As implied by its name, the autonomic nervous system, in most situations, automatically regulates breathing, heartbeat and circulation, and is difficult to control consciously.
當交感神經作用時，會產生瞳孔放大、毛髮豎立、心跳加速、心臟血液輸出量增加、腸胃蠕動減緩、血壓升高、周邊血管收縮、腎上腺素分泌增加等反應，副交感神經作用時則相反。