Myosin
It is thick in
diameter, and consists of a tail which terminates in two globular head. Myosin
tail consists of two long polypeptide chain coiled together. The heads are also
called cross bridges as they link the thick and thin micro-filaments together
during contraction.
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| Myosin and Actin Filaments |
Actin
It is thin in diameter.
The actin molecules are arranged in two chains which twist around each other.
Twisting around the actin chains are two strands of another protein
tropomyosin. The other major protein in the actin is troponin. It is actually
three polypeptide complex, one binds to actin, another binds to tropomyosin
while the third binds calcium ions.
Sliding Filaments
Z. Huxley and A. F.
Huxley proposed 'sliding filament model, of muscle contraction. Impulses
generated at a neuromuscular junction travel down a T- tubule to the calcium
storage sacs, and calcium is released into the muscle fiber. Now the muscle
fiber contracts as the sarcomeres within the myofibrils shorten. When a
sarcomere shortens, the thin actin filaments slide past the thick myosin
filaments and approach one another. This cause A band to shorten and the H zone
to almost or completely disappear. The movement of actin filaments in relation
to myosin filaments is called the sliding filament theory of muscle
contraction. During the sliding process the sarcomere shortens even though the
filaments themselves remain the same length. ATP supplies energy for muscle
contraction. Although the actin filaments slide past the myosin filaments, it
is the myosin filaments that do the work. Myosin filament break down ATP and
have cross bridges that pull the actin filaments toward the center of the
sarcomere.
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| The Sliding Filament Model |
ATP
supplies energy for muscle contraction. Although the actin filaments slide past
the myosin filaments, it is the myosin filaments that do the work. Myosin
filament break down ATP and have cross bridges that pull the actin filaments
toward the center of the sarcomere.
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