Muscle: Answers

Skeletal Muscle

#3 Muscle and tendon junction, Human or Rabbit

What has the electron microscope revealed about the relation of skeletal muscle to tendon?
Tendon is continuous with endomysium.

Cardiac Muscle

#17 Heart, Monkey (Mallory-Azan)

  • Are the cardiac muscle fibers of uniform diameter?
    No, they branch.
  • What is the position of the nuclei?
    Central.
  • Do the myofibrils pass through intercalated discs?
    No.
  • Are the cross striations as distinct as in skeletal muscle?
    Yes.
  • How can you distinguish cross sections of cardiac muscle fibers from those of skeletal muscle fibers?
    Central nuclei, intercalated discs, branching fibers.

Smooth Muscle

#101a Guinea pig small intestine (PAS and hematoxylin)

Why is the basal lamina stained with PAS?
The proteoglycans.

#102 Guinea pig small intestine (Bodian/silver)

Note that when the muscle cells are cut in cross section, there are interruptions in the basal laminae.
What is responsible for these discontinuities?
Gap junctions.

Questions

  1. Why do smooth muscle fibers in cross section have different diameters and why do some of these fail to show nuclei?
    Smooth muscle cells have tapered ends. Since the cells interdigitate different diameters would be revealed in a particular plane of section and the plane of section does not always go through the nucleus.
  2. Are myofibrils present in smooth muscle fibers?
    No.
    What has electron microscopy contributed to the resolution of this problem?
    The proportions and organization of myosin thick filaments and actin thin filaments are different than in striated muscle.

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