MODULE 5: Chapter 6.4

Question 1

What is the breakthrough idea regarding muscle contraction proposed in 1954?

Answer 1

The sliding filament model, explaining how muscle fibers slide past one another during contraction

Proposed by Andrew Huxley and Rolf Niedergerke, and by Hugh Huxley and Jean Hanson.

Question 2

What are the two primary proteins involved in muscle contraction?

Answer 2

• Actin
• Myosin

Question 3

What does the sliding filament model state?

Answer 3

Muscle filaments, consisting of actin and myosin proteins, slide over one another during muscle contraction using chemical energy from ATP hydrolysis.

Question 4

What are myoblasts?

Answer 4

Large fused cells that make up muscle tissue, containing many nuclei and sharing a common plasma membrane.

Question 5

What is the sarcolemma?

Answer 5

The common plasma membrane shared by myoblasts in muscle cells.

Question 6

What is the role of T tubules in muscle cells?

Answer 6

They provide the necessary exchange of extracellular O2 and nutrients required for muscle contraction.

Question 7

What is the primary active transporter in the sarcoplasmic reticulum?

Answer 7

SERCA, which is critical for modulating intracellular Ca2⁺ levels.

Question 8

What is a sarcomere?

Answer 8

The repeating unit in skeletal muscle formed by the overlapping arrangement of thick and thin filaments.

Question 9

What proteins are found in thick filaments?

Answer 9

Myosin protein subunits.

Question 10

What proteins are found in thin filaments?

Answer 10

• Actin
• Tropomyosin
• Troponin

Question 11

What is the function of Z-disk proteins in muscle fibers?

Answer 11

They serve as anchors for thick and thin filaments to permit muscle contraction.

Question 12

How do thick filaments form?

Answer 12

Individual thick filaments contain hundreds of myosin molecules arranged with fibrous tails in the middle and globular heads at either end.

Question 13

What is tropomyosin’s role in thin filaments?

Answer 13

It is a coiled coil α-helical protein that associates with actin and regulates myosin binding.

Question 14

What is the function of the troponin complex?

Answer 14

It regulates Ca2⁺ and mediates muscle contraction.

Question 15

What does the I band in a sarcomere represent?

Answer 15

The locations of thin filaments.

Question 16

What is the A band in a sarcomere?

Answer 16

A region consisting of thick filaments and overlapping thick and thin filaments.

Question 17

What happens during muscle contraction in terms of Z disks?

Answer 17

The Z-disk proteins are brought closer together as thick and thin filaments slide past one another.

Question 18

What initiates muscle contraction?

Answer 18

Neuronal stimulation of muscle cells at neuromuscular junctions, leading to Ca2⁺ release from the sarcoplasmic reticulum.

Question 19

What is the role of Ca2⁺ in muscle contraction?

Answer 19

It binds to troponin, causing conformational changes that allow myosin binding to actin.

Question 20

How does ATP hydrolysis affect muscle contraction?

Answer 20

It drives conformational changes in myosin, allowing it to bind to actin.

Question 21

What are the five steps of the actin–myosin reaction cycle?

Answer 21

• Myosin heads bind to actin subunits
• Release of Pi causes power stroke
• ADP is released
• ATP binds to myosin, causing release from actin
• ATP hydrolysis induces recovery conformation

Question 22

What happens during muscle relaxation?

Answer 22

Ca2⁺ levels decrease, blocking myosin binding sites on actin.

Question 23

What causes myosin to disengage from the actin filament?

Answer 23

ATP binds to the myosin head

This binding causes a conformational change that leads to myosin detaching from actin.

Question 24

What is the role of ATP hydrolysis in the myosin head?

Answer 24

Induces the recovery conformation

This prepares the myosin head for another round of the reaction cycle.

Question 25

What leads to muscle relaxation?

Answer 25

Termination of neuromuscular signals and decrease in Ca2⁺ levels ## Footnote This is facilitated by SERCA-mediated active transport.

Question 26

What happens to myosin binding sites on actin during muscle relaxation?

Answer 26

They are blocked ## Footnote This blocking occurs when Ca2⁺ levels decrease.

Question 27

What occurs when the thick and thin filaments slip past one another?

Answer 27

The sarcomere returns to its elongated state ## Footnote This process is aided by the uncoiling of the titin spring protein.

Question 28

What are the distinct molecular events in the actin–myosin reaction cycle?

Answer 28

ATP binding, hydrolysis, dissociation of ADP and Pi ## Footnote Each step has unique functions in muscle contraction.

Question 29

What does ATP binding to the myosin head cause?

Answer 29

Dissociation from the actin thin filament ## Footnote This is the first step in the actin–myosin reaction cycle.

Question 30

What happens after ATP hydrolysis in the myosin head?

Answer 30

A conformational change occurs ## Footnote This change positions the myosin head for the next contraction.

Question 31

Under what condition are myosin binding sites on actin uncovered?

Answer 31

When Ca2⁺ levels in the cell are elevated ## Footnote This is initiated by neuromuscular signaling.

Question 32

What is released during the power-stroke conformation of the myosin head?

Answer 32

Pi (inorganic phosphate) ## Footnote This release is crucial for muscle contraction.

Question 33

What does the myosin head do during the power stroke?

Answer 33

Pulls the actin filament 70 Å along the myosin filament ## Footnote This action is responsible for muscle contraction.

Question 34

What happens to ADP in the myosin head after the power stroke?

Answer 34

ADP dissociates from the nucleotide binding site ## Footnote This allows a new molecule of ATP to bind.

Question 35

What is the sliding filament model?

Answer 35

A model of muscle contraction where actin and myosin slide over one another ## Footnote This process uses chemical energy from ATP hydrolysis.

Question 36

Define myoblast.

Answer 36

A large fused muscle cell containing many nuclei ## Footnote Myoblasts share a common plasma membrane.

Question 37

What is a myofibril?

Answer 37

A bundle of muscle fibers within a myoblast ## Footnote Myofibrils are essential components of muscle tissue.

Question 38

What is a sarcomere?

Answer 38

An overlapping arrangement of thick and thin filaments ## Footnote Sarcomeres form a repeating structure in skeletal muscle.

Question 39

What role does tropomyosin play in muscle contraction?

Answer 39

It is a thin filament protein that blocks myosin binding sites ## Footnote Tropomyosin is arranged head-to-tail along actin polymers.

Question 40

What is troponin?

Answer 40

A heterotrimeric protein complex that mediates Ca2⁺ regulation ## Footnote Troponin plays a crucial role in muscle contraction regulation.

Question 41

What is the function of titin in muscle fibers?

Answer 41

Acts as a flexible spring connecting myosin proteins to Z-disk proteins ## Footnote Titin contributes to the elasticity of muscle fibers.