The parts of the extensive neural network in human airways and lung parenchyma studied best are
- the cholinergic system, of which acetylcholine is the neurotransmitter,
- the adrenergic or sympathetic system of which noradrenaline is the major neurotransmitter,
- the inhibitory nonadrenergic non-cholinergic (iNANC) system, where vasoactive intestinal polypeptide (VIP), nitric oxide (NO) and excitatory peptides are in all likelihood the neurotransmitters,
- and the excitatory nonadrenergic noncholinergic (eNANC) system, where neurokinin and substance P are the major neurotransmitters.
These pathways influence the function of smooth muscles, bronchial glands, airway blood vessels and even immune cells.
Afferent and efferent impulses travel via the vagal nerve. Afferent information derives from slowly adapting pulmonary stretch receptors, rapidly adapting irritant receptors, and from J-receptors. There is increasing evidence that antidromic transmission through unmyelinated C-fibers leads to local axon reflexes within the airways.