Muscle Types and Structure
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Questions and Answers

What is the shape of a smooth muscle cell?

  • Long, cylindrical
  • Star-shaped
  • Branching chains
  • Spindle (correct)

    • What is the location of the nucleus in a smooth muscle cell?

  • Peripheral, multinucleated
  • Central, single nucleus (correct)
  • Peripheral, single nucleus
  • No nucleus

    • What is the mechanism of contraction in smooth muscle?

  • Troponin C binds Ca2+, moving tropomyosin and exposing actin for myosin binding
  • Intercalated disks release Ca2+, triggering contraction
  • Actin-myosin binding occurs with myosin phosphorylation by MLCK triggered when calmodulin binds Ca2+ (correct)
  • Sarcoplasmic reticulum releases Ca2+, binding to troponin C

    • What is the function of fibroblasts similar to in smooth muscle?

    <p>Myofibroblasts and pericytes</p> Signup and view all the answers

    What is the characteristic of the sarcoplasmic reticulum in smooth muscle?

    <p>Irregular smooth ER</p> Signup and view all the answers

    What type of junctions do smooth muscle cells have?

    <p>Gap junctions</p> Signup and view all the answers

    What is a characteristic of skeletal muscle cells?

    <p>Multinucleated cells</p> Signup and view all the answers

    What is the function of the sarcoplasmic reticulum in muscle cells?

    <p>To store calcium ions</p> Signup and view all the answers

    What is the term for the cell membrane of a muscle fiber?

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

    What is the characteristic of cardiac muscle cells?

    <p>Involuntary and rhythmic</p> Signup and view all the answers

    What is the name of the structure that surrounds individual muscle fibers?

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

    What is the term for the unit of structure and function of muscle fibers?

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

    What is the characteristic of smooth muscle cells?

    <p>Fusiform and involuntary</p> Signup and view all the answers

    What is the term for the dark transverse line that bisects the I band?

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

    Where are cardiac muscle cells typically found?

    <p>In the wall of the heart and proximal portions of the aorta and the venae</p> Signup and view all the answers

    What is a characteristic of cardiac muscle cells?

    <p>Involuntary striated muscle</p> Signup and view all the answers

    What is a feature of the ultrastructure of cardiac muscle cells?

    <p>Small terminal cisternae</p> Signup and view all the answers

    What is the function of desmosomes in cardiac muscle cells?

    <p>To bind the cardiac cells together</p> Signup and view all the answers

    Where is smooth muscle typically found?

    <p>In the wall of blood vessels and hollow viscera</p> Signup and view all the answers

    What is a characteristic of smooth muscle cells?

    <p>Non-striated cells</p> Signup and view all the answers

    What is a feature of the ultrastructure of smooth muscle cells?

    <p>Rudimentary sarcoplasmic reticulum</p> Signup and view all the answers

    What is applicable to smooth muscle?

    <p>Sliding filament hypothesis</p> Signup and view all the answers

    What is the function of troponin in muscle contraction?

    <p>To inhibit the actin-myosin interaction</p> Signup and view all the answers

    What is the site of rapid conduction of impulses in muscle cells?

    <p>Transverse tubules</p> Signup and view all the answers

    What is the role of calcium ions in muscle contraction?

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

    What is the structure of a thin filament?

    <p>A filamentous polymer composed of actin, tropomyosin, and troponin</p> Signup and view all the answers

    What is the function of myosin heads in muscle contraction?

    <p>To form crossbridges with actin</p> Signup and view all the answers

    What is the site of calcium ion storage in muscle cells?

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

    What is the mechanism of muscle contraction?

    <p>Sliding filament hypothesis</p> Signup and view all the answers

    What is the structure of a triad?

    <p>A T-tubule and two terminal cisternae</p> Signup and view all the answers

    Study Notes

    Classification of Muscle Types

    • Skeletal muscle: long, multinucleated cells with cross-striations, voluntary, and forceful
    • Cardiac muscle: branched cells with cross-striations, intercalated discs, involuntary, and rhythmic
    • Smooth muscle: fusiform cells without striations, involuntary, and slow

    Specialized Terms

    • Sarcolemma: the cell membrane
    • Sarcoplasm: the cytoplasm
    • Sarcoplasmic reticulum (SER): a type of smooth endoplasmic reticulum

    Organization of Skeletal Muscle

    • Epimysium: dense connective tissue surrounding the entire muscle
    • Perimysium: thin connective tissue surrounding bundles of muscle fibers (fascicle)
    • Endomysium: reticular fiber and fibroblast surrounding individual muscle fibers

    Light Microscopic Structure of Skeletal Muscle Fibers

    • Long, cylindrical cells with multiple nuclei beneath the sarcolemma
    • Cross-striations: alternating dark and light bands
    • Myofibrils: thread-like structures arranged in parallel in the muscle fibers

    Myofibrils and Sarcomere

    • Myofibrils: long, cylindrical filamentous bundles parallel to the long axis of the muscle fibers
    • Sarcomere: the segment between two Z lines (½ I band + 1 A band + ½ I band), the unit of structure and function of muscle fibers
    • I band: bisected by a dark transverse line, Z line
    • A band: a light zone present, H band
    • H band: bisected by M line

    Myofilaments

    • Thick filaments: composed of myosin molecules with a rod and two globular heads, occupying A band
    • Thin filaments: composed of actin, tropomyosin, and troponin, inserting into the Z line and extending into the A band

    Transverse Tubules and Sarcoplasmic Reticulum

    • Transverse (T) tubules: sarcolemma tubular infoldings at A- and I- band boundaries, responsible for rapid conduction of impulses
    • Sarcoplasmic reticulum: storing calcium ions, located between two adjacent T tubules
    • A triad: a T tubule + 2 terminal cisternae, triggering Ca2+ release

    Mechanism of Contraction

    • Sliding filament hypothesis: myosin heads combine with actin to form cross-bridges, then pivot, moving thin filaments toward the sarcomere center

    Cardiac Muscle

    • Found in the heart and proximal portions of the aorta and venae
    • Involuntary, striated muscle, contracting spontaneously and displaying a rhythmic beat
    • Short, columnar, and branched cells with one or two centrally placed nuclei
    • Cross-striations and myofibrils, but less distinct
    • Intercalated disks link cells to one another

    Ultrastructure of Cardiac Muscle

    • Discrete myofibril bundles are not present
    • Larger T tubules at Z line level
    • Sarcoplasmic reticulum: not well-developed, small terminal cisternae
    • Dyads: T tubule and terminal cisternae on one side
    • More mitochondria, glycogen, and lipid droplets

    Intercalated Disks

    • Transverse and lateral portions
    • Desmosomes: bind cardiac cells together
    • Fascia adherens: supply anchoring sites for actin filaments
    • Gap junctions: provide ionic continuity between adjacent cells

    Smooth Muscle

    • Seen in blood vessels and hollow viscera (visceral muscle)
    • Arranged in layers
    • Enclosed by a network of type I and type III collagen fibers
    • Nonstriated cells

    Light Microscopic Structure of Smooth Muscle

    • Spindle-shaped cells with an oval, centrally located nucleus
    • A range of diameters
    • Without striations

    Electron Microscopic Structure of Smooth Muscle

    • Gap junctions
    • Caveolae
    • A rudimentary sarcoplasmic reticulum
    • Myofibrillar arrays: thick filaments, thin filaments, dense bodies, and intermediate filaments (desmin)

    Other Functions of Smooth Muscle

    • Supplement fibroblast activity: synthesizing collagen, elastin, and proteoglycans
    • Be similar to other contractile cells, such as myofibroblasts and pericytes

    Comparative Features

    • Skeletal muscle: long, cylindrical, multinucleated, peripheral
    • Cardiac muscle: branching, central, single nucleus
    • Smooth muscle: spindle, central, single nucleus

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    Description

    This quiz covers the characteristics and functions of skeletal, cardiac, and smooth muscles, including their cellular structure and properties.

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