Vagal afferent nerves regulating the cough reflex.

Coughing is initiated by activation of mechanically and chemically sensitive vagal afferent nerves innervating the airways. All afferent nerve subtypes innervating the airways can modulate the cough reflex. Rapidly adapting and slowly adapting stretch receptors (RARs and SARs, respectively) innervating the intrapulmonary airways and lung may enhance and facilitate coughing. Activation of intrapulmonary C-fibers has been shown to inhibit coughing in anesthetized animals. Extrapulmonary C-fibers and RARs can initiate coughing upon activation. C-fiber-dependent coughing is uniquely sensitive to anesthesia. Tracheal and bronchial C-fibers may also interact with other afferents to enhance coughing. Recent studies in anesthetized guinea pigs have identified a myelinated afferent nerve subtype that can be differentiated from intrapulmonary RARs and SARs and play an essential role in initiating cough. Whether these "cough receptors" are the guinea pig equivalent of the irritant receptors described in the extrapulmonary airways of other species is unclear.