Smooth muscle cells line the walls of hollow organs such as the airways and control their dimensions and mechanical functions. As in striated muscle cells, actomyosin interactions produce sliding of actin and myosin filaments but, because there are no obvious repeating contractile units of myofibrils (sarcomeres) in smooth muscle cells, it is not clear how filament sliding translates into cell shortening. Chun Seow and co-workers now link structure to function in porcine airway smooth muscle and propose that smooth muscle cells have a malleable sarcomeric structure composed of contractile units assembled in series and parallel (see ). The authors measure force, velocity and power in muscle cells adapted to different lengths and examine their ultrastructure under different conditions. Their results fit a model in which the geometric organization of the contractile units within smooth muscle cells and the dimensions of individual units alter when the muscle cells adapt to different lengths. This flexibility, the authors conclude, underlies the unique functional and structural characteristics of smooth muscle.
