Smooth Muscle Structure and Function
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Smooth Muscle Structure and Function

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Questions and Answers

Which type of muscle has a poorly developed sarcoplasmic reticulum?

  • All types of muscle
  • Smooth muscle (correct)
  • Cardiac muscle
  • Skeletal muscle

    • Which muscle type can contract without nerve stimulation?

  • Skeletal muscle
  • Smooth muscle
  • Both B and C (correct)
  • Cardiac muscle

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

  • Bind to Ca2+ to activate myosin light-chain kinase (correct)
  • Inhibit muscle contraction
  • Facilitate nerve stimulation
  • Store calcium ions

    • What type of muscle contains intercalated discs?

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

    Which muscle type is characterized by striations and the presence of troponin?

    <p>Both skeletal and cardiac muscle</p> Signup and view all the answers

    What happens to skeletal muscle if denervation occurs?

    <p>It results in muscle atrophy</p> Signup and view all the answers

    In smooth muscle, where can calcium ions enter the cytoplasm from?

    <p>From extracellular fluid, sarcoplasmic reticulum, and mitochondria</p> Signup and view all the answers

    Which statement about muscle fibers in skeletal muscle is true?

    <p>They are stimulated independently.</p> Signup and view all the answers

    What shape are smooth muscle cells?

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

    Which of the following is true regarding the arrangement of myofilaments in smooth muscle?

    <p>They are not arranged into sarcomeres.</p> Signup and view all the answers

    What is the primary factor that allows smooth muscle to contract when stretched?

    <p>Absence of sarcomeres</p> Signup and view all the answers

    How does smooth muscle contraction differ from skeletal muscle contraction?

    <p>Smooth muscle contraction is involuntary.</p> Signup and view all the answers

    Which structural component is absent in smooth muscle but present in skeletal muscle?

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

    What initiates or modulates smooth muscle contraction?

    <p>Membrane potential changes</p> Signup and view all the answers

    What type of ion channels are essential for smooth muscle contraction?

    <p>L-type Ca2+ channels</p> Signup and view all the answers

    What is the role of calmodulin in smooth muscle cells?

    <p>It replaces troponin to regulate contraction.</p> Signup and view all the answers

    What is the main function of gap junctions in smooth muscle?

    <p>They create low-resistance channels for nerve impulse transmission.</p> Signup and view all the answers

    Which type of smooth muscle exhibits pacemaker activity?

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

    What initiates the contraction of smooth muscle cells?

    <p>Increase in cytoplasmic calcium levels.</p> Signup and view all the answers

    How does the regulation of multiunit smooth muscle differ from single-unit smooth muscle?

    <p>Multiunit smooth muscle lacks gap junctions.</p> Signup and view all the answers

    Which protein complex is formed when calcium combines with calmodulin in smooth muscle?

    <p>Calcium-calmodulin (Ca2+-CaM) complex</p> Signup and view all the answers

    What is primarily responsible for cross-bridging during smooth muscle contraction?

    <p>Phosphorylation of myosin by myosin light-chain kinase.</p> Signup and view all the answers

    What happens during the relaxation phase of smooth muscle contraction?

    <p>Calcium is pumped back into the sarcoplasmic reticulum.</p> Signup and view all the answers

    Which of the following statements about the autonomic nervous system's role in smooth muscle is true?

    <p>Multiunit smooth muscle contraction is dependent on the autonomic nervous system.</p> Signup and view all the answers

    What is the approximate ratio of thin to thick filaments in smooth muscle cells?

    <p>16 to 1</p> Signup and view all the answers

    How do smooth muscle cells differ in filament arrangement from striated muscle cells?

    <p>Smooth muscle myosin proteins are stacked vertically.</p> Signup and view all the answers

    Which structure in smooth muscle is analogous to the Z discs found in striated muscle?

    <p>Dense bodies</p> Signup and view all the answers

    What is a primary functional difference between smooth muscle and striated muscle during stretching?

    <p>Smooth muscles maintain the ability to contract even when stretched.</p> Signup and view all the answers

    In which of the following locations is single-unit smooth muscle predominantly found?

    <p>Gastrointestinal tract</p> Signup and view all the answers

    What allows smooth muscle cells to function effectively when stretched?

    <p>Lack of organized sarcomeres</p> Signup and view all the answers

    What biochemical role does Ca2+ play in smooth muscle contraction?

    <p>It initiates a biochemical cascade.</p> Signup and view all the answers

    What percentage of the volume of a smooth muscle cell do myofilaments and dense bodies occupy?

    <p>90%</p> Signup and view all the answers

    What is the primary role of sympathetic nerves in the fight-or-flight response?

    <p>Pupillary dilation and bronchial dilation</p> Signup and view all the answers

    How does the innervation of smooth muscles differ from that of skeletal muscles?

    <p>Smooth muscles contain neurotransmitter receptors on their entire surface.</p> Signup and view all the answers

    What are varicosities in the context of autonomic nerve fibers?

    <p>Regions of autonomic fibers that release neurotransmitters</p> Signup and view all the answers

    In terms of neurotransmission, what does 'synapses en passant' refer to?

    <p>Synapses formed with smooth muscle cells along stretches of autonomic fibers</p> Signup and view all the answers

    Which of the following statements accurately describes the roles of the sympathetic and parasympathetic nerves?

    <p>Sympathetic nerves generally stimulate while parasympathetic nerves typically inhibit.</p> Signup and view all the answers

    Where are neurotransmitter receptor proteins primarily located in skeletal muscle fibers?

    <p>Only at the neuromuscular junction</p> Signup and view all the answers

    What physiological effect does sympathetic stimulation have on intestinal motility?

    <p>It decreases motility to conserve energy.</p> Signup and view all the answers

    What mechanism do smooth muscle cells use to receive signals from autonomic nerves?

    <p>Release of neurotransmitters along stretches of nerve fibers</p> Signup and view all the answers

    Study Notes

    Smooth Muscle Structure

    • Smooth muscle, also known as visceral muscle, is arranged in layers around hollow organs and blood vessels.
    • Its contraction reduces the size of these structures.
    • Smooth muscle cells are spindle-shaped.
    • Actin-myosin myofilaments are not arranged into sarcomeres resulting in a non-striated appearance.
    • Smooth muscle can contract even when significantly stretched, unlike skeletal muscle where contraction is limited by length.
    • The sarcoplasmic reticulum is loosely arranged within the cells and lacks T tubules.
    • Dense bodies, analogous to Z discs in skeletal muscle, are present.

    Smooth Muscle Function

    • Smooth muscle is involuntary.
    • Contraction is slow and sustained due to slow ATPase activity.
    • Circular and longitudinal layers produce peristaltic waves in the digestive tract, ureters, etc, to move contents.
    • Smooth muscle responds to various stimuli such as hormones, local factors, and mechanical stretch.
    • Its contraction is triggered by membrane depolarization that opens Ca2+ channels allowing influx of calcium (Ca2+).
    • Ionotropic receptors or membrane oscillators can induce depolarization.

    Types of Smooth Muscle

    • Single-unit (visceral) smooth muscle: This is the predominant type found in the GI tract, bladder, uterus, and ureters. Its low-resistance channels between cells form a functional syncytium enabling coordinated contractions. Slow wave potentials generate spike potentials and trigger contraction. The activity is adjusted by the autonomic nervous system, and rhythmic fluctuations of membrane potential occur, serving as a pacemaker function.
    • Multiunit smooth muscle: This type is located in the iris, ciliary muscle, arrector pili, and vas deferens. Each muscle fiber is innervated and functions independently. Gap junctions are absent, and activity is regulated by the autonomic nervous system.

    Mechanism of Smooth Muscle Contraction

    • Slow waves can stimulate spike potentials, triggering contraction in single-unit smooth muscle cells.
    • Initial phase of contraction from intracellular Ca2+ released from the sarcoplasmic reticulum ( similar to skeletal ).
    • Sustained contraction by Ca2+ influx via voltage-gated channels from interstitial fluid.
    • Calcium combines with calmodulin to form a complex, activating myosin light-chain kinase (MLCK).
    • MLCK phosphorylates myosin cross-bridges, exposing actin binding sites, allowing cross bridge formation and subsequent contraction.
    • Relaxation occurs when calcium is pumped back to its storage sites, the complex dissolves, and the muscle relaxes.

    Regulation of Contraction

    • Smooth muscle has inherent pacemaker activity
    • Sympathetic and parasympathetic autonomic nerves regulate smooth muscle activity in various organs simultaneously. Examples of actions include pupil dilation, coronary artery dilation, and reduced intestinal motility resulting from sympathetic stimulation.
    • Parasympathetic effects are often the opposite of sympathetic for example increased intestinal motility.

    Autonomic Innervation

    • Smooth muscle cells have neurotransmitter receptors across their entire surface.
    • Neurotransmitters are released along the length of autonomic nerve fibers.
    • These fibers branch into varicosities (bulges) releasing neurotransmitters, each stimulating multiple smooth muscle cells in a localized way.

    Smooth Muscle vs. Skeletal/Cardiac Muscle (Comparison)

    • Key differences in contraction mechanisms, innervation, structure, and responsiveness to stimuli are noted in a comparison table (note: a table isn't requested but you should be able to piece one together).

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    Related Documents

    Smooth Muscles PDF

    Description

    This quiz explores the structure and function of smooth muscle, also known as visceral muscle. It discusses the unique characteristics of smooth muscle cells, their contraction mechanisms, and their role in various bodily functions. Test your knowledge on how smooth muscle differs from other muscle types and its importance in organ function.

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