Box 1 | The structure of a skeletal-muscle fibre

From the following article:

Molecular mechanisms of muscular dystrophies: old and new players

Kay E Davies & Kristen J Nowak

Nature Reviews Molecular Cell Biology 7, 762-773 (October 2006)

doi:10.1038/nrm2024

Skeletal muscle is the muscle attached to the skeleton. Hundreds or thousands of muscle fibres bundle together to make up an individual skeletal muscle. Muscle fibres (myofibres) are long, cylindrical structures that are bound by a plasma membrane (the sarcolemma) and an overlying basal lamina and when grouped into bundles (fascicles) they make up muscle. The sarcolemma forms a physical barrier against the external environment and also mediates signals between the exterior and the muscle cell.

The sarcoplasm is the specialized cytoplasm of the striated-muscle fibre that contains the usual subcellular elements along with the Golgi apparatus, abundant myofibrils, a modified endoplasmic reticulum known as the sarcoplasmic reticulum (SR), glycogen granules, myoglobin and mitochondria. Transverse (T)-tubules invaginate the sarcolemma (see the figure), which allows action-potential signals to penetrate the cell and activate the SR. As shown in the figure, the SR forms a network around the myofibrils, storing and providing the Ca2+ that is required for muscle contraction.

Molecular mechanisms of muscular dystrophies: old and new players 

Myofibrils are contractile units that consist of an ordered arrangement of longitudinal myofilaments. Myofilaments can be either thick filaments (comprised of myosin) or thin filaments (comprised of actin). Together they produce movement by contraction through the sliding filament model. The characteristic 'striations' of skeletal and cardiac muscle are readily observable by light microscopy as alternating light and dark bands on longitudinal sections (see the figure). The light band, (known as the I-band) is made up of thin filaments and is isotropic in polarized light, whereas the dark band (known as the A-band) is made up of thick filaments and is anisotropic. The M-line (for 'mittelscheibe' or middle disc) bisects the A-band and the Z-line ('zwischenscheibe,' or between disc; also known as the Z-disk or Z-band) defines the lateral boundary of each sarcomeric unit. Contraction of the sarcomere occurs when the Z-lines move closer together, making the myofibrils contract, and therefore the whole muscle cell and then the entire muscle contracts.

Protein assemblies known as costameres, which are located on the peripheral myofibrils of the myofibre, line up with the Z-disk. Costameres physically link the sarcomeres, which produce force through contraction, with the sarcolemma and, are proposed to transmit this force across the sarcolemma to the extracellular matrix and on to neighbouring muscle cells. So, during both contraction and relaxation, sarcomere length remains consistent between muscle cells within skeletal muscle.