What happens when you have a concussion?

Each year in the United States, players of sports and recreational activities receive between 2.5 and 4 million concussions. How dangerous are all those concussions? The answer is complicated and lies in how the brain responds when something strikes it. Clifford Robbins explains the science behind concussions. [Directed by Rémi Cans, narrated by Addison Anderson, music by Nicolas Martigne].