Module 4: Section 2

Question 1

The sliding filament mechanism

Answer 1

• During contraction, the thin filaments move inward over the thick filaments
• The z-lines move closer together
• The length of the filaments don’t change, just the degree of overlap

Question 2

Concentric contraction

Answer 2

The whole muscle shortens because of the overlap between filaments

Question 3

The power stroke

Answer 3

• Interaction between myosin and actin that leads to shortening of the sarcomere
• The cross-bridge bends, pulling the thin filament inward toward the centre of the thick filament

Question 4

Binding 1 - Step 1 of the cross-bridge cycle

Answer 4

Myosin cross-bridge binds to actin molecule

Question 5

Power stroke - Step 2 of the cross-bridge cycle

Answer 5

The myosin head bends pulling the thin myofilament inward

Question 6

Detachment - Step 3 of the cross-bridge cycle

Answer 6

Cross-bridge detaches at end of power stroke and returns to original conformation

Question 7

Binding 2 - Step 4 of the cross-bridge cycle

Answer 7

• Cross-bridge binds to more distal actin molecule
• Cycle repeats

Question 8

The result of the power stroke

Answer 8

• The actin molecules are being pulled closer to the centre of myosin molecules
• On each cross-bridge cycle the actin is pulled even more

Question 9

Structure of myosin and actin

Answer 9

• Each myosin molecule is surrounded by 6 actin molecules on each end
• Myosin has 2 heads and only one may be attached to actin at any given time

Question 10

Note about cross-bridges

Answer 10

• Not all cross-bridges are actively pulling actin
• Some just hold the actin in position while others prepare for the next power stroke

Question 11

Excitation-contraction coupling

Answer 11

Process of converting an electrical signal into a contraction

Question 12

2 membrane structures in skeletal muscles that help transmit signal to muscle fibres

Answer 12

• Sarcoplasmic reticulum
• T-tubules

Question 13

Sarcoplasmic reticulum

Answer 13

• Runs parallel to the fibres
• At its end, the lateral sacs of the SR are close to the T-tubules
• SR is a storage site for Ca2+

Question 14

T-tubules

Answer 14

• Folds of the plasma membrane
• At the junction of A and I bands, T-tubules dip into the fibre and run perpendicular to the fibre

Question 15

Relationship between SR and T-tubules - Step 1

Answer 15

• SR runs lengthwise with segments that expand to form the lateral sacs lying adjacent to T-tubules
• T-tubules dip into muscle fibres at junctions between A and I bands of myofibrils

Question 16

Relationship between SR and T-tubules - Step 2

Answer 16

• Dihydropyridine receptors are on the surface of T-tubules
• These are voltage-sensors that sense the wave of depolarization as it travels down T-tubules

Question 17

Relationship between SR and T-tubules - Step 3

Answer 17

Opposite to the dihydropyridine receptors are ryanodine receptors on the SR

Question 18

Relationship between SR and T-tubules - Step 4

Answer 18

• When the wave of excitation enters the T-tubules it is sensed by the dihydropyrindine receptors which signal the ryanodine receptors to undergo a conformational change
• When ryanodine receptors are active they open and Ca2+ enters the cytoplasm

Question 19

What is the primary trigger to allow skeletal muscles to contract?

Answer 19

Ca2+

Question 20

In relaxed muscles, why can’t contractions take place?

Answer 20

• Tropomysoin and troponin are positioned in a way to prevent cross-bridge formation
• They block the binding sites on the actin molecules

Question 21

Excited muscles

Answer 21

• Ca2+ enters the muscle fibre
• Ca2+ binds to troponin and causes a conformational change
• Causes tropomyosin to move and expose the myosin binding sites on actin molecules

Question 22

What is the cause of muscle relaxation?

Answer 22

• Decreased nerve activity at the neuromuscular junction
• Acetylcholinesterase removes leftover acetylcholine, stopping action potentials in the muscle fibre

Question 23

Calcium reuptake into the sarcoplasmic reticulum

Answer 23

• Without action potentials the SR stops releasing Ca²⁺
• Ca²⁺-ATPase pump actively move calcium from the cytosol back into the SR for storage

Question 24

How do low calcium levels cause relaxation?

Answer 24

• When Ca²⁺ levels drop, the troponin-tropomyosin complex covers actin again
• Prevents cross-bridge formation and causing the muscle to relax and lengthen

Question 25

Step 1 - How muscles contract

Answer 25

- ATPase site binds ATP and splits it into ADP and inorganic phosphate (Pi) - When Pi is removed stored energy is released and transferred to the myosin cross-bridge

Question 26

2 sites on the myosin head

Answer 26

1. Actin binding site 2. ATPase site

Question 27

Step 2 - How muscles contract

Answer 27

- In the presence of Ca2+ the troponin-tropomyosin complex exposes the actin molecules - The cross-bridge can bind with the actin molecules - Cross-bridge swings causing the power stroke

Question 28

Step 3 - How muscles contract

Answer 28

In the absence of Ca2+ the cross-bridge remains "cocked" and contraction will not occur

Question 29

Step 4 - How muscles contract

Answer 29

- During the power stroke Pi is released - When power stroke is complete ADP is released leaving the ATPase site empty - Cross-bridge is still bound to the actin

Question 30

Step 5 - How muscles contract

Answer 30

Binding of a new ATP molecule causes the cross-bridge to detach and return to its "un-cocked" shape

Question 31

Step 6 - How muscles contract

Answer 31

- After death rigorous mortis occurs - Ca2+ concentrations increase in cells - The cross-bridges were already "cocked" so muscles will contract until they run out of ATP, causing them to remain contracted - After several days, as muscle proteins decay, relaxation occurs again

Question 32

Latent period

Answer 32

- A single skeletal muscle action potential lasts only 1–2 milliseconds, but there’s a short delay before contraction begins - latent period is when, the action potential finishes before contraction starts, and cross-bridge cycling is just beginning

Question 33

Contraction time and peak tension

Answer 33

- After cross-bridge cycling begins, actin slides along myosin to create increasing tension - Peak tension produced before relaxation, occurs for around 40–120 milliseconds

Question 34

Factors affecting contraction time

Answer 34

The length of contraction varies depending on muscle fiber type (fast twitch vs slow twitch) and the muscle’s location in the body

Question 35

Relaxation time

Answer 35

- Contractile response does not end until all Ca2+ has been removed - Takes 50-200 milliseconds from peak tension