Question 1

Which best describes a "myofibril"?

Accepted Answer

A myofibril is a cylindrical cellular organelle that is composed of myofilaments, specifically actin and myosin. It is responsible for the contraction of skeletal muscle cells.

Question 2

During an isometric twitch in a skeletal muscle:

Accepted Answer

During an isometric twitch, tension is generated by the muscle but the load on the muscle remains constant, resulting in no significant shortening of the sarcomeres. Therefore, choice C is the best answer. Choices A, B, D, and E are all incorrect as they describe events that do not occur during an isometric twitch.

Question 3

What protein is the principal component of skeletal muscle thick filaments?

Accepted Answer

Myosin is the principal component of skeletal muscle thick filaments, while actin is the principal component of the thin filaments. Troponin and tropomyosin are regulatory proteins that are involved in muscle contraction, and calmodulin is a calcium-binding protein involved in a variety of cellular processes.

Question 4

Which best describes "rigor mortis"?

Accepted Answer

Rigor mortis is the stiffening of muscles after death due to the lack of ATP to remove myosin cross-bridges from actin, causing continued contraction.

Question 5

An action potential in the motor end plate rapidly spreads to the interior regions of a muscle cell by means of the:

Accepted Answer

The transverse tubules (T-tubules) are invaginations of the sarcolemma that penetrate deep into the muscle fiber, allowing the action potential to quickly spread throughout the entire cell. The Z lines, sarcoplasmic reticulum, H zone, and pores in the plasma membrane do not play a direct role in conducting the action potential throughout the muscle cell.

Question 6

Which of these occurs during an isometric contraction of a skeletal muscle?

Accepted Answer

Isometric contractions occur when the tension generated by the muscle remains constant, while the length of the sarcomere does not change significantly. Options A, B, and D describe the changes that occur during isotonic contractions, where the muscle length changes. Option E is incorrect as it describes a state of muscle contraction after death and not a normal physiological process.

Question 7

Which occurs FIRST as a result of ATP binding to myosin?

Accepted Answer

ATP binding to myosin causes the dissociation of actin from myosin, which is the first step in the cycle of muscle contraction.

Question 8

What is the definition of a "motor unit"?

Accepted Answer

A motor unit consists of a single motor neuron and all the muscle fibers it innervates. When the motor neuron sends a signal, all the muscle fibers in the motor unit contract together.

Question 9

Which of the following statements regarding skeletal muscle contraction is true?

Accepted Answer

During every muscle contraction, tension is developed in the muscle. While statements A and B may be partially true in certain circumstances, they are not always true for all types of muscle contractions. Statement C is false, as some muscle contractions may not result in a visible change in muscle length (such as isometric contractions). Statement E is also false, as cross-bridge cycling is slower during isometric contractions.

Question 10

Binding of ________ to myosin permits cross-bridge _____________ between actin and myosin in skeletal muscle cells.

Accepted Answer

Binding of ATP to myosin is necessary for the detachment of the myosin head from actin, which is a crucial step in the muscle contraction cycle. This allows the cycle of cross-bridge formation and breakdown to continue, enabling muscle contraction and relaxation.

Question 11

During concentric isotonic contraction of a skeletal muscle fiber, which of these occurs?

Accepted Answer

During concentric isotonic contraction, the force generated by the muscle fiber is greater than the opposing load, causing the I bands to shorten as the actin filaments slide over the myosin filaments towards the center of each sarcomere. This shortening of the I bands results in a shorter overall length of the muscle fiber. The A bands, which contain the myosin filaments, do not change in length during a concentric contraction. The thin filaments, specifically the actin filaments, do not shorten but move towards the M line in the center of the sarcomere, resulting in the shortening of the I bands.

Question 12

The removal of calcium ions from the cytosol of skeletal muscle causes

Accepted Answer

The removal of calcium ions causes tropomyosin to cover the myosin binding sites on actin, preventing muscle contraction.

Question 13

How does the chemical curare affect skeletal muscle function?

Accepted Answer

Curare is a nicotinic acetylcholine antagonist that binds to the nicotinic acetylcholine receptors on the motor end plate, preventing acetylcholine from binding to the receptors and thus blocking synaptic transmission at the neuromuscular junction. This results in paralysis of the skeletal muscles.

Question 14

Myasthenia gravis is an autoimmune disease in which the immune system gradually destroys the receptors for acetylcholine at the neuromuscular junction. Which of the following drugs might initially be useful in treating the symptoms of this disease?

Accepted Answer

A drug that inhibits acetylcholinesterase would be useful in treating the symptoms of myasthenia gravis because it would increase the amount of acetylcholine available to bind to the remaining receptors at the neuromuscular junction. This would improve muscle strength and reduce symptoms of weakness.

Question 15

Which of the following statements regarding the motor end plate of a skeletal muscle fiber is TRUE?

Accepted Answer

Acetylcholinesterase, an enzyme in the end plate membrane, breaks down acetylcholine in the synaptic cleft, terminating the signal transmission between the motor neuron and muscle fiber. Muscarinic receptors are found in other parts of the body and not in the motor end plate. Temporal summation occurs at the level of the muscle cell membrane and not specifically in the motor end plate. Sympathetic nerve fibers do not terminate on the motor end plate, but instead release norepinephrine to modulate muscle contraction. Voltage-gated Na+ and K+ channels are found in the muscle cell membrane but not specifically in the motor end plate.

Question 16

In skeletal muscle cells, calcium initiates contraction by binding to:

Accepted Answer

Calcium binds to troponin, which causes tropomyosin to move out of the way, exposing the binding sites on actin. Myosin then binds to actin and initiates contraction.

Question 17

Rigor mortis is caused by:

Accepted Answer

Rigor mortis is caused by the lack of ATP, which is required for muscle relaxation. Without ATP, the muscle fibers are unable to detach from each other and remain contracted, leading to stiffness.

Question 18

Which of the following statements regarding contraction of a skeletal muscle fiber is TRUE?

Accepted Answer

Contracting muscle fibers can generate tension while remaining the same length or even lengthening, depending on the type of contraction (isometric, concentric, or eccentric). This means that muscle fibers do not always shorten when they contract.

Question 19

What is the function of the transverse tubules in a skeletal-muscle fiber?

Accepted Answer

The transverse tubules (T-tubules) are invaginations of the muscle cell membrane that allow action potentials to rapidly propagate deep into the center of skeletal muscle fibers. This allows for a coordinated contraction of the entire muscle fiber.

Question 20

Which of the following statements is TRUE regarding skeletal-muscle contraction?

Accepted Answer

The binding of myosin to actin occurs when the concentration of [Ca²⁺] increases in the cytosol. This initiates the cross-bridge cycle and leads to muscle contraction. Option A is incorrect because multiple cross-bridge cycles can occur as long as [Ca²⁺] remains elevated. Option B is incorrect because ATP hydrolysis products do not need to be removed from myosin before it can bind to actin. Option D is incorrect because a twitch lasts longer than the action potential that causes it. Option E is incorrect because the powerstroke of the cross-bridge cycle occurs after ATP is hydrolyzed into ADP and P_i.

Quiz 9: Muscle