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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).
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