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 (C)</p> Signup and view all the answers

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

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

What type of junctions do smooth muscle cells have?

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

What is a characteristic of skeletal muscle cells?

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

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

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

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

<p>Sarcolemma (A)</p> Signup and view all the answers

What is the characteristic of cardiac muscle cells?

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

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

<p>Endomysium (C)</p> Signup and view all the answers

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

<p>Sarcomere (B)</p> Signup and view all the answers

What is the characteristic of smooth muscle cells?

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

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

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

What is a characteristic of cardiac muscle cells?

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

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

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

What is the function of desmosomes in cardiac muscle cells?

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

Where is smooth muscle typically found?

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

What is a characteristic of smooth muscle cells?

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

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

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

What is applicable to smooth muscle?

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

What is the function of troponin in muscle contraction?

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

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

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

What is the role of calcium ions in muscle contraction?

<p>To trigger muscle contraction (D)</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 (C)</p> Signup and view all the answers

What is the function of myosin heads in muscle contraction?

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

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

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

What is the mechanism of muscle contraction?

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

What is the structure of a triad?

<p>A T-tubule and two terminal cisternae (C)</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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