Photomicrograph of a skeletal muscle, a. This low-magnification photomicrograph shows skeletal muscle in longitudinal section. Muscle fibers (cells) are arranged in parallel; they are vertically oriented, and the length of each fiber extends beyond the upper and lower edge of the micrograph. The fibers appear to be of different thicknesses. This is largely a reflection of the plane of section through
Muscle is classified on the basis of the appearance of the contractile cells
Two principal types of muscle are recognized:
• Striated muscle, in which the cells exhibit cross-stria-tions at the light microscope level
• Smooth muscle, in which the cells do not exhibit cross-striations
Striated muscle tissue is further subclassified on the basis of its location:
• Skeletal muscle is attached to bone and is responsible for movement of the axial and appendicular skeleton and for maintenance of body position and posture. In addition, skeletal muscles of the eye (extraocular muscles) provide precise eye movement.
the muscle fibers. Note on the left the epimysium, the sheath of dense connective tissue surrounding the muscle. xl60. b. At higher magnification, cross-striations of the muscle fibers are readily seen. The nuclei of skeletal muscle fibers are located in the cytoplasm immediately beneath the plasma membrane. x360.
• Visceral striated muscle is morphologically identical with skeletal muscle but is restricted to the soft tissues, namely, the tongue, pharynx, lumbar part of the diaphragm, and upper part of the esophagus. These muscles play essential roles in speech, breathing, and swallowing.
• Cardiac muscle is a type of striated muscle found in the wall of the heart and in the base of the large veins that empty into the heart.
The cross-striations in striated muscle are produced largely by the specific cytoarchitectural arrangement of both thin and thick myofilaments. This arrangement is the same in all types of striated muscle cells. The main differences between skeletal muscle cells and cardiac muscle cells are in their size, shape, and organization relative to one another.
Smooth muscle cells do not exhibit cross-striations because the myofilaments do not achieve the same degree of order in their arrangement. In addition, the myosin-containing myofilaments in smooth muscle are highly labile. Smooth muscle is restricted to the viscera and vascular system, the arrector pili muscles of the skin, and the intrinsic muscles of the eye.
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