Skeletal Muscle and Fatigue Overview

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Questions and Answers

What does the length-tension relationship in muscle fibers indicate about muscle contraction?

Muscle tension is only dependent on the number of muscle fibers activated.

Muscle fibers contract with maximal force regardless of length.

Muscle tension decreases with increased muscle length.

Muscle tension is affected by the sarcomere length before contraction. (correct)

According to the sliding filament theory, what is critical for generating muscle tension?

The energy source used by the muscle fibers.

The number of motor units recruited.

The number of crossbridges formed between thick and thin filaments. (correct)

The speed of muscle contraction.

What happens to muscle tension when there is too much overlap or too little overlap between thick and thin filaments?

Muscle tension can become unstable.

Muscle tension remains unchanged.

Muscle tension decreases. (correct)

Muscle tension increases significantly.

What range of sarcomere length is considered optimal for muscle contraction?

2 to 2.4 μm Signup and view all the answers

Which of the following factors does NOT affect muscle tension?

The environmental temperature. Signup and view all the answers

What is the primary definition of muscle fatigue?

The muscle's inability to generate or sustain force of contraction Signup and view all the answers

Which of the following factors does NOT affect muscle fatigue?

Dietary intake of minerals Signup and view all the answers

What type of fatigue usually precedes peripheral fatigue?

Central fatigue Signup and view all the answers

Which of the following is a proposed mechanism for central fatigue?

Psychological effects Signup and view all the answers

Which statement about muscle fatigue is true?

Muscle fatigue can be both centrally and peripherally related. Signup and view all the answers

What primarily generates ATP in slow-twitch oxidative fibers (Type I)?

Aerobic cellular respiration Signup and view all the answers

Which muscle fiber type is known for its capacity for sustained contraction and endurance activities?

Slow-twitch oxidative fibers (Type I) Signup and view all the answers

Which type of muscle fiber has the largest fiber diameter and is adapted for intense anaerobic glycolytic ATP generation?

Fast-twitch glycolytic fibers (Type IIb) Signup and view all the answers

What distinguishes fast-twitch oxidative-glycolytic fibers (Type IIa) from slow-twitch fibers (Type I)?

Type IIa fibers fatigue more quickly Signup and view all the answers

What is the primary function of fast-twitch glycolytic fibers (Type IIb)?

Rapid, intense movements of short duration Signup and view all the answers

Which attribute is a characteristic of slow-twitch oxidative fibers (Type I)?

Dark red color due to high myoglobin Signup and view all the answers

How does ATPase activity differ between type I and type IIb muscle fibers?

Type IIb has the fastest ATPase activity Signup and view all the answers

Which muscle fiber type is most capable of undergoing sustained contraction?

Type I fibers Signup and view all the answers

What primarily influences the force of muscle fiber contraction?

Rate of nerve impulse frequencies Signup and view all the answers

What is the refractory period in muscle fibers?

The time during which the muscle is unable to contract at all Signup and view all the answers

What is the phenomenon called when a second stimulus leads to a stronger contraction after the refractory period?

Wave summation Signup and view all the answers

At what rate of stimulation does unfused or incomplete tetanus occur?

20 to 30 times per second Signup and view all the answers

What action describes 'complete tetanus' in muscle fibers?

The muscle does not relax at all and maintains steady tension Signup and view all the answers

What comprises a motor unit?

A motor neuron and all the muscle fibers it innervates Signup and view all the answers

What happens to muscle tension during fatigue?

It decreases even with ongoing stimuli Signup and view all the answers

Why are not all motor units of a muscle contracted at the same time?

To delay muscle fatigue and sustain contraction Signup and view all the answers

Which type of muscle fibers is typically resistant to fatigue?

Slow-twitch oxidative fibers Signup and view all the answers

What is the maximum magnitude of tension that can be developed by a muscle called?

Peak tension Signup and view all the answers

What factor determines the classification of skeletal muscle fibers?

The speed of contraction and resistance to fatigue Signup and view all the answers

What results from applying two stimuli very close in time to a muscle fiber?

A single contraction without response to the second stimulus Signup and view all the answers

Which type of muscle fiber is primarily used by marathon runners?

Slow-twitch oxidative fibers Signup and view all the answers

How does muscle plasticity affect skeletal muscle fibers?

It modifies speed of contraction and resistance to fatigue with training Signup and view all the answers

What term describes the rate at which tension develops in muscle fibers?

Force slope Signup and view all the answers

Which type of muscle fiber is most suitable for producing fine and precise movements?

Smaller motor units Signup and view all the answers

In muscle recruitment for specific tasks, which type of motor unit is preferentially recruited?

Smaller motor units for specific motor tasks Signup and view all the answers

What is primarily indicated when central fatigue occurs during maximal exertion?

Failure of neural connections Signup and view all the answers

What mechanism is often associated with muscle fatigue?

Neuromuscular junction failure Signup and view all the answers

During muscle contraction, which of the following is most directly responsible for muscle relaxation?

Decrease in Ca2+ signal Signup and view all the answers

If a muscle cannot contract despite strong stimulation, which phase is this likely to occur in?

Absolute refractory period Signup and view all the answers

What primarily triggers the protective reflex associated with muscle fatigue?

Increase in acid production Signup and view all the answers

Central fatigue may be related to which of the following factors?

Depletion of neurotransmitters Signup and view all the answers

Which conclusion can be drawn about muscle fatigue mechanisms?

They may involve psychological effects Signup and view all the answers

Which statement best describes the role of the Ca2+ signal in muscle contraction?

Duration of the signal affects contraction strength Signup and view all the answers

Study Notes

Skeletal Muscle Fatigue: Motor Unit and Muscle Fatigue

The lecture covers skeletal muscle, motor units, and muscle fatigue.
Objectives include understanding length-tension relationships, factors affecting muscle tension control, skeletal muscle fiber types, and causes of muscle fatigue.

Length-Tension Relationships for Muscle Fibers

Tension developed during a muscle twitch is directly related to the length of individual sarcomeres before contraction.
Sarcomere length is related to the overlap between thick and thin filaments.
Sliding filament theory: Tension generated is directly proportional to the number of crossbridges formed between thick and thin filaments during contraction.
Optimal length (2 to 2.4 µm) for maximum tension to be generated. Decreased or increased from optimal length leads to decreased tension.

Control of Muscle Tension

Action potentials are always the same size within a given neuron or muscle fiber.
Force/Tension generated varies depending on: rate (frequency) of nerve impulses at the neuromuscular junction, amount of stretch (passive tension) before contraction, energy substrates, and oxygen availability, and the number of muscle fibers contracting in unison.
Twitch: contraction-relaxation from a single stimulation (single action potential)
- Lag phase, Contraction phase, Relaxation phase.
Wave summation: if a second stimulus occurs after the refractory period and before the muscle fiber has fully relaxed, the second contraction will be stronger than the first.
Unfused (incomplete) tetanus: Stimulation at a rate of 20-30 times per second leads to a sustained but wavering contraction.
Complete tetanus: Stimulation at a rate of 80-1000 times per second results in a sustained contraction without relaxation.

Control of Muscle Tension (Motor Unit Recruitment)

A motor unit comprises a motor neuron and all the muscle fibers it innervates.
A muscle can contain numerous motor units.
Motor units fire signals (action potential), and all muscle fibers within that unit contract.
The number of muscle fibers in a motor unit varies.
Muscle fibers within a single unit are of the same type.
Typically, not all motor units in a muscle contract simultaneously, aiding in delay of muscle fatigue and enabling for sustained contractions.

Diversity of Skeletal Muscle Fibers

Skeletal muscle fibers are broadly classified into three types: slow-twitch oxidative, fast-twitch oxidative-glycolytic, and fast-twitch glycolytic.
Fiber classification is based on contraction speed and resistance to fatigue.
Different fiber types are needed for varying demands: strength, speed, and fatigability.

Slow-twitch Oxidative Fibers (Type I)

Smallest in diameter, least powerful.
High myoglobin content (dark red color).
High density of blood capillaries and mitochondria.
ATP generation primarily via aerobic cellular respiration.
Low glycogen content.
Relatively slow ATPase activity.
Fatigue-resistant; capable of sustained contractions.
Primarily involved in maintaining posture and endurance activities.

Fast-twitch Oxidative-Glycolytic Fibers (Type IIa)

Intermediate diameter; moderate velocity of muscle shortening.
Moderate myoglobin content (pink-red color).
Good capillary supply and significant mitochondria.
ATP generation through aerobic glycolysis (converts glucose to pyruvate).
Moderate glycogen content.
Moderate to high fatigue resistance.
Primarily involved in activities like walking and sprinting.

Fast-twitch Glycolytic Fibers (Type IIb)

Largest diameter and contain most myofibrils; fastest velocity of muscle shortening.
Low myoglobin content (pale white color).
Few capillaries and mitochondria.
ATP generation primarily via glycolysis.
Highest glycogen content.
Rapid ATPase activity.
Adapted for intense anaerobic glycolytic activity, quick contractions.
Fatigue quickly
Used for short duration activities like weightlifting

Muscle Fatigue

Defined as the inability of a muscle to generate or maintain a force of contraction.
Factors contributing to muscle fatigue include: intensity of contractile activity, duration of contractile activity, aerobic vs. anaerobic cellular metabolism, muscle fiber composition, fitness level, psychological effects.
Central fatigue: often precedes peripheral fatigue, potentially protective reflex to prevent tissue injury, could be due to neural connection failure or the depleting of neurotransmitters.
Peripheral fatigue: possible causes include increased acid production.

Sample Questions & Answers (from slides)

Question 1: What happens to sarcomere length during muscle contraction?

Answer: Sarcomere length decreases progressively.

Question 2: What is the time interval after a stimulus where the muscle cannot contract, regardless of stimulus strength?

Answer: Absolute refractory period.

Question 3: When does a muscle group generate maximum tension?

Answer: During complete tetanus

Question 4: What is a group of muscle fibers activated by the same motor neuron called?

Answer: Motor unit

Question 5: Which muscle fiber type is most susceptible to fatigue?

Answer: Type IIb.

Question 6: What results when skeletal muscle fibres are stimulated at 20-30 times per second?

Answer: A sustained but wavering contraction (unfused tetanus).

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Description

Explore the fundamentals of skeletal muscle, motor units, and the mechanisms behind muscle fatigue. This quiz covers essential topics including length-tension relationships and various factors affecting muscle tension control. Understand the intricacies of muscle fiber types and how they contribute to fatigue dynamics.