Hello IMark,
N-acetylcysteine can be turned into cysteine by removal of the N-acetyl group. Cysteine is a smaller molecule and can presumably cross the BBB. Cysteine and N-acetylcysteine are different chemicals.
Don't ask me why they use NAC rather than cysteine itself, there must be a reason!! Found it, the acetyl group is used in NAC to speed absorbtion by the body, once quickly absorbed, the NAC is quickly hydrolysed to cysteine. So you are doing the right thing taking NAC, it boosts the level of glutathione a powerful natural antioxidant.

If you want to see the formulae of the 2 compounds, go to
http://www.benbest.com/nutrceut/NAC.html

Ron