Box 1 | The structure of the vertebrate striated muscle

From the following article:

Titin: properties and family relationships

Larissa Tskhovrebova & John Trinick

Nature Reviews Molecular Cell Biology 4, 679-689 (September 2003)

doi:10.1038/nrm1198

The beautiful near-crystalline appearance of striated muscle (a; micrograph courtesy of R. Craig, University of Massachusetts, USA) results directly from the highly ordered and hierarchical organization of the proteins that form the contractile apparatus, principally actin and myosin. These both self-associate into filamentous polymers known as myofilaments — myosin forms bipolar thick filaments, and actin forms polar thin filaments. Remarkably, in most cases, both filament types contain an exact numbers of subunits, so there are probably 294 myosin molecules in most vertebrate thick filaments. The myofilaments assemble into elemental contractile units, sarcomeres, which are concatenated to form MYOFIBRILS. The myofibrils are cross-connected and aligned in register, and fill most of the interior of muscle cells. The resulting highly ordered network provides the basis for rapid generation of macroscopic levels of force and displacement.

Titin: properties and family relationships 

Within the sarcomere (b), the myofilaments are assembled into parallel bundles with exactly aligned ends. The thick filament bundle is located in the centre of the sarcomere and appears as a dark band — the anisotropic A-band. On both sides, the thick filaments overlap with thin filaments that extend from the Z-lines at the ends of the sarcomere. The thin-filament region that is not overlapped by thick filaments is termed the light, isotropic I-band. A very dense, narrow Z-line bisects the I-band. A central, less dense region of the A-band is known as the H-band, and this, in turn, is bisected by a dark M-line. The assembly of thick and thin filaments into bundles in register is mediated by a number of crosslinking proteins that are located at the centre of the sarcomere in the M-line and within the Z-lines.

Except in a small central region that is called the bare zone, myosin heads are distributed in helical arrays over the surface of each half of the thick filament, ready to interact with actin. The inner two-thirds of these regions are termed the C-zones, because C-protein and its homologues, H- and X-proteins, all of which are composed of Ig and fibronectin domains, are located here. The functions of these proteins are unknown, but they bind to myosin and are spaced at 43-nm intervals along the filament, which is also the repeat distance of the helix that describes the myosin head spacing. The ends of thick filaments do not have these other proteins and are known as the D-zones. Activation of muscle initiates the interaction of myosin heads with actin filaments. The force produced by this interaction results in relative sliding of the thin and thick filaments and sarcomere contraction (see Fig. 5).