Innervation of the Airways Quiz

• The innervation of the lungs includes right and left plexuses that follow the trachea and bronchial tree.
• The pulmonary plexuses provide sensory and motor innervation to the smooth muscle and glands.
• Parasympathetic (cholinergic) and sympathetic (adrenergic) fibers provide weak motor effect on airway diameter, causing vasoconstriction and inhibition of gland secretion.
• Visceral afferents travel with CNX fibers and the non-adrenergic/non-cholinergic (NANC) system provides inhibition, including bronchodilation, which is the predominant balance to the excitatory cholinergic system.
• The pulmonary plexus includes autonomic and visceral afferent fibers from CNX and thoracic nerves.
• Vagal parasympathetic fibers are secretomotor, glands, and bronchoconstrictive – smooth muscle with postganglionic parasympathetic innervation being a major source of unitary smooth muscle contraction.
• Sympathetic innervation from fibers of T1 to T4 inhibit secretion from bronchial glands, vasoconstrict of pulmonary vessels, and inhibit bronchoconstriction of smooth muscle, while weakly bronchodilating.
• Visceral afferent fibers transmit sensory information, which are sparse compared to somatic structures, and are nociceptive and reflexive.
• Hormonal release of epinephrine and norepinephrine during "flight or fight" circumstances results in relaxation of airway smooth muscle, with β2 adrenergic receptors widely distributed in smooth muscle and combining with GPCRs in the plasma membrane of myocytes to cause smooth muscle relaxation and bronchiole dilation.
• Mast cells, which are heavily granulated and reside in the connective tissues of airways, contain secretory granules filled with inflammatory mediators, including histamine, proteoglycans, lysosomal enzymes, and metabolites of arachidonic acid, which can induce bronchoconstriction, stimulate mucus secretion, and increase permeability of bronchial vessels when degranulation occurs.
• Smooth muscle in airways contains significant amounts of collagen and elastic connective tissue fibers, with walls of bronchi and bronchioles having a thicker layer of smooth muscle relative to the diameter of the bronchiole lumen.
• The respiratory epithelium descending the tracheobronchial tree has varying heights and complexities, with the trachea and bronchus having ciliated pseudostratified columnar epithelium, conducting bronchioles having ciliated simple columnar epithelium, and terminal bronchioles having ciliated simple cuboidal epithelium, which is replaced by club cells.
• Club cells, also known as nonciliated bronchiolar exocrine cells, secrete proteins, including surfactant apoproteins A,B, and D, lipids, glycoproteins, and modulators of inflammation, act as progenitors for club cells and ciliated epithelial cells, and destroy airborne toxins that reach bronchioles.
• An acinus is a part of the lung supplied by a single 1st order respiratory bronchiole, with all structures to and including the alveolar sac, and all parts of an acinus are involved in gas exchange.
• Alveoli are composed mainly of a thin layer of squamous Type I epithelial cells, which are specialized for gas exchange, and Type II alveolar cells, which are cuboidal epithelium and produce surfactant, with surfactant reducing surface tension and produced in lamellar bodies derived from the Golgi.
• The respiratory zone is the segment of airways completely lined with alveoli, alveolar ducts, and sacs, and each respiratory bronchiole gives rise to 2-11 alveolar ducts, with alveolar ducts being elongated airways with walls completely lined with alveoli and having very small amounts of smooth muscle and elastic tissue.
• An acinus is part of the lung supplied by a single 1st order respiratory bronchiole and all structures to and including the alveolar sac, and all parts of an acinus are involved in gas exchange.
• Alveoli are composed mainly of a thin layer of squamous Type I epithelial cells and large, thin, flattened Type II alveolar cells, with Type II cells producing surfactant.
• Alveoli have a coverage of 10% alveolar surface by Type II alveolar cells, which function as stem cells, and free-ranging phagocytic alveolar macrophages monitor and remove debris and microorganisms.
• The adult human alveolus contains prominent cells, including capillaries, the interstitium, and the alveolar epithelium, with intimately related capillaries, the interstitium, and the alveolar epithelium.

Test your knowledge on the innervation of the lungs and airways, including the role of pulmonary plexuses, parasympathetic and sympathetic fibers, and the non-adrenergic/non-cholinergic system. Explore how these systems influence airway diameter, vasoconstriction, and gland secretion.