Musculoskeletal Disorders. Sean Gallagher
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type IIb, or fast‐twitch fibers or “white” fibers, have an opposite metabolic profile. They are also large in size. They have abundant glycogen for the generation of APT via anaerobic metabolism, but fewer mitochondria and less myoglobin, giving them a pale whitish color. As a consequence, they depend on anaerobic glycolysis for energy and are adapted mainly for rapid contractions, although they undergo rapid physiological fatigue. These fibers are recruited during short‐duration high‐intensity activity, such as short sprints and maximum weight lifting.
Physiologically intermediate between slow and fast fibers are type IIa or intermediate fibers. They are also intermediate in size. They have many mitochondria and high myoglobin content and also contain a high amount of glycogen. Since they utilize both oxidative metabolism and anaerobic glycolysis, they are adapted for both rapid contracts and short bursts of energy. They are also intermediate in color and energy metabolism. There is high percentage of type IIa fibers in muscles used during sustained power activities, such as sprinting 400 m.
Experiments using myosin heavy‐chain isoform immunostaining has also revealed an additional type of fiber, type IIx, that does not stain with antibodies against type I or II antibodies (and are thus unstained, as shown in Figure 3.5) (Pierobon‐Bormioli, Sartore, Libera, Vitadello, & Schiaffino, 1981; Schiaffino, 2010). Interestingly, if nerves to slow and fast type fibers are exchanged experimentally, the fibers change their morphological and physiological features to conform to the innervating nerve.
Extracellular matrix
Collagen is the major structural protein in skeletal muscle extracellular matrix. It accounts for 1–10% of a muscle’s dry weight (Dransfield, 1977; Schiaffino & Reggiani, 2011). Fibrillar types of collagen I and III predominate in the adult endomysium, perimysium, and epimysium (Listrat et al., 2000; Marvulli, Volpin, & Bressan, 1996). Type V collagen, another fibril‐forming collagen, associates with types I and III and may form a core for type I collagen fibrils in the perimysium and endomysium (Fitch, Gross, Mayne, Johnson‐Wint, & Linsenmayer, 1984). Elastin is also part of the fascial layers for the provision of structural elasticity. In contrast, the basement membranes of muscle fibers consist of a branched network structure of type IV collagen (Sanes, 1982). Many glycoproteins function as linker molecules between type IV collagen in the basement membrane and sarcolemma (Ervasti & Campbell, 1993). Interactions between these glycoproteins provide potential mechanisms for the transmission of lateral forces from myofibers (Grounds, Sorokin, & White, 2005).
Organization
Skeletal muscle is a hierarchically organized tissue that employs a bundling technique to develop its structure (Figure 3.6). Myofilaments are the chains of filamentous proteins located inside myofibrils. Groupings of myofibrils are bundled together into long cylindrically shaped muscle fibers (multinuclear muscle cells) by a plasma membrane that surrounds the cytoplasm (termed sarcolemma and sarcoplasm in muscles, respectively). Note, the sarcolemma is the plasma membrane of a striated muscle fiber; however, the sarcoplasmic reticulum, to be discussed shortly, is the smooth endoplasmic reticulum within muscle fibers. Next, the individual muscle fibers are bundled together into parallel bundles termed fascicles by a typically small amount of delicate connective tissue called the endomysium. The endomysium also occupies the space between individual muscle fibers (Figures 3.3 and 3.6). Most muscles are composed of multiple fascicles bundled together by a slightly denser collagenous connective tissue called the perimysium. These connective tissues contain cells (the most numerous being fibroblasts and macrophages), fibrous proteins (collagen and elastin), and ground substance in roughly equal parts. They are also flexible, well vascularized, innervated, and not very resistant to stress. Lastly, the entire muscle is surrounded by a dense external sheath of connective tissue called the epimysium (Figures 3.3 and 3.6). These various muscle‐related connective tissue sheaths are linked together by thin septa of connective tissue that typically contain a small amount of collagen I and III and elastin. These connective tissue sheaths can undergo thickening as a consequence of increased collagen deposition in processes termed scarring or fibrosis, processes described further in Chapter 11.
Contractile proteins and the sarcomere
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.