Novel insights in the neurochemistry and function of pulmonary sensory receptors.

Afferent nerves in the airways and lungs contribute to optimisation of the breathing pattern, by providing local pulmonary information to the central nervous system. Airway sensory nerve terminals are consequently tailored to detect changes readily in the physical and chemical environment, thereby leading to a variety of respiratory sensations and reflex responses. Most intrapulmonary nerve terminals arise from fibres travelling in the vagal nerve, allowing a classification of "sensory airway receptors", based on their electrophysiologically registered action potential characteristics. Nowadays, at least six subtypes of electrophysiologically characterised vagal sensory airway receptors have been described, including the classical slowly and rapidly adapting (stretch) receptors and C-fibre receptors. The architecture of airways and lungs makes it, however, almost impossible to locate functionally the exact nerve terminals that are responsible for transduction of a particular intrapulmonary stimulus. With the advances in immunohistochemistry in combination with confocal microscopy, airway sensory receptor end organs can now be examined and evaluated objectively. Based on their "neurochemical coding", morphology, location and origin, three sensory receptor end organs are currently morphologically well characterised: smooth muscle-associated airway receptors (SMARs), neuroepithelial bodies (NEBs) and visceral pleura receptors (VPRs). The present information on the functional, morphological and neurochemical characteristics of these sensory receptors leads to important conclusions about their (possible) function. Currently, ex vivo lung models are developed that allow the selective visualisation of SMARs, NEBs and VPRs by vital staining. The described ex vivo models will certainly facilitate direct physiological studies of the morphologically and neurochemically identified airway receptors, thereby linking morphology to physiology by identifying in situ functional properties of a given receptor end organ.