"The sympathetic nervous system (SNS) mediates the body's response to arousing circumstances, for example, producing the famed "fight or flight" stress response. To use the feeble joke told to first-year medical students, the SNS mediates the "four Fs--fear, fight, flight, and sex." Particular midbrain/brain-stem nuclei send long SNS projections down the spine and on to outposts throughout the body, where the axon terminals release the neurotransmitter norepinephrine."