Afferent neural pathways in cough and reflex bronchoconstriction

Cough and bronchoconstriction are airway reflexes that protect the lung from inspired noxious agents. These two reflexes can be evoked both from the larynx and tracheobronchial tree and also from some extrarespiratory sites. Within the airways, certain sites are particularly sensitive to stimulation of cough (larynx and points of proximal airway branching), whereas bronchoconstriction can be triggered from the whole of the tracheobronchial tree. In the larynx, "irritant" receptors with myelinated afferents mediate cough and bronchoconstriction. Little seems to be known about laryngeal nonmyelinated afferents and their reflexes. In the tracheobronchial tree and lung, slowly adapting stretch receptors (SARs) and rapidly adapting stretch receptors (RARs) have opposing effects on airway tone, the former mediating bronchodilation and the latter bronchoconstriction. In cough, on the other hand, they operate concurrently, a mediatory role for RARs and a facilitatory role for SARs. C-fiber endings (bronchial and pulmonary) mediate bronchoconstriction. Inhalation of so-called "selective" C-fiber stimulants induces cough, but excitation of RARs has not been eliminated, and the possibility also exists that the cough is secondary to other lung actions mediated by these nerve endings. Although cough and bronchoconstriction may be mediated by the same type of receptor, they seem to have separate afferent neural pathways.