Information on how excercise effects the body?

I just finished the 100 push up challenge, and am trying to get a more rounded understanding of exercise physiology. I got a decent grasp of how strength training and muscle building work (certain % of your 1 rep max for a certain number of reps, eat your protein, get appropriate amounts of rest, & basically how the muscle rebuilds itself), and have no trouble finding information on that stuff. What i'm trying to find out more about is the little bit of stuff I've seen about the parasympathetic nervous system or something, moving your body through space as opposed to moving a weight. Also, what adding instability to a workout really does (ie using straps or rings for dips/push ups), besides making them more difficult (what effect do all these things have on the body/neural activity/connections?).