Neuromuscular Junction and Muscle Contraction
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Neuromuscular Junction and Muscle Contraction

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Questions and Answers

What is the role of troponin in muscle contraction?

  • It facilitates the breakdown of ATP for energy.
  • It binds calcium and prevents tropomyosin from uncovering G actin active sites. (correct)
  • It anchors tropomyosin to myosin.
  • It prevents calcium release from the sarcoplasmic reticulum.

    • Which sequence best describes the steps of muscle relaxation?

  • Acetylcholine release, myosin head attachment, shortening of muscle.
  • Calcium release, acetylcholine release, cross-bridge formation.
  • Cessation of action potential, calcium diffusion from troponin, tropomyosin covering G actin. (correct)
  • Action potential propagation, calcium diffusion, tropomyosin uncovering.

    • During contraction, what role do myosin heads play?

  • They serve as the primary energy source for muscle contraction.
  • They bind to active sites on actin molecules to form cross bridges. (correct)
  • They transport calcium ions into the muscle fibers.
  • They control the release of neurotransmitters at the neuromuscular junction.

    • What occurs during isometric contractions?

    <p>Muscle tension increases without a change in length.</p> Signup and view all the answers

    What happens at the triad during excitation-contraction coupling?

    <p>Electrical signals trigger the release of calcium from the sarcoplasmic reticulum.</p> Signup and view all the answers

    Which of the following describes muscle atrophy?

    <p>Age-related reduction in muscle mass</p> Signup and view all the answers

    Which mechanism is NOT involved in muscle fatigue?

    <p>Increased motor neuron activity</p> Signup and view all the answers

    What is a common cause of muscle soreness after vigorous exercise?

    <p>Inflammatory chemical influx into the muscle fibers</p> Signup and view all the answers

    What causes skeletal muscle cramps?

    <p>Ion imbalance</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 type of disease is Myasthenia Gravis?

    <p>Autoimmune disease</p> Signup and view all the answers

    Duchenne Muscular Dystrophy primarily affects which group of muscles initially?

    <p>Hip muscles</p> Signup and view all the answers

    Which diagnostic test is NOT commonly used for Duchenne Muscular Dystrophy?

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

    Which statement accurately describes a feature of myosin?

    <p>The head of myosin molecule binds to an active site on G actin</p> Signup and view all the answers

    What structure invaginates into the interior of skeletal muscle fibers?

    <p>T tubule</p> Signup and view all the answers

    What type of muscle contraction occurs when a weight lifter is unable to move the weight?

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

    In the process of muscle contraction, which event occurs first?

    <p>Troponin binds to calcium</p> Signup and view all the answers

    What is the role of ATPase found in muscle contraction?

    <p>Hydrolyzing ATP to provide energy for muscle contraction</p> Signup and view all the answers

    What happens after calcium ions bind to troponin?

    <p>Troponin pulls tropomyosin away from actin</p> Signup and view all the answers

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

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

    Which role does actin play during muscle contraction?

    <p>Actin serves as a site for myosin binding</p> Signup and view all the answers

    What is the first step in the initiation of muscle contraction at the neuromuscular junction?

    <p>Acetylcholine is released into the synaptic cleft by exocytosis</p> Signup and view all the answers

    Which molecule prevents tropomyosin from uncovering the G actin active sites in a relaxed muscle?

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

    What occurs after acetylcholine binds to the ligand-gated sodium channels on the motor end plate?

    <p>Sodium enters the muscle fiber</p> Signup and view all the answers

    What role does acetylcholinesterase play in muscle contraction?

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

    Which component of the actin myofilament serves to cover active sites on G actin subunits when the muscle is relaxed?

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

    What is the primary role of acetylcholine at the neuromuscular junction?

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

    Which of the following correctly describes the synaptic cleft?

    <p>A gap separating axonal ends and muscle fibers</p> Signup and view all the answers

    What initiates the opening of voltage-gated calcium channels at the neuromuscular junction?

    <p>Depolarization caused by action potential</p> Signup and view all the answers

    The muscle plasma membrane at the neuromuscular junction is referred to as what?

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

    Which structure contains synaptic vesicles that hold acetylcholine?

    <p>Axonal endings</p> Signup and view all the answers

    What condition is primarily affected at the neuromuscular junction in myasthenia gravis?

    <p>Loss of acetylcholine receptors</p> Signup and view all the answers

    What occurs after the action potential reaches the presynaptic terminal?

    <p>Calcium influx triggers neurotransmitter release</p> Signup and view all the answers

    What is the main function of the muscular system as mentioned in the learning outcome?

    <p>To enable movement and maintain posture</p> Signup and view all the answers

    Study Notes

    Neuromuscular Junction

    • The neuromuscular junction is where nerve cells stimulate muscle cells.
    • It is formed by axonal endings, containing synaptic vesicles with acetylcholine, and the motor end plate, which is the muscle plasma membrane in the area of the junction with acetylcholine receptors.
    • Axonal ends and muscle fibers are separated by a gap called the synaptic cleft.

    Physiology of Muscle Contraction

    • Muscle contraction at the neuromuscular junction occurs in 9 steps:
      • Action potential arrives, opening voltage-gated calcium channels in the presynaptic membrane.
      • Calcium ions initiate the release of acetylcholine from synaptic vesicles.
      • Acetylcholine is released into the synaptic cleft by exocytosis.
      • Acetylcholine binds to ligand-gated sodium channels on the motor end plate.
      • Ligand-gated sodium channels open, allowing sodium to enter the muscle fiber.
      • Acetylcholine detaches from the ligand-gated channels which close.
      • Acetylcholinesterase removes acetylcholine from the synaptic cleft.
      • Choline is transported back into the presynaptic terminal with sodium.
      • Acetylcholine is reformed inside the presynaptic terminal.

    Actin and Myosin Myofilament Structure

    • Actin myofilaments are composed of globular (G) actin, which forms fibrous (F) actin, tropomyosin, which covers the active sites on G actin in relaxed muscles, and troponin, which anchors to actin, prevents tropomyosin from uncovering G actin sites, and binds calcium.
    • Myosin myofilaments are made up of myosin molecules with two heavy chains, two heads that bind to active sites on actin forming cross bridges, and a rod portion attached to the heads by a hinge region.
    • Myosin heads are also ATPase enzymes.

    Excitation-Contraction Coupling

    • Excitation-contraction coupling occurs at the triad.
    • In order to contract, skeletal muscle must be stimulated by a nerve ending, propagate action potential along its sarcolemma, and have an increase in intracellular calcium levels which trigger contraction.

    Cross-Bridge Movement

    • Calcium binds to troponin, causing tropomyosin to move and expose the active sites on actin.
    • The myosin heads bind to the actin, forming cross bridges.
    • The myosin heads then pivot, pulling the actin filaments towards the center of the sarcomere.
    • ATP is used to detach the myosin heads from the actin, and the process repeats as long as calcium is present.
    • Muscle relaxation happens when acetylcholine stops being released at the neuromuscular junction, action potential ceases on the sarcolemma, calcium diffuses away from troponin, tropomyosin covers G actin active sites, and cross bridge formation stops.

    Types of Muscle Contractions

    • Isometric contractions involve muscle tension increasing without shortening.
    • Isotonic contractions involve muscle shortening with constant tone, and are further categorized into concentric and eccentric contractions.

    Disorders of the Muscular System

    • Aging: Muscle atrophy, a reduction in muscle mass and regulation of muscle function, is a physiological condition that occurs with age.
    • Muscle Fatigue: Fatigue is a temporary state of reduced work capacity that can be caused by acidosis, ATP depletion, oxidative stress, and local inflammatory responses.
    • Muscle Soreness: Soreness after exercise is caused by an inflammatory chemical influx into the muscle fibers.
    • Cramps: Painful, spastic contractions of skeletal muscle can occur due to dehydration and ion imbalances.
    • Tendinitis: Inflammation of a tendon or its attachment point due to overuse.
    • Fibromyalgia: Chronic widespread pain in skeletal muscles with no known cure.
    • Myasthenia Gravis: Autoimmune disease in which autoantibodies bind to acetylcholine receptors.
    • Duchenne Muscular Dystrophy: Inherited disorder involving mutations in the dystrophin gene, leading to progressive muscle weakness and contractures.

    Diagnostic Tests of the Muscular System

    • Electromyography
    • Serum creatinine
    • Muscle biopsies
    • Immunohistochemical studies

    Formative Assessment

    • The answer to question 1 is e. Secretability.
    • The answer to question 2 is b. The head of myosin molecule binds to an active site on G actin.
    • The answer to question 3 is a. T tubule.
    • The answer to question 4 is b. Isometric contraction.
    • The answer to question 5 is c. 2,3,7.

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    Description

    This quiz covers the neuromuscular junction and the physiology of muscle contraction in detail. Learn about the roles of acetylcholine and the step-by-step process of muscle activation. It's essential for understanding how nerve signals stimulate muscle movement.

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