Pulmonary Stretch Receptors, Irritant Receptors, and Juxtacapillary Receptors Can Influence the Rhythm of Breathing

Three types of receptors with vagal afferent nerves have been identified within the lung: (1) slowly adapting stretch receptors and (2) irritant receptors, both of which have mye­linated afferent nerves, and (3) C fibers with unmyelinated axons.

Slowly adapting stretch receptors are nerve endings associated with smooth muscle in the trachea and main bronchi, but to a lesser degree in the smaller intrapulmonary airways. They are stimulated by deformation of the wall of larger airways, as when intrathoracic airways are stretched dur­ing lung inflation. Because firing rates from these receptors increase progressively as the lung inflates, they are thought to be responsible for the inhibition of breathing caused by lung inflation (Hering-Breuer reflex). Termination of input from these receptors by vagotomy leads to a slowing of respiration and an increase in tidal volume. Slowly adapting stretch receptors may be responsible in part for adjustments in the rate and depth of respiration to minimize the work of the respiratory muscles.

Irritant receptors, or rapidly adapting stretch receptors, are thought to be myelinated nerve endings ramifying between epithelial cells in the larynx, trachea, large bronchi, and intra­pulmonary airways. They are stimulated by mechanical defor­mation of the airways, such as the deformation that occurs during lung inflation, bronchoconstriction, and mechanical irritation of the airway surface. Irritant gases, dusts, histamine release, and a variety of other stimuli can also cause these receptors to respond. Stimulation of rapidly adapting irritant receptors leads to cough, bronchoconstriction, mucus secre­tion, and rapid, shallow breathing (Iiyperpnea), all of which are protective responses to clear irritant materials from the respiratory system. These receptors may initiate the sighs that are thought to redistribute pulmonary surfactant over the alveolar surface.

C fibers ramify in the pulmonary interstitium close to pulmonary capillaries (juxtacaρillary receptors), where they may monitor blood composition or the degree of distention of the interstitium. Similar fibers also occur in the walls of the airways. C-fiber activation may be responsible for the increase in res­piratory rate (tachypnea) that accompanies allergic, infectious, or vascular diseases.

In addition to intrapulmonary receptors, receptors exist in the upper airway. Stimulation of receptors in the nasal cavity causes sniffing and sneezing, whereas stimulation of laryngeal and pharyngeal receptors may cause cough, apnea, or broncho­constriction. Temperature receptors in the pharynx that are cooled by airflow alert the animal if inappropriate effort is being exerted by the inspiratory muscles so that adjustments can be made.