In all my research I have never come across such a figure as that. A source would be appreciated, *edit*. Cheers.

RE. CSF. You have to picture the brain as suspended in the fluid, both fluid and brain fill your skull wholly. It is not open, or with air, so the fluid sloshes around, but is a complete volume.

With most knocks there will remain a cushion, or portion of the CSF volume between brain and skull. With a forceful enough blow, or with great momentum, this cushion is not soft put provides a tense/hard surface. With great enough force/momentum the brain can hit against the skull.

To test this: get a bottle/jar of water. put in a pingpong ball (brain) with a weight attached (brain stem) and fill completely with water (CSF). Have a shake, using diff forces. This will give you a good idea...but remember we are more sophisticated and have evolved this process for protection!

However...those twitches? I get them. Though I think of them as tremours, and this is related to my motor issues. It was unnerving but now is kinda annoying and a bit embarrassing. It is not damaging my brain.

A couple of nights ago I smacked my head rather hard againts the rail of my sons bed--went to put something on the desk and stood up in the wrong lace, duh. Symtoms flared, my headache worsened but I did not injure my brain.