Muscle Contraction

Question 1

How does motor stimulation of muscle fiber work?

Answer 1

AP –> opens VG CA++ –> CA++ triggers release of ACh into cleft –> continues AP on sarcolemma –> activates myofilaments –> spreads to T tubules –> CA++

Question 2

What are the 4 phases of muscle contraction?

Answer 2

1. excitation
2. excitation-contraction coupling
3. contraction
4. relaxation

Question 3

Excitation

Describe the mechanism | Does it require ATP?

Muscle Contraction

Answer 3

1. AP arrives at lower motor neuron, opening VG Ca++ channels
2. Ca++ enters presynaptic terminal, causes fusion of vesicles (filled with ACh) to merge with the membrane
3. ACh is released at the NMJ
4. ACh binds onto postsynaptic receptors in muscle, triggers passage of cations into cell

Does not require ATP

Question 4

Excitation-contraction coupling

Describe the mechanism | Does it require ATP?

Muscle Contraction

Answer 4

1. cations enter muscle fibers
2. triggers propogation of signal down T-tubules in muscle
3. causes SR to release calcium inside the sarcomere

Does not require ATP

Question 5

Contraction

Describe the mechanism | Does it require ATP?

Muscle Contraction

Answer 5

1. Ca++ binds to troponin on thin filaments -> causes propomyosin to shift -> reveal binding sites on thin filaments (G-actin)
2. allows binding of myosin from thick filaments onto actin
3. allows for “cock-back” or “power-stroke” action

Requires ATP

Question 6

Muscle Contraction

Relaxation

Describe the mechanism | Does it require ATP?

Answer 6

• Relaxation of ACh stops
• ACh esterase clears ACh out from NMJ
• Ca++ is pumped into SR -> removes troponin
• Tropomyosin re-blocks actin filaments
• Muscle returns to original length

Requires ATP