Physiological and pathophysiological implications of upper airway reflexes in humans

The upper airway is a vital part of the respiratory tract. Although the upper airway serves several functions, protection of the airway and preservation of airway patency are the most essential functions subserved by upper airway reflexes. Various types of nerve endings have been identified in and under the epithelium of the upper airway, and afferent nerve endings are the natural starting of all reflex activity. The upper airway reflexes consist of many different types of reflex responses such as sneezing, apnea, swallowing, laryngeal closure, coughing, expiration reflex, and negative pressure reflex. Although the activation of upper airway reflexes does not necessarily occur at one particular site of the respiratory tract, individual reflex response is usually considered to be highly specific for the particular respiratory site which has been affected. The upper airway reflexes are modified by many factors such as sleep, anesthesia, and background chemical ventilatory drive. Both depression and exaggeration of upper airway reflexes cause clinical problems. Depression of upper airway reflexes enhances the chance of pulmonary aspiration and compromises the maintenance of the airway, whereas exaggeration of airway reflexes such as laryngospasm and prolonged paroxysm of cough can be harmful and dangerous. In this review, various aspects of upper airway reflexes are discussed focusing on the functions of upper airway reflexes in humans and some pathophysiological problems related to clinical medicine.