Afferent properties and reflex functions of bronchopulmonary C-fibers

Bronchopulmonary C-fiber afferents are characterized by their distinct sens itivity to chemical stimuli in the airways or pulmonary circulation. Respon ses evoked by activating these afferents are mediated by both central refle x pathways and by local or axon reflexes involving the release of tachykini ns from sensory endings. Bronchopulmonary C-fiber stimulation reflexly redu ces tidal volume and increases respiratory rate, constricts the airways, in creases mucus secretion in the airways, and is associated with coughing. Ca rdiovascular effects include bradycardia, a fall in cardiac output, and bro nchial vasodilation that increases airway blood flow despite systemic hypot ension. In animals, C-fiber stimulation inhibits skeletal muscle activity, and in humans, is accompanied by burning and choking sensations in the thro at and upper chest. Recent studies have identified additional physiologic a nd pharmacologic stimuli to these afferents, such as hydrogen ions, adenosi ne, reactive oxygen species, and hyperosmotic solutions. Furthermore, incre asing evidence indicates that the excitability of these afferents is enhanc ed by the local release of certain autocoids (e.g. PGE(2)) during airway in flammation. These findings further indicate that vagal C-fiber endings in t he lungs and airways play an important role in regulating the cardiopulmona ry functions under both normal and abnormal physiologic conditions. (C) 200 1 Elsevier Science B.V. All rights reserved.