1. Sliding Filament Theory
When muscle fibre contracts actin and myosin fibres do NOT change their length.
Each sarcomere is shortened by an increase in the amount actin and myosin overlap.
The H zone and I band become smaller because the actin and myosin filaments have slid over each other.

2. Actin
Each actin filament is made up of two strands of actin molecules.
They twist around each other.
Along the actin are myosin binding sites, here myosin can form chemical bonds with actin.
Two proteins are also associated with the strands of actin.
These are globular troponin and two strands of tropomyosin
The troponin-tropomysin-actin complex conceal the biding sites for actin.

3. Myosin Each myosin filament is made up of bundles of myosin molecules.
Each myosin molecule has a globular ‘head’ and a rod like ‘tail’.
Myosin is bundled together so that the ‘tails’ form a central stalk and the globular ‘heads’ stick out to the side.

4. Muscle Contraction Muscle fibres get stimulated by the nervous system.
Myosin ‘heads’ attach to their binding sites on actin filaments to form temporary cross bridges.
In one contraction each cross bridge could attach and detach up to 100 times.
After it has detached a new cross bridge is formed further along the actin filament.

5. Sliding Filament Theory
Myosin ‘heads’ contain ATPase enzyme which hydrolyses ATP to ADP and inorganic phosphate (ATP ADP+Pi)
Calcium ions (entering sarcoplasm during wave of depolarisation) bind to the troponin-tropomyosin-actin complex
This causes the myosin binding sites to be revealed and release ATPase activity releasing energy.
Using the energy each myosin ‘head’ forms a new cross bridge with the exposed binding site.

6. Sliding Filament Theory
Using the same energy the shape of the myosin ‘head’ changes.
The ‘head’ bends causing myosin to pull on the actin filament (the ‘power stroke’).
The Z line moves closer to the myosin filament.
During this ADP and Pi are released from the myosin.
A new molecule of ATP attaches to the myosin ‘head’ and the actin-myosin bridge breaks.

7. Sliding Filament Theory
The new molecule of ATP hydrolyses and changes the shape of the myosin ‘head’ to the original position.
It is now ready to attach to the actin filament for the next ‘power stroke’.
This cycle can continue as long as calcium ions and ATP are present in the cell.
Myosin heads act like little oars pulling the actin filaments into the A band.

8. Sliding Filament Theory
When the nervous system stops stimulating the muscle calcium ions are taken up by the sarcoplasmic reticulum.
This means cross bridges wont reform after the myosin head has detached from the actin.
Muscle relaxes
If ATP runs out attached cross bridges can’t detach.
Muscles become stiff and unable to relax.
An example is rigor mortis.

9. Sliding Filament Theory
During vigorous use of muscle stored ATP is used up within 10 seconds.
This could stop contraction however another compound is used instead.
Phosphocreatine is used to generate more ATP.
Phosphocreatine is stored in muscle cells.
Phosphocreatine breaks down to creatine and an inorganic phosphate group, which releases energy.
This reaction is coupled with the resynthesis of ATP from ADP.