Block 2 Pace Quiz Skeletal Muscle: Physiology

1. Choose the correct sequence of events during excitation/contraction coupling:

Action potential, calcium release, depolarization of the t-tubules, contraction, calcium reuptake

Action potential, depolarization of the t-tubules, calcium release, contraction, calcium reuptake

Action potential, depolarization of the t-tubules, calcium re-uptake, contraction, calcium release

Action potential, calcium release, contraction, depolarization of the t-tubules, calcium reuptake

During excitation/contraction coupling, the sequence of events is as follows: first, an action potential is generated. This action potential then leads to the depolarization of the t-tubules. Subsequently, calcium is released from the sarcoplasmic reticulum. This calcium release triggers the contraction of the muscle fibers. Finally, after contraction, calcium is reuptaken back into the sarcoplasmic reticulum.

Explanation

During excitation/contraction coupling, the sequence of events is as follows: first, an action potential is generated. This action potential then leads to the depolarization of the t-tubules. Subsequently, calcium is released from the sarcoplasmic reticulum. This calcium release triggers the contraction of the muscle fibers. Finally, after contraction, calcium is reuptaken back into the sarcoplasmic reticulum.

2. Thick filaments are anchored to Z disks by what protein?

Nebulin

C protein

Titin

Alpha-actinin

Myomesin

Titin is a large protein that plays a crucial role in muscle contraction. It is responsible for anchoring thick filaments to the Z disks in muscle cells. This protein provides structural support and stability to the sarcomere, which is the basic unit of muscle contraction. Therefore, titin is the correct answer for the protein that anchors thick filaments to Z disks.

Explanation

Titin is a large protein that plays a crucial role in muscle contraction. It is responsible for anchoring thick filaments to the Z disks in muscle cells. This protein provides structural support and stability to the sarcomere, which is the basic unit of muscle contraction. Therefore, titin is the correct answer for the protein that anchors thick filaments to Z disks.

3. While you are holding a tray piled with dishes, an additional 5-pounds of dishes is placed on it. Your muscles that are hoIding the tray increase their force of contraction through an increase in which of the following?

Length of the muscle.

Number of motor units activated

Strength of each cross-bridge interaction with actin .

Maximum shortening velocity of the muscle

When an additional 5-pounds of dishes is placed on the tray, the muscles holding the tray need to increase their force of contraction in order to support the added weight. This is achieved by activating more motor units. Motor units are made up of a motor neuron and the muscle fibers it innervates. By activating more motor units, more muscle fibers are recruited to contract, leading to an increase in force generation. Therefore, the correct answer is the number of motor units activated.

Explanation

When an additional 5-pounds of dishes is placed on the tray, the muscles holding the tray need to increase their force of contraction in order to support the added weight. This is achieved by activating more motor units. Motor units are made up of a motor neuron and the muscle fibers it innervates. By activating more motor units, more muscle fibers are recruited to contract, leading to an increase in force generation. Therefore, the correct answer is the number of motor units activated.

4. An action potential propagating along the sarcolemma of a skeletal muscle cell initiates a mechanical contraction within the muscle fiber. All of the following steps are essential to initiate this increase in tension within the muscle fiber except which?

Calcium binding to troponin allows myosin heads to bind to actin molecules

Calcium ATPase actively transports Ca 2 + back into the sarcoplasmic reticulum (SR)

Dihydropyridine (DHP) receptors tug on ryanodine receptors (RyR) causing the release of Ca 2 + through the RyR

Cross-bridge cycling

Membrane depolarization causes the DHP receptor to open

The action potential propagating along the sarcolemma of a skeletal muscle cell initiates a mechanical contraction within the muscle fiber by triggering a release of calcium ions from the sarcoplasmic reticulum (SR) into the cytoplasm. This calcium then binds to troponin, allowing myosin heads to bind to actin molecules and initiate cross-bridge cycling. The DHP receptors play a role in triggering the release of calcium from the SR. Membrane depolarization causes the DHP receptor to open, allowing calcium to enter the cell. However, the role of Calcium ATPase is to actively transport calcium back into the SR, which helps to relax the muscle fiber and decrease tension. Therefore, the step of calcium ATPase actively transporting calcium back into the SR is not essential to initiate the increase in tension within the muscle fiber.

Explanation

The action potential propagating along the sarcolemma of a skeletal muscle cell initiates a mechanical contraction within the muscle fiber by triggering a release of calcium ions from the sarcoplasmic reticulum (SR) into the cytoplasm. This calcium then binds to troponin, allowing myosin heads to bind to actin molecules and initiate cross-bridge cycling. The DHP receptors play a role in triggering the release of calcium from the SR. Membrane depolarization causes the DHP receptor to open, allowing calcium to enter the cell. However, the role of Calcium ATPase is to actively transport calcium back into the SR, which helps to relax the muscle fiber and decrease tension. Therefore, the step of calcium ATPase actively transporting calcium back into the SR is not essential to initiate the increase in tension within the muscle fiber.

5. A severe laceration to a wrist completely severed a major muscle tendon . To suture the ends of the tendon together, the surgeon had to overlap the severed ends by 7.5 cm. What change if any in passive and maximum active tension would you expect? PASSIVE MAXIMUM ACTIVE

Decrease Decrease

No change No change

When a muscle tendon is completely severed and the ends are sutured together with an overlap, it can lead to a decrease in maximum active tension. This is because the overlap in the tendon reduces the effective length of the muscle, resulting in a decrease in the maximum force that can be generated by the muscle. Additionally, the severe laceration and injury to the wrist can also impact the muscle's ability to generate tension, leading to a decrease in both passive and maximum active tension.

Explanation

When a muscle tendon is completely severed and the ends are sutured together with an overlap, it can lead to a decrease in maximum active tension. This is because the overlap in the tendon reduces the effective length of the muscle, resulting in a decrease in the maximum force that can be generated by the muscle. Additionally, the severe laceration and injury to the wrist can also impact the muscle's ability to generate tension, leading to a decrease in both passive and maximum active tension.

6. Assuming two skeletal muscle fibers have the same myosin ATPase activity, and both are producing an equal force, but one fiber is longer than the other one . Which of the following best describes their shortening velocities?

Long fiber has a higher velocity during isotonic contraction than the short one

Long fiber has a higher velocity during isometric contraction than the short one

Long fiber has a lower velocity during isotonic contraction than the short one

Long fiber has a lower velocity during isometric contraction than the short one

Both fibers have the same velocity during isotonic contraction

The length of the muscle fiber affects its shortening velocity during isotonic contraction. A longer fiber will have a higher velocity because it has a greater distance to cover in the same amount of time. Therefore, the long fiber will have a higher velocity during isotonic contraction compared to the short fiber.

Explanation

The length of the muscle fiber affects its shortening velocity during isotonic contraction. A longer fiber will have a higher velocity because it has a greater distance to cover in the same amount of time. Therefore, the long fiber will have a higher velocity during isotonic contraction compared to the short fiber.

7. A certain skeletal muscle is at rest. What is the state of the myosin heads within that muscle's myocytes?

High energy, bound to ADP and phosphate

Low energy, bound to ATP

High energy, with an open binding site

Low energy, with an open binding site

High energy, bound to ATP

The myosin heads within the muscle's myocytes are in a high energy state and are bound to ADP and phosphate. This is because during muscle contraction, the myosin heads hydrolyze ATP to ADP and phosphate, releasing energy that allows them to bind to actin and generate force. Therefore, in a resting muscle, the myosin heads are still in a high energy state as they are bound to the products of ATP hydrolysis, ADP, and phosphate.

Explanation

The myosin heads within the muscle's myocytes are in a high energy state and are bound to ADP and phosphate. This is because during muscle contraction, the myosin heads hydrolyze ATP to ADP and phosphate, releasing energy that allows them to bind to actin and generate force. Therefore, in a resting muscle, the myosin heads are still in a high energy state as they are bound to the products of ATP hydrolysis, ADP, and phosphate.

8. Which of the following characteristics is typical of type lIb skeletal muscle but not type I skeletal muscle?

Use of ATP to provide power to myosin heads

Extensive capacity for oxidative breakdown of glucose

Relatively large stores of glycogen within the myocytes

Myosin ATPase that is faster than that of smooth muscle

1. Red: Type I
slow but repetitive, not easily fatigued. Smallest diameter. Rich myoglobin. Largest density of mitochondria, oxidative.

2. White: Type IIB.
fast but easily fatigued, largest diameter, Poor myoglobin, least density of mitochondria, glycolysis (phosphorylases and ATPase).

3. Intermediate: Type IIA
intermediate
All muscle contain different ratios of the three types of fibers.

Explanation

1. Red: Type I
slow but repetitive, not easily fatigued. Smallest diameter. Rich myoglobin. Largest density of mitochondria, oxidative.

2. White: Type IIB.
fast but easily fatigued, largest diameter, Poor myoglobin, least density of mitochondria, glycolysis (phosphorylases and ATPase).

3. Intermediate: Type IIA
intermediate
All muscle contain different ratios of the three types of fibers.

9. Person beginning athletic training begins both weight training and yoga. If his skeletal muscle subsequently gains 20% more sarcomeres in length and doubles its number of sarcomeres in parallel through hypertrophy, what effect will this have on his maximum force generation?

Increases 20%

Increases 100%

Increases 120%

Increases 200%

Increases 240%

When the skeletal muscle gains 20% more sarcomeres in length and doubles its number of sarcomeres in parallel through hypertrophy, it means that both the length and width of the muscle fibers have increased. This results in an overall increase in the cross-sectional area of the muscle. According to the physiological principle known as the size principle, an increase in muscle cross-sectional area leads to an increase in force generation. Since the cross-sectional area has doubled, the maximum force generation will also increase by 100%.

Explanation

When the skeletal muscle gains 20% more sarcomeres in length and doubles its number of sarcomeres in parallel through hypertrophy, it means that both the length and width of the muscle fibers have increased. This results in an overall increase in the cross-sectional area of the muscle. According to the physiological principle known as the size principle, an increase in muscle cross-sectional area leads to an increase in force generation. Since the cross-sectional area has doubled, the maximum force generation will also increase by 100%.

10. Over the physiological (in vivo) range of muscle lengths, tension increases with length because as length increases which of the following occurs?

Thin filaments overlap increasing numbers of cross-bridges.

Thin filaments overlap decreasing numbers of cross-bridges

There is increasing overlap of thin filaments attached to opposing Z-disks.

There is decreasing overlap of thin filaments attached to opposing Z-disks

There is increasing coupling of cross-bridge activation.

As the muscle length increases, there is a decreasing overlap of thin filaments attached to opposing Z-disks. This is because the Z-disks move further apart, causing the sarcomeres to lengthen. As a result, the thin filaments also stretch and slide apart, reducing their overlap with the thick filaments. This decrease in overlap leads to a decrease in the number of cross-bridges formed between the thin and thick filaments, resulting in a decrease in tension.

Explanation

As the muscle length increases, there is a decreasing overlap of thin filaments attached to opposing Z-disks. This is because the Z-disks move further apart, causing the sarcomeres to lengthen. As a result, the thin filaments also stretch and slide apart, reducing their overlap with the thick filaments. This decrease in overlap leads to a decrease in the number of cross-bridges formed between the thin and thick filaments, resulting in a decrease in tension.