Muscles work by contracting and relaxing. During |contraction| they shorten their length to bring the bone closer to their points of attachment on two different bones. Every muscle movement, therefore, is a pull. This pulling action is accomplished by the |fibers| and fibrils of the muscle. All skeletal muscles are made up of small |fibers|. The |fibers| are cylindrical in shape and are several centimeters long, with regular bands (striations) dividing them into sections. The |fibers| are made up of many cylindrical subunits called fibrils. These are the structures that actually contract. Muscular |fibers| are able to shorten 30% to 40% in length during muscle |contraction|. Fibrils are made of two types of |protein|: actin and myosin. These |proteins| are in the form of long filaments. The filaments made of myosin are thicker than the filaments made of actin. These filaments interlock and are able to slide over each other, shortening the length of the muscle. When the muscle is stretched the filaments tend to be pulled apart. During shortening (contraction), they shorten by sliding into one another. It appears that during |contraction| several cross-links are made between the actin and myosin filaments. By the process of making and breaking these cross-links, the two filaments move towards one another and the whole muscle shortens. This process is very rapid. Cardiac muscle has a similar appearance to skeletal muscle. It has striations and is thought to contract in the same manner as the skeletal muscle. |Smooth muscle| has no striations and is composed of small spindle-shaped cells totally lacking in filaments. Researchers still do not understand the mechanism of |smooth muscle| |contraction|.
