Muscle Contraction and Neuromuscular Junction
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Muscle Contraction and Neuromuscular Junction

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Questions and Answers

What is the primary role of troponin in muscle contraction?

  • Binding to calcium (correct)
  • Releasing acetylcholine
  • Contributing to muscle shortening
  • Facilitating cross-bridge formation

    • Which event is essential for the excitation-contraction coupling in skeletal muscle?

  • Release of myosin heads from actin
  • Propagation of an action potential (correct)
  • Calcium leaving the muscle fibers
  • Increased glycogen breakdown

    • What happens during muscle relaxation?

  • Calcium levels increase in the muscle cell
  • Action potentials continue at the neuromuscular junction
  • Tropomyosin uncovers G actin active sites
  • Calcium diffuses away from the troponin molecules (correct)

    • In isotonic contractions, what happens to the muscle?

    <p>Muscle shortens while tone remains constant</p> Signup and view all the answers

    Which statement describes isometric contractions?

    <p>Muscle does not undergo any length change</p> Signup and view all the answers

    What initiates the release of acetylcholine from synaptic vesicles at the neuromuscular junction?

    <p>Exocytosis of synaptic vesicles</p> Signup and view all the answers

    What happens to acetylcholine after it binds to the ligand-gated sodium channels?

    <p>It unbinds and allows the channels to close</p> Signup and view all the answers

    Which component of actin myofilaments covers the active sites on G actin subunits in relaxed muscle?

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

    What role does acetylcholinesterase play at the neuromuscular junction?

    <p>It removes acetylcholine from the synaptic cleft</p> Signup and view all the answers

    Which process is involved in the reformation of acetylcholine within the presynaptic terminal?

    <p>Symport of choline with sodium</p> Signup and view all the answers

    What is a physiological change that occurs with aging in the muscular system?

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

    What is a primary cause of muscle cramps?

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

    Which condition is characterized by chronic widespread pain in skeletal muscles?

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

    What diagnostic method is used for confirming Myasthenia Gravis?

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

    What type of muscular issue often follows vigorous exercise?

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

    Which of the following is NOT a common mechanism involved in muscle fatigue?

    <p>Chronic inflammation</p> Signup and view all the answers

    What genetic mutation is associated with Duchenne Muscular Dystrophy?

    <p>Dystrophin gene</p> Signup and view all the answers

    What is a primary cause of tendinitis in skeletal muscles?

    <p>Overuse of muscles</p> Signup and view all the answers

    What is the primary role of the neuromuscular junction in muscle physiology?

    <p>Point of stimulation of muscle cells by nerves</p> Signup and view all the answers

    Which component of the neuromuscular junction contains acetylcholine receptors?

    <p>Motor end plate</p> Signup and view all the answers

    What occurs first during the process of muscle contraction at the neuromuscular junction?

    <p>Action potential arrives at the presynaptic terminal</p> Signup and view all the answers

    What happens at the presynaptic membrane when an action potential arrives?

    <p>Voltage gated calcium channels open</p> Signup and view all the answers

    Which structure separates the axonal endings and the muscle fibers at the neuromuscular junction?

    <p>Synaptic cleft</p> Signup and view all the answers

    What is the function of synaptic vesicles in the neuromuscular junction?

    <p>To store and release acetylcholine</p> Signup and view all the answers

    In the context of muscle physiology, what does the term 'action potential' refer to?

    <p>The electrical impulse that causes muscle contraction</p> Signup and view all the answers

    Which of the following is NOT a component of the neuromuscular junction?

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

    Which statement correctly describes the action of myosin during muscle contraction?

    <p>The head of myosin molecule binds to G actin.</p> Signup and view all the answers

    What is the primary function of the T tubule within skeletal muscle fibers?

    <p>It facilitates the transmission of action potentials.</p> Signup and view all the answers

    In which type of muscle contraction is the muscle unable to visibly shorten despite exerting force?

    <p>Isometric contraction</p> Signup and view all the answers

    Which event occurs first during muscle contraction activation?

    <p>Calcium ions bind to troponin.</p> Signup and view all the answers

    What is the role of tropomyosin in muscle contraction?

    <p>It blocks the binding sites on actin.</p> Signup and view all the answers

    Which structure is primarily responsible for calcium ion storage in muscle cells?

    <p>Sarcoplasmic reticulum</p> Signup and view all the answers

    What happens to tropomyosin in response to calcium ion binding?

    <p>It moves away from the active site on actin.</p> Signup and view all the answers

    Which statement about the myosin head during contraction is incorrect?

    <p>Myosin head binds to tropomyosin for contraction.</p> Signup and view all the answers

    Study Notes

    Neuromuscular Junction

    • Stimulation of muscle cell by a nerve happens at a neuromuscular junction.
    • The neuromuscular junction is formed from axonal endings and motor end plate.
    • Axonal endings have small membranous sacs called synaptic vesicles containing acetylcholine.
    • Motor end plate is the muscle plasma membrane in the area of the junction, and contains acetylcholine receptors.
    • Axonal endings and muscle fibers are separated by a space called the synaptic cleft.

    Muscle Contraction

    • The process of muscle contraction begins when an action potential arrives at the presynaptic terminal.
    • Voltage gated calcium channels in the presynaptic membrane open.
    • Acetylcholine is then released into the synaptic cleft by exocytosis.
    • Acetylcholine binds to ligand-gated sodium channels on the motor end plate.
    • Ligand-gated sodium channels open and sodium enters the muscle fiber. An action potential is generated if depolarization passes threshold.
    • Acetylcholine unbinds from the ligand-gated channels, which then close.
    • Acetylcholinesterase removes acetylcholine from the synaptic cleft.
    • Choline is symported with sodium in the presynaptic terminal.
    • Acetylcholine is reformed within the presynaptic terminal.

    Actin and Myosin Myofilament Structure

    • Actin Myofilament Component: Globular (G) actin forms a strand called fibrous (F) actin. Tropomyosin covers the active sites on the G actin subunits in relaxed muscle. Troponin anchors to the actin, and binds calcium.
    • Myosin Myofilament Component: Myosin molecules consist of two myosin heavy chains and two myosin heads which bind to active sites on actin molecules to form cross bridges.

    Excitation-Contraction Coupling

    • Excitation-contraction coupling occurs at the triad, which links the electrical component of muscle contraction to the mechanical component.
    • A skeletal muscle must be stimulated by a nerve ending, propagate an action potential along its sarcolemma, and have a rise in intracellular calcium levels in order to contract.
    • Calcium release from the sarcoplasmic reticulum is triggered by the action potential traveling down the T tubules, activating voltage-sensitive receptors that open calcium release channels.

    Cross-bridge Movement

    • Calcium ions bind to troponin, causing tropomyosin to move away from the active sites on actin.
    • Myosin heads bind to the exposed active sites on actin, forming cross-bridges.
    • The myosin heads pivot, pulling the thin filaments past the thick filaments. This is powered by ATP hydrolysis.
    • The myosin heads detach from actin, then bind to a new active site further down the actin filament, repeating the cycle.

    Types of Muscle Contractions

    • Isometric contractions occur when the muscle does not shorten, but tension increases.
    • Isotonic contractions occur when the muscle shortens and tone remains constant throughout the contraction.
    • Concentric contractions occur when the muscle shortens to move a load.
    • Eccentric contractions occur when the muscle lengthens while resisting a load.

    Disorders of the Muscular System

    • Aging: Physiologic muscle atrophy is a age-related reduction in muscle mass and regulation of muscle function. Loss of muscle fibers begins as early as 25 years old.
    • Muscle fatigue: This is a temporary state of reduced work capacity due to factors such as acidosis, ATP depletion, oxidative stress and local inflammatory response.
    • Muscle soreness: Caused by inflammatory chemical influx into the muscle fibers.
    • Cramps: Painful spastic contractions of skeletal muscle, caused by dehydration or ion imbalance.
    • Tendonitis: Inflammation of a tendon or its attachment point due to overuse.
    • Fibromyalgia: Non-life-threatening chronic widespread pain in skeletal muscles with no known cure.
    • Myasthenia gravis: Autoimmune disease in which autoantibodies bind to acetylcholine receptors at the neuromuscular junction.
    • Duchenne muscular dystrophy: Mutations in the dystrophin gene on the x chromosome causing progressive muscle weakness and muscle contractures.

    Diagnostic Tests for Muscular System Disorders

    • Electromyography: Measures electrical activity of muscles.
    • Muscle biopsies: Are used to examine muscle tissue structure.
    • Immunohistochemical studies: Detect specific proteins or antibodies in muscle tissue.
    • Serum creatinine: Measures the level of creatinine in the blood, which can be elevated in muscle damage or disease.

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    Unit 5.2 The Muscular System PDF

    Description

    Explore the intricate processes involved in muscle contraction and the critical role of the neuromuscular junction. This quiz covers how nerve stimulation leads to muscle action and the functions of synaptic vesicles and acetylcholine. Test your knowledge on the mechanisms of action potentials and synaptic transmission.

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