Dabashire -BubbaRayGracie - I'd be less worried about the potential effects of chokes and the occasional accidental knee to the head in BJJ than what could happen if you land on your head/neck wrong after a throw or during a sweep or have a strong guy give you hard neck crank before you know what's going on or something like that.
That said, I've never had a doctor, including neurologists, express concern to me about head injuries through BJJ, and those same folks have been pretty open to me about their concerns about head injuries with boxing and MMA. That doesn't mean you're not right to want to look into the potential of brain damage through BJJ/sub grappling though.
BJJ is no worse than any of the other grappling specialties. Judo would have a slight increase due to potential of landing on your head more often from a throw, but overall what Bubba said is pretty accepted.
As a person who has had a brain surgery and trains, I can say there is a neurological effect or 'stress', but no worse than taking a fall skiing as far as throws and take downs go...if anything, it improves your ability to take a fall and NOT get hurt.
Chokes increase CSF and blood pressure in your skull, but I doubt there is a means of measuring that definitively. Picture squeezing a balloon at one end. I seriously doubt there is any noticeable long-term damage as far as training goes...most people don't wait to black out to tap.
I do know that too much pressure on your brain will cause brain damage, but that would be pressure far greater than you'd experience in training.
Unless you get choked four or five times until you pass out every time you train, for a few years, your body is built to take the rest. I'd be a lot more worried about your joints
I totally agree with what you said about BJJ and other grappling arts like Judo or wrestling. I only used BJJ in my post because it was what was referenced in the thread title.
Out of curiousity, why did you have brain surgery, if you don't mind me asking?