Module 4 Section 2

Question 1

Describe the sliding filament theory.

Answer 1

Cross-bridge formation forms the basis of the sliding filament mechanism which brings about muscular contraction.

Question 2

Describe what happens during the power stroke.

Answer 2

Power stroke: the interaction b/w myosin and actin that leads to a shortening of the sarcomere. It occurs when the cross-bridge bends, pulling the thin myofillament inward toward the centre of the thick filament.

Question 3

Explain what is meant by excitation-contraction coupling.

Answer 3

It refers to the process of converting an electrical signal into an actual contraction.

ACh is released into the neuromuscular junction where it causes permeability changes and initiates an action potential that is conducted across the entire muscle membrane.

Skeletal muscle cells have the sarcoplasmic reticulum and t-tubules that help to transmit this signal to the muscle fibres.

Question 4

Explain the role of Ca in skeletal muscle contraction.

Answer 4

The sarcoplasmic reticulum (SR) is a storage site for Ca. The ryanodine receptors (receptors on the SR) are a form of Ca channels. When they are activated, they open and Ca enters the cytoplasm.

Membrane depolarization in T-tubules results in the release of Ca from the SR. The release of Ca is important since it’s the primary trigger to allow skeletal muscles to contract.

When the muscle is excited, Ca enters the muscle fibre and binds to troponin which results in a conformational change that moves tropomyosin out of the way, thus exposing the myosin binding sites on the actin molecules.

Question 5

Fill in the blanks with the following terms:

• troponin (x2)
• tropomyosin (x2)
• calcium (x2)
• T-tubules
• ATP
• sarcoplasmic reticulum
• myosin

1) Action potential generated in response to binding of ACh and subsequent end-plate potential is propogated across surface membrane and down ___ of muscle cell.
2) Action potential T-tubule triggers ___ release from sarcoplasmic reticulum.
3) Calcium ions released from lateral sacs bind to ___ on actin filaments; leads to ___ being physically moved aside to uncover cross-bridge binding sites on actin.
4) ___ cross-bridges attach to actin and bend, pulling actin filaments toward the centre of the sarcomere; powered by energy provided by ___.
5) Calcium is actively taken up by the ___ when there is no longer an action potential.
6) With ___ no longer bound to ___, ___ slips back to its blocking position over binding sites on actin; contraction ends; actin slides back into its original resting position.

Answer 5

1) Action potential generated in response to binding of ACh and subsequent end-plate potential is propogated across surface membrane and down T-tubules of muscle cell.
2) Action potential T-tubule triggers calcium release from sarcoplasmic reticulum.
3) Calcium ions released from lateral sacs bind to troponin on actin filaments; leads to tropomyosin being physically moved aside to uncover cross-bridge binding sites on actin.
4) myosin cross-bridges attach to actin and bend, pulling actin filaments toward the centre of the sarcomere; powered by energy provided by ATP.
5) Calcium is actively taken up by the sarcoplasmic reticulum when there is no longer an action potential.
6) With calcium no longer bound to troponin, tropomyosin slips back to its blocking position over binding sites on actin; contraction ends; actin slides back into its original resting position.