Fig. 2. Schematic diagram of airway irritant reflex-induced bronchoconstriction by tracheal intubation. The upper airway is well innervated by afferent sensory pathways synapsing in the nucleus of the solitary tract (nTS), which projects excitatory glutaminergic and inhibitory γ-aminobutyric acid-A (GABA)-ergic neurons to the airway-related vagal preganglionic neurons (AVPN). Parasympathetic preganglionic efferents travel in the vagus nerve to release acetylcholine onto M3muscarinic receptors on airway smooth muscle inducing bronchoconstriction.