The sarcomere is the basic contractile unit of a skeletal muscle fiber. It is part of the myofibril, lying between consecutive Z bands. Therefore, each myofibril represents a continuous series of sarcomeres joined at Z lines. When it is relaxed, it measures between 2 and 3 μ (micron), contracting to roughly half its resting length as it pulls its Z lines closer together.
A sarcomere contains two sets of contractile filaments; actin and myosin, which are two types of proteins. Actin is a thin filament, while a myosin is a thick one. These filaments are longitudinally oriented both in the sarcomere and the myofibril. Half the actins are attached to the Z lines. Muscular contraction is the result of inward sliding of the thin filaments (actin) along the fixed thick ones, until they nearly meet in the middle of the sarcomere. This sliding action shortens the distance between the Z lines.
Aside from the actin and the myosin, the sarcomere also contains titin (also called connectin), which is a protein but with a high molecular weight. Titin molecules are long enough to connect an M line with a Z line. At one end, they have a domain that is incorporated into a thick filament, and at the other end they have a spring-like elastic domain which connects the thick filament with the Z lines. Comparatively rigid lattices and ring-like arrangements of intermediate filaments provide further internal and external support for the sarcomere.
High resolution electron micrograph of a sarcomere in skeletal muscle fiber. The sarcomere extends from Z line to Z line.
Below, a picture of a sarcomere, showing the different parts that make up its whole structure.
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