Page 79 - Headlines Histology2024_Neat

• When the muscle fiber is stimulated to contract sliding of the thin and thick
filaments occurs without change in the length of either filament.

• The Z lines of all sarcomeres are drawn toward each other resulting in
contraction.

3) SMOOTH MUSCLE (VISCERAL MUSCLE)

• Smooth muscles are found in the walls of hollow viscera and blood vessels.
• It is called smooth because it has no cross striations, involuntary because its

contraction cannot be elicited at will and visceral because they are found in
visceral organs.
Structure of smooth myofibers

• At the LM level, the smooth muscle cell is elongated, spindle-shaped with
pointed ends. It has a diameter of 3-10 ?m with length ranges between 30-
500 ?m.

• The cytoplasm is acidophilic and contains only one nucleus. The nucleus is
elongated and centrally located in the cytoplasm at the widest part of the
cell. During contraction, the nuclei may appear spiral in shape.

• The smooth muscle fibers are bound together into irregular branching
bundles. Within the bundles, individual muscle fibers are arranged parallel
to each other with the thick part of the cell lying against the thin parts of
adjacent cells.

• A sheath of delicate connective tissue containing capillaries and few nerve
fibers invests each muscle bundle.

• In cross sections, the spindle shaped cells is sectioned at different levels
along their length. The cells appear spherical or oval with a differing
diameter. Nuclei are only included when the fiber cut through their widest
portion and is located centrally.

• At the EM level, the plasma membrane contains numerous flask-shaped
invaginations called caveole. They are regarded as the probable
counterparts of T tubules. Mitochondria, Golgi saccules, glycogen and

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