Mechanisms involved in airway obstruction: the role of smooth muscle

Structural changes in the airway walls that are probably driven by mediator s released as a consequence of chronic allergic inflammation are prominent features of asthma. However, it is not clear how each of the many changes t hat occur in the airway wall contribute to altered airway function in asthm a. Collagen deposition in the subepithelial matrix, around and inside the s mooth muscle, would be expected to oppose the effect of smooth-muscle contr action. Conversely, geometric factors would result in exaggerated airway na rrowing for a given degree of smooth-muscle shortening; decreased airway wa ll stiffness and increased airway narrowing for a given amount of force gen erated by the smooth muscle. Degradation of the matrix may alter the coupli ng between muscle and lung recoil, allowing exaggerated smooth-muscle short ening. Increase in muscle mass associated with preservation of its contract ile capacity could be the most important contributor to exaggerated airway narrowing.