The innervation of the upper airways depicted in a highly schematic diagram

Sympathetic and parasympathetic nerves innervate the airways. Sympathetic nerves innervate blood vessels in the airways and mediate vasoconstriction. Parasympathetic nerves innervate the vasculature as well as the mucus glands of the airways. When activated, parasympathetic nerves initiate mucus secretion and dilatation of airway blood vessels. Parasympathetic nerves also innervate airway smooth muscle. The opposing actions of the sympathetic and parasympathetic nerves on nasal blood flow likely serves to optimize the air conditioning function of the nasal passages. Afferent nerves innervating the nose regulate such homeostatic reflexes as well as defensive reflexes like mucus secretion and sneezing. Upper airway afferent nerves, which arise from the trigeminal ganglion may also initiate reflex effects in the lower airways and may regulate respiration. These interactions between the upper and lower airways are facilitated by the proximity of CNS structures regulating upper and lower airway function.