A2 Human Biology – Unit 4
Muscle Contraction
Knowing the structure of the sarcomere enables us to understand what happens when a muscle
contracts. The mechanism of muscle contraction can be deduced by comparing electron micrographs
of relaxed and contracted muscle:
The Sliding Filament Theory
These show that each sarcomere gets shorter when the muscle contracts, so the whole muscle gets
shorter. But the dark band, which represents the thick filament, does not change in length. This
shows that the filaments don’t contract themselves, but instead they slide past each other.
A2 Human Biology – Unit 4
The Sliding Filament Theory
What makes the filaments slide past each other?
Energy is provided by the splitting of _______________, and the ATPase that does this splitting is
located in the _______________ cross bridge head. These cross bridges can also attach to
_______________, so they are able to cause the filament sliding by "walking" along the thin filament.
This cross bridge walking is called the cross bridge cycle, and it has 4 steps. One step actually
causes the sliding, while the other 3 simply reset the cross bridge back to its starting state.
The Sliding Filament Theory
1. The cross bridge swings out from the thick filament and attaches to the thin filament. [Put oars in
water.]
2. The cross bridge changes shape and rotates through 45°, causing the filaments to slide. The
energy from ATP splitting is used for this "power stroke" step, and the products (ADP + Pi) are
released. [Pull oars to drive boat through water.]
3. A new ATP molecule binds to myosin and the cross bridge detaches from the thin filament. [push
oars out of water.]
4. The cross bridge changes back to its original shape, while detached (so as not to push the
filaments back again). It is now ready to start a new cycle, but further along the thin filament. [push
oars into starting position.]
One ATP molecule is split by each cross bridge in each cycle, which takes a few milliseconds. During
a contraction, thousands of cross bridges in each sarcomere go through this cycle thousands of
times. Fortunately the cross bridges are all out of synch, so there are always many cross bridges
attached at any time to maintain the force.
A2 Human Biology – Unit 4
How is the cross bridge cycle switched off in a relaxed muscle?
This is where the regulatory proteins on the thin filament, troponin and tropomyosin, are involved.
_______________ is a long thin molecule, and it can change its position on the thin filament. In a
relaxed muscle it is on the outside of the filament, covering the _______________ molecules so that
myosin cross bridges can’t attach. This is why relaxed muscle is compliant: there are no connections
between the thick and thin filaments. In a contracting muscle the tropomyosin has moved into the
groove of the double helix, revealing the _______________ molecules and _______________ the cross
bridges to attach.
Controlling Contraction
1. An action potential arrives at the end of a motor neurone, at the _______________.
2. This causes the release of the _______________ acetylcholine.
3. This initiates an _______________ _______________in the muscle cell membrane.
4. This action potential is carried quickly throughout the large muscle cell by invaginations in the cell
membrane called _______________.
5. The action potential causes the sarcoplasmic reticulum (large membrane vesicles) to release its
store of _______________ into the myofibrils.
6. The calcium binds to _______________ on the thin filament, which changes shape, moving
tropomyosin into the groove in the process.
7. Myosin cross bridges can now attach and the cross bridge cycle can take place.
A2 Human Biology – Unit 4
Cross-section of actin filament
Here calcium has
bound to the
troponin to move
tropomysin and
allow the myosin
heads to bind.
Troponin
Tropomysin
Actin
Myosin head binding site
The Neuromuscular Junction
• A muscle is supplied with only one nerve fibre
• There is no threshold for stimulation – if an impulse arrives at the motor end plate it will always
stimulate the muscle fibre.
T-System
• Transverse system (T-tubules) – an infolding of the sarcolemma (membrane surrounding muscle
fibre) that spreads across the muscle fibre.
• Sarcoplamic Reticulum – accumulates calcium ions in a resting muscle.
• The depolarisation of the muscle spread across the sarcolemma and into the T-tubules and the
sarcoplasmic reticulum.
• Calcium ions are released into the sarcoplasm (cytoplasm of muscle fibre).
A2 Human Biology – Unit 4