This might help.

http://faculty.washington.edu/chudler/chnt1.html

This too.

http://users.rcn.com/jkimball.ma.ult...ges/P/PNS.html

The immune system, central nervous system and the body in general are COMPLICATED. One thing leads to another leads to another. Reading about it can drive you batty or it can make you want more. It's all quite fascinating.