Muscle anatomy refers to the structure and organization of muscles in the body. Muscles are made up of bundles of fibers that contract to produce movement. There are three main types of muscle tissue: skeletal muscle, which is responsible for voluntary movements; smooth muscle, found in internal organs and responsible for involuntary movements; and cardiac muscle, which makes up the heart and is also involuntary. Each muscle is composed of muscle fibers, connective tissue, blood vessels, and nerves. Muscles are attached to bones by tendons, allowing for movement of the skeleton. The basic functional unit of a muscle is the sarcomere, which contains the proteins actin and myosin that interact to enable contraction.