This audio file is from Talk of the Nation, originally published on March 15th, 2013. It is a very concise piece that explores if even one concussion can have lasting impacts on the brain. It also talks about whether helmets actually can protect against concussion.
Have a listen >
For those without Flash Player, http://www.npr.org/2013/03/15/174409382/can-just-one-concussion-change-the-brain