The lengths of the filaments

Electron microscopy provides evidence that the filaments do not change in length when the muscle is stretched or allowed to shorten. This is just what we would expect if such length changes involve sliding of the filaments past each other. Measurements on frog muscles suggest that the thick filaments are 1.6 fxm long, and that the thin filaments extend for 1.0 fxm on each side of the Z line.

Electron microscopy has to be performed on muscle tissue which has been fixed and stained. There is a need, therefore, for some length-measuring method which can be applied directly to the living muscle. This is provided by the technique of X-ray diffraction, in which a beam of X-rays is passed through a muscle and the resulting diffraction pattern indicates the distances between repeat units in the muscle structure.

In the thick filaments the X-ray diffraction pattern suggests that there are structures which repeat axially at 14.3 nm and helically at 42.9 nm. In the thin filaments there seem to be structures arranged on helices with pitches of 5.1, 5.9 and about 37 nm.

The structural basis of these repeat distances we shall return to later. What is important in our present context is that they do not change when the muscle is lengthened or shortened, either when it is resting or during contraction.

Fig. 10.10. The isometric tension (active increment) of a frog muscle fibre at different sarcomere lengths. The numbers 1 to 6 refer to the myofilament positions shown in Fig. 10.11. From Gordon, Huxley and Julian (1966).

This provides further evidence that the filaments themselves do not shorten or lengthen during the corresponding changes in muscle length.

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