Bone continually degenerates and builds up. Smooth muscle cells are spindle-shaped and involuntary, and they line organs, push food through the digestive tract, and regulate blood flow and pressure. Cardiac muscle cells, in the heart, are striated and involuntary and are joined by intercalated disks into a branching pattern. Skeletal muscle cells are striated, voluntary, and move bones. Sliding protein filaments in muscles provide movement in diverse organisms.
Tendons attach an intact whole skeletal muscle to bones.
Within a muscle are bundles of muscle fibers. Each muscle fiber is a long cylindrical cell composed primarily of thick and thin myofilaments. The heads project outward from each end of a myosin bundle. The thin myofilaments are composed of actin plus troponin and tropomyosin. A muscle fiber cell is a chain of many contractile units called sarcomeres.