Question 1

Which of the following best describes the role of the sarcoplasmic reticulum (SR) in skeletal muscle contraction?

Accepted Answer

It stores and releases calcium ions ($$\mathrm{Ca$$^{2+}$$}) $$necessary for muscle contraction.

Question 2

During the sliding filament model of muscle contraction, what happens to the I band and H zone as the sarcomere shortens?

Accepted Answer

Both the I band and H zone decrease in width.

Question 3

Which event directly triggers the release of acetylcholine (ACh) from the axon terminal at the neuromuscular junction?

Accepted Answer

Influx of $$\mathrm{Ca$$^{2+}$$}$$ into the axon terminal through voltage-gated channels.

Question 4

A patient is diagnosed with myasthenia gravis. Which of the following best explains the muscle weakness observed in this disease?

Accepted Answer

Autoimmune destruction of ACh receptors reduces the ability of ACh to stimulate muscle fibers.

Question 5

What is the primary function of T tubules in skeletal muscle fibers?

Accepted Answer

To conduct action potentials deep into the muscle fiber, ensuring simultaneous contraction.

Question 6

Which of the following best describes the sequence of events at the neuromuscular junction leading to muscle fiber excitation?

Accepted Answer

Action potential arrives at axon terminal → $$\mathrm{Ca$$^{2+}$$}$$ enters terminal → ACh released → ACh binds to receptors → Na$$^+$$ enters muscle fiber

Question 7

Which step in the cross bridge cycle is directly powered by the hydrolysis of ATP?

Accepted Answer

Cocking of the myosin head into a high-energy state

Question 8

A muscle fiber is unable to relax after contraction. Which of the following is the most likely cause?

Accepted Answer

Failure to pump $$\mathrm{Ca$$^{2+}$$}$$ back into the sarcoplasmic reticulum

Question 9

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

Accepted Answer

ATP is no longer produced, so myosin heads cannot detach from actin.

Question 10

Which of the following is true regarding the propagation of an action potential along the sarcolemma?

Accepted Answer

It is initiated by the opening of voltage-gated Na$$^+$$ channels once threshold is reached.

Question 11

During muscle contraction, which of the following does NOT change in length?

Accepted Answer

A band

Question 12

Which of the following best describes the function of acetylcholinesterase at the neuromuscular junction?

Accepted Answer

It breaks down acetylcholine, terminating the signal at the synapse.

Question 13

A patient is exposed to a toxin that blocks voltage-gated Na$$^+$$ channels in skeletal muscle. What is the most likely effect on muscle contraction?

Accepted Answer

Action potentials cannot propagate, so muscle contraction is prevented.

Question 14

Which of the following is the correct order of events in excitation-contraction coupling?

Accepted Answer

Action potential travels along sarcolemma → Down T tubules → Ca$$^{2+}$$ released from SR → Cross bridge formation

Question 15

Which of the following best explains why the muscle fiber cannot be stimulated during the refractory period?

Accepted Answer

Voltage-gated Na$$^+$$ channels are inactivated and cannot reopen until repolarization is complete.

Question 16

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

Accepted Answer

Tropomyosin moves away from actin's myosin-binding sites.

Question 17

A patient presents with muscle weakness and drooping eyelids. Laboratory tests reveal antibodies against ACh receptors. Which process is most directly impaired?

Accepted Answer

Generation of end plate potentials in the muscle fiber

Question 18

Which of the following best describes the role of the Na$$^+/K$^+$$ pump in skeletal muscle fibers?

Accepted Answer

It restores ionic conditions after an action potential by pumping Na$$^+$$ out and K$$^+ in.$$

Question 19

Which of the following is a characteristic of chemically gated ion channels at the neuromuscular junction?

Accepted Answer

They open in response to binding of acetylcholine.

Question 20

Which of the following best explains why ATP is required for both contraction and relaxation of skeletal muscle?

Accepted Answer

ATP is needed for cross bridge detachment and for pumping Ca$$^{2+}$$ back into the SR.

Skeletal Muscle Contraction: Structure, Function, and Mechanisms

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