Muscle Physiology: EC Coupling Overview
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Questions and Answers

What is the primary function of calmodulin in smooth muscle contraction?

  • It acts as a second messenger to increase ATP production.
  • It directly initiates the action potential in muscle fibers.
  • It facilitates the release of calcium from the extracellular fluid.
  • It binds to calcium and activates myosin-light-chain-kinase. (correct)
  • What characterizes malignant hyperthermia?

  • Decreased heart rate and muscle contractions.
  • Absence of gap junctions in cardiac muscles.
  • Mutation in RyR leading to increased body temperature and muscle rigidity. (correct)
  • Improved performance of muscle fibers during exercise.
  • Which type of channels are involved in calcium entry for smooth muscle contraction?

  • Voltage-gated, ligand-gated, second messenger-gated, and mechanically-gated channels. (correct)
  • Only ligand-gated channels.
  • Only mechanically-gated channels.
  • Calcium channels that only respond to electrical stimuli.
  • How do cardiac muscle cells effectively communicate with one another?

    <p>Using gap junctions that form an electrical syncytium.</p> Signup and view all the answers

    What distinguishes the T tubules in cardiac muscles from those in skeletal muscles?

    <p>Cardiac T tubules have a larger diameter and are well developed.</p> Signup and view all the answers

    What initiates the contraction of a muscle cell during excitation-contraction coupling?

    <p>Release of calcium ions from the sarcoplasmic reticulum</p> Signup and view all the answers

    What structure is responsible for transmitting the action potential deep into the muscle fiber?

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

    Which process follows the depolarization of the T-tubule membrane?

    <p>Opening of calcium channels in the sarcoplasmic reticulum</p> Signup and view all the answers

    What is the role of sodium ions during the excitation phase of muscle contraction?

    <p>They are responsible for the formation of the action potential.</p> Signup and view all the answers

    How does excitation-contraction coupling primarily link excitation to contraction?

    <p>By linking depolarization of the membrane to calcium release.</p> Signup and view all the answers

    What anatomical feature facilitates the connection between the T-tubule and the sarcoplasmic reticulum?

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

    What happens during muscle fiber relaxation?

    <p>Calcium ions are reabsorbed into the sarcoplasmic reticulum.</p> Signup and view all the answers

    Which type of muscle is characterized by a different mechanism of excitation-contraction coupling compared to skeletal muscle?

    <p>Cardiac muscle</p> Signup and view all the answers

    What does the DHPR (dihydropyridine receptor) primarily function as in muscle contraction?

    <p>A voltage sensor</p> Signup and view all the answers

    What triggers the release of calcium ions during muscle contraction?

    <p>Conformational change of DHPR</p> Signup and view all the answers

    What role does troponin C play in muscle contraction?

    <p>It binds to calcium and initiates contraction</p> Signup and view all the answers

    What effect does the calcium-troponin complex have on the tropomyosin protein?

    <p>It induces a change that exposes myosin binding sites</p> Signup and view all the answers

    What process occurs to shorten the sarcomere during muscle contraction?

    <p>Release of ADP and inorganic phosphate</p> Signup and view all the answers

    Which pump is responsible for muscle relaxation by moving calcium back into the sarcoplasmic reticulum?

    <p>SERCA pump</p> Signup and view all the answers

    In an excited muscle, how does the presence of calcium ions in the cytosol affect muscle contraction?

    <p>It promotes muscle contraction by allowing myosin to bind to actin</p> Signup and view all the answers

    What is the condition of calcium levels in the cytosol during a resting muscle state?

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

    Study Notes

    Excitation-Contraction Coupling (EC Coupling)

    • EC coupling is the link between muscle excitation and muscle contraction.
    • Action potentials trigger muscle cell contraction.
    • Calcium ions control whether or not contraction occurs.
    • The link between muscle excitation and calcium release is excitation-contraction coupling.

    Objectives

    • Understand EC coupling.
    • Understand the passage of impulses.
    • Understand the function of T-tubules.
    • Understand how calcium is released.
    • Know the muscle contraction sequence.
    • Understand muscle relaxation.
    • Compare smooth and cardiac muscle differences.
    • Understand applied aspects of EC coupling.

    What is EC Coupling?

    • Action potentials initiate muscle contraction.
    • Calcium regulates contraction.
    • Excitation-contraction coupling links muscle excitation to calcium release from the sarcoplasmic reticulum.

    The Passage of the Impulse

    • Nerve impulses travel to the post-synaptic membrane via neuromuscular junctions.
    • Conformational changes result in sodium ion influx into muscle fibers.
    • Sodium accumulation depolarizes the membrane, creating an end-plate potential.
    • The potential increases towards the action potential threshold.

    The T-tubule

    • T-tubules are invaginations of the sarcolemma that extend into the muscle fiber.
    • T-tubule lumen is continuous with the extracellular fluid (ECF).
    • Depolarization during action potentials occurs across the T-tubule membrane.
    • Terminal cisterns, swellings of sarcoplasmic reticulum (SR), are on each side of T-tubules.

    How Depolarization Opens a Calcium Channel

    • Dihydropyridine receptor (DHPR) is a voltage sensor in the T-tubule membrane.
    • DHPR is closely associated with the foot of the calcium channel.
    • The action potential passes to the L-tubules, containing the ryanodine receptor (RyR).
    • The voltage sensor changes shape when depolarized, causing the RyR to open, releasing calcium.

    The Contraction Sequence

    • Accumulated calcium initiates and maintains sarcomere contraction.
    • Free calcium binds to troponin C on thin actin filaments.
    • This forms the active calcium-troponin complex.

    Binding of Calcium to Troponin C

    • Calcium binds to troponin C.
    • This causes a conformational change in tropomyosin.
    • This exposes myosin binding sites on actin filaments.

    Myosin Head Binding

    • Myosin heads bind to actin.
    • ADP and IP release trigger power strokes, shortening sarcomeres.

    Relaxation

    • SERCA pump (Sarcoplasmic/Endoplasmic Reticulum Calcium ATPase pump) actively pumps calcium back into the sarcoplasmic reticulum.

    Differences in Smooth Muscles

    • Different calcium channels (voltage-gated, ligand-gated, second messenger-gated, or mechanically-gated) exist depending on smooth muscle type.
    • Calcium can enter from the extracellular fluid.
    • IP3 (inositol triphosphate) is a second messenger that opens channels and releases calcium from the SR.
    • Calcium binds to calmodulin.
    • Myosin light chain kinase (MLCK) activates the myosin head through phosphorylation.

    Differences in Cardiac Muscles

    • Cardiac muscle cells form a syncytium (electrically connected).
    • Gap junctions, made of connexins, link cells.
    • T-tubules are well-developed.
    • Only one terminal cistern per T-tubule (a diad).

    Applied Aspects (e.g., Malignant Hyperthermia)

    • Malignant hyperthermia is caused by RyR mutations in L-tubules.
    • Symptoms include increased body temperature, muscle contractions, rigidity, and heart rate with high fever.

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    Description

    Explore the intricate process of excitation-contraction coupling, where electrical impulses lead to muscle contraction. This quiz covers essential concepts such as the function of T-tubules, calcium release, and differences between muscle types. Enhance your understanding of how muscle contraction and relaxation occur through this vital physiological mechanism.

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