5.3.1 Subcellular organisation of muscle fibres
Meat & Livestock Australia March 5, 2021
The muscle fibre or cell is the fundamental organisational unit of muscle. In isolation it can contract if stimulated and perform all the functions of muscle, although it normally operates in larger functional units.
It is composed of smaller sub units called myofibrils, which comprise the majority of the volume of the cell (80-87%). Each myofibril contains a number of (smaller) long thin filaments called myofilaments. These are actin and myosin. Other cell elements include mitochondria and a membrane system, the sarcoplasmic reticulum (SR). When skeletal muscle is examined using a light microscope, alternating light and dark bands are seen (Figure 5.5) indicating that it contains a repeating structure.
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.
The distances between Z-discs varies as muscles contract or lengthen thereby changing the sarcomere length. The basis of muscle contraction resides in the back and forth movement of thick and thin filaments (myosin and actin) relative to each other. These filaments are not actively interacting in relaxed muscle.
As rigor mortis sets in following death, the sarcomeres shorten and assume a fixed length. Final sarcomere length in post mortem muscle is a major determinant of meat tenderness.
Thick and thin filaments slide over each other during contraction and relaxation of the muscle. They do not shorten individually, but move past one another and the banding patterns change. See Figures 5.8 and 5.9.
