Do you know of any source for the precise explanation. I can’t find anything that resolves the question of whether it’s the twisting of the brain stem/spinal cord, or concussion. As a side note, you can knock a person out with a sufficiently hard blow to the solar plexus, which would not involve a concussion at all.
There are several nerve/plexuses that you can strike to cause unconsciousness without concussion. The solar plexus is the easiest one to get at, but there’s there’s another at the base of the nose/upper lip and a couple in the the neck. There are also various other points such as the carotid sinuses that will cause instant knockout if they are struck correctly. The tricky bit in most cases is actually hitting these spots with enough force to have the desired effect.
I’ve been knocked out before. It was a work accident. My experience is that it takes a really, really hard blow to go out for even a few seconds.
I was working on an offshore rig and a cable under tension snapped. It whipped across the deck and caught me at an angle roughly from my lower left jaw up to just behind my right eye socket. It dislocated my jaw, chipped and/or removed several teeth, and left a (surprisingly) small gash on my upper cheek. My shipmates said it was like I’d been “turned off” and just fell to the deck. They said I was out for less than a minute. After awakening, I had to convince them to let me sit up and spit out my teeth (I was starting to choke on all the little bits).
Due to weather, my only choice was to ride a supply ship for a couple of days back to land. During the trip (did I mention the bad weather?), I got to experience not only headaches from the concussion, but seasickness and vomiting on and over the exposed nerves of my broken teeth. Due to my jaw I couldn’t eat the whole time (not that I wanted to). Spent a couple of weeks on the beach getting my teeth repaired, and returned to work without even a decent scar to show for my effort. 
FWIW, I’ve never had any adverse effects from the unconsciousness. It’s been 25 years, so I suppose any longterm problems would have shown up by now.
It seems that a single knock-out probably has very little long term effect. What leads to dementia is an accumulation of head blows that take a cumulative effect. The brain seems to adapt to minor injuries and can override the effect. A boxer, over the course of training, gets hit in the head hundreds of times with blows that would otherwise knock out a normal person. The brain adapts to the blows, finds a way to maintain consciousness and allows the boxer to proceed. Boxers get up off the canvas after severe head blows and can comeback and win. The resilience is admirable but it also can lead to a long term life of misery.
That adaptability of the brain is what leads to the long term problem. Everything seems fine but after time dementia sets in. Terry Bradshaw is the latest notable. For Troy Aikman it’s only a matter of time. Muhammad Ali is there. Jim McMahon is on his way to dementia. The head trauma often leads to depression, substance abuse, and suicide.
One severe knock-out isn’t always the problem. Accumulation of blows to the head can ruin a life and no amount of money earned can correct that.
Muhammad Ali does not have dementia. He does suffer from Parkinson’s Syndrome, which is associated with blows to the head. It is severe, and he is probably suffering secondary effects from the drugs he needs to take.
ETA: If I could change the world the way he did, I would gladly suffer the consequences.
Parkinson’s Syndrome is a form of dementia. Dementia is a catch-all. Parkinson’s, Alzheimer’s, etc. are forms of dementia. Muhammad Ali is suffering from dementia. There is no question. Muhammad Ali does have dementia.
Parkinson’s may lead to dementia, but dementia is a cognitive disorder. There is a term (which I cannot recall at the moment) for the condition of boxers and others who have suffered repeated head trauma. It has the word dementia in it, but it is not dementia, which refers to cognitive disorders.
Dementia pugilistica
Parkinson’s and/or Parkinson’s Syndrome are a subset of dementia. Alzheimer’s is a subset of dementia. People with Alzheimer’s don’t necessarily have Parkinson’s or vice versa. Yet, both subsets are suffering a form of dementia.
Reference this:
http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0001748/
Whether it’s:
- Parkinson's disease
- Multiple sclerosis
- Huntington's disease
- Pick's disease
- Progressive supranuclear palsy
- Infections that can affect the brain, such as HIV/AIDS and Lyme disease
- Azheimer's
It’s still dementia.
Read your own cite: