by immunofluorescence in a rat flexor digitorum muscle."/>
Figure 3.5 Distribution of different myosin heavy chains detected by immunofluorescence in a rat flexor digitorum muscle. This muscle is composed predominantly of type IIb (myosin heavy‐chain type II) fast fibers. The banding seen is the myosin chains. A smaller number of fibers are type I, slow fibers. This muscle contains several intermediate fibers that stain for both types I and II myosin heavy chains. There are also some type IIx fibers that are unstained.
The connective tissue sheaths of muscles, described further below, contain fibroblasts, endothelial cells associated with blood vessels, nerve axons, and sometimes adipose cells and myofibroblasts. Muscles also contain resident macrophages that secrete various growth factors for the maintenance of the muscle fibers. The sites of neuromuscular junctions are the sites where nerve axons terminate on muscle fibers, and muscle spindles and Golgi tendon apparati are sensory receptors located within muscles and myotendinous junctions, respectively. These sites contain glial cells, the Schwann cells, and glial satellite cells, in additional to nerve axon terminals.
Metabolic subtypes
Muscle tissue obtains energy as adenosine triphosphate (ATP) and phosphocreatine kinase from both the aerobic metabolism of fatty acids and glucose and the anaerobic glycolysis. Skeletal muscle cells can be divided into three subtypes based on their metabolic and histochemical characteristics as well as their myosin heavy‐chain subcomponents. Type I fibers, also known as slow‐twitch fibers or “red” fibers, are small in size, contain many mitochondria and large amounts of myoglobin and cytochromes, and have high type I myosin heavy‐chain content (Figure 3.5). Their glycolytic enzyme content is low. Myoglobin is an iron protein that binds O2 and is the feature that makes these fibers appear dark red in color. Type I fibers obtain their energy primarily from aerobic oxidative phosphorylation of fatty acids. As a consequence, they are adapted for slow contractions over prolonged periods. Postural muscles of the back contain many type I fibers.
In contrast, 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).
