I'm wondering what is (or isn't) happening in the brain that causes it not to heal.

Example: A person had an accident a year ago. There's obviously physical damage to the brain. Then for a week he overdoes it. This causes a setback. Is the setback a result of chemical damage in the brain or what? I get the physical trauma causing symptoms.

I've heard that excessive stress kills the brain. Is this kind of what is happening in the brain during a setback? Is there too much stimulus that it causes stress chemicals to flood the brain?

Thanks!