Parasympathetic control of airway submucosal glands: central reflexes and the airway intrinsic nervous system.

Airway submucosal glands produce the mucus that lines the upper airways to protect them against insults. This review summarizes evidence for two forms of gland secretion, and hypothesizes that each is mediated by different but partially overlapping neural pathways. Airway innate defense comprises low level gland secretion, mucociliary clearance and surveillance by airway-resident phagocytes to keep the airways sterile in spite of nearly continuous inhalation of low levels of pathogens. Gland secretion serving innate defense is hypothesized to be under the control of intrinsic (peripheral) airway neurons and local reflexes, and these may depend disproportionately on non-cholinergic mechanisms, with most secretion being produced by VIP and tachykinins. In the genetic disease cystic fibrosis, airway glands no longer secrete in response to VIP alone and fail to show the synergy between VIP, tachykinins and ACh that is observed in normal glands. The consequent crippling of the submucosal gland contribution to innate defense may be one reason that cystic fibrosis airways are infected by mucus-resident bacteria and fungi that are routinely cleared from normal airways. By contrast, the acute (emergency) airway defense reflex is centrally mediated by vagal pathways, is primarily cholinergic, and stimulates copious volumes of gland mucus in response to acute, intense challenges to the airways, such as those produced by very vigorous exercise or aspiration of foreign material. In cystic fibrosis, the acute airway defense reflex can still stimulate the glands to secrete large amounts of mucus, although its properties are altered. Importantly, treatments that recruit components of the acute reflex, such as inhalation of hypertonic saline, are beneficial in treating cystic fibrosis airway disease. The situation for recipients of lung transplants is the reverse; transplanted airways retain the airway intrinsic nervous system but lose centrally mediated reflexes. The consequences of this for gland secretion and airway defense are poorly understood, but it is possible that interventions to modify submucosal gland secretion in transplanted lungs might have therapeutic consequences.