Personally I've found some sort of link between mental and physical exhaustion, though I'm not sure which causes which.
I think that being struck in the head probably leads to a loss of control and an overall inability to remain cool more than it affects mental capacity. People panic when someone's trying to punch them. That said, the more experienced (and physically fit) a boxer, the longer they can last before becoming overwhelmed. I don't think it's as simple as purely a mechanical reaction to being punched in the head.
http://www.sciencedaily.com/releases/2007/05/070502093035.ht...
http://www.bmj.com/content/335/7624/809.full
Both from 2007. First one says that amateur boxers do get brain damage, second one says they don't. (there are the top google hits, I looked into this deeper about 5 years ago; at that time there was very little research done, let alone consensus.)
Point being, the question is: does getting hit in the head pose linear risks/ I.e, simplified, do a number of cells die for each punch or kick to the head? Or does it require a certain threshold of damage for there to be any cognitive effects?
I don't compete in any sport but I've trained plenty of full contact sports over the last 10 or 15 years. I've been punched and kicked in the face a lot, with and without gloves, been knocked out briefly once, been choked out several times. Still my head/brain has taken nowhere near the amount of punishment a pro or even good amateur fighter gets; my brains gets plenty of time between trainings to recover. I can never know for sure if my training has caused brain damage, and if it did, in what form that will manifest itself (or has already, maybe), but I don't have reasons to believe it affected my cognitive functions. Maybe it did and the damage caused decreased intelligence, making me believe that it didn't :)
After suffering a blow to the head, I was given a battery of cognitive tests: Repeat this sequence of numbers, and the like. Three days out I was failing the tests rather spectacularly. At five days everything was back to normal.
This is particularly true for head hits, cause beyond the pain, head hits really do make you dizzy and think fuzzy. I know in my experience, hitting smacked hard in my head will make my vision explode into white fuzz, and also temporarily make me feel like I just shot back 4 shots of rum.