How does myosin bind to actin?
A. upon release of sodium ions from storage sacs
B. upon release of potassium ions from storage sacs
C. by means of cross-bridges only
D. upon release of calcium ions from storage sacs and by means of cross bridges
E. none of the above
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option D is the correct answer.
explanation is as below:
Sliding Filament Model:
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 (with reversed polarity). 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.
Physiologically, this contraction is not uniform across the sarcomere; the central position of the thick filaments becomes unstable and can shift during contraction. However the actions of elastic proteins such as titin are hypothesised to maintain uniform tension across the sarcomere and pull the thick filament into a central position.
Pre-process of movement
If the process of movement were to continue constantly, all muscles would constantly be contracted. Therefore, the body needs a way to control the ability of myosin to bind to the actin. This is accomplished by the introduction of calcium into the cytoplasm of the muscle cell.
- When the muscle does not need to contract (is in a resting state), thin strands of a protein called tropomyosin are wrapped around the actin filaments, blocking the myosin binding sites. This inhibits the myosin from binding to actin, and therefore causes a chain of events leading to muscle relaxation.
- Molecules called troponin are attached to the tropomyosin.
- When calcium is introduced into the muscle cell (fiber), calcium ions bind to troponin molecules.
- Calcium binding changes the shape of troponin, causing tropomyosin to be moved deeper into the groove of the actin dimer, therefore causing the myosin binding sites on the actin to be exposed.
- Myosin binds to the now-exposed binding sites, and muscles contract via the sliding-filament mechanism.
Nerve impulses affect the way in which calcium bonds to the troponin.
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