My current childbirth fascination is with the hormones that start labor. Released by the brain, oxytocin is the hormone that causes the uterus to contract. Pitocin, the synthetic oxytocin used to induce contractions, is introduced directly into the veins by IV. So while Pitocin mimics oxytocin, it bypasses the brain, and the chain reaction release of endorphins that follows the release of oxytocin - the body's own painkiller - doesn't happen.
Incidentally, oxytocin is also released by the brain after you've had a good meal. So a delicious dinner and great sex makes for one fantastic evening! It's another good reason to eat in labor.
Another excellent discussion of the hormones involved in creating and sustaining a spontaneous labor is in the book Gentle Birth, Gentle Mothering, but Sarah Buckley.