Smooth muscle

This electron micrograph of a smooth muscle fiber shows the external lamina surrounding the entire fiber, as well as sarcolemmal invaginations called caveoli.  The sarcoplasm is filled with myofilaments, with the majority of the other cellular organelles located at the poles of the nucleus.  7000x

This electron micrograph of a smooth muscle fiber shows the external lamina surrounding the entire fiber, as well as sarcolemmal invaginations called caveoli.  The sarcoplasm is filled with myofilaments, with the majority of the other cellular organelles located at the poles of the nucleus.  7000x

This electron micrograph of a smooth muscle fiber shows the external lamina surrounding the entire fiber, as well as sarcolemmal invaginations called caveoli.  The sarcoplasm is filled with myofilaments, with the majority of the other cellular organelles located at the poles of the nucleus.  7000x

Smooth muscle fibers lack T tubules; however, numerous invaginations of the plasma membrane, termed caveoli, are thought to represent rudimentary T tubules.  The caveoli are often associated with SER profiles in the cytoplasm, suggestive of a primitive sarcoplasmic reticulum.

Actin and myosin myofilaments fill the cytoplasm of smooth muscle fibers but are not organized into distinct myofibrils.  The overlapping actin and myosin myofilaments criss-cross through the cytoplasm and the actin filaments are anchored in cytoplasmic and plasma membrane-associated dense bodies composed of alpha-actinin.

Actin and myosin myofilaments fill the cytoplasm of smooth muscle fibers but are not organized into distinct myofibrils.  The overlapping actin and myosin myofilaments criss-cross through the cytoplasm and the actin filaments are anchored in cytoplasmic and plasma membrane-associated dense bodies composed of alpha-actinin.

The majority of the organelles which are not directly involved with contraction are clustered at the poles of the nucleus.