They are shaped as long cylinders that can be as long as the muscle to which they belong. They are multinucleated, and their many nuclei are placed at the fiber periphery, right under the plasma membrane. They are striated as when observed with a microscope they show characteristic transverse bands in their cytoplasm.
Cardiac muscle fibers
They are short cylinders that have one or two nuclei placed in the center of the cell. Their cytoplasm shows transverse bands (it is striated), similarly to that of the skeletal muscle. Their fibers adhere to each other at their ends by many intercellular junctions. Each group of junctions (shown in the diagram by dark lines) is called an intercalated disc. These discs are also called scalariform discs because they are sometimes seen as steps of a ladder.
Smooth muscle fibers
Their name derives from the fact that their cytoplasm does not show transverse striations when longitudinal setions are observed in a microscope. They are spindle-shaped and their nucleus occypies a central position in the cytoplasm.
The observation of the position of the nuclei in longitudinal and tranverse sections of muscle fibers helps to distinguish their types.
