Abstract
Myosin filaments in vertebrate striated muscle have a long roughly cylindrical backbone with cross-bridge projections on the surfaces of both halves except for a short central bare zone. In the middle of this central region the filaments are cross-linked by the M-band which holds them in a well-defined hexagonal lattice in the muscle A-band. During muscular contraction the M-band-defined rotation of the myosin filaments around their long axes influences the interactions that the cross-bridges can make with the neighbouring actin filaments. We can visualise this filament rotation by electron microscopy of thin cross-sections in the bare-region immediately adjacent to the M-band where the filament profiles are distinctly triangular. In the muscles of teleost fishes, the thick filament triangular profiles have a single orientation giving what we call the simple lattice. In other vertebrates, for example all the tetrapods, the thick filaments have one of two orientations where the triangles point in opposite directions (they are rotated by 60° or 180°) according to set rules. Such a distribution cannot be developed in an ordered fashion across a large 2D lattice, but there are small domains of superlattice such that the next-nearest neighbouring thick filaments often have the same orientation. We believe that this difference in the lattice forms can lead to different contractile behaviours. Here we provide a historical review, and when appropriate cite recent work related to the emergence of the simple and superlattice forms by examining the muscles of several species ranging back to primitive vertebrates and we discuss the functional differences that the two lattice forms may have.
Highlights
The Sarcomere, A-Band, Simple Lattice and SuperlatticeOnce the sliding filament model of muscle contraction had been proposed [1,2], the problem of how muscle contraction takes place became one of determining what makes the actin and myosin filaments in the muscle sarcomere slide past each other (Figure 1a)
Huxley [4] came up with the idea of the myosin heads swinging on actin and detaching in a kind a rowing movement powered by ATP hydrolysis
The purpose of the myosin filaments in muscle is to interact with the actin filaments to generate shortening forces
Summary
Once the sliding filament model of muscle contraction had been proposed [1,2], the problem of how muscle contraction takes place became one of determining what makes the actin and myosin filaments in the muscle sarcomere slide past each other (Figure 1a). Brown [9] clearly showed that the myosin filaments have strong axial repeats of 14.3 nm and 42.9 nm and they suggested a possible helical arrangement of the myosin heads to explain these observed spacings. They thought they could see evidence of a particular set of rotations between adjacent myosin filaments. 2-stranded myosin filaments contained filaments with three different rotations distributed in a systematic way to give a so-called superlattice structure where the repeating unit cell had a side of a√3 where a is the centre-to-centre separation of nearest neighbour myosin filaments (Figure 1e). Sampling in the frog pattern (b,d) is different on different layer lines and shows a superlattice. (c,d) adapted from [10] with permission from Elsevier
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