The myofilaments fill most of the cytoplasm. They are not organized into sarcomeres. There is no banding, little sarcoplasmic reticulum, and no T tubules. Smooth muscle myosin is different from that in striated muscle, and it forms a different type of thick filament that permits more overlap with actin and thus a greater degree of contraction. Thick filaments are visible only under certain circumstances, whereas the thin filaments are more readily seen.
Smooth muscle contains extensive amounts of intermediate filaments, primarily of the desmin type.
The dense bodies contain a-actinin and act as attachment points for both actin and intermediate filaments intracellularly and at the cell surface. They are analogous to the Z disc. The actin filaments also form connections with the cell surface. Smooth muscle cells in a tissue are connected by gap junctions , which serve to transmit depolarization. Smooth muscle cell contraction is regulated by calcium, but the mechanism is somewhat different than that in skeletal muscle since there is no troponin in smooth muscle.