sciencetalk

Airway smooth muscle cells in the large segmental or lobar bronchi are similar to those in trachealis. Muscle bundles encase the bronchi in a roughly circular orientation but appear devoid of a specific anatomic arrangement where they branch and interdigitate with plates of cartilage and other connective tissue structures within the airway wall. More distally, in the peripheral noncartilaginous airways the smooth muscle bundles surround the airway in a helix–antihelix pattern, with extensive crisscrossing of fibers between bundles giving an overall geodesic arrangement (Figure 1). The helical pitch of these bundles varies with lung inflation and airway level but is in the range±13–30° in noncartilaginous airways. However, because of the regularity of crossover of fibers from one bundle to another, the magnitude of this obliquity may not result in physiologically important changes in airway length during bronchoconstriction. Although absolute amounts of airway smooth muscle are less in peripheral airways compared with proximal airways, the proportion of airway wall structure occupied by muscle is higher in periphery (15–25%) than in proximal airways (2–4%).