The momenta of individual objects changes. The total momentum remains constant. I have to disagree. If you have two cars that collide head on, the momentum of both vehicles stops. The ENERGY created by the impact causes usually, some reverse momentum but the momentum is lost.
As the SNS continues to trigger physical reactions, it causes a wear-and-tear on the body. It's not so much what chronic stress does to the nervous system, but what continuous activation of the nervous system does to other bodily systems that become problematic.
Additionally, the autonomic nervous system can be further subdivided into two divisions: the sympathetic nervous system and the parasympathetic nervous system. Both of these systems control the same group of body functions, but they have opposite effects on the functions that they regulate. The sympathetic nervous system prepares the body for intense physical activity and is often referred to as the fight-or-flight response. The parasympathetic nervous system has almost the exact opposite effect and relaxes the body and inhibits or slows many high energy functions. The effects of the parasympathetic nervous system can be summarized by the phrase ' rest and digest '.