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The arrangement of contractile proteins in skeletal muscle fibers gives rise to their light microscopic appearance of having cross‐striations and thus the name “striated muscle” (Figures 3.4 and 3.6). These striations are alternating light and dark bands that are perpendicular to the long axes of muscle cells. The darker bands are called A bands and are visible due to their optical properties in polarized light (they are anisotropic); the lighter bands are called I bands (they are isotropic and do not alter their appearance in polarized light). Transmission electron microscopes reveal that each I band is bisected by a dark transverse line, termed the Z‐disc (German, Zwischenscheibe, for “between the discs”). The repetitive functional subunit of the muscle fibers contractile apparatus is the sarcomere (Greek, sarkos + mere, part) and extends from Z disc to Z disc (Figure 3.7).

The sarcomere is filled with long cylindrical filamentous bundles termed myofibrils. Myofibrils have a diameter of 1–2 μm and run parallel to the long axis of the muscle fibers. Myofibrils consist of an end‐to‐end chain‐like arrangement of sarcomeres that contain two types of myofilaments, thick and thin, that lie parallel to the long axis of the myofibril (Figure 3.6). These myofilaments are the contractile proteins of the myofibril. Each thin filament is composed of F‐actin, tropomyosin, and troponin complexes. Troponin is a complex of three subunits: TnT that attaches to tropomyosin, TnC that binds calcium ions, and TnI that inhibits the actin–myosin interaction. Troponin complexes are attached at regular intervals along each tropomyosin molecule. Each thick filament is composed of many myosin heavy‐chain molecules bundled together along their rod‐like tails, with their heads exposed and directed toward neighboring thin filaments. The thick myofilament bundles are held in place by myosin‐binding proteins along the M line (German, Mittelscheiber, middle line). Small globular projections on one end of each heavy chain form the myosin heads that have ATP and actin‐binding sites and the enzymatic capacity to hydrolyze ATP (Figure 3.10). Cross bridges are formed between the thin and thick filaments by the head of the myosin molecules plus a short part of its rod‐like portion. These cross bridges are involved in the conversion of chemical energy to mechanical energy (Brunello et al., 2014). This structural biology generates the force necessary for the contraction of individual muscle fibers, which when bundled together into an entire muscle drive movement of the skeleton (via attachment of muscles and tendons to bones).

Figure 3.6 Structure of a skeletal muscle.

Tortora, G. J., & Derrickson, B. H. (Eds.), (2010). Muscle. In Introduction to the human body, 11th ed., Wiley.

Figure 3.7 A simplified schematic of a sarcomere is shown. A sarcomere is composed of actin and myosin filaments that are organized into a characteristic pattern displaying A‐, I‐, M‐, and Z‐bands.

Modified from Nayak, A. & Amrute‐Nayak, M. (2020). SUMO system – A key regulator in sarcomere organization. FEBS Journal, 287, 2176–2190. https://doi.org/10.1111/febs.15263.