Muscle Physiology: Thick Filaments Quiz
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Questions and Answers

Which of the following statements about thick filaments is true?

  • Thick filaments have myosin heads present only in areas of myosin-actin overlap. (correct)
  • Thick filaments are composed entirely of actin.
  • Thick filaments do not play a role in muscle contraction.
  • Thick filaments consist of two strands of actin subunits.

    • Troponin is one of the proteins found in thin filaments.

    True

    What are the primary components of a thick filament?

    Many myosin molecules with heads protruding from opposite ends.

    Each thin filament consists of two strands of ______ subunits twisted into a helix.

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

    Match the following components with their respective functions or characteristics:

    <p>Myosin heads = Bind to actin during contraction Troponin = Regulates muscle contraction Actin = Forms the core structure of thin filaments Tropomyosin = Blocks myosin binding sites on actin</p> Signup and view all the answers

    What is the primary structure that generates force during muscle contraction?

    <p>Cross bridges</p> Signup and view all the answers

    Thin filaments slide past thick filaments during muscle contraction.

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

    Name the two types of filaments involved in the sliding filament model.

    <p>Actin and myosin</p> Signup and view all the answers

    The _____ disc marks the boundary of a sarcomere.

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

    Match the following parts of a muscle fiber with their functions:

    <p>I band = Region of thin filaments only A band = Region of overlap of thick and thin filaments H zone = Zone of thick filaments only M line = Middle of the sarcomere</p> Signup and view all the answers

    What must occur for muscle fiber shortening to happen?

    <p>Tension generated by cross bridges must exceed opposing forces</p> Signup and view all the answers

    In a relaxed muscle state, thin and thick filaments completely overlap.

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

    During contraction, actin and myosin _____ more.

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

    What initiates the sliding filament mechanism of muscle contraction?

    <p>Binding of myosin heads to actin</p> Signup and view all the answers

    During muscle contraction, the A bands shorten.

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

    What is the role of intracellular Ca2+ levels during muscle contraction?

    <p>They must rise briefly to trigger contraction.</p> Signup and view all the answers

    The action potential is propagated along the ______ after it is generated.

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

    Match the following components with their functions in muscle contraction:

    <p>Z discs = End points of the sarcomere H zone = Region without thin filaments I band = Region containing only thin filaments A band = Region containing both thick and thin filaments</p> Signup and view all the answers

    What happens to the Z discs during muscle contraction?

    <p>They are pulled closer to the M line</p> Signup and view all the answers

    Excitation-contraction coupling does not involve ion permeability changes.

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

    How do cross bridges contribute to muscle contraction?

    <p>They form and break, ratcheting thin filaments toward the center.</p> Signup and view all the answers

    What ion primarily enters the cell during the depolarization phase of action potential generation?

    <p>Na+</p> Signup and view all the answers

    During the action potential, voltage-gated K+ channels open before Na+ channels.

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

    What generates the end plate potential at the neuromuscular junction?

    <p>Acetylcholine (ACh) binding to receptors.</p> Signup and view all the answers

    The spread of local depolarization current along the sarcolemma opens __________ channels.

    <p>voltage-gated sodium</p> Signup and view all the answers

    Match the following channels with their states during an action potential:

    <p>Na+ Channel = Open during depolarization K+ Channel = Open during repolarization Ca2+ Channel = Open at the axon terminal Voltage-gated Channel = Triggered by local depolarization</p> Signup and view all the answers

    What follows the opening of voltage-gated sodium channels in action potential generation?

    <p>K+ exits the cell</p> Signup and view all the answers

    The local depolarization wave is responsible for starting new action potentials in adjacent areas of the sarcolemma.

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

    What is the role of acetylcholine (ACh) in muscle cell activation?

    <p>It binds to receptors and initiates depolarization.</p> Signup and view all the answers

    What causes the detachment of the cross bridge in the cross bridge cycle?

    <p>The attachment of ATP to the myosin head</p> Signup and view all the answers

    The myosin head pivots and bends to pull the actin filament toward the M line.

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

    What happens to the myosin head during the 'cocking' phase of the cross bridge cycle?

    <p>It moves into a high-energy state</p> Signup and view all the answers

    In the absence of ATP, myosin heads will not detach, causing __________.

    <p>rigor mortis</p> Signup and view all the answers

    Match the following steps of the cross bridge cycle with their descriptions:

    <p>Cross bridge formation = Energized myosin head attaches to actin Power stroke = Myosin head pivots and pulls actin filament Cocking of myosin head = Myosin head moves to high-energy state Cross bridge detachment = ATP binds to myosin head</p> Signup and view all the answers

    Which molecule is hydrolyzed to provide energy for the myosin head to move to a high-energy state?

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

    What triggers the release of Ca2+ from the sarcoplasmic reticulum?

    <p>Change in shape of sarcoplasmic reticulum</p> Signup and view all the answers

    Troponin prevents myosin from binding to actin until calcium binds to it.

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

    ADP and Pi are released during the cocking phase of the cross bridge cycle.

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

    What is the role of calcium ions (Ca2+) in the cross bridge cycle?

    <p>To expose binding sites on actin filaments</p> Signup and view all the answers

    What happens when calcium binds to troponin?

    <p>Troponin changes shape and removes the blocking action of tropomyosin.</p> Signup and view all the answers

    The process where myosin binds to actin to form ___________ is known as contraction.

    <p>cross bridges</p> Signup and view all the answers

    Match the components in muscle contraction with their roles:

    <p>Ca2+ = Signals release of troponin Troponin = Binds calcium and moves tropomyosin Tropomyosin = Blocks active sites on actin Myosin = Forms cross bridges with actin</p> Signup and view all the answers

    What is the outcome of myosin binding to exposed active sites on actin?

    <p>Cross bridge cycling and contraction</p> Signup and view all the answers

    The contraction process occurs before E-C coupling is complete.

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

    What does the term E-C coupling refer to?

    <p>The process of excitation-contraction coupling in muscle fibers.</p> Signup and view all the answers

    Study Notes

    Muscular System Overview

    • Muscles comprise nearly half of the body's mass.
    • Muscles transform chemical energy (ATP) into directed mechanical energy, creating force.
    • Muscles are classified into three types: skeletal, cardiac, and smooth.
    • Prefixes like "myo," "mys," and "sarco" are frequently used in muscle-related terminology.

    Muscle Tissue Types

    Skeletal Muscle

    • Skeletal muscles are attached to bones and skin.
    • Their cells are elongated, called muscle fibers, and are striated (striped).
    • Skeletal muscle contractions are voluntary (consciously controlled).
    • These muscles contract quickly but tire easily.
    • They require nervous system stimulation for contraction.

    Cardiac Muscle

    • Found only in the heart, forming the bulk of its walls.
    • Cardiac muscle cells are striated.
    • These muscles can contract without nervous system stimulation.
    • Contraction of cardiac muscle is involuntary.

    Smooth Muscle

    • Found in the walls of hollow organs (e.g., stomach, urinary bladder, airways).
    • Smooth muscle cells lack striations.
    • Smooth muscle contractions are involuntary.
    • Smooth muscle can contract without nervous system stimulation.

    Muscle Tissue Comparisons

    • A table comparing characteristics of skeletal, cardiac, and smooth muscle is provided.
      • Key distinctions include body location, cell shape/appearance, myofibrils/sarcomeres, and presence of T tubules and/or gap junctions.
      • Different types also have various contraction regulation mechanisms.

    Special Characteristics of Muscle Tissue

    • Excitability (responsiveness): The ability to receive and respond to stimuli.
    • Contractility: The ability to shorten forcibly when stimulated.
    • Extensibility: The ability to be stretched.
    • Elasticity: The ability to recoil to resting length.

    Muscle Functions

    • Movement: Movement of bones or fluids (e.g., blood).
    • Maintaining posture and body position.
    • Stabilizing joints.
    • Heat generation: Primarily skeletal muscles.
    • Additional functions: Protecting organs, forming valves, controlling pupil and lumen size, and causing "goosebumps."

    Skeletal Muscle Structure and Function

    • Each skeletal muscle is served by one nerve, one artery, and one or more veins.
    • The connective tissue sheaths (epimysium, perimysium, and endomysium) surround and support muscle fibers.
    • Muscles attach in at least two places:
      • Insertion (movable bone)
      • Origin (immovable or less movable bone)
      • Some attachments are direct, others indirect (tendons or aponeuroses).

    Skeletal Muscle Fiber Structure

    • Skeletal muscle fibers are long, cylindrical, multinucleate cells with peripheral nuclei.
    • Sarcolemma: The plasma membrane of the muscle fiber.
    • Sarcoplasm: The cytoplasm of the muscle fiber containing glycosomes and myoglobin (oxygen-binding protein).
    • Myofibrils: Contractile organelles within the muscle fiber, containing sarcomeres (the functional units of contraction).
    • Sarcoplasmic reticulum: Specialized smooth endoplasmic reticulum that regulates calcium ion levels within the muscle fiber.
    • T tubules: Tubular infoldings of the sarcolemma that penetrate through the muscle fiber, bringing the action potential from the surface membrane into the interior of the cell.

    Myofibrils and Sarcomeres

    • Myofibrils are densely packed, rod-like elements that compose roughly 80% of the muscle cell volume.
    • They are composed of repeating subunits called sarcomeres.
    • Sarcomeres exhibit striations (alternating light and dark bands).
    • The structure of thick and thin filaments within sarcomeres accounts for the banding patterns.

    Sliding Filament Model of Contraction

    • In a relaxed muscle, thin and thick filaments overlap only at the ends of the A band.
    • During contraction, thin filaments slide past thick filaments, causing the sarcomere to shorten.
    • Myosin heads bind to actin, forming cross bridges, pulling the thin filaments toward the center of the sarcomere.
    • ATP is crucial for cross-bridge detachment and myosin head recocking.

    Physiology of Skeletal Muscle Fibers

    • Excitation: A nervous system signal is required before contraction can occur.
    • Excitation-contraction coupling: The events that connect the nerve signal to the muscle contraction.
    • AP propagated along the sarcolemma and down into T tubules, which eventually causes SR to release calcium ions.

    Neuromuscular Junction (NMJ)

    • A specialized area where a motor neuron synaptic terminal meets a muscle fiber.
    • Synaptic vesicles contain acetylcholine, a neurotransmitter involved in signal transmission across the synapse.

    Events at the Neuromuscular Junction

    • Nerve impulse arrives at the axon terminal.
    • ACh is released into the synaptic cleft.
    • ACh diffuses across the cleft and binds to receptors on the sarcolemma.
    • This binding triggers a local electrical event (end-plate potential).
    • The end-plate potential triggers an action potential in the muscle fiber.

    Destruction of Acetylcholine

    • Acetylcholinesterase breaks down ACh in the neuromuscular junction.
    • This termination of ACh activity prevents continuous muscle fiber contraction.

    Channels in Muscle Contraction

    • Voltage-gated channels are central to the process.
    • ACh binds to receptors, opening ligand-gated channels allowing Na+ and K+ flow.
    • Voltage-gated Na+ channels open leading to depolarization (action potential).
    • Voltage-sensitive proteins in T tubules help trigger Ca2+ release.

    Role of Calcium in Contraction

    • At low Ca2+ levels, tropomyosin blocks active sites on actin, preventing myosin from binding.
    • At higher Ca2+ levels, Ca2+ binds to troponin, moving tropomyosin away, exposing active sites, and allowing myosin binding.

    Cross Bridge Cycle

    • The cycle of cross-bridge formation, working stroke, and detachment continually pulls thin filaments inward.
    • Requires energy from ATP.
    • Crucial to muscle contraction.

    Muscle Mechanics

    • Isometric contractions: Muscle tension increases but does not exceed the load, no shortening occurs.
    • Isotonic contractions: Muscle shortens because muscle tension exceeds the load.
    • Force and duration of contraction vary: Based on stimulus frequency and intensity.
    • Motor units: A motor neuron and all the muscle fibers it supplies. Smaller motor units enable fine control

    Homeostatic Imbalances

    • Myasthenia gravis: Autoimmune disease where antibodies block ACh receptors leading to progressive muscle weakness.
    • Rigor mortis: When death occurs, muscle fibers run out of ATP causing cross-bridge detachment failure, resulting in muscle stiffening.

    Muscular Dystrophies

    • Duchenne muscular dystrophy (DMD): Inherited sex-linked disorder, characterized by a deficiency or absence of dystrophin that supports the sarcolemma.

    Polio

    • Polio is a viral infection that destroys motor neurons.

    Muscle Action Types

    • Agonists: Main movers in joint actions
    • Antagonists: Muscles that oppose or reverse agonist actions
    • Synergists: Aid agonists in a movement
    • Fixators: Stabilize bones involved in the movement.

    Muscle Names

    • Location: Reflecting the muscles' location (e.g., frontalis, pectoralis).
    • Size: Indicating muscle size (e.g., maximus, minimus).
    • Shape: Based on muscle shape (e.g., deltoid, trapezius).
    • Direction of fibers: Muscle fibers' orientation (e.g., rectus, oblique).
    • Number of origins: Based on the number of attachments to the bone (e.g., bicep, tricep).
    • Attachments: The location and attachment points on bones.
    • Movement actions: The type of movement the muscle performs (e.g., flexors, extensors).

    Muscle Fiber Types

    • Speed of contraction (slow/fast twitch): Classified based on the speed of myosin ATPase activity.
    • Metabolic pathways: Classified based on how they generate ATP (aerobic vs. anaerobic).
    • Oxidative fibers: Use aerobic pathways to generate ATP, suited for endurance activities.
    • Glycolytic fibers: Use anaerobic glycolysis to generate ATP, more suited for short, powerful bursts of activity.

    Exercise Types

    • Isotonic: Muscles change in length during contraction (e.g., lifting weights).
    • Isometric: Muscle tension increases but no change in length (e.g., holding a heavy object).
    • Anaerobic: Respiration without oxygen, can result in lactic acid buildup.
    • Aerobic: Respiration using oxygen, a longer term energy source.

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    Test your knowledge on thick filaments in muscle physiology with this quiz. Explore the essential components of thick filaments, their functions, and the sliding filament model. Stay sharp on terms like sarcomere and muscle contraction mechanisms.

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