Actin  

Actin is the major constituent of the thin filaments which exists in 2 forms. In solutions  of  low  ionic  strength  it  present  as  a  42  kDa  monomer which termed as G-actin because of its globular shape. Since the ionic strength of the solution rises to that at the physiological level, G-actin polymerizes into a fibrous form called F-actin that resembles the thin filaments found in muscle. Although actin, such as myosin, is an ATPase,  the  hydrolysis  of  ATP  is  not  involved  in  the  contraction relaxation cycle of muscle but rather in the disassembly and assembly of the actin filament. Cross-bridges   emerge  from  the  filament  axis  on  the  thick  filaments,  in  a regular  helical  array towards  either end, whereas  there is a bare region  in the middle  that is devoid  of cross-bridges. In muscle  depleted  of ATP, the myosin cross-bridges interact with the surrounding actin filaments. The absolute

(a)

Structure of myosin (a) showing the association of the two heavy and two pairs of light chains, (b) showing the proteolytic fragmentation of myosin, and (c) showing the hinge regions between domains.

direction of the  myosin and actin  molecules  reverses  halfway  between  the  Z lines. therefore, as the 2 thin filaments that bind the cross-bridges at either end of a thick filament  move  towards  each other,  sliding  over the thick filament,  the distance between the muscle contracts and the Z lines shortens.

Schematic diagram indicating the interaction of the myosin thick filaments and the actin thin filaments during skeletal muscle contraction.