4b

Question 1

How does a muscle contract?

Answer 1

Contraction = Activation of force-generating actin-myosin cross-bridge cycling within muscle fibres.
After contraction = Relaxation

Mechanism?
=> Sliding-filament mechanism (caused by cross-bridge cycling)
When force generation produces shortening of skeletal muscle fibres => Overlapping thick (myosin)
and thin (actin) filaments in each sarcomere move past each other => movement of the cross-bridge.
=> Sliding-filament mechanism.

The sliding filament mechanism is the result of the action potential stimulating skeletal, smooth, and
cardiac muscles.
In all three types of muscle, myosin and actin interactions are regulated by the availability of calcium
ions.
Changes in the membrane potential of muscles are linked to internal changes in calcium release (and
contraction).

Question 2

Structure of the actin molecule

Answer 2

globular molecule = one single polypeptide (‘long chain of amino
acids’). They polymerize to generate a long actin filament => two intertwined helical chains.
Each actin molecule contains a binding site for the thick filament myosin

1 actin molecule = 1 globule
1 actin helix = many globules linked together

Regulatory molecules: Troponin and
Tropomyosin

Question 3

What is troponin?

Answer 3

In relaxed skeletal muscle, tropomyosin blocks the cross-bridge binding site on actin.
- Partially covers the myosin-binding sites on each actin molecules
Each tropomyosin molecule is hold in such inhibitory binding position by the troponin molecule

The troponin has three subunits: T, I, C.
1. T, tropomyosin = Interaction with the tropomyosin molecule
2. I, Inhibitory = Inhibitory grip that prevents tropomyosin from moving along actin.
3. C, Calcium = Binding site for calcium.
Both tropomyosin and troponin cooperatively block any interaction between myosin cross-bridge and
actin filament in a resting myofibre.

Question 4

Structure of the myosin molecule

Answer 4

Two large polypeptide heavy chains + four smaller light chains.
They combine to generate a large molecule with two globular heads and one long tail.
These globular heads are the site where a cross-bridge with actin forms

Each globular head
Two binding sites
1. One for actin
2. One for ATP

Heavy chain:
- 1 tail, 1 head
Light chain:
- Specialized proteins that
stabilize the head (also a
site of regulation)

Question 5

ATPase

Answer 5

Energy for contraction
The myosin molecules in the two ends of each thick filament are oriented in opposite directions => The
power strokes of the cross-bridges move the thin filaments toward the centre of the sarcomere

Molecular basis of skeletal muscle contraction
The sequence of events:
= Cross-bridge cycle
1) binding between the myosin head and the thin
filament,
2) movement of the cross-bridge (Power stroke),
3) detachment of the cross-bridge: detach myosin
from the thin filament,
4) Re-energize the myosin head so it can re-attach to
a thin filament to repeat the cycle.

Question 6

3 slides explain on power point