Question 1

Which of the following best describes the sliding filament theory of muscle contraction?

Accepted Answer

Actin and myosin filaments slide past each other, shortening the sarcomere without changing the length of the filaments.

Question 2

What is the primary role of calcium ions ($$\mathrm{Ca$$^{2+}$$}) in $$skeletal muscle contraction?

Accepted Answer

To bind to troponin, causing tropomyosin to move and expose myosin-binding sites on actin.

Question 3

During the cross-bridge cycle, what event immediately follows the release of inorganic phosphate ($$\mathrm{P_i}) $$from the myosin head?

Accepted Answer

The power stroke occurs, pulling actin toward the M line.

Question 4

Which of the following best explains why rigor mortis occurs after death?

Accepted Answer

ATP is unavailable, so myosin heads remain bound to actin.

Question 5

What is the function of myosin ATPase during muscle contraction?

Accepted Answer

It hydrolyzes ATP, providing energy to cock the myosin head.

Question 6

Which of the following sequences correctly describes the order of events in the cross-bridge cycle?

Accepted Answer

ATP binds to myosin → Myosin detaches from actin → ATP hydrolysis cocks myosin head → Myosin binds to actin → Power stroke → ADP and $$\mathrm{P_i}$$ released

Question 7

A patient is exposed to a toxin that prevents the release of calcium from the sarcoplasmic reticulum. What is the most likely effect on muscle contraction?

Accepted Answer

Muscle contraction will not occur because myosin-binding sites on actin remain covered by tropomyosin.

Question 8

Which statement about the interaction between myosin and actin during contraction is correct?

Accepted Answer

Each myosin thick filament can interact with multiple actin thin filaments, and each actin can be contacted by several myosin heads.

Question 9

Which of the following best describes the role of troponin in muscle contraction?

Accepted Answer

Troponin binds calcium, causing a conformational change that moves tropomyosin away from myosin-binding sites on actin.

Question 10

If a mutation caused tropomyosin to permanently block myosin-binding sites on actin, what would be the result?

Accepted Answer

Muscle contraction would not occur, even if calcium and ATP are present.

Question 11

Which event marks the transition from the rigor state to the relaxed state in the cross-bridge cycle?

Accepted Answer

Binding of ATP to the myosin head.

Question 12

A drug that inhibits myosin ATPase would have what effect on muscle contraction?

Accepted Answer

It would prevent the myosin head from cocking, inhibiting contraction.

Question 13

Which of the following is true regarding the power stroke in muscle contraction?

Accepted Answer

It occurs when the myosin head releases $$\mathrm{P_i}$$ and pulls actin toward the M line.

Question 14

Which of the following best explains why ATP is necessary for muscle relaxation?

Accepted Answer

ATP binds to myosin, causing it to detach from actin and end the rigor state.

Question 15

In a scenario where ATP is present but calcium is not released from the sarcoplasmic reticulum, what will happen to the muscle?

Accepted Answer

The muscle will not contract because myosin-binding sites on actin remain covered.

Question 16

Which of the following steps in the cross-bridge cycle is directly powered by the hydrolysis of ATP?

Accepted Answer

Cocking of the myosin head into a high-energy position.

Question 17

A patient with a genetic defect in troponin C cannot bind calcium efficiently. What is the most likely consequence?

Accepted Answer

Tropomyosin will not move, preventing muscle contraction.

Question 18

Which of the following best describes the rigor state in the cross-bridge cycle?

Accepted Answer

Myosin is tightly bound to actin with no ATP or ADP present.

Question 19

Which of the following is a direct result of the power stroke during muscle contraction?

Accepted Answer

Actin filaments are pulled toward the M line, shortening the sarcomere.

Skeletal Muscle Contraction: Sliding Filament Theory and the Cross-Bridge Cycle

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