Sympathetic Nervous System - Friend or Foe?

What is the autonomic nervous system and how does it effect our horses? 

In this podcast I talk about the flight and fight, rest and digest elements of horse behaviour and ask the question - is the being in a high arousal state always a concern? Can it be beneficial in rehab and training. I have a lot to learn about the topic of motivation and ideas such as flow state, and how they can be applied to horses but I think there is so much that can translated. What do you think?