A muscle fiber (cell) has special terminology and distinguishing characteristics:
- The sarcolemma, or plasma membrane of the muscle cell, is highly invaginated by transverse tubules (T tubes) that permeate the cell.
- The sarcoplasm, or cytoplasm of the muscle cell, contains calcium‐storing sarcoplasmic reticulum, the specialized endoplasmic reticulum of a muscle cell.
- Striated muscle cells are multinucleated. The nuclei lie along the periphery of the cell, forming swellings visible through the sarcolemma.
- Nearly the entire volume of the cell is filled with numerous long myofibrils. Myofibrils consist of two types of filaments, shown in Figure 1:
- Thin filaments consist of two strands of the globular protein actin arranged in a double helix. Along the length of the helix are troponin and tropomyosin molecules that cover special binding sites on the actin.
- Thick filaments consist of groups of the filamentous protein myosin. Each myosin filament forms a protruding head at one end. An array of myosin filaments possesses protruding heads at numerous positions at both ends.
Figure 1.Two types of filaments
Within a myofibril, actin and myosin filaments are parallel and arranged side by side. The overlapping filaments produce a repeating pattern that gives skeletal muscle its striated appearance. Each repeating unit of the pattern, called a sarcomere, is separated by a border, or Z disc (Z line), to which the actin filaments are attached. The myosin filaments, with their protruding heads, float between the actin, unattached to the Z disc.