The parasympathetic nervous system regulates the sympathetic adrenal medullary system by reducing cardiac output through direct innervation of the sinoatrial node by parasympathetic nerve fibres.

• 副交感神經系統，是透過直接分佈在竇房結上的副交感神經纖維來降低心臟輸出，用此機轉來調節交感腎上腺髓質系統產生的生理現象。