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Non-striated Muscle

As the name implies, non-striated muscle lacks visible striations.  Smooth muscle is the only muscle of this type and without striations, other less obvious characteristics are used to identify this tissue.  Before we examine smooth muscle in longitudinal or cross-section, it might be worth our while to determine why some myofibers are striated while others are not.

Myofilaments are elongate contractile filaments within myofibers that contain the proteins actin and myosin.  Although other proteins can be involved in the contractile process, it is the interaction between actin and myosin myofilaments that ulltimately brings about contraction.  These myofilaments are arranged in muscles in two different ways to provide contractility with different capabilities.

Smooth muscle is found in walls of hollow organs and blood vessels where contractility and stretch are both specific needs.  Think of the bladder or the uterus of a pregnant female.  To accommodate substantial stretch and still retain contractility, actin and myosin myofilaments are arranged randomly in smooth muscle.  For this reason, there are no repeating areas of myofilament overlap.

Key Points for Smooth Muscle

  • In the longitudinal section of this diagram, note the elongate nuclei and spindle shape of cells.
  • in the cross-section see how some cells are cut in the center and contain nuclei whereas other cells are cut at their tips
  • myofilaments arranged randomly within smooth muscle fibers enable cells to stretch a great deal and still have overlap of actin and myosin.

Smooth muscle can be identified in cross-section or longitudinal sections by keeping some structural features of these cells in mind.  Smooth muscle cells are spindle-shaped.  In longitudinal sections, this spindle shape may not be immediately obvious because membranes of smooth muscles do not stand out.  However, elongate "cigar-shaped", central nuclei are visible within cells.  Watch for these elongate nuclei arranged parallel to one another as only smooth muscle and  dense regular connective tissues  tend to exhibit this attribute.  Also, keep in mind that smooth muscles are found in the walls of organs and blood vessels.  There are always small arteries and veins in most tissue specimens.  Look for these and you will find smooth muscles encircling the vessel lumens.  These can be used as a convenient reference or comparison to another tissue block you think is smooth muscle.  If the slide you are viewing is a hollow body organ, you should be able to find smooth muscles in its walls.

Now let's take a look at some smooth muscles in tissues!

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