Myosin is a molecular motor that acts like an active ratchet. Chains of actin proteins form high tensile passive 'thin' filaments that transmit the force generated by myosin to the ends of the muscle. Myosin also forms 'thick' filaments. Each myosin 'paddles' along an actin filament repeatedly binding, ratcheting and letting go, sliding the thick filament over thin filament.
All muscle cells are composed of a number of actin and myosin filaments in series. The basic unit of organisation of these contractile proteins in striated muscle cells (i.e., the cells that compose cardiac and skeletal muscle, but not in smooth muscle tissue) is called the sarcomere. It consists of a central bidirectional thick filament flanked by two actin filaments, orientated in opposite directions. When each end of the myosin thick filament ratchets along the actin filament with which it overlaps, the two actin filaments are drawn closer together. Thus, the ends of the sarcomere are drawn in and the sarcomere shortens. Sarcomeres are connected together by so-called 'Z lines', which anchor the ends of actin filaments in such a way that the filaments on each side of the Z line point in opposite directions. By this means, sarcomeres are arranged in series. When a muscle fiber contracts, all sarcomeres contract simultaneously so that force is transmitted to the fiber ends.
Nerve impulses affect the way in which calcium bonds to the troponin.
A model of cross-bridge attachment to actin in the A-M-ATP state based on x-ray diffraction from permeabilized rabbit psoas muscle
Apr 01, 2002; ABSTRACT A model of cross-bridges binding to actin in the weak binding A.M-ATP state is presented. The modeling was based on the...