Sarcomeres and the Sliding Filament Theory
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Sarcomeres and the Sliding Filament Theory

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Questions and Answers

What occurs within the sarcomeres during muscle contraction?

  • Rearrangement of Z-discs
  • Change in distance between Z-discs (correct)
  • Thickening of the I-band
  • Lengthening of the A-band

    • Which statement correctly reflects the properties of the A-band during skeletal muscle contraction?

  • A-band stays the same (correct)
  • A-band disappears completely
  • A-band becomes shorter
  • A-band enlarges significantly

    • What is a primary way to gather evidence for the sliding filament theory?

  • Histological staining of muscle tissue
  • Microscopy of contracted and stretched muscle fibres (correct)
  • Electrophysiological measurements
  • Biochemical analysis of muscle proteins

    • Which property of the I-band is observed during muscle contraction?

    <p>I-band decreases in thickness</p> Signup and view all the answers

    What experimental condition is necessary for observing the structure of sarcomeres?

    <p>Muscle fibres should be stretched and then rapidly prepared</p> Signup and view all the answers

    In the context of whole muscle contraction, what implication does the length change of sarcomeres have?

    <p>It determines the overall force output of the muscle</p> Signup and view all the answers

    What structural feature of skeletal muscle provides the primary mechanism for force generation?

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

    What element plays a crucial role in controlling skeletal muscle force?

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

    What is primarily responsible for the shortening of the sarcomere during muscle contraction?

    <p>The sliding of thick filaments over thin filaments</p> Signup and view all the answers

    In the context of the sliding filament theory, what does the term 'cross-bridge' refer to?

    <p>The binding of myosin heads to actin filaments</p> Signup and view all the answers

    Which factor influences the maximum force production by muscle fibers according to the sliding filament theory?

    <p>The initial length of the sarcomere at stimulation</p> Signup and view all the answers

    How do muscle fibers produce force at various lengths?

    <p>By changing the number of cross-bridges formed</p> Signup and view all the answers

    Why is it significant that muscle fibers are bundled together as fascicles?

    <p>It aids in the overall force transfer to the tendon</p> Signup and view all the answers

    Which statement best describes the relationship between muscle fibers and sarcomeres?

    <p>Muscle fibers contain multiple sarcomeres arranged in parallel</p> Signup and view all the answers

    What mechanical effect does muscle contraction have on the attached bone?

    <p>Muscle contraction creates torque, allowing movement around joints</p> Signup and view all the answers

    What role does ATP hydrolysis play in muscle contraction?

    <p>It powers the conformational change in the myosin head</p> Signup and view all the answers

    What initiates the action potential at the neuromuscular junction?

    <p>Sodium ion influx</p> Signup and view all the answers

    Which receptors are primarily responsible for opening Ca2+ channels in the sarcoplasmic reticulum?

    <p>Voltage-sensitive DHP receptors</p> Signup and view all the answers

    What occurs to sodium influx when muscle relaxation begins?

    <p>It stops, leading to repolarization.</p> Signup and view all the answers

    What role does SERCA play in muscle relaxation?

    <p>It pumps calcium ions back into the SR.</p> Signup and view all the answers

    What mechanism allows the continued formation of cross-bridges within the sarcomeres?

    <p>High calcium ion concentration</p> Signup and view all the answers

    What is the effect of repolarization on DHP receptors?

    <p>They return to a relaxed state.</p> Signup and view all the answers

    What is the function of calsequestrin in the sarcoplasmic reticulum?

    <p>It holds calcium ions in the sarcoplasmic reticulum.</p> Signup and view all the answers

    What happens to calcium ions after the cessation of muscle contraction?

    <p>They must be pumped back into the sarcoplasmic reticulum.</p> Signup and view all the answers

    What is the primary function of calcium ions in muscle contraction?

    <p>To bind to troponin and allow cross-bridge formation</p> Signup and view all the answers

    How does the sliding filament mechanism achieve controlled muscle contraction?

    <p>Via calcium binding to troponin which displaces tropomyosin</p> Signup and view all the answers

    What role do T-tubules play in muscle contraction?

    <p>They facilitate the movement of calcium ions into the cytoplasm</p> Signup and view all the answers

    What structural change occurs in the I-band during muscle contraction?

    <p>It becomes narrower or decreases in length</p> Signup and view all the answers

    What is the significance of the geometry and joint angle in muscle force generation?

    <p>They influence the force-length properties of sarcomeres</p> Signup and view all the answers

    Which statement accurately describes the action of tropomyosin during muscle contraction?

    <p>It blocks cross-bridge formation until calcium is present</p> Signup and view all the answers

    Which compound is primarily responsible for the energy supply during muscle contraction?

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

    What is a consequence of calcium ion movement in muscle contraction?

    <p>It initiates cross-bridge cycling between actin and myosin</p> Signup and view all the answers

    Study Notes

    Sarcomeres and the Sliding Filament Theory

    • The Sliding Filament Theory describes how muscle contraction occurs.
    • Sarcomere length changes during muscle contraction.
    • The I-band (thin filament only) shortens during contraction.
    • The A-band (both thin and thick filaments) remains the same length.
    • Z-discs move closer together during contraction.
    • The myosin head binds to the actin thin filament forming a cross-bridge.
    • The myosin head undergoes a conformational change pulling on the thin filament.
    • Repeated cycles of cross-bridge formation and myosin head attachment cause the sarcomere to shorten.
    • Myosin and actin filaments slide past one another, shortening the sarcomere but not the filaments themselves.

    Muscle Contraction and Force

    • Muscle fibres can be stimulated to contract at different lengths (sarcomere lengths) by an external force.
    • The force generated by a muscle fibre is greatest at a specific range of sarcomere lengths.
    • Muscle fibres are bundled into fascicles and ultimately the force produced by the muscles is transferred to the tendon.
    • There are specific joint angles (postures) which maximize force generated.

    Control of Skeletal Muscle Force and Role of Calcium

    • Calcium ions are crucial for regulating cross-bridge formation between thick (myosin) and thin (actin) filaments within sarcomeres.
    • In the absence of calcium, tropomyosin blocks the binding site on actin, preventing myosin from attaching.
    • When calcium binds to troponin, it causes a conformational change in tropomyosin, exposing the binding site on actin.
    • This allows myosin heads to bind to actin, forming cross-bridges and initiating the sliding filament mechanism.

    T-Tubules, Sarcoplasmic Reticulum, and Calcium Ion Movement

    • Action potentials (AP) generated at the neuromuscular junction travel along the muscle fiber's sarcolemma and down into T-tubules – invaginations of the sarcolemma.
    • The arrival of an AP at T-tubules triggers the release of calcium from the sarcoplasmic reticulum (SR), a network of internal membranes within muscle cells.
    • Voltage-sensitive dihydropyridine (DHP) receptors in T-tubules are linked to ryanodine receptors (RyR) on the SR.
    • When DHP receptors undergo a conformational change due to the AP, they open RyR channels, releasing calcium into the sarcoplasm.
    • The high concentration of calcium within the sarcomeres facilitates continued cross-bridge formation, leading to muscle contraction.

    Muscle Relaxation and Calcium Replenishment

    • The cessation of neural stimulation ends action potentials, halting the influx of sodium into the sarcoplasm.
    • The sarcolemma repolarizes, causing DHP receptors to return to their relaxed state, closing RyR channels and stopping calcium release.
    • Calcium is pumped back into the SR by the sarco/endoplasmic reticulum Ca2+ ATPase (SERCA) protein pump.
    • SERCA uses ATP energy to move calcium against its concentration gradient, facilitating muscle relaxation.
    • Calsequestrin, a protein within the SR, helps store calcium ions.

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    Description

    This quiz explores the Sliding Filament Theory and its relation to muscle contraction. It covers the changes in sarcomere length, the role of myosin and actin filaments, and how muscle fibers generate force. Test your knowledge on the mechanics of muscle contraction and the functions of sarcomeres.

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