domain, nuclei are lost from fibers when a reduction in muscle fiber size occurs (i.e., muscle atrophy)Beneath the sarcolemma lies the sarcoplasm (also called cytoplasm), which contains the cellular proteins, organelles, and myofibrils. Myofibrils are numerous threadlike structures that contain the con tractile proteins (Fig. 8.2). In general, myofibrils are composed of two major types of protein filaments (1) thick filaments composed of the protein myosin and (2) thin filaments composed primarily of the protein actin. The arrangement of these two protein filaments gives skeletal muscle its striated appearance (Fig. 8.2). Located on the actin molecule itself are two additional proteins: troponin and tropo myosin. These proteins make up only a small portion of the muscle, but they play an important role in the regulation of the contractile process.