Afferent pathways for the cough reflex.

Cough is most often a reflex act elicited by the stimulation of nervous receptors in the respiratory tract. The most important tussigenic areas are at the level of the larynx and the more distal portion of the trachea with its bifurcation. Laryngeal rapidly adapting receptors with myelinated fibres are promptly activated by tussigenic stimuli, but it is unlikely that they are the only ones responsible for laryngeal coughing. In fact, some evidence supports the involvement of laryngeal C-fibre receptors. Cough in response to inhalation of aqueous aerosols containing low chloride concentrations may involve the same category of receptors with myelinated fibres that mediate apnoea in the newborn. Within the tracheobronchial tree, there are rapidly-adapting irritant receptors responsive to several tussigenic stimuli and concentrated at tussigenic sites. Animal species without this type of ending do not cough. However, some results for the tracheobronchial tree suggest a role for other types of receptors. Slowly adapting stretch receptors may play an indirect role through their stimulatory effect on expiratory muscles. A role for C-fibre receptors is proposed considering the activation of these endings by substances (capsaicin, lobeline, bradykinin, SO2) capable of inducing cough. Bronchial C-fibre receptors, present also in proximal airways, respond to light mechanical probing. Contrary to an involvement of pulmonary C-fibres is the observation that cough is not part of the lung chemoreflex, although during this reflex syndrome (apnoea followed by rapid and shallow breathing, bradycardia and hypotension) cough can be induced by mechanical irritation of the trachea.