Smooth Muscle Contraction Mechanisms
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Questions and Answers

Which type of signaling is associated with the release of hormones from enteroendocrine cells in the GI tract?

  • Hydroxyl signaling
  • Cyclic AMP signaling
  • Calcium signaling (correct)
  • Nuclear signaling
  • What is true about the mechanisms of contraction within the GI tract?

  • Complex movements involve several sites and can be influenced by hormones. (correct)
  • They solely depend on central nervous system inputs.
  • Contractions occur through short reflex pathways.
  • Only smooth muscle is involved in GI tract contractions.
  • How do depolarization and hyperpolarization affect GI tract activity?

  • Both processes have no significant effect on GI tract functions.
  • Hyperpolarization initiates contractions in the GI tract.
  • Depolarization always inhibits GI activity.
  • Depolarization can trigger contractions in the GI muscles. (correct)
  • Which statement correctly describes hormonal regulation in the GI tract?

    <p>Hormonal signals often result in slow but sustained physiological effects.</p> Signup and view all the answers

    What role do neurotransmitters play in regulating gut function?

    <p>They can inhibit GI tract activity through secondary neurotransmitters.</p> Signup and view all the answers

    What is the role of NO (nitric oxide) in muscle contraction?

    <p>It increases the amount of Myosin light chain Phosphatase.</p> Signup and view all the answers

    How does the CNS aid in the relaxation of smooth muscles?

    <p>By sending a message to the ENS via the parasympathetic NS using acetylcholine.</p> Signup and view all the answers

    What occurs when opioids bind to opioid receptors in the myenteric plexus?

    <p>They inhibit neurotransmission by ENS neurons.</p> Signup and view all the answers

    What happens to calcium levels during stimulation of smooth muscle contraction?

    <p>Calcium levels increase, causing depolarization and contraction.</p> Signup and view all the answers

    What is the effect of acetylcholine on the ENS?

    <p>It enhances excitatory signals to smooth muscles.</p> Signup and view all the answers

    Which receptor type does the ENS possess for adrenergic neurotransmitters?

    <p>Alpha-adrenergic receptors.</p> Signup and view all the answers

    Which mechanism leads to the reduction of contraction size in smooth muscle tissues?

    <p>Dephosphorylation of myosin light chains.</p> Signup and view all the answers

    What is a likely mechanism behind constipation associated with opioid drug use?

    <p>Decreased muscle contractions in the gastrointestinal tract.</p> Signup and view all the answers

    What is the primary effect of the influx of Ca2+ through voltage-gated channels in the sarcolemma?

    <p>Activation of the ryanodine receptor</p> Signup and view all the answers

    How does the activation of phospholipase C by a receptor contribute to contraction?

    <p>By producing IP3 that induces Ca2+ release</p> Signup and view all the answers

    What is one mechanism through which phasic contractions can be down-regulated?

    <p>Hyperpolarization of smooth muscle cell membranes</p> Signup and view all the answers

    Which statement accurately describes the role of hormones and neurotransmitters in muscle contraction regulation?

    <p>They modulate the strength of contraction without affecting frequency.</p> Signup and view all the answers

    What must occur for actin and myosin to form cross-bridges during contraction?

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

    What effect does increasing myosin light chain phosphatase have on muscle contraction?

    <p>It dephosphorylates myosin, reducing contraction force.</p> Signup and view all the answers

    Which of the following signals contributes to the down-regulation of phasic contractions?

    <p>Hyperpolarization of smooth muscle cell membranes</p> Signup and view all the answers

    How does stress influence contraction in smooth muscle?

    <p>By the CNS signaling the ENS, leading to neurotransmitter release.</p> Signup and view all the answers

    Study Notes

    Smooth Muscle Mechanism of Contraction

    • Sarcolemma (cell membrane): Voltage gated Na+ and Ca2+ channels open during depolarization.
    • Influx of Ca2+ causes the following events
      • Activation of the ryanodine receptor, releasing Ca2+ from the sarcoplasmic reticulum
      • Increased contraction facilitated by calmodulin and MLCK

    Up-regulation Mechanism 2 – Pharmomechanical Contraction

    • Hormones & neurotransmitters bind to their receptors, activating Gαq-signaling pathway.
      • Receptor activation triggers phospholipase C activity.
      • Production of IP3 induces Ca2+ release from the sarcoplasmic reticulum, enhancing contraction.

    Down-regulation of Phasic Contractions

    • Hyperpolarization of smooth muscle cells:
      • Receptors open ion channels that hyperpolarize cells.
      • Slow waves fail to reach threshold leading to no contraction.
      • Alternatively, the number of action potentials is reduced.
    • Increased activity of Myosin Light Chain Phosphatase (MLCP): This enzyme dephosphorylates myosin, reducing the force of contraction.
    • Inhibition of excitatory ENS neurotransmission: Opioid drugs are examples.

    Regulation - Examples

    Relaxation

    • The CNS detects relaxation through senses and sends signals to the ENS via the parasympathetic NS using acetylcholine as a neurotransmitter.
    • ENS has muscarinic cholinergic receptors and sends an excitatory signal to smooth muscle.
    • Mechanism - Depolarization and increase in Ca2+

    Stress

    • CNS detects stress via senses and communicates with ENS through the sympathetic NS using adrenaline.
    • ENS has α adrenergic receptors, receiving an inhibitory signal from the ENS.
    • Mechanism - Increased MLCP activity leads to dephosphorylation of myosin and reduced crossbridge cycling.

    Opioid Drugs

    • Mechanism: Opioids bind to opioid receptors in the myenteric plexus, inhibiting neurotransmission by ENS neurons. This blocks excitatory signals from reaching the GI smooth muscle.

    Regulation - Overview

    • Hormones and neurotransmitters primarily regulate the magnitude/strength of contraction.
      • They have minimal impact on frequency, which is determined by ICC and varies by region.
    • Mechanisms of regulation:
      • To increase contraction: Depolarize or increase Ca2+
      • To decrease contraction: Hyperpolarize or decrease Ca2+
    • Increasing MLCP activity dephosphorylates myosin, reducing cross-bridge cycling
    • Blocking excitatory neurotransmission decreases contractions.

    Case Study: Elvis Presley

    • History: Elvis had a history of opioid drug use, constipation, an enlarged colon, heart disease, and an enlarged heart. These factors contributed to his death.
    • Autopsy Findings: Opioids were found in Elvis' system, along with an enlarged colon and heart disease.
    • Cause of Death: Cardiac arrest/heart attack.
    • Opioid's Impact on GI Tract: Opioid drugs can cause constipation by inhibiting the myenteric plexus, which regulates the complex movements of the GI tract.

    Myenteric Plexus

    • Location: The myenteric plexus is located within the muscularis propria of the GI tract.
    • Function: It regulates the complex movements (peristalsis) and duration of contractions of the GI tract.
    • Opioid Impact: Opioids inhibit the myenteric plexus by blocking the excitatory signals from the CNS, contributing to constipation.
    • Role in GI Contraction: The myenteric plexus is responsible for coordinating the contractions of the GI smooth muscles, essential for digestion and elimination.

    Enteroendocrine Cells

    • Function: Enteroendocrine cells are located in the GI tract and release hormones.
      • These hormones trigger a cascade of events that influence various bodily functions, including digestion and metabolism.
    • Stimuli: The release of the hormone is triggered by a variety of stimuli, including food intake and changes in pH.
    • Mechanism: Enteroendocrine cells possess apical receptors that bind to peptide hormones, leading to a slow but sustained release of hormones from the GI tract.

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    Description

    This quiz explores the various mechanisms behind smooth muscle contraction, including the roles of calcium ions, hormones, and neurotransmitters. It delves into the up-regulation of contraction through the Gαq-signaling pathway and the down-regulation that leads to hyperpolarization. Test your understanding of these intricate processes!

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