I'm sorry I don't have better news for Joe Mauer, a player I truly admire. He does so many things right, but if a hard, jarring impact brought back his symptoms even without banging his head... Man, that sounds a lot like what Corey Koske went through. Different parts of our bodies heal in different ways. A properly set bone can be stronger after it heals. A cut either heals by itself or can be stitched, and heals. The TJ ligament can be replaced, sewed together, and be stronger than the original. But the brain is like a mass of microcircuits that doesn't regrow quite right after it gets damaged. Instead, it appears to create new connections, but they aren't as good as the originals, more like a weak patch that is less robust than the original tissue. Worse, repeated concussions make the overall situation progressively worse, which Justin Morneau discovered after his concussions as a hockey goalie made him more vulnerable to that collision at second base. One thing we all know is that you don't have to be a pro athlete to have plenty of experience with repeated concussions. I've been knocked unconscious several times going back to my childhood playing football, baseball, wrestling and gymnastics. If I had any idea what was in store later on, I would have avoided all contact sports and joined the friggin chess club. The human brain simply isn't built to survive heavy impacts without suffering permanent damage.