Reflexes from the Lungs and Chest Wall

A variety of mechanical and chemical sensory receptors in the lungs and chest wall regulate breathing pattern and defend the respiratory system. Ascribing specific reflex responses to activation of particular receptors has been complicated by the difficulty in accessing and selectively activating receptor subtypes. Receptors within the lower airways and lungs are classified based on whether the sensory afferent fibers are myelinated or unmyelinated. Their axons primarily course in the vagus nerves. Receptors with myelinated axons include the slowly and rapidly adapting pulmonary stretch receptors (SARs and RARs). SARs respond to lung inflation and mediate the Breuer-Hering inspiratory inhibiting and expiratory facilitating reflexes. Activation of RARs by inhaled irritants evokes airway protective reflexes including a rapid breathing frequency, bronchoconstriction, and mucus secretion. These receptors also elicit sighs in response to a decrease in airway compliance or a reduction in lung volume below the normal resting volume. Cough is produced by a subset of receptors with small myelinated axons. Activation of receptors with unmyelinated fibers (C-fibers) also produces airway protective reflexes consisting of a shallow rapid breathing pattern, bronchoconstriction, and mucus secretion. Chest wall receptors can reflexively alter motor drive to the diaphragm but have relatively little influence on respiratory muscle activation during eupnea.