What is a Sarcomere?

A sarcomere is the basic functional unit of striated muscle, which is to say, it’s the basic building block of most muscle cells. In the human body, each muscle is made up of multiple bundles of muscle fibers. These muscle fibers, in turn, are comprised of numerous finer strands called myofibrils. It’s usually not obvious unless looking under an electron microscope, but each myofibril is primarily composed of two kinds of filaments, termed “thick” and “thin,” and each of these is organized into regular, repeating sub-units. Each sub-unit individually is known as a sarcomere, it is their patterned arrangement that gives striated muscle its characteristic banded appearance.

Though most of the body’s muscles are striated, not all are, and sarcomeres only occur in striations. So-called “smooth” muscles have a different cellular makeup that does not include bands, and doesn’t typically include bundled fibers, either. Smooth tissues are most commonly found in the blood vessels, arteries, eyeballs, and reproductive organs of both males and females.

Striation, or banding, happens as a result of cell and fiber orientation. In between sarcomeres lies the Z line, also known as the 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 with each other and look like a series of parallel dark lines running across the muscle cell. Numerous thin filaments also reach toward the center of the sarcomere from the Z line, 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 is 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 generally oriented in opposite directions, which in most cases means that the head regions line up along each end of the molecule while the tails are gathered together in the center. What this means is that the filament has roughly the shape of an elongated dumbbell, with bumpy heads projecting at the ends and a smooth region in the middle. 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. Actin molecules are roughly spherical in shape, and they tend to arrange themselves in double strands — much 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, partially overlapping with the bumpy regions of thick filaments.

Sarcomeres themselves are relatively simple. The very center typically contains only the smooth, middle region of the thick filaments. This region is termed the H zone. Similarly, in most cases the outer edges are made up only of thin filaments, at least when the muscle is at rest; this forms a narrow strip around the Z line known as the I band. The main goal of this arrangement is to enable the contraction of the sarcomere, the myofibril, and the entire muscle, which helps make muscle movement more efficient.

Muscle movement happens when the filaments slide across each other within the muscular bands. 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, myosin molecules form and release bonds with the actin molecules of neighboring filaments, effectively pulling them toward the center. 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.