OK, first of all, Ali suffers from
Parkinson's Disease, which is not, according to
Yahoo!'s health page, related to
trauma, although it is characterized by neuron
degeneration.
However, a concussion occurs when you undergo a non-penetrating blow to the skull. This can be very dangerous. It is because your brain acts like a passenger in a car accident: The skull moves violently, and crashes into the brain. If this happens hard enough, you can undergo some irreparable damage.
So, if you shake your head hard enough to whack your brain into the side of your skull, then yes, you can damage it. If you shake your arm hard enough, you can dislodge it from its socket. Moderation in all things.
That said, I have indeed noticed that my head hurts after a hard shaking, and it has caused me some concern.