Nervous System 101
Most of us musicians understand "fight or flight," or at least we've felt it, but we often lack the understanding of what the rest of the nervous system is doing. Our nervous system consists of the Central Nervous System (brain and spinal cord) and the Peripheral Nervous Stem (cranial and spinal nerves), which basically work in a feedback loop of information with the motor and sensory aspects.
Within the peripheral nervous system are the somatic, enteric, and autonomic systems. Somatic can be conscious or unconscious, and results in movement of skeletal muscles as a response to sensory input. Simply put, the somatic system moves your body. The enteric system consists of the 100 million neurons of your GI system, and the various actions needed for homeostasis.
The autonomic system is unconscious and regulates internal organs. Within the autonomic system are two more divisions: the sympathetic and the parasympathetic systems (now we're in more familiar ground!). The sympathetic system is what's responsible for our "fight or flight" response, which causes our breath to shorten, eyes to dilate, heart rate to increase, and a few other things. Most of us know this well from performing and public speaking! Our parasympathetic system is the compliment, in charge of day to day functions of organs, cellular repair, etc.