Question 1

Which of the following best explains why the resting membrane potential of a skeletal muscle cell is negative?

Accepted Answer

The plasma membrane is more permeable to K+ than to Na+ due to a greater number of K+ leak channels.

Question 2

What is the primary function of acetylcholinesterase at the neuromuscular junction?

Accepted Answer

It breaks down acetylcholine in the synaptic cleft, terminating its action on the muscle fiber.

Question 3

During the depolarization phase of a muscle fiber action potential, which ion primarily moves and in which direction?

Accepted Answer

Na+ moves into the cell through voltage-gated Na+ channels.

Question 4

Which of the following events directly triggers the release of Ca2+ from the terminal cisterns of the sarcoplasmic reticulum during excitation-contraction coupling?

Accepted Answer

A conformational change in voltage-sensitive tubule proteins in the T-tubules.

Question 5

What would happen to the cross bridge cycle if ATP were suddenly unavailable in a living muscle fiber?

Accepted Answer

Myosin heads would remain attached to actin, preventing muscle relaxation.

Question 6

Which of the following best describes the role of Ca2+ in skeletal muscle contraction?

Accepted Answer

Ca2+ binds to troponin, causing tropomyosin to move and expose myosin binding sites on actin.

Question 7

A patient is exposed to a toxin that blocks voltage-gated Ca2+ channels in the axon terminal of motor neurons. What is the most likely effect on skeletal muscle contraction?

Accepted Answer

Acetylcholine will not be released, so muscle fibers will not be excited.

Question 8

Which of the following correctly lists the sequence of events in excitation-contraction coupling?

Accepted Answer

Action potential travels along sarcolemma → enters T-tubules → triggers Ca2+ release → Ca2+ binds to troponin → cross bridge formation

Question 9

Which of the following best explains why K+ ions tend to move out of the cell at rest?

Accepted Answer

The concentration of K+ is higher inside the cell than outside, and the membrane is more permeable to K+.

Question 10

Which of the following is NOT a step in the cross bridge cycle?

Accepted Answer

Ca2+ is pumped back into the sarcoplasmic reticulum.

Question 11

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

Accepted Answer

Failure of Ca2+ pumps in the sarcoplasmic reticulum to remove Ca2+ from the cytosol.

Question 12

Which equation best represents the Nernst equation for calculating the equilibrium potential for K+ ($$E_K) in a $$skeletal muscle cell?

Accepted Answer

$$E_K = \frac{RT}{zF} \ln \left( \frac{[K^+]_{	ext{outside}}}{[K^+]_{	ext{inside}}} ight)$$

Question 13

Which of the following best describes the physiological basis of rigor mortis?

Accepted Answer

ATP is depleted after death, preventing myosin heads from detaching from actin.

Question 14

A drug that blocks acetylcholine receptors at the neuromuscular junction would have which effect?

Accepted Answer

It would prevent muscle fiber excitation and contraction.

Question 15

Which of the following is a direct result of the power stroke in the cross bridge cycle?

Accepted Answer

The actin filament is pulled toward the center of the sarcomere.

Question 16

Which of the following best describes the function of T-tubules in skeletal muscle fibers?

Accepted Answer

They conduct action potentials from the sarcolemma deep into the muscle fiber.

Question 17

Which of the following is required for the return of tropomyosin to its blocking position on actin after contraction?

Accepted Answer

Active transport of Ca2+ back into the sarcoplasmic reticulum by Ca2+ pumps.

Skeletal Muscle Physiology: Membrane Potentials, Neuromuscular Junction, and Muscle Contraction

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