Last modified 5th Jun '95 © Birkbeck College 1995

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Muscle Fibres Page 2

Structure of the sarcomere

The striped appearance of skeletal muscle as seen under the light microscope is revealed in more detail in electron micrographs. Each individual myofibril consists of alternating dark and light stripes, and the myofibrils are all aligned. The darkness of the stripes relates to the arrangement of the two different types of protein fibres.

The dark bands are called A bands while the light regions are termed I bands . The A bands are themselves striped, consisting of a lighter central band called the H zone (or AH zone) flanked by two darker regions (AI zones). At the centre of the H zone is a thin dark line, the M line (or M disc). A similar line, the Z line (or Z disc) is visible in the centre of the I band. The sarcomere is defined as the region between two Z lines; in relaxed muscle, these are 2.5 to 3.0 µm apart, but they become closer during contraction.

The appearance of the sarcomere in longitudinal section is indicated below.

Diagram of sarcomere (1)
Here is a diagram.

The dark A band consists of parallel myosin thick filaments, in a hexagonal arrangement in cross-section. The thick filaments of a single sarcomere are connected to each other by means of the M disc at their centres. The orientations of the individual myosin molecules on opposite sides of the M disc are therefore antiparallel.

The M disc arises from bulging at the centres of the thick filaments, and involves C- protein and M- protein (150 and 100 kD respectively).

The I band consists of actin-based thin filaments; these filaments also extend into the AI band, where they overlap with the thick filaments, which explains why the AI bands appear darker than the H zone (where only thick filaments are present). A cross section of the I band reveals a more complicated arrangement compared to the myosin filaments. In the overlapping region, each thick filament is surrounded by six thin filaments, while each thin filament has three thick and three thin filaments as neighbours. This is indicated in the diagram below.


Recall that actin filaments are directional, as the two strands of the double helix of monomers are parallel rather than antiparallel. Actin filaments are all connected to the Z disc at the same end, i.e. thin filaments on opposite sides of the Z disc are antiparallel. Thus there is a similar polarized arrangement of both thick and thin filaments.

Several different types of fibrous proteins comprise the Z discs. Alpha-actinin is believed to anchor the ends of the actin filaments, while another actin-binding protein beta-actinin is thought to determine the length of these thin filaments. The alignment of adjacent myofibrils is possibly effected by the fibrous (intermediate filament) proteins desmin and vimentin which are found at the disc periphery; these may also bind to the membrane of the myofibre.

To summarize, the extent of the thick filaments is represented by the A band; the extent of the thin filaments is represented by the I band + AI band. The AI band is the region where the two types of filaments overlap.

Interaction of thick and thin filaments

The globular heads of the myosin thick filaments form cross-links with the actin thin filaments. During muscle contraction, these cross-links are repeatedly broken and reformed further along the thin filaments (i.e. nearer to the Z disc) , as the myosin heads effectively "walk" along them. It is for this reason that there is a hinge between the globular heads and the fibrous tails of myosin molecules. Note that the lengths of the individual thin and thick filaments do not change.

Examine the diagrams of the sarcomere, and consider the walking motion described above. What will be the resulting change in the appearance of each sarcomere? How will the width of each band (I, A, AI, AH) change, if at all?

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J. Walshaw