The sliding filament theory of muscle contraction occurs in 5 phases: 1) A nervous impulse causes acetylcholine release at the neuromuscular junction, depolarizing the motor end plate and triggering calcium release from the sarcoplasmic reticulum. 2) Calcium binds to troponin, displacing tropomyosin and exposing actin binding sites for myosin. 3) ATP hydrolysis allows myosin to pull actin inward, shortening the sarcomere as cross-bridges detach and reattach down the actin filament. 4) Contraction continues as long as ATP and calcium levels remain high. 5) When the impulse stops, calcium reenters storage and

1. Sliding Filament Theory

2. Phase 1
A nervous impulse arrives at the neuromuscular
junction (NMJ) – this causes a release of a
chemical called acetylcholine. Acetylcholine
causes the depolarisation of the motor end
plate which travels throughout the muscle.
Calcium (Ca+) is then released from the
sarcoplasmic reticulum (this encircles each
myofibril). This calcium triggers a muscle
contraction.

3. Phase 2
If there is a lot of calcium, it binds to troponin.
This causes the troponin-tropomyosin to move
away from the actin. This means the myosin can
now attach to the actin – this forms a cross-bridge.

4. Phase 3
ATP is broken down (into ADP and P) so energy is released.
The myosin pulls the actin inwards (the Z-line/Z-discs move
closer together) and the sarcomere shortens e.g. the muscle
shortens.
The length of the thin and thick filaments do not change –
they (actin and myosin) just move closer together / overlap
and shorten the whole muscle fibre.
The release of the P triggers what we call the ‘power-stroke’
of contraction!!! This is because the ADP which is still
attached causes the myosin head to rotate, pulling the actin
filaments past the myosin filaments and towards the M-line.

5. The myosin then detaches from the actin
breaking the cross-bridge as the ATP binds again
to the myosin head.
The whole process is repeated when the myosin
head attaches to an actin-binding site further
down the actin filament - another ‘power-stroke’
occurs.
Phase 4

6. Phase 5
Muscular contraction can continue so long as
there is ATP and enough calcium in the
sarcoplasm.
If the nervous impulse STOPS, the calcium
goes back to the sarcoplasmic reticulum and the
actin returns to resting position e.g. the muscle
lengthens and relaxes.

7. TextWall
Answer the question given to you in the lesson
or the one that appears on the text wall