____ ____ ____ are arranged in ____ layers in the walls of blood vessels and bronchioles (small air passages in the lungs). Both ____ ____ ____ smooth muscle layers occur in the tubular digestive tract, the ureters (which transport urine), the ductus deferentia (which transport sperm cells), and the uterine tubes (which transport ova). The alternate contraction of circular and longitudinal smooth muscle layers in the intestine produces ____ ____, which propel the contents of these tubes in one direction.
Although smooth muscle cells do not contain ____ (which produce striations in skeletal and cardiac muscle), they do contain a great deal of actin and some myosin, which produces a ratio of thin to thick filaments of about ____ ____ ____ (in striated muscles the ratio is 2 to 1). Unlike striated muscles, in which the thin filaments are relatively short (extending from a Z disc into a sarcomere), the thin filaments of smooth muscle cells are quite long. They ____ ____ the sarcoplasm to attach either to regions of the plasma membrane of the smooth muscle cell or to cytoplasmic protein structures called ____ ____, which are analogous to the Z discs of striated muscle. This arrangement causes smooth muscle to 'scrunch' when stimulated.