The sympathetic nervous system (SNS) is part of the “fight and flight” response system. Maletic and Raison (2017) describe the activation of the system as follows. “In response to a stressor, several things happen sequentially. First parasympathetic tone is withdrawn. If this is inadequate to the perceived danger, the SNS is then activated. It begins with an increase in SNS nerve fiber signaling, with a resultant release of the neurotransmitter norepinephrine (NE). One SNS channel connects brain with the adrenal medulla, from whence epinephrine (adrenalin) is released. The release of epinephrine is much faster than the release of cortisol, but slower than the release of NE (pg. 216).”