Section 4

Question 1

Describe what is meant by fatigue as it relates to skeletal muscle.

Question 2

Explain why some muscle is dark and some muscle is pale.

Question 3

Describe the innervation of the different muscle fibre types.

Question 4

Compare and contrast the properties of the different muscle fibre types.

Question 5

What are three ways ATP is important in the contraction-relaxation process?

Answer 5

1. Splitting of ATP to provide energy for power stroke
2. Binding of new ATP to myosin head to release the cross-bridge
3. Active transport of Ca2+ back into SR

Question 6

What is the definition of fatigue in muscle physiology?

Answer 6

In muscle physiology, fatigue is when contractile activity cannot be maintained, and tension in a muscle declines.

Question 7

What are the two major classifications of fatigue, and how do they differ?

Answer 7

The two major classifications of fatigue are central fatigue and muscle fatigue.

Central fatigue is poorly understood and is of psychological origin, characterized by a slowing down or cessation of activity despite unfatigued muscle fibers.

Muscle fatigue, on the other hand, occurs at the level of the muscle and aims to protect muscle cells by reducing contractile activity before ATP supplies are depleted.

Question 8

What are some causes of muscle fatigue?

Answer 8

Muscle fatigue can result from various causes, including local accumulation of ADP and Pi from ATP hydrolysis, accumulation of lactic acid, accumulation of extracellular K+ (which affects membrane potential and excitability), and depletion of glycogen stores in muscles.

Question 9

What is the role of glycogen in muscle physiology, and how can its depletion lead to fatigue?

Answer 9

Glycogen is a form of energy storage in muscles, serving as an immediate reserve source of energy for muscle cells. Depletion of glycogen during extreme exercise can lead to muscle fatigue.

Question 10

How does the accumulation of lactic acid contribute to muscle fatigue?

Answer 10

Accumulation of lactic acid inhibits the enzymes of glycolysis, reducing ATP production and interfering with excitation-contraction coupling, contributing to muscle fatigue.

Question 11

What characterizes central fatigue, and what may be its origin?

Answer 11

Central fatigue occurs when the central nervous system (CNS) decreases its activation of motor neurons. It is not well understood and may have psychological origins, often resulting in a slowing down or cessation of activity, even if the muscle fibers are not fatigued.

Question 12

What is muscle fatigue, and why does it occur at the level of the muscle?

Answer 12

Muscle fatigue occurs at the level of the muscle and is a mechanism to protect the muscle cells. It reduces contractile activity before ATP supplies run out, preventing rigor mortis. Muscle fatigue can occur due to various reasons, such as local accumulation of ADP and Pi, accumulation of lactic acid, or the accumulation of extracellular K+.

Question 13

What are the two broad classifications of muscle fibers?

Answer 13

Muscle fibers can be broadly classified as either slow twitch (type I muscle fibers) or fast twitch (type II muscle fibers).

Question 14

What characterizes the speed of contraction in slow twitch muscle fibers?

Answer 14

Slow twitch muscle fibers contract and relax at slower rates than fast twitch fibers.

Question 15

What type of motor neurons innervate slow twitch muscle fibers, and how do they differ from those innervating fast twitch fibers?

Answer 15

Slow twitch fibers are innervated by α2 motor neurons, which are smaller, have a lower activation threshold, and slower conduction speeds compared to α1 motor neurons. This results in slower twitches in slow twitch fibers.

In addition, the ATPase located at the myosin head is also slow, correlating with a slower rate of cross-bridge cycling.

Question 16

What metabolic properties are associated with slow twitch muscle fibers, and why are they sometimes called “slow oxidative” fibers?

Answer 16

Slow twitch or type I muscle fibers are often called “slow oxidative” because they primarily produce ATP through aerobic processes, indicating their reliance on oxygen for energy production.

Question 17

What characterizes the speed of contraction in fast twitch muscle fibers?

Answer 17

Fast twitch muscle fibers contract and relax at rates much faster than slow twitch fibers.

Question 18

What type of motor neurons innervate fast twitch muscle fibers, and how do they differ from those innervating slow twitch fibers?

Answer 18

Fast twitch fibers are innervated by α1 motor neurons, which are larger, have a higher activation threshold, and faster conduction speeds compared to α2 motor neurons. This results in faster twitches in fast twitch fibers.

Question 19

What are the two types of fast twitch (type II) muscle fibers, and how do they differ in terms of metabolic properties?

Answer 19

Fast twitch (type II) muscle fibers come in two types: Fast oxidative glycolytic (FOG) fibers produce ATP through both aerobic and anaerobic metabolism, while fast glycolytic (FG) fibers primarily produce ATP through anaerobic means.

Question 20

What determines the color of muscle fibers, and how do red and white fibers differ in terms of color and energy production?

Answer 20

The color of muscle fibers is determined by how they produce energy. Red fibers, such as slow oxidative and fast oxidative glycolytic fibers, are highly vascularized, contain many mitochondria, and have myoglobin to support oxygen use, giving them a red color. In contrast, white fibers, like fast glycolytic fibers, rely on anaerobic metabolism, have fewer mitochondria, and lack myoglobin, resulting in a pale color, as seen in dark and white meat in poultry.

Question 21

What is myoglobin?

Answer 21

• An iron and oxygen binding protein found in muscle tissue of most mammals.
• It is the primary oxygen carrying mechanism of muscle tissues.
• Unlike hemoglobin, it is only found in the bloodstream after muscle injury

Question 22

Review slide 7, it has a chart of the overview of muscle fiber properties that is very helpful

Question 23

Explain why different athletes would have muscles with greater proportions of either slow
oxidative fibres or fast glycolytic fibres?

Answer 23

In muscles specialized for maintaining low-intensity contractions for a long time without fatigue, such as in a marathon runner, there would be a higher proportion of slow oxidative fibres. Examples of specific muscles would be the back and legs as they need to support the weight of the body.

In contrast, muscles adapted for performing rapid or forceful movements do fatigue, such as leg
muscle used for sprinting. These muscles would have more fast glycolytic fibres.