Muscle structure consists of three main types: skeletal, smooth, and cardiac muscle. Skeletal muscle is attached to bones and enables voluntary movement, while smooth muscle is found in internal organs and operates involuntarily. Cardiac muscle makes up the heart and is also involuntary, working continuously to pump blood. Each muscle is made up of bundles of fibers called muscle fibers, which contain myofibrils. Myofibrils are further divided into smaller units called sarcomeres, the basic functional units of muscle contraction. These structures work together to allow muscles to contract and relax, facilitating movement and various bodily functions.