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

In the lower airways, two distinct parasympathetic pathways have been identified: cholinergic and noncholinergic. Cholinergic parasympathetic nerves mediate bronchospasm while noncholinergic parasympathetic nerves mediate bronchodilatation. Reflexes differentially regulate these distinct parasympathetic pathways. This differential regulation may be facilitated by the distinct origin of preganglionic parasympathetic nerves arising from regions of the nucleus ambiguous (nA) or the dorsal motor nucleus of the vagus nerves (dmnX), and perhaps through differential processing of afferent input through the nucleus tractus solitarius (nTS). Sympathetic-adrenergic innervation of airway smooth muscle in humans is sparse or nonexistent. Circulating catecholamines are thus likely to be the primary endogenous ligand for b-adrenoceptors on airway smooth muscle. Vagal afferent nerves regulate autonomic reflexes in the lower airways. Multiple subtypes of afferent nerves innervating the airways and lungs have been identified. These afferent nerves regulate autonomic nerve activity, respiration and defensive reflexes such as cough.