Lecture 7 - Skeletal Muscle: Contraction

Question 1

Myofilaments

Answer 1

Located inside myofibril (recall myofibril is the contractile organelle within a muscle fibre)

Question 2

Sacromeres

Answer 2

Repeating units that describes how myofilaments are organized in a myofibril

Question 3

Describe what the thin filaments of a sacromere are. What proteins are associated with thin filaments?

Answer 3

Twisted strands built from actin and other proteins

Actin - organized in a double-helical fibre
Actinin - at the z-line
Tropomyosin and troponin - wrap around the actin helix

Question 4

F-actin

Answer 4

Actin filaments - they are linear polymers of gobular actin (G-actin)

Question 5

What is a thick filament and what is its function?

Answer 5

Thick filaments are composed of myosin protein and each myosin monomer has a mobile head that can bind actin

Question 6

What happens during skeletal muscle contraction

Answer 6

Myosin and actin filaments slide past each other in a repeating cycle

Question 7

How does an excited muscle remain contracted?

Answer 7

An excited muscle begins to contract after Ca2+ levels increase, and will continue to contract as long as Ca2+ remains elevated

Question 8

Function of Ca2+ in muscle contraction

Answer 8

It interacts with the toponin-tropomyosin complex of the thin filament, revealing the myosin-binding active site on actin

Question 9

Contraction cycle

Answer 9

0. Ca2+ is released and binds with troponin, revealing actin’s binding site for myosin
1. Myosin heads bind to actin, creating a cross-bridge
2. Power stroke occurs and the myosin neck pivots, moving the thin and thick filaments past each other

3 and 4. ATP binding allowed myosin to detach from actin (cross-bridges detach)

5. ATP hydrolysis occurs to recock the myosin head and the myosin neck resets

Question 10

Does the “power stroke” step of the contraction cycle require ATP?

Answer 10

No! ATP is hydrolyzed (ie. used) to reset the myosin neck, not while it’s pulling on actin during the “power stroke”. Myosin can freely change the orientation of its neck and head (but not reset it)

Question 11

Describe the sliding filament model and how tension is produced

Answer 11

The interaction of sliding filaments causes the sarcomere to shorten, creating tension in the tendons. Tension can only be produced by myosin heads that overlap with (and thus can bind) actin

Therefore, the total amount of “pull” a thick filament can make on a thin filament depends on how many of its myosin heads can grab onto actin

Question 12

How can maximum tension be produced?

Answer 12

Produced at an intermediate sarcomere length because of the maximum number of cross-bridges formed

Question 13

What happens to tension if the muscle is too compressed or too stretched out?

Answer 13

If it is too compressed, no tension can form when thick filaments meet Z lines and sarcomere cannot shorten

If too stretched out, there is zero zone of overlap and results to zero tension because of the lack of interactions between thin and thick filaments

Question 14

How can tension be produced

Answer 14

It can only be produced when active sites are available, therefore, tension also depends on Ca2+ levels

Question 15

Muscle twitch

Answer 15

The contractile/tension response of a muscle fibre in response to a single action potential (the stimulus)

Question 16

3 distinct phases of muscle twitch

Answer 16

1. Latent period - muscle excitation (the amount of time it takes to go through the excitation steps)
2. Contraction phase - Ca2+ build-up
3. Relaxation phase - Ca2+ removal

Question 17

Summation

Answer 17

Occurs because more Ca2+ enters the cytosol before all the original Ca2+ is removed (residual), revealing more active sites

Question 18

Tetanus

Answer 18

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

Question 19

Why does summation/tetanus occur?

Answer 19

Because ion transport by pumps is slower than movement through ion channels

(for more info) less Ca2+ are being taken up by the sarcoplasmic reticulum than there are being released, which is why it’s likely for there to be Ca2+ buildup that can result in summation/tetanus

Question 20

Concentric (shortening) contractions

Answer 20

Occurs when the external load is less than internally generated tension

Note: heavier loads = slower contraction rates

Ex. Produces movement of a joint

Question 21

Isometric contractions

Answer 21

Occurs when the muscles produced just enough tension to balance an external load - tension is produced without shortening

These contractions are used to stabilize a joint

Question 22

Eccentric (lengthening) contractions

Answer 22

Occurs when the external load is higher than the internally generated tension. Despite the muscle being activated (produces internal tension), it gets longer, not shorter

Can be used like a “brake”, slowing down movement at a joint produced by external forces