The structure of the myofibril

Each muscle fibre contains a large number of thin longitudinal elements called myofibrils. They have characteristic banding patterns, and the bands on adjacent myofibrils are transversely aligned so that the whole fibre appears striated. In order to see the striations by light microscopy it is necessary to fix and stain the fibres, or to use phase-contrast, polarized light or interference microscopy.

Fig. 10.7« shows the striation pattern as seen by these methods and by low-power electron microscopy. The two main bands are the dark, strongly birefringent A band and the lighter, less birefringent I band. These bands alternate along the length of the myofibril. In the middle of each I band is a dark line, the Z line. In the middle of the A band is a lighter region, the H zone, which is bisected by a darker line, the M line. A lighter region in the middle of the H zone, the L zone, can sometimes be distinguished. (The letters used to describe the striation pattern are mostly the initials of names which are now no longer used.) The unit of length between two Z lines is called the sarcomere.

In the 1950s the use of the techniques of electron microscopy and thin sectioning, especially by H. E. Huxley and his colleagues, enabled the structural basis of the striation pattern to be discerned (Figs 10.7b, 10.8). The myofibrils are composed of two sets of filaments, thick ones about 11 nm in diameter and

Fig. 10.8. Thin longitudinal section of a glycerol-extracted rabbit psoas muscle fibre. Notice, particularly, the cross-bridges between the thick and thin filaments. Photograph supplied by Dr H. E. Huxley.

nebulin

actin

myosin

titin

Fig. 10.9. The locations of titin and nebulin within the sarcomere. Nebulin is associated with the thin filaments. Each titin molecule is attached at one end to the Z line and at its other end is bound to a thick filament; the section in the I band is highly extensible. From Bagshaw (1993), Muscle Contraction, Fig. 4.16, p. 56, with kind permission from Kluwer Academic Publishers.

thin ones about 5 nm in diameter. The thick filaments run the length of the A band. The thin filaments are attached to the Z lines and extend through the I bands into the A bands. The H zone is the region of the A band between the ends of the two sets of thin filaments, and the M line is caused by cross-links between the thick filaments in the middle of the sarcomere. The thick filaments have projections from them except in a short central region which corresponds to the L zone. In the overlap region these projections may be attached to the thin filaments so as to form cross-bridges between the two sets of filaments.

The two major proteins of the myofibril are myosin and actin. When myofibrils are washed in a solution that dissolves myosin the A bands disappear, and further washing with a solution that dissolves actin causes the I bands to disappear. This suggests that the thick filaments are composed largely of myosin and the thin filaments largely of actin. Thin filaments also contain the proteins tropomyosin and troponin, which are concerned with the control of contraction, as we shall see later.

The Z line contains the protein a-actinin, which binds to the actin filaments. Two large filamentous proteins also occur in the myofibril: nebulin, which runs parallel to the thin filaments, and titin, which runs from the Z line to near the middle of the sarcomere and is bound to the thick filaments in the A band (Fig. 10.9). Titin is a very large molecule (molecular weight about 3 million) and very elastic; it probably serves to keep the thick filaments in the middle of the sarcomere when the myofibril is stretched.

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