Skeletal Muscle

Question 1

Describe stages of initiation of skeletal muscle contraction

Answer 1

1) Action pot arrives - Ach is released by exo from motor neurone axon terminal and diffuses across synaptic cleft
2) binds to ACh receptors proteins in motor end plate - contain voltage gated Na+ channels - open and Na+ enters the cell.
3) General depolarisation (end plate pot) spreads over the plasma membrane and via the T tubules
4) EPP above threshold for triggering act pot in muscle fibre
5) In T-tubules, voltage-gated Ca2+ channels activate RyR on the SR
6) Ca2+ released from SR in to the sarcoplasm - binds to troponin - tropomyosin to be pulled aside, exposing binding sites
7) myosin head attaches to binding sites on actin - crossbridge formed - contraction initiated as long as Ca2+ levels high.
8) Decreases in Ca2+ (goes back into SR) - unbind from troponin allowing tropomyosin to recover the binding sites
9) Muscle relaxes

Question 2

Describe the 5 stages of contraction cycle

Answer 2

1) Attachment - myosin head tightly bound to actin (rigor state).
2) Release - ATP binds to myosin head and induces the release from actin - muscle now relaxed.
3) Bending - ATP causes further changes to myosin head - bends forwards. Bending movement initiates hydrolysis of ATP —> ADP + Pi - both remain in myosin head.
4) Energy released from hydrolysis - myosin head bind to new site (crossbridge) and Pi released - increases binding affinity of myosin for actin. Myosin head gens force to straighten up and in doing so forces thin filament along thick filament, creating power stroke and shortening sarcomere. ADP lost from myosin head.
5) Reattachment - myosin head to actin + rigour state is re-established. (until another ATP binds and cycle starts again)

Question 3

What would happen to muscle without ATP?

Answer 3

muscle would remain in a state of rigour.