Question 1

Which structure in a skeletal muscle fiber is responsible for storing and releasing calcium ions ($$Ca^{2+}) $$during muscle contraction?

Accepted Answer

Sarcoplasmic reticulum

Question 2

What is the primary function of the neuromuscular junction (NMJ)?

Accepted Answer

To transmit action potentials from the motor neuron to the muscle fiber

Question 3

During the sliding filament model of contraction, which event directly causes the shortening of the sarcomere?

Accepted Answer

Thin filaments slide past thick filaments

Question 4

Which protein blocks the myosin binding sites on actin when intracellular $$[Ca$$^{2+}$$] is $$low?

Accepted Answer

Tropomyosin

Question 5

A patient presents with muscle stiffness several hours after death. What is the physiological cause of rigor mortis?

Accepted Answer

Failure to synthesize ATP, leading to inability to pump $$Ca^{2+}$$ back into the SR

Question 6

Which of the following best describes a motor unit?

Accepted Answer

A motor neuron and all the muscle fibers it innervates

Question 7

What is the role of acetylcholine at the neuromuscular junction?

Accepted Answer

It depolarizes the motor end plate, leading to an action potential in the muscle fiber

Question 8

Which event marks the beginning of excitation-contraction coupling in skeletal muscle?

Accepted Answer

Propagation of the action potential along the sarcolemma

Question 9

If a muscle fiber is stimulated by a single action potential, what is the resulting contraction called?

Accepted Answer

Muscle twitch

Question 10

Which of the following statements about cross bridge cycling is correct?

Accepted Answer

ATP is required for both attachment and detachment of myosin heads from actin

Question 11

A muscle fiber is unable to contract because the myosin binding sites on actin are blocked. Which molecule is most likely responsible for this inhibition?

Accepted Answer

Tropomyosin

Question 12

Which of the following best explains why muscle fibers have a system of T-tubules?

Accepted Answer

To rapidly transmit action potentials deep into the muscle fiber

Question 13

During muscle contraction, what happens to the H zone within the sarcomere?

Accepted Answer

It disappears

Question 14

Which of the following is true regarding the all-or-none response of a muscle fiber?

Accepted Answer

Once threshold is reached, the action potential is unstoppable and the muscle fiber contracts fully

Question 15

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

Accepted Answer

Troponin binds $$Ca^{2+}$$, causing a conformational change that moves tropomyosin away from myosin binding sites on actin

Question 16

A muscle fiber is observed to contract slowly and hold its contraction for a longer period. Which type of muscle is most likely being described?

Accepted Answer

Large, fleshy muscles such as the calf muscles

Question 17

Which of the following is a direct consequence of increased intracellular $$[Ca$$^{2+}$$] in a $$muscle fiber?

Accepted Answer

$$Ca^{2+}$ binds to troponin, leading to exposure of myosin binding sites on actin

Question 18

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

Accepted Answer

To prevent muscle fatigue and allow continuous contraction

Question 19

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

Accepted Answer

Action potential propagates along sarcolemma → $$Ca^{2+}$$ released from SR → $$Ca^{2+}$$ binds to troponin → Tropomyosin moves → Myosin binds to actin

Question 20

A researcher is studying the effect of a drug that inhibits the reuptake of $$Ca^{2+}$$ into the sarcoplasmic reticulum. What is the most likely effect on muscle contraction?

Accepted Answer

Muscle relaxation will be prolonged due to sustained high $$[Ca$$^{2+}$$]$$

Skeletal Muscle Fiber Microanatomy and Neuromuscular Junction: Structure and Function

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