Smooth Muscle Structure and Types
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Questions and Answers

What role does calcium play in smooth muscle contraction?

  • It is only important for muscle relaxation.
  • It can induce contraction through action potentials and receptor binding. (correct)
  • It exclusively inhibits muscle contraction.
  • It is not involved in smooth muscle contraction.
  • Which neurotransmitter is known to cause constriction of smooth muscle via M-cholinergic receptors?

  • Acetylcholine (correct)
  • Epinephrine
  • Norepinephrine
  • Nitric oxide
  • What distinguishes tonic muscle contraction from phasic muscle contraction?

  • There is no difference between tonic and phasic muscle contractions.
  • Tonic muscle shows rhythmic contractions; phasic muscle does not.
  • Tonic muscle produces constant contraction, while phasic muscle shows rhythmic contraction. (correct)
  • Phasic muscle produces constant contraction; tonic muscle does not.
  • Which of the following statements about G-protein coupled receptors (GPCR) is correct?

    <p>They are crucial for the action of a-adrenergic and b-adrenergic receptors.</p> Signup and view all the answers

    How does myosin contribute to the contraction of smooth muscle?

    <p>Myosin binds to actin after phosphorylation.</p> Signup and view all the answers

    What type of muscle is characterized by discreet motor units and is always activated to produce tonic contractions?

    <p>Multi-unit smooth muscle</p> Signup and view all the answers

    Which feature differentiates single-unit smooth muscle from multi-unit smooth muscle?

    <p>Cells function as a single unit</p> Signup and view all the answers

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

    <p>Activating myosin light chain kinase</p> Signup and view all the answers

    Which type of smooth muscle is known for its phasic contractions and is commonly found in the gastrointestinal tract?

    <p>Visceral smooth muscle</p> Signup and view all the answers

    What is the role of myosin light chain phosphatase in smooth muscle physiology?

    <p>Deactivating contractions by dephosphorylation</p> Signup and view all the answers

    Which of the following statements about smooth muscle contraction is true?

    <p>Tonic contraction can be maintained indefinitely.</p> Signup and view all the answers

    What type of smooth muscle shows rhythmic contractions and is influenced by action potentials?

    <p>Phasic smooth muscle</p> Signup and view all the answers

    Which of the following hormones or neurotransmitters commonly affects smooth muscle contraction?

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

    What role do Interstitial Cells of Cajal (ICC) play in smooth muscle contractions?

    <p>They are pacemaker cells triggering depolarization.</p> Signup and view all the answers

    What is the primary mechanism by which slow waves propagate through smooth muscle cells?

    <p>Gap junctions allowing ionic passage.</p> Signup and view all the answers

    During the depolarization phase of the smooth muscle action potential, which type of channels primarily opens?

    <p>L-type calcium channels.</p> Signup and view all the answers

    Which intracellular protein does calcium bind to initiate smooth muscle contraction?

    <p>Calmodulin.</p> Signup and view all the answers

    What is the effect of the opening of Kir channels during smooth muscle action potential?

    <p>It maintains resting membrane potential.</p> Signup and view all the answers

    In the context of excitation-contraction coupling, what does MLCK stand for?

    <p>Myosin light chain kinase.</p> Signup and view all the answers

    What is the typical amplitude and duration of the slow waves generated by ICC in smooth muscle?

    <p>~10-15mV lasting for 1-2 seconds.</p> Signup and view all the answers

    What triggers the contraction of smooth muscle cells ultimately?

    <p>Increase in intracellular calcium concentration.</p> Signup and view all the answers

    Study Notes

    Smooth Muscle Structure

    • Smooth muscle (SM) is found in various tissues, including blood vessels, airways, alimentary tract, bladder, and uterus.
    • Cells are not striated and controlled by autonomic nerves, hormones, and local mediators.
    • SM contractions are slower than striated muscle and can be maintained for extended periods.
    • Some SM cells are phasic and show rhythmic contractions.
    • Responsible for smooth muscle contractions are the thick filaments (Myosin) and thin filaments (Actin), which are attached to dense bodies.

    Multi-Unit Muscle

    • Discreet motor units like skeletal muscle, and each unit must be activated independently,
    • This type of muscle always produces smooth, tonic contractions and is not rhythmic.
    • Found in the eye, large blood vessels, and airways.

    Single-Unit Muscle

    • Cells function as a single unit with gap junctions between cells.
    • Found in the bladder, small blood vessels, and the gastrointestinal tract.
    • The cells can show slow waves or basal electrical rhythms and exhibit tonic contractions in tissues like the bladder and small blood vessels.
    • Display phasic (rhythmic) contractions in the gut and uterus.

    Pacemaker Activity and Slow Waves

    • SM can exhibit rhythmic contractions as a result of "pacemaker" activity or "slow waves".
    • The slow waves are generated by interstitial cells of Cajal (ICC) in tissues of the gut.
    • These slow waves are not action potentials, but rather sub-threshold depolarizations that can trigger action potentials, allowing the cells to contract.
    • The slow waves are transmitted from ICC to neighboring muscle (SM) cells via gap junctions, resulting in synchronized contractions.

    Action Potentials in Smooth Muscle

    • Action potentials in SM are triggered by the opening of voltage-gated calcium channels (L-type channels).
    • These calcium channels open when the membrane potential is depolarized, allowing calcium to enter the cell.
    • The entry of calcium causes further depolarization, which leads to a positive feedback loop.
    • Repolarization occurs when potassium channels open, causing potassium to leave the cell.
    • The resting potential is maintained due to the activity of potassium channels (Kir).

    Excitation-Contraction Coupling in Smooth Muscle

    • Calcium plays a key role in smooth muscle contraction.
    • In response to an action potential, intracellular calcium ([Ca2+]i) increases.
    • Calcium binds to calmodulin, which activates myosin light chain kinase (MLCK).
    • MLCK phosphorylates myosin, which allows it to bind to actin and initiate contraction.
    • Dephosphorylation of myosin by myosin light chain phosphatase (MLCP) terminates contraction.

    Neural Control of Smooth Muscle

    • Neural control of smooth muscle can be influenced by the neurotransmitters acetylcholine (ACh) and noradrenaline (NA).
    • ACh typically causes smooth muscle contraction by acting on muscarinic (M) receptors.
    • In the gut, ACh can also induce relaxation via the release of nitric oxide.
    • Noradrenaline can cause either relaxation or contraction, depending on the type of receptor it interacts with.
    • Typically, noradrenaline causes contraction via alpha (α) adrenergic receptors and relaxation via beta (β) adrenergic receptors.

    Smooth Muscle Contraction

    • Calcium influx can be triggered by various mechanisms, including:
      • Action potentials (e.g., gut)
      • Receptor binding (e.g., blood vessels and adrenaline)
    • The source of calcium for contraction can vary depending on the type of smooth muscle.
    • Some smooth muscles rely primarily on intracellular calcium stores (e.g., airways, blood vessels).
    • Others rely mainly on extracellular calcium (e.g., gut).
    • Contraction occurs due to the interaction of Actin and Myosin.

    Summary

    • Three main types of muscle exist: smooth muscle, skeletal muscle, and cardiac muscle.
    • Smooth muscle contraction is controlled by the binding of calcium to calmodulin, which activates MLCK (Myosin light chain kinase).
    • MLCK phosphorylates Myosin, allowing it to bind to Actin and initiate contraction.
    • Tonic muscle produces constant contraction, like in blood vessels.
    • Phasic muscle shows rhythmic contractions, like in the gut.
    • Slow waves are generated in the gut by ICC.
    • Gut muscle contraction is triggered by opening L-type calcium channels, allowing direct calcium entry.
    • ACh usually causes contraction but can also relax via the release of nitric oxide.
    • Noradrenaline relaxes via b-receptors and stimulates via a-receptors.

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    Explore the fascinating structure and types of smooth muscle in this quiz. Learn about smooth muscle's unique characteristics, including the differences between multi-unit and single-unit muscle types. This quiz covers various functions and examples of smooth muscle found in the body.

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