Question 1

Which event initiates the release of acetylcholine (ACh) at the neuromuscular junction?

Accepted Answer

Influx of $$Ca^{2+}$$ into the synaptic knob

Question 2

During muscle fiber excitation, what is the threshold membrane potential required to trigger an action potential?

Accepted Answer

$$-65mV$$

Question 3

Which step in the cross-bridge cycle directly requires ATP binding to myosin?

Accepted Answer

Detachment of myosin from actin

Question 4

A patient is unable to produce sufficient ATP. Which phase of the cross-bridge cycle would be most directly affected, leading to muscle rigidity?

Accepted Answer

Detachment of myosin from actin

Question 5

Which energy system is primarily used during a 100-meter sprint lasting about 12 seconds?

Accepted Answer

Direct phosphorylation (creatine phosphate)

Question 6

What is the main reason rigor mortis occurs after death?

Accepted Answer

Calcium remains in the cytosol and ATP is unavailable for myosin detachment

Question 7

During the upward phase of a bicep curl, which contraction modality is occurring?

Accepted Answer

Concentric isotonic

Question 8

Which molecule binds to troponin to initiate the exposure of myosin-binding sites on actin?

Accepted Answer

Calcium ($$Ca^{2+})$$

Question 9

If a muscle is attempting to lift an immovable object and its length does not change, which type of contraction is occurring?

Accepted Answer

Isometric

Question 10

Which process is responsible for the rapid depolarization of the sarcolemma during muscle excitation?

Accepted Answer

Influx of $$Na^{+}$$ ions

Question 11

A muscle cell is exposed to a toxin that blocks voltage-gated calcium channels in the terminal cisternae. Which process will be directly impaired?

Accepted Answer

Calcium binding to troponin

Question 12

Which metabolic pathway produces lactic acid as a byproduct during muscle contraction?

Accepted Answer

Anaerobic respiration

Question 13

During repolarization of the muscle fiber, which ion exits the cell to restore the resting membrane potential?

Accepted Answer

$$K^{+}$$

Question 14

Which of the following best describes the efficiency and duration of aerobic respiration in muscle contraction?

Accepted Answer

High efficiency, hours

Question 15

A muscle fiber is exposed to a drug that prevents ATP hydrolysis. Which step in the cross-bridge cycle will be directly inhibited?

Accepted Answer

Re-energization of myosin head

Question 16

Which sequence correctly represents the order of events in muscle contraction?

Accepted Answer

ACh release → Sarcolemma depolarization → T-tubule propagation → SR calcium release → Troponin/tropomyosin shift → Cross-bridge cycle

Question 17

Which contraction modality is characterized by muscle lengthening while tension remains constant, such as the controlled lowering of a weight?

Accepted Answer

Eccentric isotonic

Question 18

If only one molecule of acetylcholine binds to a receptor on the motor end plate, what is the likely outcome?

Accepted Answer

No channel opening occurs

Question 19

Which of the following best explains why aerobic respiration yields more ATP than anaerobic respiration?

Accepted Answer

Aerobic respiration utilizes $$O_2$$, allowing complete breakdown of glucose

Question 20

A muscle fiber is exposed to a toxin that prevents the rotation of tropomyosin. What is the immediate effect on muscle contraction?

Accepted Answer

Myosin heads cannot bind to actin

Physiology of Muscle Contraction & Energy Systems

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