Breath-taking complexity of vagal C-fibre nociceptors: implications for inflammatory pulmonary disease, dyspnoea and cough.

摘要

Vagal sensory nerves are traditionally considered as the afferent pathways that detect physiological information from visceral organs, including airways and lungs, while spinal afferents are involved in the detection of 'noxious' information. More recently, however, subsets of vagal afferents have also been implicated in processing potentially harmful stimuli (nociceptors) (Kollarik et al.2010).Recent reviews describe at least seven electrophysiologically characterized vagal sensory airway receptors, including slowly adapting receptors (SARs), rapidly adapting receptors (RARs), bronchial and pulmonary C-fibre receptors (CFRs), high threshold Adelta-receptors (HTARs), cough receptors and neuroepithelial bodies (NEBs) (Adriaensen et al. 2006; Widdicombe, 2009; Yu, 2009). Similarly, many respiratory sensations have been reported (pain/ache, irritation, tightness, urge-to-cough, air-hunger, sense of effort, sense of lung volume/airflow, temperature sense, etc.). The often reported 'dyspnoea' or its near equivalent 'breathlessness' includes air-hunger, sense of effort and tightness. Several of these sensations unmistakably originate from the lungs and lower respiratory tract but, unfortunately, hard evidence for these sensations being evoked by any of the airway sensors is still lacking (Widdicombe, 2009). Both HTARs, connecting to thin myelinated fibres, and CFRs, linked to non-myelinated fibres, are considered as subgroups of vagal airway nociceptors (Yu et al. 2007; Yu, 2009).