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Samenvatting Neuromuscular junction uitbreiding college via Guyton
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Neuromuscular junction uitbreiding college via Guytonblz 74
Muscle contraction occurs by a sliding filament
mechanism.
The myosin molecule is composed of 2 heavy chains,
and 4 light chains. The 2 heavy chains wrap spirally
around each other to form a double helix, which is
called the tail of the myosin molecule. One end of
each of these chains is folded bilaterally into a
globular polypeptide structure called a myosin head.
Thus, there are 2 free heads at one end of the double-
helix myosin molecule. The four light chains are also
part of the myosin head, two to each head. These light
chains help control the functions of the head during
muscle contraction.
The tails of the myosin molecules are bundled
together to form the body of the myosin filament.
Also, part of the body of each myosin molecule hangs
to the side along with the head, thus providing an arm
that extends the head outward from the body. The
protruding arms and heads together are called cross-bridges.
Each cross-bridge is flexible at two points: called the hinges: one
where the arm leaves the body of the myosin filament, and the
other where the head attaches to the arm. The hinged arms
allow the heads either to be extended far outward from the
body of the myosin filament, or to be brought close to the body.
The hinged heads in turn participate in the actual contraction
process.
Another feature of the myosin head is that it functions as an
ATPase enzyme: essential for muscle contraction. This property
allows the head to cleave ATP and to use the energy derived
from the ATPs high energy phosphate bond to energize the
contraction process.
A pure actin filament without the presence of the troponin-tropomyosin complex, binds instantly and
strongly with the heads of the myosin molecules. If this is present, the binding between actin and
myosin cannot take place. In the presence of large amounts of calcium ions, the inhibitory effect of
the troponin-tropomyosin on the actin filaments is itself inhibited. When calcium ions combine with
troponin C, the troponin complex undergoes a conformational change, which causes uncovering of
the active sites of the actin, thus allowing these to attract the myosin cross-bridge heads and cause
contraction to proceed this is a hypothetical mechanism.
… see presentation.
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