Airway afferent nerves

The airways are a conduit for inhaled air to the gas exchange regions of the lungs. The ∼10,000L of air an adult breathes every day includes a complex mixture of particles and substances that, depending on the environment, may be harmful and potentially lethal. Therefore, in addition to gas exchange, aspects of airway physiology also serve important host-defense functions. The sensory nervous system is a component of this airway defense. Befitting of playing a major role in host defense, the majority of sensory nerves innervating the airways are polymodal nociceptive C-fibers. Sensory innervation to the respiratory tract also comprises faster-conducting A-fibers, most of which are low-threshold mechanosensors. Irrespective of phenotype, the afferent nerves “sense” their environment and send signals (action potentials) to the brain. Subsequently, integrated input leads to outputs, including sensations such as the urge to cough and dyspnea and defensive reflexes such as apnea and cough. Activation of airway sensory nerves also regulates autonomic efferent activity, mainly the parasympathetic system that controls heart rate, airway caliber, and airway secretions. Activation of subsets of sensory nerves can also subconsciously modulate the rate and depth of breathing. This review provides a brief overview of the major subtypes of sensory nerves in the airways and discusses the physiological outcomes of their activation. For more information on this topic, the reader is directed to two review articles listed at the end of the document. Although “airways” technically include nasal passages, this review focuses on the innervation of the lower airways, the trachea and bronchi.