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The Sliding Filament Model
The striped appearance of voluntary muscles is different when relaxed or contracted.
The span from one Z line to another Z line is one sacromere; in a relaxed state it is 2.5
µm long. Z lines are closer together during contraction because the length of the I band
and the H zone are reduced. The A band does not change in length during contraction.
Types of Protein Filaments Involved in Contraction
Thin filaments are two
strands, made chiefly of the
protein actin, coiled around
each other like a twisted
double string. Each strand is
composed of G actin (globular
Tropomyosin molecules coil
around the F actin, reinforcing it. A troponin complex consists of 3 polypeptides;
one binds to actin, one binds to tropomyosin(to keep it in place) and one binds to
Thick filaments are bundles of the protein myosin. Each myosin molecule
consists of a tail and two protuding heads. Each thick filament consists of many
myosin molecules whose heads stick out from opposite ends of the filament.
The Power Stroke
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In muscle contraction;
1. Myosin head groups attach to the surrounding actin filaments forming a
2. The head then bends, causing the thin filament to be pulled along and so
overlap more with the thick filament. This is the power stroke. ADP and Pi are
3. The cross-bridge is then broken as a new ATP attaches to the myosin head.
4. The head moves backwards as the ATP is hydrolysed to ADP and Pi.…read more
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When there is no action potential; calcium ions are actively transported back into the
sarcoplasmic reticulum by carrier proteins. This leads to muscle relaxation.
The Role of ATP in The Power Stroke
When the myosin head attaches to the actin binding site and `bends', the molecules are in
their most stable form. Energy from ATP is required in order to break the cross-bridge
and to re-set the myosin head forwards.…read more