Two types of myofilaments are associated with cell contraction.  Thin filaments (6 to 8 nm in diameter, 1.0 m long) are composed primarily of the protein actin. Each thin filament of fibrous actin (F-actin) is a polymer formed from globular actin molecules (G-actin). Thick filaments (15 nm in diameter, 1.5 m long) are composed of the protein myosin II. Each thick filament consists of 200 to 300 myosin II molecules. The long, rod-shaped tail portion of each molecule aggregates in a regular parallel but staggered array, whereas the head portions project out in a regular helical pattern.
The two types of myofilaments occupy the bulk of the cytoplasm, which in muscle cells is also called sarcoplasm [Gr. sarcos, flesh; plasma, thing ]. Actin and myosin are also present in most other cell types (although in considerably smaller amounts), where they play a role in cellular activities such as cytokinesis, exocytosis, and cell migration. In contrast, muscle cells contain a large number of aligned contractile filaments that the cells use for the single purpose of producing mechanical work.
Muscle is classified according to the appearance of the contractile cells.
Two principal types of muscle are recognized:  Striated muscle, in which the cells exhibit cross-striations at the light microscope level, and  Smooth muscle, in which the cells do not exhibit cross-striations.
FIGURE 11.1 • 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 the muscle fibers. Note on the left the epimysium, the sheath of dense connective tissue surrounding the muscle. 160. 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. 360.
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.
 Visceral striated muscle is morphologically identical to 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.
A skeletal muscle cell is a multinucleated syncytium.