Question 1

Which of the following best describes the role of myosin II in skeletal muscle contraction?

Accepted Answer

Myosin II binds to actin filaments and, through ATP hydrolysis, pulls the thin filaments toward the Z line, resulting in muscle contraction.

Question 2

In the sliding filament model of muscle contraction, what happens to the length of the A band during contraction?

Accepted Answer

The A band remains the same length during contraction.

Question 3

Which protein acts as a calcium-sensitive switch that regulates myosin’s access to actin in skeletal muscle?

Accepted Answer

Troponin

Question 4

What is the immediate effect of acetylcholine release at the neuromuscular junction?

Accepted Answer

It opens Na$$^+$$ channels on the muscle cell membrane, causing depolarization.

Question 5

Which of the following steps in the muscle contraction cycle is out of order?

Accepted Answer

Myosin binds to thin filament and releases Pi before ATP binds to myosin.

Question 6

What is the primary function of ATP in the cross-bridge cycle of muscle contraction?

Accepted Answer

To detach myosin from actin, allowing the cycle to repeat.

Question 7

How does calcium regulate contraction in smooth muscle cells?

Accepted Answer

Calcium binds to calmodulin, which activates myosin light-chain kinase (MLCK) to phosphorylate myosin.

Question 8

Which of the following best explains why muscle relaxation occurs after contraction?

Accepted Answer

Calcium ions are actively pumped back into the sarcoplasmic reticulum by ATP-dependent pumps.

Question 9

Which of the following is a key structural difference between skeletal and smooth muscle cells?

Accepted Answer

Smooth muscle cells have dense bodies instead of Z lines for anchoring filaments.

Question 10

During skeletal muscle contraction, what is the role of the sarcoplasmic reticulum?

Accepted Answer

It stores and releases Ca$$^{2+}$$ ions in response to nerve impulses.

Question 11

Which of the following statements about myosin is TRUE?

Accepted Answer

Myosins are ATP-dependent motors that generally move toward the plus end of actin filaments.

Question 12

What would happen if the ATP-dependent Ca$$^{2+}$$ pumps in the sarcoplasmic reticulum were inhibited?

Accepted Answer

Muscle contraction would be prolonged due to sustained high cytosolic Ca$$^{2+}.$$

Question 13

Which of the following best describes the difference in calcium regulation between skeletal and smooth muscle?

Accepted Answer

Skeletal muscle uses troponin, while smooth muscle uses calmodulin.

Question 14

A patient is exposed to a toxin that prevents the release of Pi from myosin during the contraction cycle. What is the most likely effect on muscle contraction?

Accepted Answer

The power stroke will not occur, and muscle contraction will be impaired.

Question 15

Which of the following is NOT a function of myosin motors?

Accepted Answer

DNA replication

Question 16

Which of the following equations correctly represents the ATP hydrolysis reaction catalyzed by myosin during muscle contraction?

Accepted Answer

$$ \mathrm{ATP} + \mathrm{H}_2\mathrm{O} ightarrow \mathrm{ADP} + \mathrm{P}_i + 	ext{energy} $$

Question 17

Which of the following best explains why smooth muscle contraction is slower and more sustained than skeletal muscle contraction?

Accepted Answer

Smooth muscle contraction depends on a phosphorylation cascade involving MLCK, which is slower than the troponin-tropomyosin mechanism.

Question 18

Which of the following is true regarding the orientation of actin filaments in a sarcomere?

Accepted Answer

The plus ends are anchored at the Z line.

Question 19

A mutation causes troponin to lose its ability to bind Ca$$^{2+}. $$What is the most likely consequence for skeletal muscle function?

Accepted Answer

Myosin-binding sites on actin will remain blocked, preventing contraction.

Microfilament-Based Motility and Muscle Contraction

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