Afferent nerve subtypes innervating the airways and lungs of rats

Subtypes can be identified based on their responsiveness to chemical stimuli such as capsaicin, their activity during eupneic breathing, and their response to sustained lung inflation. Three broad classes of airway afferent nerves have been identified. A) Bronchopulmonary C-fibers are relatively unresponsive to mechanical stimulation but are activated by noxious chemical stimuli such as capsaicin, acid, hypertonic saline and inflammatory mediators including bradykinin and serotonin (5-HT). B) and C) Airway mechanoreceptors are exquisitely sensitive to the dynamic and sustained mechanical forces put upon the lung during respiration. Subtypes of airway mechanoreceptors can be identified based on their adaptation to sustained lung inflation. Activation of airway C-fibers and rapidly adapting mechanoreceptors reflexly increases airway parasympathetic nerve activity and regulates airway defensive reflexes such as cough. By contrast, activation of slowly adapting mechanoreceptors reflexly reduces airway parasympathetic nerve activity and may play only a modulatory role in cough. Slowly adapting receptors also play a primary role in regulating respiratory pattern.