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SLIDING FILAMENT THEORY |FINDYOURSELF
SLIDING FILAMENT THEORY
What are Cross-bridges?
• With an electron microscope, fine cross bridges can
be seen extending from each thick filament to the thin filament. These are
formed by the arm and head of the myosin molecules projecting outward from the
tail, and pointing towards the thin filaments.
SLIDING FILAMENT THEORY
Definition:
When a muscle cell contracts, the
thin filaments slide past the thick filaments, and the sarcomere shortens. This
process comprised of several steps is called the Sliding Filament Theory. It is
also called the Walk Along Theory or the Ratchet Theory.
After the ATP has bound to the myosin head, the binding of Myosin to
Actin molecule takes place:
Once the actin active sites are uncovered, the myosin binds to it:
It has the following steps:
1.
Before
contraction begins, An ATP molecule binds to the myosin head of the
cross-bridges.
2.
The ATPase
activity of the myosin head immediately cleaves the ATP molecule but the
products (ADP+P) remains bound to the head. Now the myosin head is in a high
energy state and ready to bind to the actin molecule.
3.
When the troponin-tropomyosin
complex binds with calcium ions that come from the sarcoplasmic
reticulum, it pulls the tropomyosin so that the active sites on the actin
filaments for the attachment of the myosin molecule are uncovered.
4.
Myosin head binds to the active site on the actin
molecule.
5.
The bond
b/w the head of the cross bridges(myosin) & the actin filaments causes a
the bridge to change shape bending 45° inwards as if it was on a hinge,
stroking towards the centre of the sarcomere, like the stroking of a boat oar.
This is called a POWER STROKE.
6.
This power
stroke pulls the thin filament inward only a small distance.
7.
Once the
head tilts, this allows release of ADP & phosphate ions.
8.
At the site
of release of ADP, a new ATP binds. This binding causes the detachment of the
myosin head from the actin.
9.
A new cycle
of attachment-detachment-attachment begins.
10.
Repeated
cycles of cross-bridge binding, bending and detachment complete the shortening
and contraction of the muscle.
11.
Participant Will bind to:
1. Myosin ATP,
Actin
2. Actin Myosin,
Troponin
3. Tropomyosin Troponin
4. Troponin Calcium,
Actin , Tropomyosin
5. ATP Myosin
6. Calcium ions Troponin
• Through the
attachment-detachment-attachment cycle, the myosin heads or cross bridges “walk”
along an actin filament to pull it inward relative to the stationary thick
filament.
• Because of the way the myosin molecules are
oriented within a thick filament, all the cross-bridges stroke towards the
center of the sarcomere.
• At any time
during contraction, part of the cross bridges are attached to the thin filaments
and are stroking, while others are returning to their original conformation in
preparation for binding with another actin molecule. Thus, some cross-bridges
“hold on” while others “let go”. Otherwise, the thin filaments would slip back
to their resting position b/w strokes.
• The
detachment of the myosin head from the actin cannot take place until and unless
a new ATP does not attach to the myosin head. This is important when death
occurs, no more ATP is available and thus, rigor mortis occurs.
Shortening
of the Muscle:
• The thick
and thin filaments DO NOT shorten.
• Contraction
is accomplished by the thin filaments from opposite sides of each sarcomere
sliding closer together or overlapping the thick filaments further.
• The H-zone
becomes smaller as the thin filaments approach each other.
• The I band
becomes smaller as the thin filaments further overlap the thick filaments.
• The width
of the A band remains unchanged as it depends on the thick filaments and the
thick filaments do not change length.
