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.
The sympathetic nervous system has an active pushing function, the parasympathetic has mainly a relaxing function. The sympathetic nervous system is located to the sympathetic chain, which connects to skin, blood vessels and organs in the body cavity. The sympathetic chain is located on both sides of the spine and consists of ganglias.