Fig. 1. (A) The sarcomere length (SL)–tension relationship for striated muscle, showing how the degree of thick (myosin) and thin (actin) filament overlap determines the potential availability of cross-bridges and thus tension. Details of the cardiac sarcomeric filaments are given in C and are drawn schematically here in the insets a–d, where the thick line with cross hatching is myosin and the thin line is actin. (a) The position of actin and myosin at short SLs, when myosin comes in contact with the Z-line. There is a rapid decline in tension as SLs decreases (to the left of the arrow). The region between b and c is the range of SLs where the potential availability of cross-bridges remains constant during sarcomere stretch because the central cross-bridge head-free zone of the myosin filament (M-line) is progressively uncovered. SLs to the left of b show how tension decreases when the thin filaments from opposite ends of the sarcomere overlap at the M-line. (d) Tension declines toward zero when the sarcomere is stretched such that there is no overlap between thick and thin filaments. A key feature of the cardiac SL–tension ascending limb is that it is shifted in relation to the skeletal curve, and that force increases over the SL range where myofilament overlap remains constant. This means mechanisms other than overlap play an important role in cardiac muscle tension (see text). The figure is adapted from Gordon et al. (Gordon et al., 1966) and Bers (Bers, 2002). (B) Illustration of a cross section through striated muscle showing the effect of stretch on myofilament lattice spacing. Light grey circles represent thick (myosin) filaments and black circles represent thin (actin) filaments. (C) Schematic diagram of a cardiac sarcomere. The sarcomere is the fundamental unit of contraction and is defined as the region between two Z-lines. Each sarcomere consists of a central A-band (thick filaments) and two halves of the I-band (thin filaments). The I-band from two adjacent sarcomeres meet at the Z-line. The central portion of the A-band is the M-line, which does not contain actin. Also shown are the positioning of titin, actin (thin) and myosin (thick) filaments. The coloured bars at the top indicate key segments of the titin molecule and show the regions bound to the contractile filaments and the extensible region. Figure is adapted from Granzier and Labeit (Granzier and Labeit, 2004).