Q: What is the correct sequence of events at the neuromuscular junction leading to muscle fiber excitation?

Action potential arrives at axon terminal, voltage-gated Ca\textsuperscript{2+} channels open, Ca\textsuperscript{2+} enters axon terminal, ACh is released, ACh diffuses across synaptic cleft and binds to receptors, Na\textsuperscript{+} enters muscle fiber.

Question 1

Which of the following best describes the role of chemical-gated ion channels at the neuromuscular junction?

Accepted Answer

They open when neurotransmitters such as acetylcholine bind to their receptors.

Question 2

What is the correct sequence of events at the neuromuscular junction leading to muscle fiber excitation?

Accepted Answer

Action potential arrives at axon terminal, voltage-gated Ca	extsuperscript{2+} channels open, Ca	extsuperscript{2+} enters axon terminal, ACh is released, ACh diffuses across synaptic cleft and binds to receptors, Na	extsuperscript{+} enters muscle fiber.

Question 3

During the generation of an action potential across the sarcolemma, what is the primary effect of Na	extsuperscript{+} influx?

Accepted Answer

It causes the interior of the sarcolemma to become more positive, leading to depolarization.

Question 4

Which protein does Ca	extsuperscript{2+} bind to during excitation-contraction coupling to initiate muscle contraction?

Accepted Answer

Troponin

Question 5

What is the function of tropomyosin in a relaxed muscle fiber?

Accepted Answer

It blocks the active sites on actin, preventing myosin attachment.

Question 6

Which step in the cross bridge cycle directly requires ATP?

Accepted Answer

Cross bridge detachment

Question 7

A patient is unable to relax their muscles after death due to lack of ATP. What is the physiological explanation for this phenomenon?

Accepted Answer

ATP is required for detachment of myosin heads from actin during cross bridge cycling.

Question 8

Which type of muscle contraction occurs when muscle tension increases but the muscle does not change length?

Accepted Answer

Isometric contraction

Question 9

What is the definition of a motor unit?

Accepted Answer

A motor neuron and all the muscle fibers it innervates.

Question 10

Which phase of a muscle twitch corresponds to the time when cross bridges are forming and tension increases?

Accepted Answer

Period of contraction

Question 11

What is wave (temporal) summation in muscle contraction?

Accepted Answer

Multiple stimuli are received in rapid succession, and muscle fibers do not have time to relax between stimuli.

Question 12

Which of the following best describes fused (complete) tetanus?

Accepted Answer

Muscle tension reaches a plateau and contractions fuse into one smooth, sustained contraction.

Question 13

Which factor does NOT directly affect the force of muscle contraction?

Accepted Answer

Type of neurotransmitter released at the neuromuscular junction

Question 14

Which type of isotonic contraction occurs when a muscle shortens and does work, such as lifting a book?

Accepted Answer

Concentric contraction

Question 15

If a muscle fiber is stimulated at a frequency that does not allow complete relaxation between stimuli, what is the result?

Accepted Answer

Wave (temporal) summation

Question 16

Which of the following is true regarding the relationship between muscle fiber size and force of contraction?

Accepted Answer

Larger fibers generate more force due to increased cross-sectional area.

Question 17

What is the effect of increasing the frequency of stimulation on muscle contraction?

Accepted Answer

It increases muscle tension due to summation of contractions.

Question 18

Which metabolic pathway is primarily used for ATP synthesis during prolonged, low-intensity muscle activity?

Accepted Answer

Aerobic respiration

Question 19

During the cocking of the myosin head in cross bridge cycling, which molecule is hydrolyzed?

Accepted Answer

ATP

Question 20

Which of the following best explains why muscle tension declines to zero during the period of relaxation in a muscle twitch?

Accepted Answer

Ca	extsuperscript{2+} is pumped back into the sarcoplasmic reticulum, ending cross bridge formation.

Muscle Fiber Contraction and Whole Muscle Physiology

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