You use muscles every day to do activities. This woman is using muscles to breathe, circulate blood, and move her hand to take notes. Your cardiac and smooth muscle tissues are involuntary. You do not consciously control their actions. Skeletal muscle works under voluntary control. Skeletal muscles are composed of bundles of muscle fibers. Muscle fibers are long cylindrical cells containing several nuclei. Muscles will contract or relax when they receive signals from the nervous system. A neuromuscular junction is the site of the signal exchange. This is where the synaptic bulb of an axon terminal and muscle fiber connect. Muscle fibers are composed of many myophibrals. A myofibral contains contractile units called sarcomeres. Sarcomeres run adjacent to one another down the length of the myofibral. 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 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. Although the action between the filaments is described as sliding, the myosin filament actually pulls the actin along its length. The cross bridges of the myosin filaments attach to the actin filaments and exert force on them to move. This action is known as the sliding filament mechanism of muscle contraction. In this model, the sarcomeres shorten without the thick or thin filaments changing in length. A contraction begins when a bound ATP is hydrolyzed to ADP and inorganic phosphate. This causes the myosin head to extend and can attach to a binding site on actin forming a cross bridge. An action called the power stroke is triggered, allowing myosin to pull the actin filament toward the M line thereby shortening the sarcomere. ADP and inorganic phosphate are released during the power stroke. The myosin remains attached to actin until a new molecule of ATP binds, freeing the myosin to either go through another cycle of binding and more contraction or remain unattached to allow the muscle to relax. Muscle contractions are controlled by the actions of calcium. The thin actin filaments are associated with regulatory proteins called troponin and tropomyosin. When the muscle is relaxed, tropomyosin blocks the cross bridge binding sites on actin. When calcium ion levels are high enough and ATP is present, calcium ions bind to the troponin which displaces tropomyosin, exposing the myosin binding sites on actin. This allows myosin to attach to a binding site on actin forming a cross bridge. Calcium ions are stored in the sarcoplasmic reticulum and are released in response to signals from the nervous system to contract. Neurotransmitter molecules are released from a neuron and bind to receptors which depolarizes the membrane of the muscle fiber. The electrical impulse travels down the T tubules and opens calcium stores. Calcium ions flow to the myofibrils where they trigger a muscle contraction. As the actin and myosin slide along each other, the entire sarcomir shortens as the z lines draw closer to the m line. As the sarcomirs in myofibrils contract, the entire muscle fiber will shorten. When muscle fibers contract in unison, a muscle can produce enough force to move the body, allowing you to take notes.