Muscle tissue is composed of long cells called muscle fibers that contract to cause body movements. Shown here are the three types of muscle tissue found in vertebrate animals. Skeletal muscles are made up of unbranched fibers. They are striated muscles found attached to bones. They are responsible for the voluntary movements of the body. Cardiac muscles are formed of branched fibers. The heart is made up of these striated muscles. They are responsible for the contraction of the heart. Smooth muscles are made up of spindle-shaped cells. They are unstriated muscles found in the digestive tract, arteries, and bladder. They are responsible for the contraction of many internal organs.