Download In the sliding filament model, the thick and thin filaments

In the sliding filament model, the thick and thin filaments past each
other, shortening the sarcomere.
LEARNING OBJECTIVE [ edit ]
Describe the sliding filament model of muscle contraction
KEY POINTS [ edit ]
ATP activates myosin, bringing it to a higher energy state, ready to form cross­bridges with the
thin actin filament.
Myosin acts as a molecular ratchet by binding to an actin filament and changing shape, thereby
pulling the actin filament toward the A­band.
ATP binds again, destabilizing the myosin filament and enabling it to bind to another site along
the actin filament, increasing the strength of contraction.
All the myosin heads contract simultaneously, shortening all the sarcomeres, causing the muscle
to contract.
The myosin heads pull the A­band toward the Z­lines at the end of each sarcomere, shortening the
length of the I­band.
TERMS [ edit ]
cross­bridge
the bond between the globular head of a myosin molecule and the actin filament, repeatedly
formed during muscle contraction to draw it into the A band of a sarcomere
A­band
contains the entire length of a single thick filament
I­band
the zone of thin filaments that is not superimposed by thick filaments
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Sliding Filament Model of
Contraction
Muscles contract when sarcomeres
shorten. The thin and thick filaments that
compose sarcomeres do not shorten;
instead, they slide past one another,
causing the sarcomere to shorten while
the filaments remain the same length. The
sliding filament theory of muscle
contraction is the binding of myosin to
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actin, forming cross­bridges that generate filament movement .
Sliding filament model
When (a) a sarcomere (b) contracts, the Z­lines move closer together and the I band gets smaller. The A­
band stays the same width and, at full contraction, the thin filaments overlap.
Process of Movement
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 the
thin filament.
1. ATP binds to myosin and is hydrolyzed by ATPase into ADP and phosphate. The energy
released by this process activates the myosin head and cocks it into a high­energy,
extended position.
2. The cocked myosin head binds to a newly­exposed active site on the thin filament,
generating a cross­bridge between actin and myosin.
3. Myosin releases the ADP and phosphate, returning to a low­energy position, pulling the
thin filament along; this movement is called a power stroke. Shortening occurs when the
extensible region pulls the filaments across each other (like the shortening of a spring).
Myosin remains attached to the actin.
4. The binding of ATP destabilizes the myosin­actin bond, allowing myosin to detach from
actin. While detached, ATP hydrolysis occurs, "recharging" the myosin head. If the
actin­binding sites are still available, myosin can bind actin again.
5. The collective bending of numerous myosin heads (all in the same direction), combine
to move the actin filament relative to the myosin filament. This results in muscle
contraction.
Contraction in all Sarcomeres
The sarcomere consists of a central bidirectional thick filament flanked by two actin
filaments, oriented 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. The I­bands contract toward the A­bands, thereby shortening the I­band as
the myosin overlaps with the actin. Thus, the ends of the sarcomere are drawn in and the
sarcomere shortens. Sarcomeres are connected by so­called 'Z­lines', which anchor the ends
of actin filaments in such a way that the filaments on each side of the Z­line point in opposite
directions (with reversed polarity). When a muscle fiber contracts, all sarcomeres contract
simultaneously so that force is transmitted to the fiber ends.