Q: Which of the following best explains why the Sodium-Potassium Pump must transport $$3\ Na^+$$ out of the muscle cell for every $$2\ K^+ it $$brings in?

To preserve the negative resting membrane potential ($$-90\text{ mV}) by $$creating an electrochemical gradient.

Question 1

Which of the following best explains why the Sodium-Potassium Pump must transport $$3\ Na^+$$ out of the muscle cell for every $$2\ K^+ it $$brings in?

Accepted Answer

To preserve the negative resting membrane potential ($$-90	ext{ mV}) by $$creating an electrochemical gradient.

Question 2

A patient is exposed to a toxin that blocks voltage-gated calcium channels at the neuromuscular junction. Which step in muscle contraction would be directly impaired?

Accepted Answer

Release of acetylcholine (ACh) into the synaptic cleft.

Question 3

During the cross-bridge cycle, what is the immediate consequence if ATP is not available in the muscle cell?

Accepted Answer

Myosin heads remain bound to actin, resulting in muscle stiffness (rigor mortis).

Question 4

Which of the following best describes the sequence of events leading from an action potential at the motor neuron to muscle contraction?

Accepted Answer

Action potential → Calcium influx at synaptic knob → ACh release → Sodium influx at motor end plate → T-tubule activation → SR calcium release → Troponin binding → Tropomyosin shift → Cross-bridge cycle

Question 5

If a muscle fiber's sarcolemma is experimentally held at $$-70	ext{ mV}$$, which of the following is most likely to occur?

Accepted Answer

The cell will remain below threshold and not generate an action potential.

Question 6

Which energy pathway provides the greatest amount of ATP per glucose molecule during prolonged muscle activity?

Accepted Answer

Aerobic respiration.

Question 7

A muscle is contracting but not changing in length while holding a heavy object stationary. What type of contraction is this?

Accepted Answer

Isometric contraction.

Question 8

Why is ATP required for both contraction and relaxation of skeletal muscle?

Accepted Answer

ATP is required to pump calcium back into the sarcoplasmic reticulum and to detach myosin heads from actin.

Question 9

Which of the following best explains how the body increases the force of muscle contraction to lift heavier objects?

Accepted Answer

By recruiting more motor units to participate in the contraction.

Question 10

A muscle fiber is exposed to a drug that prevents the hydrolysis of ATP by myosin. What is the most immediate effect on the cross-bridge cycle?

Accepted Answer

Myosin heads cannot be re-cocked into the energized position.

Question 11

Which of the following events directly triggers the release of calcium from the sarcoplasmic reticulum during muscle contraction?

Accepted Answer

Depolarization of the T-tubule membrane.

Question 12

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

Accepted Answer

Troponin binds to calcium, causing a conformational change that moves tropomyosin away from actin's binding sites.

Question 13

Which of the following best explains why lactic acid accumulates in muscle during intense exercise?

Accepted Answer

Anaerobic glycolysis produces lactic acid when oxygen is insufficient for aerobic respiration.

Question 14

A muscle fiber is unable to return to its resting membrane potential after contraction. Which process is most likely impaired?

Accepted Answer

Function of the sodium-potassium pump.

Question 15

Which of the following is the correct formula for the sodium-potassium pump cycle?

Accepted Answer

$$3\ Na^+_{	ext{in}} + 2\ K^+_{	ext{out}} + ATP ightarrow 3\ Na^+_{	ext{out}} + 2\ K^+_{	ext{in}} + ADP + P_i$$

Question 16

Which of the following best describes the difference between isotonic concentric and isotonic eccentric contractions?

Accepted Answer

Concentric contractions shorten the muscle, while eccentric contractions lengthen the muscle under tension.

Question 17

A person is running a marathon and relies on the most efficient ATP production pathway. Which pathway is primarily being used, and how many ATP are produced per glucose molecule?

Accepted Answer

Aerobic respiration; $$30$$–$$36$$ ATP per glucose.

Question 18

Which of the following is the best explanation for why two molecules of acetylcholine (ACh) must bind to each receptor at the motor end plate?

Accepted Answer

To open the chemically gated ion channel and allow sodium influx, generating the end-plate potential.

Question 19

Which of the following best describes depolarization in a muscle cell?

Accepted Answer

A shift in membrane potential from $$-90	ext{ mV}$$ toward $$+75	ext{ mV}$$ due to sodium influx.

Question 20

Which of the following best explains the phenomenon of rigor mortis?

Accepted Answer

Calcium leaks into the cytosol and ATP is depleted, preventing myosin from detaching from actin.

Mechanics of Muscle Contraction: From Electrical Stimulation to Mechanical Movement

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