Myosin is a family of motor proteins that work to elongate muscle fibers to create movement and force. Grouped together, myosin heavy chains refer to the filaments that fit along the muscle fiber similar to the way a pawl, or lever catch, fits into the teeth of a ratchet or gear. One of two proteins responsible for all motor movement, myosin takes the form of a molecule with a head and tail resembling two threads twisted together; these extend into a double head on one side, wrapped in myosin light chains. This hammer-shaped filament moves along the muscle fiber, energized by adenosine triphosphate (ATP), binds one phosphate on to the filament, pulls the fiber, and then wriggles away with an adenosine diphosphate (ADP).
Muscles are made of many complex components working together. The fibers that comprise muscle are grouped together like cables, lengthening and shortening while turning several types of energy into mechanical force. Forces employed depend on the job required; they adjust to pick up a pencil or a suitcase, or to keep a heart beating and lungs breathing steadily. Groups of muscle fibers are recruited only when needed. These fibers act like cables sliding against each other, sometimes a few at a time, sometimes many, and sometimes all together, depending on the load.
A muscle acts like a deck of cards. When the deck is placed on a table and spread into a long line using a finger, the finger acts as the myosin filament molecule. The process continues as the card player spreads the deck out, raising the finger and putting it back down again over and over, to the maximum expansion allowed by the size of the deck and limited by the width of the cards. These cycles occur about five times per second and are multiplied many times throughout the myosin heavy chains of recruited muscle fibers. Myosin heavy chains move muscle actions through three types of forces: contraction, holding, and controlled release.
There are a number of different types of myosin heavy chains. For example, a myosin II filament consists of two heavy chains and four light chains. All four chains are knotted together at the head; its long tail consists of the two heavy chains twisted together into a helix. These interact with accordion-like contractile units of muscle fibers called myofibrils and sarcomeres.
Together with the other protein responsible for muscle movement — actin — myocin chains attach to disc-shaped Z discs. If the myosin were an umbrella handle, the actin would be the umbrella spokes. Holding two umbrellas handle-to-handle and opening them in opposite directions, all these elements work together to result in the lengthening and shortening of muscle fibers.
Comprising about half of all the muscle proteins in the body when combined with actin, myocin forms the molecule actomyosin. The myosin heavy chains function in all three types of muscle: skeletal muscle; cardiac, or heart muscle; and smooth muscle, such as that in irises and blood vessels. A polypeptide consisting of about 2,000 amino acids, myosin heavy chains have an N-terminal, or head domain; a leveraging neck, or N-domain; and a C-terminal, which is the tail.
Predetermining factors involving myosin heavy chains may influence fast and slow muscle twitch responses. Muscles have limiting factors of their length and elasticity. This affects the power to contract or spring into action. Individual muscle twitch properties, however, can be adapted or trained for increased cross-sectional area of the muscle, and motor unit recruitment, as for improved athletic performance.