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. 2023 May 13;44(3):153–163. doi: 10.1007/s10974-023-09642-8

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© The Author(s) 2023

Open AccessThis article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.

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Fig. 1 — Vertebrate striated muscle sarcomere and the myosin filament superlattice. (a) Electron micrograph of frog sartorius muscle sarcomere and (b) schematic diagram of the sarcomere. The sarcomere is bounded by the Z-bands from which emanate actin filaments which overlap with the myosin filaments in the A-band. Contraction of muscle occurs when crossbridges on the myosin filaments interact with actin filaments to bring them closer to the centre of the A-band. The bipolar myosin filaments are crosslinked at the centre by the M-band assembly (b,c) which determines their relative rotation about the long axis. (c,d,e) Show schematic views of cross-sections in the M-band, bare region and the crossbridge region, respectively. (f) Shows electron micrographs of transverse sections through the M-band (left), bare region (centre) and crossbridge region (right). In the crossbridge region, the filament cross-section profile is indistinct hence the axial rotations of the filaments cannot be determined. In the bare region between the A-band and the start of the crossbridge region, the myosin filaments have a triangular profile (d and f centre) and this enables determination of the axial rotations of the filaments within 60^o. In a hexagonal lattice we can define a simple unit cell (c, dashed outline). In certain vertebrate muscles, the myosin filaments tend to have identical orientations for second-next nearest neighbours (d) and so are arranged on a superlattice (dashed outline). Parts b,c,d,e adapted from Millane et al. (2021)