In martial arts, there's a lot of talk about the "nerve clusters" as preferred striking points. People refer to spots in the neck and the shoulders (most typically) as places where you can hit someone and temporarily paralyze them. I've always thought it was a bit of a crock: something dreamed up by people who have spent too much time watching Star Trek and convinced themselves that the Vulcan Death Grip was real. As a physician, I found the anatomy dubious -- sure, there are the brachial plexus and cervical plexus -- but the idea that you could strike them and paralyze someone, to me, seemed like so much magical thinking.
Which isn't to say that the neck isn't a great target. It is. It hurts like crazy when you strike it, it's a vital place where you can really injure someone, and if you control the neck you control the body. So I have tended to accept the nonsense talk about nerve clusters as just meaning "a good place to hit" while quietly not believing the hand-waving superstitious belief in "nerve clusters."
Then I saw this video off of the "Fail Blog:"
It's hard to watch that and not conclude that something funny neurologically just happened. The guy has an almost instantaneous total body spasm or convulsion, and drops as if poleaxed. It's not a centrally mediated reflex (like a vagal response); it's too fast. I'm not sure I can really explain it, medically, but damn.
I guess the old masters may have actually known a thing or two after all.
One thing about karate -- it does have a way of keeping you humble.