A sarcomere is the basic functional unit of striated muscle. In the human body, each muscle is made up of multiple bundles of muscle fibers, or cells. The muscle fibers, in turn, are comprised of numerous finer strands called myofibrils. When viewed under an electron microscope, it can be seen that each myofibril is primarily composed of two kinds of filaments — termed thick and thin — organized into regular, repeating sub-units. These sub-units are the sarcomeres, and it is their patterned arrangement that gives striated muscle its characteristic banded appearance.
In between sarcomeres lies the Z line, or Z disc. When stained and viewed microscopically, the Z line appears as a dark, distinct border. The Z lines of adjacent myofibrils generally line up, appearing as a series of parallel dark lines across the muscle cell. Arising from the Z lines, numerous thin filaments reach toward the center of the sarcomere, where they overlap slightly with the free-floating thick filaments. These filaments together represent the active structures.
Thick filaments are made up of hundreds of molecules of the protein myosin. Myosin molecules are characterized by long, fibrous, tail regions that run along the axis of the filament, and globular head regions that project outwards along the axis. Individual myosin molecules within a filament are oriented in opposite directions, resulting in the head regions lining up along each end of the molecule, with the tails gathered together in the middle. Overall, the filament has the shape of an elongated dumbbell, with bumpy heads projecting at the ends, and a smooth region in the center. The interior region of sarcomeres, corresponding to the length of the thick filaments, is called the A band.
Thin filaments are approximately half the diameter of thick filaments, and contain primarily the protein actin. Roughly spherical, actin molecules arrange in double strands like a beaded necklace, with each strand twisting around the other to form a helix. Thin filaments project inward from the Z lines at each end of the sarcomere, partially overlapping with the bumpy regions of thick filaments.
The very center of the sarcomere typically will only contain the smooth, middle region of the thick filaments. This region is termed the H zone. Similarly, the outer edges will generally only include thin filaments when the muscle is at rest, forming a narrow strip around the Z line known as the I band. In its entirety, this arrangement enables the contraction of the sarcomere, the myofibril, and the entire muscle.
Muscles contract according to the sliding filament model of muscular contraction. Upon contraction, the distance between Z lines becomes shorter, but the filaments themselves do not change length. Instead, the two types of filaments slide longitudinally against each other, increasing the degree of overlap and thereby shortening the length of the sarcomere. When this occurs, the I bands shorten, the H zone disappears, and the length of the A bands remains constant.
The sliding filament model is made possible by the interaction of the actin and myosin proteins within the filaments. Using energy from the energy-transfer nucleotide adenosine triphosphate (ATP,) myosin molecules form and release bonds with the actin molecules of neighboring filaments, effectively pulling them toward the center of the sarcomere. Under optimal conditions, the process typically continues at a rate of five bonds per molecule per second. Bond formation is generally regulated by the concentration of calcium ions within the cell, and can also be affected by the amount of available glycogen and creatine phosphate.