Skeletal Muscle Plasticity

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Questions and Answers

How does the type of myosin expressed in a muscle fiber influence its speed of contraction?

The type of myosin determines the rate at which ATP is utilized, which directly affects the speed of cross-bridge cycling and, consequently, the speed of contraction.

Explain how oxidative versus glycolytic energy production differs in muscle fibers and its impact on sustained activity.

Oxidative fibers use mitochondria to generate ATP continuously with oxygen, allowing for sustained activity. Glycolytic fibers produce ATP quickly from glycogen but fatigue faster due to limited glycogen stores.

How does the presence of SERCA pumps influence muscle fiber tension?

The type of SERCA pump affects the rate of calcium clearance from the sarcoplasm into the sarcoplasmic reticulum, thereby influencing the rate of tension decrease of the muscle fibers.

What are the key differences between Type I and Type II muscle fibers in terms of fatigue resistance and energy metabolism?

<p>Type I fibers are fatigue resistant and rely on oxidative metabolism, while Type II fibers fatigue more easily and primarily use glycolytic metabolism.</p> Signup and view all the answers

What role does myoglobin play in slow oxidative muscle fibers?

<p>Myoglobin in slow oxidative fibers binds and stores oxygen, facilitating oxygen delivery to the mitochondria for ATP production.</p> Signup and view all the answers

How does the distribution of muscle fiber types vary among individuals, and what implications does this have for athletic performance?

<p>The proportion of fiber types varies and is genetically influenced; a higher proportion of Type I fibers supports endurance, while more Type II fibers favor power and sprint activities.</p> Signup and view all the answers

How do endurance and strength training induce different adaptations in muscle fibers?

<p>Endurance training increases oxidative capacity with more mitochondria and capillaries, facilitating sustained activity. Strength training leads to hypertrophy, resulting in increased fiber diameter and more cross-bridges.</p> Signup and view all the answers

What factors influence muscle fiber type distribution in the body?

<p>Motor unit firing rate, genetics, training, disease, space flight and aging all affect muscle fiber type distribution.</p> Signup and view all the answers

Describe the influence of motor unit size on force generation.

<p>Larger motor units, which innervate more muscle fibers, generate more force than smaller motor units.</p> Signup and view all the answers

Explain how the nervous system regulates muscle force through 'motor units'.

<p>The nervous system controls muscle force by modulating the activity of motor units, which consist of a motor neuron and the muscle fibers it innervates.</p> Signup and view all the answers

What is meant by the term 'recruitment' in muscle physiology, and how does it contribute to force generation?

<p>Recruitment refers to the activation of additional motor units to increase muscle force, allowing for incremental increases in strength.</p> Signup and view all the answers

Describe the 'size principle' of motor unit recruitment. Why is it important?

<p>The size principle states that motor units are recruited from smallest to largest, allowing fine graded control of small forces, preventing fatigue, and efficiently increasing force production.</p> Signup and view all the answers

What is 'summation' in the context of muscle force production, and how is it achieved?

<p>Summation is increasing the rate of activity of each unit, resulting from high rates of stimulation where SERCA cannot clear calcium between twitches, resulting in continuous max force.</p> Signup and view all the answers

Contrast isometric, concentric and eccentric muscle contractions, providing a brief description of each.

<p>Isometric contractions generate force without changing muscle length, concentric contractions shorten the muscle while generating force, and eccentric contractions lengthen the muscle while generating force.</p> Signup and view all the answers

Explain the length-tension relationship in skeletal muscle and its physiological significance.

<p>The length-tension relationship describes how the force a muscle can generate is dependent on its length; force is maximal at an optimal length where actin and myosin overlap is ideal for cross-bridge formation.</p> Signup and view all the answers

How does the degree of actin and myosin overlap affect the tension generated in a muscle fiber?

<p>Optimal tension is generated when there is ideal actin and myosin overlap, at very short or very long lengths overlap is reduced so tension is reduced.</p> Signup and view all the answers

Predict what would occur if a muscle started at a very short length.

<p>If a muscle started at a very short length, the muscle fibers would be compressed, reducing the space for cross-bridge cycling and hindering force generation.</p> Signup and view all the answers

Describe some circumstances under which a muscle is likely to be injured.

<p>Muscles are especially prone to injury during eccentric actions because the forces involved tend to be quite high.</p> Signup and view all the answers

What happens when you try to lift something that is far too heavy?

<p>Isometric contractions, where the muscle length remains constant, occur when trying to lift something too heavy. No external world is done.</p> Signup and view all the answers

Outline the relationship between ATPase activity and contraction speed.

<p>A high ATPase activity is generally associated with fast contractions as it accelerates the cross-bridge cycle.</p> Signup and view all the answers

Define fatigue resistance and how it varies with fiber type.

<p>Type I fibers are fatigue resistant and type II fibers tend to fatigue more quickly. Fatigue resistance enables sustained activity.</p> Signup and view all the answers

Where is the location of glycolytic enzymes found?

<p>Glycolytic enzymes are found in the cytosol.</p> Signup and view all the answers

What happens when a muscle runs out of ATP?

<p>When a muscle runs out of ATP, it stays stiff.</p> Signup and view all the answers

Which muscle is made up of mixed fibers but predominantly slow twitch?

<p>The diaphragm is made up of mixed fiber types, but it is mostly slow twitch.</p> Signup and view all the answers

What are the characteristics of Type IIA fibers?

<p>Type IIA fibers have a fast form of myosin ATPase, are a mix of oxidative and glycolytic enzymes, and have intermediate fatigue/speed.</p> Signup and view all the answers

What happens when SERCA cannot clear Ca2+ between twitches?

<p>When SERCA cannot clear Ca2+ between twitches, there is no chance for muscle relaxation and max force is transmitted to the tendon continuously.</p> Signup and view all the answers

Why are distance runners disproportionately type I fibers?

<p>Distance runners require fatigue resistance over power and speed, therefore a higher proportion of Type 1 fibers, which are oxidative, are beneficial.</p> Signup and view all the answers

Why is the rate of activity in each unit important?

<p>The rate of activity is important, because high rates leads to increasing force due to summation and a sustained maximal force.</p> Signup and view all the answers

How does cross-bridge cycling contribute to the contractile properties of the muscle?

<p>The speed of cross-bridge cycling greatly affects the rate of contractions, which enables fast movements.</p> Signup and view all the answers

What are some of the results of regular training? Why is it important?

<p>Training increases oxidative capacity, resulting in increases ability for sustained activity or hypertrophy, resulting in larger muscles.</p> Signup and view all the answers

Flashcards

Key Fibre Type Differences

Skeletal muscle fibers differ in myosin type, oxidative vs. glycolytic energy production, and SERCA pump type.

Type I: Slow Oxidative Fibres

Type I fibers are slow-twitch with slow myosin ATPase, many mitochondria, rich blood supply and fatigue resistance. Use the oxidative pathway

Type IIB: Glycolytic Fibres

Type IIB fibers are fast-twitch with fast myosin ATPase and fewer mitochondria. They rely on the glycolytic pathway and fatigue quickly.

Type IIA: Intermediate Fibres

Type IIA fibers have fast myosin ATPase, a mix of oxidative and glycolytic enzymes, and intermediate speed/fatigue.

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Effects of training

Strength generates more force; endurance has sustained activity.

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Fibre Type Distribution Factors

Motor unit firing rate, disease, genetics, training, space flight and aging.

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Motor Unit

A motor neuron and all the muscle fibres it innervates.

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Recruitment

Muscle force is increased by activating more motor units.

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Summation

Muscle force is modulated by changing the rate of activity in each motor unit.

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Isometric Action

Length remains constant during contraction.

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Shortening Action

Shortening against fixed load

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Lengthening Action

Muscle lengthens during contraction

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Length-Tension Relationship

The length-tension relationship describes how muscle force varies with sarcomere length.

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Study Notes

  • This lecture covers skeletal muscle plasticity.
  • The objectives are to compare/contrast muscle fibre types, describe recruitment/summation, and understand the length-tension relationship.

Actin-Myosin Interaction Revision

  • Actin contains myosin binding sites.
  • Tropomyosin covers these sites when calcium concentration is low, preventing binding.
  • Calcium binds to troponin, causing tropomyosin to shift and expose the myosin binding sites on actin.
  • Myosin heads can then bind forming cross-bridges.

Cross-Bridge Cycle Revision

  • When calcium is high, tropomyosin exposes actin's binding sites, allowing myosin to bind.
  • If Calcium is low, binding sites are covered, which halts the process.
  • ATP is hydrolyzed into ADP + inorganic phosphate, energizing the myosin head.
  • ADP and inorganic phosphate are released, causing the head to flex and generate force.
  • ATP then binds, causing the myosin head to detach from actin.
  • Without ATP, the myosin head remains attached leading to stiffness.

Fibre Type Differences

  • There are three key differences between fiber types
  • Myosin type: affecting ATP utilization speed and cross-bridge cycling
  • Oxidative versus glycolytic energy production.
  • SERCA pump type affects calcium clearance

Type I: Slow Oxidative Fibres

  • Possess a slow form of myosin ATPase.
  • They have numerous mitochondria and high levels of oxidative enzymes, supported by a rich blood supply.
  • Type I fibres also exhibit a slow form of SERCA.
  • These muscles are fatigue resistant

Type IIB: Glycolytic Fibres

  • These fibres contain a fast form of myosin ATPase.
  • They have few mitochondria and lower levels of oxidative enzymes, accompanied by fewer capillaries.
  • These fibres also have a fast form of SERCA
  • Glycolytic Fibres are fast-twitch and fatigue Fast

Type IIA: Intermediate Fibres

  • These fibres exhibits a fast form of myosin ATPase.
  • Type IIA fibres have a mix of oxidative and glycolytic enzymes causing an intermediate speed/fatigue.

Muscle Fibre Distribution

  • Most human muscles are a mix of fibre types
  • Soleus muscles are low power and fatigue resistant
  • Higher proportion of Type I oxidative fibres
  • Muscles can be power producing and susceptible to fatigue
  • Higher proportion of Type II glycolytic fibers

Effects of Training

  • Strength training increases actin and myosin which increases fibre diameter (hypertrophy) and cross-bridges, increasing force.
  • Endurance training increases oxidative capacity which increases the ability for sustained activity
  • Endurance training Increases mitochondria and capillaries, increases muscle stores of lipid and increases ability to use lipids directly from blood

Slow and Fast Twitch Fibers

Characteristic Type I (slow-oxidative) Type IIa (fast-oxidative) Type IIb (fast-glycolytic)
Vascularization Extensive - Average
Motor unit size 100+ fibers 2-6 fibers 2-6 fibers
ATPase activity Low High High
Contraction speed Slow Fast Fast
Fatigue resistance High Intermediate Low
Myoglobin content High High Low
Fiber color Red Red White
Glycolytic enzymes Low Intermediate High
Mitochondrial content Packed - Sparse
  • Slow-oxidative fibers are useful for endurance events.
  • Fast-oxidative fibers are useful for power and sprint events.
  • Fast-glycolytic fibers are useful for power and sprint events.

Factors Affecting Fibre Type Distribution

  • Motor unit firing rate.
  • Disease.
  • Genetics.
  • Training.
  • Space flight.
  • Aging.
  • These factors influence function through changes in fibre type composition.

Active Control: Motor Units

  • Motor units are a motor neuron and all the muscle fibres it innervates.
  • All muscle fibres in one unit have the same metabolic type (fast or slow ATPase).
  • Motor unit size varies, influencing max force.
  • Muscle force is regulated by recruitment (number of active units) and summation (rate of activity).

Muscle Tension: Recruitment

  • The number of fibres activated relies on the number of active neurons.
  • Few active neurons produce low force, increasing force with more activation.
  • Recruitment is the process of activating more fibres to increase force.
  • Recruitment is orderly, from smallest to largest units.
  • Smaller units are recruited first and are more tonically active.
  • Finer control of small forces is enabled through these smaller units.
  • Larger units are automatically recruited as needed for increased force.

Active Control: Summation

  • Summation is rate modulation of motor units.
  • High rates prevent SERCA from clearing calcium between twitches.
  • A lack of relaxation results in max force transmitted continuously to the tendon.

Actions During Muscle Contraction

  • Shortening is isotonic, shortening against a fixed load.
  • Speed relies on Myosin-ATPase activity and load.
  • Isometric action indicates muscle length remains constant during contraction with no external work done.
  • Lengthening action has a higher likelihood of causing muscle injury.

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