The brain is not like an injured muscle. It does not regrow and replace cells like the rest of the body does. It can't because it needs to keep those cells because they have information stored in them. So, those cells need to heal. Some do but some don't. Slowly, some new cells grow but not to replace the injured cells. Unfortunately, the axons and myelin sheath between cells do not heal very well. They can function is a weakened state but not at the same level. This leaves the brain in a weakened state and more susceptible to dysfunction with the next concussion. This is why lowering stress is important.

But, permanent injury does not mean permanent symptoms or dysfunction. The brain is very good at adapting to injury and finding new pathways to make up for weakened pathways. It takes effort and many repetitions to help the brain establish these new pathways. It takes discipline to help the brain recover.