Figure 4.

Normalized force of skinned fibres under osmotic compression. The force generated by skinned osmotically compressed fibres was measured using small-angle X-ray fibre diffraction, a sample image of which is shown. (a) Diffraction image of skinned Manduca sexta flight muscle bundle with largely vertical equatorial axis. The dark line and dark central circle are due to a backstop protecting the detector from direct exposure to the X-ray beam. The 1,0 and 2,0 equatorial diffraction peaks, along with the 14.5 nm and 7.25 nm meridional reflections, are labelled. The boxed intensity profile along the equator is inset. Lattice spacings were estimated from the separation of the 1,0 equatorial diffraction peaks as described by Irving [16]. (b) Maximum force produced through Ca²⁺ activation of fibres stretched to the descending limb decreases as lattice spacing is shrunk through osmotic compression. Lattice spacing is displayed as normalized to the lattice spacing on the initial post-skinning activation to control for variability between fibres. Lattice spacing is shown as decreasing from left to right to facilitate comparison with the descending limb of the LT curve as conventionally depicted in figure 2. (Online version in colour.)