At right are pictures showing a part①Fully relaxed sarcomere of a muscle fiber of a skeletal muscle cell when it is relaxing and after it has contracted fully. 1. Label the following components: • Actin/thin filament • Myosin/thick filament • Sarcomere Н 2. Skeletal muscle cells are said to be striated, striped. Alternating dark and light stripes can be seen under a light microscope; more detail is shown in the electron micrographs (EMS) at right. Compare the top EM, which shows one dark vertical stripe (taking up about the middle third of the picture) flanked by two white stripes (taking up the rest of the picture). What protein's presence or absence appears to cause the dark and light stripes? ②Fully contracted sarcomere of a muscle fiber Figure: E. Marieb & K. Hoehn, Human Anatomy & Physiology (2016).