Figure 1.

Organization of the sarcomere of skeletal muscle. Electron micrograph of a longitudinal thin section through a muscle fiber, with the fiber long axis horizontal. Two complete sarcomeres are shown, and elements of the sarcoplasmic reticulum separate myofibrils in the longitudinal direction. The major bands and lines are indicated, notably the thin, distinctive Z-line flanked by two low-density half I-bands, with very dense A-bands containing the thick filaments. The relative densities of the bands in this electron micrograph are related to their content of protein. The light band on either side of the M-line shows the extent of the cross-bridge-free regions of the thick filaments. The fine transverse periodicity across the A-band, at ∼43 nm, is due to the periodic structure of the thick filament backbone, which in turn determines the position of relaxed cross-bridges on the surface of the filaments and is enhanced by the presence of accessory proteins.