Functional implications of the multiple afferent pathways regulating cough.

At least 2 airway vagal afferent nerve subtypes can directly initiate coughing upon activation. The capsaicin-insensitive, acid-sensitive mechanoreceptors innervating the larynx, trachea and large bronchi regulate coughing in both conscious and anesthetized animals. Activation of capsaicin-sensitive C-fibers innervating these airways will also produce coughing, but C-fiber dependent cough is prevented entirely by anesthesia. The different stimuli activating these afferent nerve subtypes and their differential sensitivity to anesthesia implies the existence of 2 parallel pathways for cough, and by extension, 2 types of cough, one essential and homeostatic, the second nonessential and pathophysiologic. The basic properties of the afferent nerves regulating cough, their interactions both centrally and peripherally and their responsiveness to tussive stimuli are briefly reviewed. Also reviewed is evidence against the notion of 2 completely separate types of cough regulated by parallel afferent pathways, asserting instead that multiple afferent nerve subtypes contribute to all types of cough.