Learning Objective:
• To be able to explain the sliding filament theory
using appropriate terminology.
Structure of a Muscle
 Skeletal muscle is made up of bundles of muscle
fibres.
 Fibres are bound together by connective tissue
through which run blood vessels and nerves.
 Each muscle fibre contains:
 Numerous myofibrils,
 Many mitochondria,
 An extensive network of interconnecting tubes called
the sarcoplasmic reticulum,
 Many nuclei.
Structure of a Muscle
The Microstructure of a Muscle
 Muscle fibres contain thread like myofibrils.
 Myofibrils consist of repeated units called sarcomeres.
 Each sarcomere is divided from the next by Z lines and
contains two protein filaments: the thin actin
filaments and the thick myosin filaments.
 Muscle contraction occurs as the sarcomere shortens
due to myosin filaments pulling the actin filaments
inwards.
 This results in the actin and myosin filaments being
drawn closer together and the whole muscle
shortening.
Sliding Filament Theory
In its most basic form:
 Muscle contraction occurs when the myosin and actin
filaments slide across each other.
 What actually occurs is myosin pulls the actin in.
 This shortens the muscle and is a muscle contraction.
Sarcomere and Muscle Contraction
During muscle contraction in the sarcomere:
• The Z lines come closer together,
• The width of the I band (area of actin not overlapping myosin)
decreases,
• The width of the H zone (area of myosin not overlapping with
actin) decreases,
• No change in the width of the A bands (myosin).
How does it all happen?
Video clip explaining the sliding filament theory.
1. A nerve impulse arrives at the neuromuscular
junction (place where motor neurone links to muscle
fibre).
2. At rest tropomyosin (a protein that winds around
actin) strands prevent myosin from attaching to and
pulling actin.
3. At rest the sarcoplasmic reticulum (a network of
channels that spread over the surface of the
myofibril) stores large amounts of calcium ions.
How does it happen?
4. To initiate a muscle contraction the electrical nerve
impulse travels into a myofibril via the transverse
tubules and triggers the release of calcium ions from
the sarcoplasmic reticulum.
5. The calcium ions bind to troponin (a globular
protein on actin) causing it to change shape.
6. As it does this it moves tropomyosin and frees up the
actin binding site for myosin.
7. Myosin binds to actin (forming a cross bridge),
activating myosin ATP-ase (an enzyme).
8. This enzyme causes ATP to breakdown and energy is
released.
9. The energy causes the myosin head to pull the actin
filament along (known as the power stroke).
10. Myosin detatches and reattaches further along pulling
actin in with a ratchet-like mechanism.
This process of muscular contraction can last for as long
as there is adequate ATP and Ca+ stores. Once the
impulse stops the Ca+ is pumped back to the
sarcoplasmic reticulum and the actin returns to its
resting position causing the muscle to lengthen and
relax.
Another clip to explain the sliding filament theory
Or act it out…
 Starring:
Sophie Actin
Matt Myosin
Rory and Ross ( the Calcium brothers)
Becki and Charlotte (the Tropomyosin sister’s)
On a night out…
 Matt Myosin likes Sophie Actin.
 Unfortunately Sophie Actin is with her two mates, the
Tropomyosin sisters.
 Matt’s friends Ross and Rory Calcium admirably take
one for the team and go to chat up the Tropomyosin
sisters.
 With some smooth words they move the Tropomyosin
out of the way, thus freeing Sophie Actin for Matt
Myosin to head in for the pull!!
 After a bit of ‘action’ it’s getting late so the Calcium
brothers head home meaning that the Tropomyosin
sisters return forcing Matt Myosin and Amy Actin to
go their separate ways.
Homework
 Explain the sliding filament theory in your own words.
 You can use your text book and you tube to help (just
type muscle contraction or sliding filament theory in).
 You must include the following terminology:
Muscle fibre, myofibril, actin, myosin, sarcomere, I
Band, A Band, H Zone, Z Lines, Sarcoplasmic
reticulum, troponin, tropomyosin, transverse tubules,
calcium ions, ATP, myosin ATP-ase.