In addition, cough can also be initiated voluntarily. Although widely studied for many years, there has been much debate surrounding the identity of the airway afferent nerve subtype that precipitates reflex coughing (see below). These afferent nerves provide modifying inputs to the brainstem neural elements controlling respiration, and consequently help generate the cough respiratory pattern. Reflex cough occurs when subsets of airway afferent (sensory) nerves are activated by inhaled, aspirated or locally produced substances. Coughing is generally characterized by a reflex-evoked modification of breathing pattern in response to airway irritation. Perhaps the most widely recognized neural response involved in airway protection is coughing. Airway protection relies upon specialized epithelial barriers and immune responses as well as a variety of highly co-ordinated neural reflex responses that help to limit the degree of potential harm and ultimately remove or expel the harmful substance from the airways. It is therefore not surprising that a variety of defensive mechanisms have evolved along with the normal function of the respiratory system to help protect against such threats. The basic nature of the respiratory system (i.e., inspiration of air from the surrounding environment for gas exchange), as well as the shared nature of the initial anatomical structures for the passage of food and air, places the airways and lungs under the constant threat of exposure to a variety of harmful airborne particles, organisms and other substances as well as aspirated gastric contents or accidental inhalation of foodstuffs.
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