5.3.1 Subcellular organisation of muscle fibres
The units which “form” this repeating structure are termed sarcomeres, which are typically 2 to 3 mm in length. When the muscle fibre is examined at higher magnification the structured detail becomes clearer as seen in Figure 5.6.
A sarcomere is defined as the portion of the fibre lying between two consecutive dark discs, the Z-discs (see Figure 5.6). Each sarcomere contains a number of areas, which can be distinguished under higher magnification. They include two sets of filaments, thick and thin, which overlap. Visualised in cross section, sarcomeres will reveal various patterns depending on where the section is taken. The thick filaments consist of myosin and appear dark and the thin filaments are actin and appear light. In the region where these two types of filaments overlap, there are three thick filaments surrounding each thin filament and six thin filaments surrounding each thick. These structural details are shown in Figure 5.7.