Fig. 4.

Intensity profiles across the meridian of the third myosin layer line at 14.3 nm spacing from a resting bony fish muscle (cf. Fig. 1) and b fully active muscle. The M3 position, labelled here as M3r (r for relaxed) and sampling along the layer line are indicated. b Shows the presence in active muscle of only a slightly broadened central M3 peak (marked M3m) and a new, shallow, but broad, peak labelled M3a. The remnant of the 10 row-line on the edge of the M3a peak is also shown. The images are shown as they appear in the FibreFix window apart from the labelling. Vertical scale is intensity, horizontal scale is position along the layer line. In c, d the peaks in a, b have been fitted with Gaussian profiles using in-house software. The resting M3 peak (M3r) can be fitted well with a single Gaussian function. The fitted active profile in d shows a similar, relatively sharp, meridional peak (M3m) superimposed on a much broader peak (M3a) which is also well fitted by a Gaussian