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Striated Muscle

In contrast to smooth muscle, cardiac and skeletal muscle types possess an internal ultrastructure of highly organized contractile myofilaments.  Actin and myosin myofilaments are stacked and overlapped in regular repeating arrays to form sarcomeres.  Dark A bands result where myofilaments overlap with light I bands present in areas of non-overlap.  The A and I bands repeat along the length of the muscle fibers to create the striations visible under the light microscope.    

Some Key Points About Sarcomeres

  • Sarcomeres repeat along skeletal and cardiac fibers.
  • Components between adjacent Z-lines comprise the sarcomere.
  • Sarcomeres are considered the functional units of striated muscles.
  • I Bands are lighter areas of non-overlap between actin and myosin that contain the Z-lines.
  • A Bands are areas where some overlap occurs and coincide with the length of myosin myofilaments.

Striations are a key attribute to identify skeletal and cardiac muscle types.  Although striations are usually visible in most sections of these muscle types, it can sometimes take a little adjustment of the microscope lighting to see them clearly. Also, striations are only visible in longitudinal views of myofibers so it is essential some part of the muscle section being evaluated includes this type of cut.

Remember, striations across cells indicate you are viewing cardiac or skeletal muscle.  Differentiation of these two types involves other structural features.  We will examine these now!

Skeletal Muscle

Skeletal muscle is the most abundant muscle type in the body.  Any section containing bone, cartilage, or skin should be examined carefully for this muscle type.  Since cartilage and bone are the main supportive elements that skeletal muscles attach to there is always a possibility skeletal myofibers will be found attaching to the periosteum or perichondreum of these supportive connective tissues.  In thick skin slides, skeletal muscles can often be found below the hypodermal layer.

The myofibers of skeletal muscle are long, cylindrical cells that possess multiple, peripheral nuclei.  In either longitudinal or cross-sectional views, these attributes are useful to identify this muscle type.

Skeletal Muscle Features

  • striations are typically but not always visible
  • multiple peripheral nuclei along the perimeters of cells
  • in cross-section, peripheral nuclei are more obvious but multinucleate condition may be less obvious
  • cells are typically ovoid or circular in cross-section

 Do not mistake adipose cells with skeletal muscle in cross-section.  The former appear as empty cells with single peripheral nuclei while skeletal muscle cells typically have a granular or spotted internal appearance due to the numerous myofibrils.  Take a look at adipose and skeletal muscle together now side by side!

Now, here are some views of skeletal muscle!

 Cardiac Muscle

Cardiac muscle is only found in the heart.  In views of this muscle type, striations are not as strong as those of skeletal muscle.  Other structural features are usually apparent and of use for identification if the striations are weak.

Due to constant activity cardiac muscle has a constant need for oxygen and glucose delivery.  Since these are supplied from the blood, expect to see more capillaries in cardiac muscle.  These are visible in longitudinal and cross section views.

Cardiac Muscle Features

  • branching striated myofibers
  • central nuclei
  • abundant capillaries
  • perinuclear space can appear as a clear circular space in myofibers cut in cross-section

Cardiac muscle fibers are smaller than those of skeletal muscle.  Also, cardiac myofibers branch and where their membranes meet specialized intercellular junctions are found.  These consist mainly of gap junctions, a type of intercellular junction specialized for impulse conduction.  As impulses spread across membranes of cardiac myofibers they pass quickly from one myofiber to the next across gap junctions.  The cardiac myofibers then synchronize and contract together.  Where gap junctions are abundant, membranes stain darker and histologists named these areas intercalated disks.  Cardiac myofibers also contain a single, centrally located nucleus in contrast to the multiple peripheral nuclei of skeletal muscles.

In summary then, cardiac muscle consists of branching myofibers with single, centrally placed nuclei.  Intercalated disks are visible between myofibers in longitudinal views and there are abundant capillaries in intercellular spaces.

Now, let's take a look at some views of cardiac muscle!

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