Lecture 7: Skeletal Muscle Contraction

Question 1

What is the contractile organelle within muscle fibre?

Answer 1

A myofibril which contains myofilaments?

Question 2

Myofilaments

Answer 2

• In myofibrils organized in repeating units called sarcomeres (from z line-a line)
• Includes thick and thin filaments

Question 3

Thin Filaments

Answer 3

Twisted strands (double helical fibre) built from actin and other proteins

Question 4

Actinin

Answer 4

Found at the Z-line; interconnects thin filaments

Question 5

Tropomyosin and tropnin

Answer 5

Wrap around the actin helix

Question 6

Thick Filaments

Answer 6

Composed of myosin protein
- each myosin monomer has a mobile head and neck that can bind actin

Question 7

Titin

Answer 7

Tethers the thick filament to the Z-line

Question 8

When does a muscle begin to contract?

Answer 8

Contracts after calcium levels increase and will continue to contract as long as calcium remains elevated

Question 9

What is calcium’s role?

Answer 9

Interacts with the troponin-tropomyosin complex of the thin filament, revealing the myosin-binding active site on actin

Question 10

Step 0

Answer 10

The contraction begins after calcium binds troponin, revealing the myosin-binding site

Question 11

Step 1

Answer 11

Binding
- Cross bridges form

Question 12

Step 2

Answer 12

Power stroke
- Myosin neck pivots, filaments slide
- myosin heads bind to actin, and complete the power stroke, moving the thin and thick filament past each other

Question 13

Step 3

Answer 13

ADP is released (from myosin head)

Question 14

Step 4

Answer 14

ATP Binding
- cross bridges detach
- ATP binding allows myosin to detach from actin and ATP hydrolysis ‘recocks’ the myosin head

Question 15

Step 5

Answer 15

ATP Hydrolysis
- Myosin neck resets

Question 16

Tension

Answer 16

Can only be produced by myosin heads that overlap with and bind with actin

Produced when active sites are available and depends on Ca levels

Question 17

Maximum tension

Answer 17

Muscles produce maximum tension when they are excited at an intermediate sarcomere length

Question 18

Latent Period

Answer 18

Muscle excitation

Question 19

Contraction Phase

Answer 19

Ca build up

Question 20

Relaxation Phase

Answer 20

Ca removal

Question 21

Twitch

Answer 21

One Stimulus

Question 22

Summation

Answer 22

Multiple stimuli
Occurs because more Ca enters the cytosol before all the original Ca is removed, revealing more active sites

Question 23

Tetanus

Answer 23

When persistent tension is produced by a repeatedly stimulated muscle fibre or muscle

Question 24

Concentric

Answer 24

• Shortening contractions
• occur when the external load is less than internally generated tension
• produce movement of a joint

Question 25

Isometric Contraction

Answer 25

• Muscle produce just enough tension to balance an external load
• used to stabilize a joint

Question 26

Eccentric

Answer 26

• Lengthening contractions
• occur when the external load is higher than the internally generated tension