Myofibrils, sarcomeres, and myosin. (A) The organization of the thick and thin filaments within a sarcomere. Callout shows one half of a thick filament with the P, C, and D zones. (B) Myosin in the SRX and DRX states and the associated structural states. (C) The arrangement of SRX myosin may determine the availability of individual myosins for binding to the thin filament. This determines the extent of tension along the thick filament, which in turn may regulate availability of other SRX myosin.