You are muscles every to do activities. This woman is using muscles to breath, circulate blood,and move her hand to take notes. Your cardiac and smooth muscles tissues are involuntary,you do not consciously control their actions.Skeletal muscle work under voluntary control. Skeletal muscles are composed of muscle fibers. Muscle fibers are long cylindrical cells containing several nuclei. Muscles will contact or relax when they receive signals from the nervous system. A neuromuscular junction is the site for the signal exchange. This is where the synaptic bulb of an axon terminal and muscle fiber connect. Muscle fibers are composed of many myofibrils. A myofibril contains contractile units called sarcomers. Sarcomeres run adjacent to one another down the length of the myofibril. Each sarcomere consists of alternating thick and thin protein filaments giving skeletal muscle its striated appearance. The muscle contracts when these filaments slide past each other. The thick filaments are myosin,which are anchored at the center of the sarcomere,called the M line. The thin filaments are composed of the protein actin, which are anchored to be the Z lines. On the outer edges of the sarcomere. Because the actin filaments are anchored to the Z lines, the sarcomere shortens from both sides when actin filaments slide along the myosin filaments.