL, which has been pilloried for failing to respond aggressively enough to CTE and issues of brain trauma.