Locomotion and Digestion

Each leg and foot has a total of 30 bones. .
             Animals have evolved specialized cells capable of shape changes. These cells contain numerous filaments of the proteins actin and myosin, are called muscle cells. There are three kinds of muscle cells in vertebrates: skeletal, cardiac, and smooth. Skeletal muscles make produce movement of the skeleton when they contract. Usually the 2 ends of a skeletal muscle are attached to different bones. The attachments to bone are made by means of dense connective tissue straps called tendons. One end of the muscle (the origin) attaches to the bone that remains relatively stationary during a contraction. The bone provides an object against which the muscle can pull. The insertion (the other end of the muscle) is attached to the bone that moves when the muscle contracts. Synergists are muscles that cause the same action at a joint. Antagonists are muscles that produce opposing actions at a joint. The quadriceps group is synergists; the quadriceps and hamstrings are antagonists to each other. Generally. The muscles that antagonize a given movement are relaxed when that movement is performed, allowing the body to use energy more efficiently. Muscle fibers, which are cells specialized for rapid contractions and the generation of large forces, are in each skeletal muscle. Each muscle fiber encloses a bundle of 4-20 elongated structures called myofibrils. Myofibrils have cross-striations, patterns of alternating light and dark bands that are in register with those in adjacent myofibrils, giving the muscle fiber a striped appearance. Myofibrils are built from long chains of repeating sarcomeres, contractile subunits that are bounded on each end by a disc of protein called a Z line. The light bands of skeletal muscle fiber result from the thin myofilaments, while the dark bands result from the thick myofilaments. Each thin filament is made up of mainly globular actin proteins twisted into a double helix.