Читать книгу Musculoskeletal Disorders - Sean Gallagher - Страница 71

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.