Physically smooth muscle cells are much smaller than those of either skeletal or cardiac muscle. There are no visible striations when viewed under the light microscope, although there is a fibrillar appearance to the myoplasm which runs with the long axis of the cell. Scattered throughout the cytoplasm and at certain regions of the sarcolemma are aggregates of electron dense material known as dense bodies.
Thin filaments enter the dense bodies suggesting a role similar to Z-lines in striated muscle.
Filaments are often seen terminating in these structures and it is postulated that they function as a mechanical coupling from cell to cell for transmission of force. The regulation of contractile activity has some similarities to striated muscle but some important differences. Calcium appears to be the key trigger agent in activating contractility, but the means by which it acts is novel. Smooth muscle has greatly reduced sarcoplasmic reticulum in comparison with striated muscle. There is a notable juxtaposition of elements of the SR with the surface vesicles approaching within 10-20 nm.