Muscle Fiber Types and Sarcomere Structure
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Muscle Fiber Types and Sarcomere Structure

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Questions and Answers

Which type of muscle contains gap junctions?

  • Cardiac Muscle (correct)
  • Smooth Muscle
  • Both Skeletal and Smooth Muscle
  • Skeletal Muscle

    • What is the primary source of calcium for skeletal muscle contraction?

  • Blood plasma
  • Mitochondria
  • Sarcoplasmic reticulum (correct)
  • Extracellular fluid

    • Which type of muscle can achieve tetanus?

  • Both Cardiac and Smooth Muscle
  • Smooth Muscle only
  • Cardiac Muscle only
  • Skeletal Muscle only (correct)

    • What is the smallest functional unit of muscle contraction?

    <p>Motor Unit</p> Signup and view all the answers

    Which fiber type is characterized by fast myosin ATP usage and rapid fatigue?

    <p>Type 2B fibers</p> Signup and view all the answers

    What is the main function of acetylcholinesterase in muscle contraction?

    <p>To break down acetylcholine</p> Signup and view all the answers

    Which motor unit type is economical for isometric contractions?

    <p>Type 1</p> Signup and view all the answers

    What is the effect of curare on muscle contraction?

    <p>Prevents acetylcholine from binding</p> Signup and view all the answers

    What is the optimal condition for maximal tension in muscle fibers?

    <p>Maximal actin/myosin overlap</p> Signup and view all the answers

    Which training method primarily promotes muscle hypertrophy and neural changes?

    <p>Strength training</p> Signup and view all the answers

    What is the primary energy source utilized by Type 2B muscle fibers during high-intensity activities?

    <p>Glycogen</p> Signup and view all the answers

    What is the function of creatine kinase in muscle metabolism?

    <p>Synthesize creatine phosphate</p> Signup and view all the answers

    During which phase of myogenesis do myoblasts form?

    <p>Embryonic phase</p> Signup and view all the answers

    Which type of muscle fibers are primarily involved in prolonged endurance training and utilize lipids?

    <p>Type 1 fibers</p> Signup and view all the answers

    What is the result of aging on muscle strength?

    <p>Muscle atrophy and decreased strength</p> Signup and view all the answers

    What nutritional factor is crucial for myoblast proliferation during muscle development?

    <p>Adequate protein and energy</p> Signup and view all the answers

    What characteristic helps differentiate Slow Oxidative (Type I) fibers from Fast Glycolytic (Type II-B) fibers?

    <p>Aerobic ATP generation</p> Signup and view all the answers

    What is the primary function of Fast Oxidative-Glycolytic (Type II-A) fibers?

    <p>Sprinting and walking</p> Signup and view all the answers

    Which component of the sarcomere contains only thick filaments?

    <p>H zone</p> Signup and view all the answers

    What triggers the interaction between actin and myosin during muscle contraction?

    <p>Increase in cytosolic calcium concentration</p> Signup and view all the answers

    Which protein stabilizes the thick filaments in the sarcomere?

    <p>Myomesin</p> Signup and view all the answers

    What is the most notable difference in color between Slow Oxidative (Type I) fibers and Fast Glycolytic (Type II-B) fibers?

    <p>Type I fibers are red, Type II-B fibers are white</p> Signup and view all the answers

    How does the diameter of Fast Glycolytic (Type II-B) fibers compare to that of Slow Oxidative (Type I) fibers?

    <p>Larger than Type I fibers</p> Signup and view all the answers

    In the resting state of muscle fibers, what blocks the myosin-binding sites on actin?

    <p>Tropomyosin</p> Signup and view all the answers

    What is the primary effect of decreased nutrient intake during primary and secondary myogenesis?

    <p>Decreased muscle fiber number</p> Signup and view all the answers

    Which process refers to the increase in the number of sarcomeres in muscles?

    <p>Hypertrophy</p> Signup and view all the answers

    What is one consequence of inadequate nutrition during prenatal development?

    <p>Decreased muscle mass</p> Signup and view all the answers

    Which stage of muscle fiber development involves the formation of myotome from paraxial mesodermal cells?

    <p>Embyonic Stage</p> Signup and view all the answers

    What is a common cause of muscle strain?

    <p>Injury or overexertion</p> Signup and view all the answers

    How can muscle hypertrophy be regulated?

    <p>Through the inhibition of myostatin</p> Signup and view all the answers

    What defines the process of hyperplasia in muscle growth?

    <p>Increased number of muscle fibers</p> Signup and view all the answers

    What happens to muscle when it undergoes atrophy?

    <p>Decrease in muscle size</p> Signup and view all the answers

    What is a common cause of muscle necrosis related to nutritional deficiencies?

    <p>Vitamin E and Selenium deficiency</p> Signup and view all the answers

    In dogs affected by muscular dystrophy, which of the following symptoms is typically observed in puppies?

    <p>Stunted growth</p> Signup and view all the answers

    Which condition causes paresis due to decreased release of acetylcholine?

    <p>Hypocalcemia</p> Signup and view all the answers

    What is the primary defect in muscular dystrophy that leads to fiber damage?

    <p>Deficiency of dystrophin</p> Signup and view all the answers

    Which of the following symptoms is NOT associated with muscle necrosis?

    <p>Muscle stiffness</p> Signup and view all the answers

    Which of the following is associated with altered electrical conduction due to autoimmune factors?

    <p>Myasthenia gravis</p> Signup and view all the answers

    What is a common characteristic of myotonia in animals?

    <p>Prolonged muscle contraction</p> Signup and view all the answers

    What serum markers are typically elevated in cases of muscle necrosis?

    <p>Creatine Kinase and Aspartate transaminase</p> Signup and view all the answers

    Study Notes

    Muscle Fiber Types

    • Type I (Slow Oxidative): Small diameter, high capillary density, abundant mitochondria and myoglobin, low glycogen and creatine kinase levels, red color, aerobic ATP generation, high fatigue resistance, function in posture and endurance.
    • Type II-A (Fast Oxidative-Glycolytic): Moderate diameter, high capillary density, abundant mitochondria and myoglobin, moderate glycogen and creatine kinase levels, red-pink color, aerobic and glycolytic ATP generation, moderate fatigue resistance, function in sprinting and walking.
    • Type II-B (Fast Glycolytic): Large diameter, low capillary density, low mitochondria and myoglobin, high glycogen and creatine kinase levels, white color, glycolytic ATP generation, low fatigue resistance, function in short-term bursts of activity.

    Sarcomere Structure

    • Functional unit: Region between Z lines.
    • A band: Thick and thin myofilaments overlap.
      • H zone: Contains only thick filaments.
      • M line: Contains myomesin, which stabilizes thick filaments.
    • I band: Composed of thin filaments.
    • Z line: Anchors thin actin filaments.
      • Composed of actinin and nebulin.
    • M line: Center of the sarcomere.
      • Composed of myomesin.
    • Sarcoplasmic reticulum: Contains terminal cisternae and T-tubules.
      • Triad: Two terminal cisternae and a T-tubule.

    Myofilaments

    • Thick filaments: Composed primarily of myosin.
      • Myosin heads interact with actin during muscle contraction.
    • Thin filaments: Composed of actin, troponin, and tropomyosin.
      • Actin provides the binding site for myosin.

    Muscle Contraction

    • Process Overview:
      • Arrival of a nerve impulse.
      • Release of calcium ions.
      • Interaction between actin and myosin.
      • Muscle shortening and contraction.

    Resting State (No Contraction)

    • Troponin and tropomyosin block actin binding sites.
    • Low calcium levels in the cytosol.

    Nerve impulse & Calcium Release

    • Action potential travels down a motor neuron to the neuromuscular junction.
    • Gap junctions:
      • Skeletal muscle: None.
      • Cardiac muscle: Yes (intercalated discs).
      • Smooth muscle: Yes (only in single-unit).

    Calcium Sources for Contraction

    • Skeletal muscle: Sarcoplasmic reticulum.
    • Cardiac muscle: Sarcoplasmic reticulum and extracellular fluid.
    • Smooth muscle: Primarily extracellular fluid.

    Contraction Characteristics

    • Skeletal muscle: Rapid onset, can tetanize.
    • Cardiac muscle: Slow onset, cannot tetanize.
    • Smooth muscle: Slow onset, may tetanize.

    Key Definitions

    • Motor unit: The smallest functional unit of muscle contraction, consisting of a single motor neuron and all the muscle fibers it innervates.
    • Muscle force gradation: Nervous system control of muscle by:
      • Recruitment: Increasing the number of active motor units for greater force.
      • Frequency: Increasing the frequency of nerve impulses for greater force.
    • All-or-none principle: All fibers in a motor unit contract simultaneously when stimulated.
    • Types of muscle fibers: All fibers in a motor unit are the same type.

    Types of Motor Units

    • Slow motor units: Myosin uses ATP slowly, many mitochondria for ATP production, economical for isometric contractions and efficient for repetitive slow isotonic contractions.
    • Fast motor units: Myosin uses ATP quickly, higher power output.
      • Type 2A fibers: Sustained power.
      • Type 2B fibers: Fast, non-oxidative, and fatigue rapidly.

    Muscle Contraction Types

    • Isometric: Muscle length is sustained.
    • Isotonic: Muscle length changes.

    Sequence of Muscle Contraction

    • Neuromuscular junction:
      • Transmitter is synthesized and stored in vesicles.
      • Action potential reaches the synaptic terminal.
      • Depolarization opens voltage-gated Ca²⁺ channels.
      • Influx of Ca²⁺ causes vesicles to fuse with the synaptic membrane.
      • Transmitter (acetylcholine) is released into the synaptic cleft.
      • Binds to receptors on the postsynaptic membrane, causing excitatory / inhibitory potentials.
      • Acetylcholine is broken down by acetylcholinesterase to end the signal à relaxation.

    Acetylcholine

    • Curare: Blocks nicotinic receptors, preventing muscle contractions (frogs).
    • Clostridium botulinum: Produces a toxin that prevents acetylcholine release, leading to paralysis (bacteria).

    Force / Load / Length Relationship

    • Length-tension relationship: Maximal tension is reached when there is the most actin / myosin overlap.
    • Force-load relationship: Increasing load decreases the velocity of shortening.

    Plasticity of Muscles

    • Muscles adapt to habitual demand.
    • Regular exercise enhances muscle strength and endurance.
    • Aging may result in muscle atrophy and decreased strength.
    • Endurance training: Improves cardiovascular function, mitochondrial density, and fuel storage.
    • Strength training: Promotes muscle hypertrophy, neural changes, and muscle fiber changes.

    Muscle Energy Metabolism

    • Creatine phosphate: Form of excess energy storage in muscles, concentration is 2-6x more than ATP, creatine kinase is the enzyme involved in rapid synthesis, longer/sustained energy source.
    • Glycolysis: 2 ATP per glucose, used by Type 2B fibers, short-term energy source, utilizes glycogen.
    • Oxidative phosphorylation: Most ATP produced (~36), used by Type 1 fibers, lipids involved in prolonged endurance training.

    Myogenesis and Nutrition

    Embryonic Phase of Myogenesis

    • Key Events:
      • Formation of myoblasts from mesodermal cells.
      • Myoblasts fuse to form myotubes.
      • Initial muscle fiber differentiation occurs.
    • Nutrition Impact:
      • Adequate protein and energy are crucial for myoblast proliferation.
      • Deficiencies can lead to impaired muscle development.

    Phases of Myogenesis

    • Conception to birth:
      • Mesoderm: Develops into myotomal cells and stem cells.
        • Myotomal cells: Develop into early muscle.
        • Progenitor (stem) cells: Can develop into various muscle types.
      • Embryonic myoblasts: Early muscle cells.
      • Primary myotubules: Early muscle fibers.
        • Primary fibers: Contain sarcomeres and other structures.
        • Type 1 fibers: Develop along with type 2 fibers in fast muscles.
      • Fetal myoblasts: Later stage muscle cells.
      • Decreased nutrient intake:
        • During primary and secondary myogenesis: Decreases myogenesis (# of fibers), harder to recover from.
        • During hypertrophy: Decreases hypertrophy and birth weight, theoretically possible to recover from postnatally.

    Skeletal Muscle Growth

    • Hypertrophy: Increased number of sarcomeres.
      • Sarcomeres are arranged in parallel.
      • ↑ # of sarcomeres = stronger muscle.
    • Hyperplasia: Parallel alignment of muscle fibers, primarily in early development.
      • Formation of new myocytes.
      • ↑ force produced.
      • Can occur in adults through satellite cell activity during tear / laceration repair.
    • Lengthening: Addition of sarcomeres to both sides of the muscle's length.
      • ↑ velocity.
      • ↑ shortening capacity.

    Summary of Myogenesis

    • Embryonic stage: Paraxial mesodermal cells form myotome and progenitor cells.
    • Fetal stage: Primary and secondary myogenesis lead to muscle fiber formation.
    • Postnatal stage: Muscle fibers undergo hypertrophy primarily through satellite cell activity.

    Nutrition

    • Critical during prenatal development for fiber number and mass.
    • Affects mass postnatally.

    Muscle Disorders

    Types of Muscle Disorders

    • Muscle cramps / spasms: Involuntary contractions, often painful.
      • Cramps last longer (up to 15 minutes) à “Charley horse”.
      • Causes: injury, overuse, CNS issues.
    • Muscle strain: Tearing of muscle fibers, normally at the myotendinous junction.
      • Caused by overexertion or excessive stretching.
      • Common in large muscles (e.g., hamstrings, quadriceps).
      • Less common in postural muscles (slow twitch, less force).
    • Muscle hypertrophy: Increase in muscle size due to increased sarcomeres.
      • Case: Double Muscling:
        • Myostatin plays an important role in growth regulation (inhibits muscle cell hypertrophy & hyperplasia). - Expression is limited to skeletal muscle.
        • Mutation in myostatin = ↑ muscle mass.
        • Causes problems in calving with affected calves.
    • Muscle atrophy: Decrease in muscle size due to disuse or malnutrition.
      • Neurogenic:
        • Denervation: Lysosomal protein degeneration à 50% ↓ muscle mass + EMG abnormalities. - Common: Pinched nerve à can be easily resolved (even if slow).
      • Myogenic: Malnutrition, cachexia (secondary to other conditions), corticosteroid excess. - Slow progression, normal EMG, only type 2 fibers.
    • Muscle necrosis and regeneration: Death of muscle fibers followed by regeneration.
      • Muscle necrosis / rhabdomyolysis: May affect the whole fiber or subgroup of sarcomeres within a fiber.
        • Symptoms: Muscle pain, contracture, increased RR, sweating, ↑ Creatine Kinase & Aspartate transaminase in serum, myoglobinuria (released into bloodstream when muscle breaks down and filtered by kidneys into urine).
        • Causes: - Nutritional: Vitamin E and Selenium deficiency ("white muscle disease" à degeneration of cardiac and skeletal muscle), hypokalemia (low K+), toxins. - Infectious. - Immune related. - Metabolic.
    • Muscular dystrophy: Progressive degeneration of skeletal muscles (in small mammals and often inherited).
      • Dogs: Puppies: stunted growth, elbow abduction, bunny hop gait. Adults: plantigrade stance.
      • Due to deficiency of dystrophin (protein that anchors sarcolemma to actin of cytoskeleton) à fiber damage.
    • Altered electrical conduction:
      • Altered motor neuron firing:
        • Hypocalcemia (cows): ↓ Ach release = ↓ neuronal firing = paresis.
      • Altered motor end plate depolarization:
        • Myasthenia gravis (dogs): - Congenital: Deficiency in the number of Ach receptors. - Acquired: Autoantibodies to Ach receptors.
        • Botulism: Clostridium botulinum toxin à ↓Ach release = ↓ neuronal firing = paresis.
      • Altered sarcolemma excitability:
        • Myotonia (dogs, horses, goats): Muscle hypertrophy, stiffness, rigidity, prolonged muscle contraction. - Goats: Autosomal dominant mutation in Cl- channel = ↓ Cl- conductance = ↑ K+ in T-tubules = ↑ contraction.

    Muscle Disorders 2.0 (Simplified)

    • Muscle cramps / spasms: Short, involuntary muscle contractions, often painful.
    • Muscle strain: A tear in a muscle, often near the attachment of the tendon to the muscle.
    • Muscle hypertrophy: Increase in muscle size due to an increase in the size of muscle fibers.
    • Muscle atrophy: Decrease in muscle size due to disuse or lack of nutrients.
    • Muscular dystrophy: A group of genetic disorders that cause progressive muscle weakness and degeneration.
    • Myasthenia gravis: A neuromuscular disorder caused by antibodies that block the transmission of nerve impulses to muscles.
    • Botulism: A type of food poisoning caused by a toxin that blocks the release of acetylcholine, leading to muscle paralysis.
    • Myotonia: A condition characterized by prolonged muscle contractions.

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    Description

    Explore the different types of muscle fibers, including Type I, Type II-A, and Type II-B, along with their unique characteristics and functions. Additionally, delve into the sarcomere structure, understanding the functional units that contribute to muscle contraction. This quiz will enhance your knowledge of muscle physiology.

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