Question 1

Which structural feature of the myosin II molecule is responsible for its ability to form bipolar thick filaments in muscle cells?

Accepted Answer

The coiled-coil tail region

Question 2

During muscle contraction, what is the primary function of the Z disc in the sarcomere?

Accepted Answer

It anchors the plus ends of actin filaments

Question 3

Which step in the myosin II ATPase cycle directly results in the power stroke that moves the actin filament?

Accepted Answer

Release of inorganic phosphate ($$\mathrm{P_i}) $$after ATP hydrolysis

Question 4

What triggers the opening of Ca$$^{2+}$$ channels in the sarcoplasmic reticulum during muscle contraction?

Accepted Answer

Depolarization of the T-tubule membrane by an action potential

Question 5

Which accessory protein blocks the myosin-binding site on actin filaments in the absence of Ca$$^{2+}$$?

Accepted Answer

Tropomyosin

Question 6

A mutation in the troponin complex that prevents Ca$$^{2+}$$ binding would most likely result in which of the following?

Accepted Answer

Inability of tropomyosin to move, preventing muscle contraction

Question 7

Which of the following best describes the sliding-filament mechanism of muscle contraction?

Accepted Answer

Actin and myosin filaments slide past each other without shortening

Question 8

If a drug inhibits ATP binding to myosin, which step of the contraction cycle would be directly affected?

Accepted Answer

Release of myosin from actin (rigor state)

Question 9

Which equation best represents the relationship between muscle contraction force ($$F$$), the number of sarcomeres ($$n$$), and the force generated by a single sarcomere ($$f_s$$)?

Accepted Answer

$$F = n 	imes f_s$$

Question 10

Which of the following events occurs FIRST during the excitation-contraction coupling in skeletal muscle?

Accepted Answer

Release of acetylcholine at the neuromuscular junction

Question 11

Which of the following statements about the myosin II molecule is TRUE?

Accepted Answer

It contains two heavy chains and four light chains

Question 12

In a muscle cell, what would happen if the Ca$$^{2+}$$ channels in the sarcoplasmic reticulum failed to open in response to an action potential?

Accepted Answer

Tropomyosin would remain in place, blocking myosin-binding sites

Question 13

Which of the following best explains why muscle cells return to the rigor state after death?

Accepted Answer

ATP is no longer available to release myosin from actin

Question 14

Which of the following is a direct consequence of Ca$$^{2+}$$ binding to troponin during muscle contraction?

Accepted Answer

Tropomyosin moves, exposing myosin-binding sites on actin

Question 15

A patient with a genetic mutation in the tropomyosin gene is likely to experience which of the following symptoms?

Accepted Answer

Uncontrolled muscle contraction due to exposed myosin-binding sites

Question 16

Which of the following best describes the role of ATP in the myosin-actin interaction during muscle contraction?

Accepted Answer

ATP binding causes myosin to detach from actin

Question 17

Which of the following is NOT a function of the sarcoplasmic reticulum in muscle cells?

Accepted Answer

Initiation of action potentials

Question 18

Which of the following statements about the sliding-filament mechanism is correct?

Accepted Answer

The length of the sarcomere decreases during contraction

Question 19

Which of the following best describes the sequence of events leading to muscle contraction after a nerve impulse?

Accepted Answer

Acetylcholine release → Na$$^+$$ influx → T-tubule depolarization → Ca$$^{2+}$$ release → Troponin activation

22:The Cytoskeleton II: Myosins, Muscle Contraction, and the Sliding Filament Mechanism

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