Topic 10: Skeletal Muscle Physiology - Molecular Basis of Skeletal Muscle Contraction

Question 1

What are the characteristics in a relaxed muscle? (2)

Answer 1

• tropomyosin covers myosin binding sites on actin
• the myosin head is activated
  so if neuron sends signal to muscle fiber, it is ready to go

Question 2

Myosin head activation diagram

Answer 2

Question 3

What happens once actin binding sites on actin is exposed?

Answer 3

• myosin binds

Question 4

What are the steps to a muscle contraction? (3)

Answer 4

1) excitation of muscle fiber (electrical event)
2) Excitation-contraction coupling (electrical to mechanical event)
3) Contraction (mechanical) = sliding filament mechanism

Question 5

What occurs in excitation of muscle fiber? (3)

Answer 5

• sarcolemma is depolarized
• EPP to AP
• then AP propagates down to T-tubules to deep within fiber
  note that T-tubules has ECF flowing through it

Question 6

What occurs in excitation-contraction coupling? (4)

Answer 6

• electrical to mechanical event
• AP in T-tubules causes release of Ca++ (coupling agent) from terminal cisternae of sarcoplasmic reticulum via mechanically gated channels
• Ca2+ binds to troponin
• Troponin-tropomyosin complex moves, exposing myosin binding sites on actin

Question 7

In lecture, why is Ca++ important as a coupling agent?

Answer 7

going to lead to contraction

Question 8

What occurs in contraction, AKA sliding filament mechanism? (4 steps, 5 points)

Answer 8

1) Activated myosin heads attach to binding sites on actin (Cross bridge formation)
2) Energy stored in myosin head is released, myosin head pivots (POWER STROKE)
- ADP + Pi slide over myosin toward center of sarcomere (M-line)
3) ATP attaches to myosin head, causing its release from actin and unpivot (RECOVERY STROKE)
4) Myosin head reactivates (ATP to ADP+Pi)

Question 9

What happens if calcium in cytosol remains high during contraction?

Answer 9

steps repeat
cycle repeats many times to shorten the sarcomere

Question 10

In the sliding filament mechanism, what happens when the sarcomeres shorten? (2)

Answer 10

• H zone and I band shorten
• A band remains the same length

Question 11

In the sliding filament mechanism, what happens when the myofibrils shorten?

Answer 11

muscle shortens

Question 12

In the sliding filament mechanism, what happens t the thin (actin) and thick (myosin) filaments’ length?

Answer 12

remain the same length
(thats why it slides)