Question 1

Which of the following best describes the sequence of events that occurs at the neuromuscular junction (NMJ) leading to muscle contraction?

Accepted Answer

Acetylcholine is released from the somatic motor neuron, binds to receptors on the sarcolemma, causing an action potential that leads to calcium release from the sarcoplasmic reticulum.

Question 2

What is the primary role of the DHP (dihydropyridine) receptor in excitation-contraction coupling?

Accepted Answer

It acts as a voltage sensor in the t-tubule, triggering the opening of ryanodine receptors on the sarcoplasmic reticulum.

Question 3

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

Accepted Answer

Activation of the DHP receptor, which then activates the ryanodine receptor

Question 4

During muscle relaxation, which process is responsible for lowering cytosolic calcium concentration?

Accepted Answer

Calcium is actively transported back into the sarcoplasmic reticulum by the SERCA pump using ATP.

Question 5

A patient is given a drug that blocks the ryanodine receptor. What is the most likely effect on skeletal muscle contraction?

Accepted Answer

Decreased calcium release from the sarcoplasmic reticulum, leading to reduced or absent muscle contraction

Question 6

Which of the following best defines the latent period in excitation-contraction coupling?

Accepted Answer

The time between the muscle action potential and the beginning of muscle tension development

Question 7

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

Accepted Answer

Acetylcholine release → muscle action potential → DHP receptor activation → ryanodine receptor opening → calcium release → troponin binding

Question 8

Which of the following best explains why calcium channel blockers (such as DHP antagonists) are used clinically to lower blood pressure?

Accepted Answer

They inhibit calcium entry into smooth muscle cells, reducing contraction and causing vasodilation.

Question 9

A mutation causes the SERCA pump to become nonfunctional. What would be the most immediate effect on muscle physiology?

Accepted Answer

Muscle fibers would be unable to relax properly due to sustained high cytosolic calcium levels.

Question 10

Which of the following statements about the role of calcium in muscle contraction is correct?

Accepted Answer

Calcium binds to troponin, causing a conformational change that moves tropomyosin and exposes myosin-binding sites on actin.

Question 11

Which of the following best describes the function of the ryanodine receptor in skeletal muscle?

Accepted Answer

It is a calcium release channel on the sarcoplasmic reticulum activated by the DHP receptor.

Question 12

A muscle twitch is defined as:

Accepted Answer

A single contraction-relaxation cycle in a muscle fiber.

Question 13

Which of the following would most likely occur if acetylcholinesterase activity was inhibited at the neuromuscular junction?

Accepted Answer

Prolonged muscle contraction due to continued presence of acetylcholine in the synaptic cleft.

Question 14

Which of the following best explains why the latent period exists during excitation-contraction coupling?

Accepted Answer

It is the time required for the action potential to trigger calcium release and for calcium to bind to troponin.

Question 15

Which of the following is a direct effect of calcium binding to troponin during muscle contraction?

Accepted Answer

Tropomyosin moves, exposing myosin-binding sites on actin.

Question 16

Which of the following best describes the clinical use of ryanodine as an insecticide?

Accepted Answer

It blocks the ryanodine receptor, preventing calcium release and causing paralysis in insects.

Question 17

Which of the following would most likely occur if the DHP receptor was nonfunctional?

Accepted Answer

The ryanodine receptor would not open, preventing calcium release from the sarcoplasmic reticulum.

Question 18

Which of the following equations best represents the ATP-dependent process responsible for muscle relaxation?

Accepted Answer

$$ \mathrm{Ca$$^{2+}$$ + ATP \xrightarrow{SERCA} Ca$$^{2+}$$_{SR} + ADP + P_i} $$

Excitation-Contraction Coupling in Skeletal Muscle

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