Sliding Filament Theory + Excitation Contraction coupling

Question 1

What is Excitation contraction coupling?

Answer 1

The sequence of events in which a nerve impulse reaches the muscle membrane + leads to muscle shortening by cross-bridge activity.

Question 2

Outline 8 the steps of Excitation contraction coupling.

Answer 2

1. An action potential (AP) travels down the motor nerve fibre to the neuromuscular junction.
2. Vesicles in the Synaptic knob are stimulated by the AP causing the release of acetylcholine (ACh) into the synaptic cleft.
3. ACh diffuses across the synaptic cleft + binds to ACh receptors on the motor end plate causing a conformational change. Sodium ions enter the muscle fibre. This creates an End plate potential.
4. The AP is transferred along the sarcolemma until it reaches the invaginations of the transverse tubules.
5. It will cause a conformational change in the voltage-gated DHP receptor which pulls out the RYR receptor, opening calcium channels in the terminal cisternae of the sarcoplasmic reticulum.
6. Calcium ions will diffuse into the sarcoplasm + bind to calcium binding sites of troponin.
7. This shifts tropomyosin making active sites available for actin + myosin filaments to bind to each other, which changes intramuscular forces + forms crossbridges.
8. The pulling of myosin when attached to actin causes it to slide over each other.

Question 3

Outline the 3 steps of the ‘Walk along’ theory in terms of the sliding filament theory.

Answer 3

1. When a head attaches to an active site, this causes changes in the intramuscular forces between the head + arm of the cross bridge.
2. The new forces cause the head to tilt toward the arm + pull the actin.
3. The head breaks away from its active site, ready to combine with an active site further along the actin.

Question 4

What does the sliding filament theory result in?

Answer 4

• Sarcomere will shorten.
• Protein filaments slide over each other.
• Reduction in distance between Z - line.

Question 5

What will power these processes?

Answer 5

• The breakdown of ATP to ADP + Pi
• The release of energy from this process will energise the myosin cross bridges.
• They will pull the actin over myosin + shorten the muscle.