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+}$$}) to $$initiate contraction.

Question 2

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

Accepted Answer

The H zone and I band both decrease in width.

Question 3

A patient presents with muscle stiffness several hours after death. Which of the following best explains the physiological basis of rigor mortis?

Accepted Answer

ATP is no longer produced, so $$\mathrm{Ca$$^{2+}$$}$$ cannot be pumped back into the SR, resulting in sustained cross-bridge formation.

Question 4

Which protein directly blocks the myosin-binding sites on actin in a relaxed muscle fiber?

Accepted Answer

Tropomyosin

Question 5

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

Accepted Answer

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

Question 6

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

Accepted Answer

An action potential propagates along the sarcolemma.

Question 7

A muscle fiber is exposed to a single action potential. Which of the following best describes the resulting contraction?

Accepted Answer

A muscle twitch, which is a brief and rapid contraction followed by relaxation.

Question 8

Which of the following statements about the neuromuscular junction (NMJ) is correct?

Accepted Answer

Acetylcholine is released into the synaptic cleft and binds to receptors on the motor end plate.

Question 9

Which of the following best explains why only a fraction of myosin heads are actively pulling at any one time during muscle contraction?

Accepted Answer

To ensure smooth and sustained contraction by allowing some heads to attach while others detach.

Question 10

A motor unit is defined as:

Accepted Answer

A single motor neuron and all the muscle fibers it innervates.

Question 11

Which of the following best describes the molecular event that triggers the power stroke during cross-bridge cycling?

Accepted Answer

Release of inorganic phosphate ($$\mathrm{P_i}) $$from the myosin head.

Question 12

If a drug blocks acetylcholine receptors at the neuromuscular junction, what is the most likely effect on muscle contraction?

Accepted Answer

Muscle contraction will be inhibited because the action potential cannot be generated in the muscle fiber.

Question 13

Which of the following is a direct consequence of calcium ions binding to troponin during muscle contraction?

Accepted Answer

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

Question 14

A patient with a genetic defect in the protein alpha-actinin would most likely have problems with which of the following?

Accepted Answer

Anchoring thin filaments to the Z-line.

Question 15

Which of the following best explains why ATP is required for both the attachment and detachment of myosin heads during cross-bridge cycling?

Accepted Answer

ATP binding to myosin causes detachment from actin, and ATP hydrolysis re-cocks the myosin head for the next cycle.

Question 16

Which of the following changes would you expect in the sarcomere during maximal muscle contraction?

Accepted Answer

The distance between Z-discs decreases.

Question 17

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

Accepted Answer

Failure of the sarcoplasmic reticulum to reclaim $$\mathrm{Ca$$^{2+}$$}.$$

Question 18

Which of the following best describes the 'all-or-none' response in skeletal muscle fibers?

Accepted Answer

Once threshold is reached, the muscle fiber contracts maximally in response to a single action potential.

Question 19

Which of the following is a real-life example of a rapid, brief muscle contraction (twitch) in the human body?

Accepted Answer

A quick blink of the eyelid in response to a sudden bright light.

Question 20

A researcher measures the force generated by a single muscle fiber in response to repeated action potentials at increasing frequency. Which phenomenon is most likely to be observed as the frequency increases?

Accepted Answer

Summation of twitches leading to a stronger, sustained contraction (tetanus).

2D) Skeletal Muscle Fiber Microanatomy and Neuromuscular Junction: Structure and Function

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