Nerve Excitability and Refractory Periods
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What happens to the excitability of a nerve fiber during the absolute refractory period?

  • Excitability is completely lost. (correct)
  • Excitability is slightly above normal.
  • Excitability is fully restored.
  • Excitability can be influenced by external stimuli.

    • What percentage of excitability is associated with the relative refractory period?

  • 0-100% (correct)
  • Less than 50%
  • Above 100%
  • 0%

    • During which phase can weaker stimuli still excite the nerve?

  • Supernormal phase of excitability (correct)
  • Absolute refractory period
  • Subnormal phase of excitability
  • Relative refractory period

    • What is the status of sodium channels during the relative refractory period?

    <p>Some sodium channels are still inactive.</p> Signup and view all the answers

    Which statement is true regarding the excitability during the subnormal phase?

    <p>Excitability is below normal but may respond to strong stimuli.</p> Signup and view all the answers

    What characterizes the stimuli required during the supernormal phase of excitability?

    <p>Only minimal stimuli are necessary.</p> Signup and view all the answers

    What happens to potassium channels during the relative refractory period?

    <p>They are usually wide open.</p> Signup and view all the answers

    What is the appropriate stimuli strength during the absolute refractory period?

    <p>No stimulus can induce a response.</p> Signup and view all the answers

    How does warming affect the metabolic reactions needed for the Na-K pump?

    <p>It increases metabolic reactions needed for Na-K pump.</p> Signup and view all the answers

    What is the effect of alkalosis on excitability?

    <p>It decreases free calcium, leading to increased excitability.</p> Signup and view all the answers

    What is the primary effect of local anesthetics on sodium channels?

    <p>They block sodium channels, reducing permeability.</p> Signup and view all the answers

    What impact does cooling have on metabolic reactions related to the Na-K pump?

    <p>It decreases metabolic reactions needed for the Na-K pump.</p> Signup and view all the answers

    How does deep pressure affect nerve fiber excitability?

    <p>It decreases excitability.</p> Signup and view all the answers

    What effect does hypoxia have on nerve fiber excitability?

    <p>It decreases excitability.</p> Signup and view all the answers

    What is the result of electrotonic effects at the anode?

    <p>Increased hyperpolarization, reducing excitability.</p> Signup and view all the answers

    What effect does the extracellular potassium concentration have on resting membrane potential (RMP) and excitability?

    <p>Increased potassium leads to increased RMP and decreased excitability.</p> Signup and view all the answers

    What characterizes the excitability during the supernormal phase?

    <p>Excitability is above normal.</p> Signup and view all the answers

    What type of stimuli is required during the relative refractory period to excite a nerve fiber?

    <p>Strong stimuli above threshold.</p> Signup and view all the answers

    During which phase is the nerve completely resistant to further stimulation regardless of stimulus strength?

    <p>Absolute refractory period</p> Signup and view all the answers

    Why does excitability decrease during the subnormal phase?

    <p>Potassium channels have excess flow out.</p> Signup and view all the answers

    What happens to the sodium channels during the relative refractory period?

    <p>Some remain in an inactivated state.</p> Signup and view all the answers

    What is true about the excitability during the absolute refractory period?

    <p>Excitability is completely lost.</p> Signup and view all the answers

    Which stimuli are needed to excite a nerve fiber during the supernormal phase?

    <p>Mild stimuli can initiate excitation.</p> Signup and view all the answers

    Which phase is characterized by a percentage of excitability less than 100%?

    <p>Relative refractory period</p> Signup and view all the answers

    What is the effect of decreasing extracellular sodium concentration on nerve excitability?

    <p>It decreases excitability due to reduced Na permeability.</p> Signup and view all the answers

    How does the application of negative charges at the cathode influence nerve excitability?

    <p>It causes hyperpolarization and decreases excitability.</p> Signup and view all the answers

    What effect does acidosis have on nerve fiber excitability?

    <p>It increases free Ca concentration, decreasing excitability.</p> Signup and view all the answers

    What is the relationship between temperature and the Na-K pump activity in nerve fibers?

    <p>Warming enhances metabolic reactions, increasing Na-K pump activity.</p> Signup and view all the answers

    How do local anesthetics like cocaine and Novocaine affect nerve excitability?

    <p>They block Na channels, resulting in decreased excitability.</p> Signup and view all the answers

    Which condition is likely to increase nerve excitability?

    <p>Alkalosis leading to lowered free calcium levels.</p> Signup and view all the answers

    What occurs in nerve fibers during the process of electrotonus at the anode?

    <p>Hyperpolarization decreases excitability.</p> Signup and view all the answers

    What effect does increased extracellular calcium have on nerve excitability?

    <p>It decreases excitability by stabilizing the membrane.</p> Signup and view all the answers

    Describe the difference between the absolute and relative refractory periods in terms of excitability.

    <p>During the absolute refractory period, excitability is completely lost, while in the relative refractory period, excitability is partially recovered and stronger stimuli are needed.</p> Signup and view all the answers

    What factors contribute to the decreased excitability during the subnormal phase?

    <p>In the subnormal phase, excitability is below normal due to a higher threshold for activation and changes in ion channel states.</p> Signup and view all the answers

    How does the supernormal phase of excitability differ from both the absolute and relative refractory periods?

    <p>The supernormal phase allows excitability above normal levels, which contrasts sharply with the absolute and relative refractory periods where excitability is either completely or partially impaired.</p> Signup and view all the answers

    Explain the role of sodium and potassium channels during the relative refractory period.

    <p>During the relative refractory period, some sodium channels remain inactivated while potassium channels are open, leading to an excess positive flow of potassium ions outside the fiber.</p> Signup and view all the answers

    What is the consequence of applying a stronger stimulus during the relative refractory period?

    <p>Applying a stronger stimulus during the relative refractory period can potentially excite the nerve, as some excitability has returned.</p> Signup and view all the answers

    In what way do the ascending and descending limbs of the action potential correspond to the different phases of excitability?

    <p>The early part of the descending limb corresponds to the absolute refractory period, while the late part corresponds to relative excitability during the relative refractory period.</p> Signup and view all the answers

    Discuss the significance of the negative afterpotential in nerve excitability.

    <p>The negative afterpotential period further decreases excitability by requiring stronger stimuli to reach the threshold for activation.</p> Signup and view all the answers

    Why are weaker stimuli ineffective during the relative refractory period?

    <p>Weaker stimuli are ineffective during the relative refractory period because excitability is not fully restored, necessitating stronger stimuli for activation.</p> Signup and view all the answers

    Explain how mechanical factors such as deep pressure affect nerve excitability.

    <p>Deep pressure reduces nerve excitability by decreasing metabolic reactions needed for the Na-K pump.</p> Signup and view all the answers

    What is the effect of alkalosis on the excitability of nerve fibers?

    <p>Alkalosis increases excitability due to decreased free calcium levels in the extracellular fluid.</p> Signup and view all the answers

    Describe how electrotonic potentials affect excitability when applying positive charges at the anode.

    <p>Applying positive charges at the anode hyper-polarizes the membrane, which decreases excitability.</p> Signup and view all the answers

    How does hypoxia influence the excitability of nerve fibers?

    <p>Hypoxia decreases excitability in nerve fibers by impairing their metabolic function and ion balance.</p> Signup and view all the answers

    What happens to the resting membrane potential (RMP) and excitability when extracellular potassium concentration decreases?

    <p>Decreasing extracellular potassium concentration increases the RMP, which decreases excitability.</p> Signup and view all the answers

    Discuss the impact of Ca concentration on excitability under acidic conditions.

    <p>Acidosis leads to increased free calcium levels, which decreases the excitability of nerve fibers.</p> Signup and view all the answers

    What is the role of local anesthetics in neurotransmission related to sodium channels?

    <p>Local anesthetics block sodium channels, leading to decreased permeability to sodium ions and diminished excitability.</p> Signup and view all the answers

    Clarify the relationship between temperature and the Na-K pump activity in terms of nerve excitability.

    <p>Increasing temperature enhances Na-K pump activity, thereby increasing nerve excitability due to heightened metabolic reactions.</p> Signup and view all the answers

    What distinguishes the supernormal phase from the subnormal phase in terms of excitability?

    <p>The supernormal phase exhibits excitability above normal levels, while the subnormal phase shows excitability below normal levels.</p> Signup and view all the answers

    How does the activity of sodium channels affect excitability during the relative refractory period?

    <p>During the relative refractory period, some sodium channels remain inactivated, reducing excitability.</p> Signup and view all the answers

    What is the primary factor affecting excitability during the absolute refractory period?

    <p>During the absolute refractory period, nerve fibers cannot be excited regardless of stimulus strength because they are completely resistant to further stimulation.</p> Signup and view all the answers

    Describe the nature of stimuli required to excite nerve fibers during the supernormal phase.

    <p>During the supernormal phase, weaker stimuli than normal can effectively excite nerve fibers.</p> Signup and view all the answers

    In the context of excitability, how does the late part of the descending limb of the spike potential correlate with the relative refractory period?

    <p>The late part of the descending limb corresponds to the relative refractory period where excitability is partially restored.</p> Signup and view all the answers

    What physiological condition occurs with the potassium channels during the relative refractory period?

    <p>Potassium channels are usually wide open during the relative refractory period, allowing excess positive charge to flow out of the fiber.</p> Signup and view all the answers

    What role do stronger stimuli play during the relative refractory period?

    <p>Stronger stimuli than the threshold are required to excite the nerve during the relative refractory period.</p> Signup and view all the answers

    How does the excitability during the subnormal phase compare to typical levels?

    <p>The excitability during the subnormal phase is below normal and requires stronger stimuli to elicit a response.</p> Signup and view all the answers

    How does increased extracellular potassium concentration affect resting membrane potential (RMP) and excitability?

    <p>Increased extracellular potassium concentration raises the RMP, resulting in decreased excitability.</p> Signup and view all the answers

    What is the effect of deep pressure on nerve excitability?

    <p>Deep pressure decreases nerve excitability by acting as a mechanical factor that reduces metabolic reactions.</p> Signup and view all the answers

    What role does calcium play in determining nerve excitability during acidosis?

    <p>During acidosis, increased free calcium leads to decreased nerve excitability due to the alteration of sodium channel permeability.</p> Signup and view all the answers

    Explain the difference in excitability between the anode and cathode during electrotonus.

    <p>At the anode, application of positive charges results in hyperpolarization and decreased excitability, while at the cathode, negative charges lead to depolarization and increased excitability.</p> Signup and view all the answers

    How does hypoxia affect the excitability of nerve fibers?

    <p>Hypoxia reduces the excitability of nerve fibers, leading to diminished responsiveness to stimuli.</p> Signup and view all the answers

    In what way does alkalosis influence nerve excitability?

    <p>Alkalosis causes a decrease in free calcium, which increases nerve excitability.</p> Signup and view all the answers

    Describe how cooling affects the Na-K pump activity and nerve excitability.

    <p>Cooling decreases the metabolic activity needed for the Na-K pump, resulting in decreased excitability.</p> Signup and view all the answers

    What is the impact of local anesthetics on sodium channel permeability?

    <p>Local anesthetics block sodium channels, decreasing sodium permeability and thus reducing nerve excitability.</p> Signup and view all the answers

    Study Notes

    Factors Affecting Excitability

    • Excitability of a nerve fiber can vary
      • Absolute refractory period: The nerve fiber is completely resistant to stimulation, regardless of the stimulus strength, as its excitability is lost.
      • Relative refractory period (RRP): The nerve fiber's excitability is partially recovered, requiring a stronger-than-threshold stimulus to elicit a response.
      • Supernormal phase: Excitability is above normal, requiring even weaker stimuli to elicit a response.
      • Subnormal phase: Excitability is below normal, requiring stronger stimuli than usual to initiate a response.

    Refractory Period Explained

    • The RRP is caused by:
      • Sodium channels: Some sodium channels remain inactivated
      • Potassium channels: These channels are widely open, allowing excessive potassium ions to flow outwards.

    Physical Factors Affecting Excitability

    • Temperature:

      • Warming: Increases excitability by boosting metabolic reactions required for the sodium-potassium pump.
      • Cooling: Decreases excitability by reducing the metabolic reactions needed for the sodium-potassium pump.

    • Mechanical factors:

      • Pressure, trauma, and injury: Reduce excitability.

    Chemical Factors Affecting Excitability

    • Local anesthetics (e.g., cocaine, Novocain, xylocaine):

      • Block sodium channels, reducing sodium permeability, which:
        • Decreases depolarization leading to reduced excitability and conductivity.
        • Increases the resting membrane potential (RMP) further reducing excitability.

    • Sodium (Na+) and Potassium (K+) Ion Concentration:

      • Increased extracellular sodium: Decreases potassium efflux, leading to a less negative RMP and increased excitability.
      • Decreased extracellular sodium: Increases potassium efflux, leading to a more negative RMP and decreased excitability.
      • Increased extracellular potassium: Decreases excitability and conductivity.
      • Decreased extracellular potassium: Increases excitability and conductivity.

    • Calcium (Ca2+):

      • Decreased calcium: Increases excitability by binding to sodium channel proteins and reducing sodium permeability
      • Increased calcium: Decreases excitability by increasing sodium permeability.

    • pH changes:

      • Alkalosis: Decreases free calcium, leading to increased excitability.
      • Acidosis: Increases free calcium, leading to decreased excitability.

    • Hypoxia, Increased CO2, and Ischemia:

      • Reduce nerve fiber excitability.

    Electrical Factors Affecting Excitability

    • Electrotonic potentials (electrotonus): Temporary, passive electrical changes in the nerve caused by subthreshold stimulation.
      • Anodal: Application of positive charges to the outer surface hyperpolarizes the membrane, decreasing excitability.
      • Cathodal: Application of negative charges to the outer surface depolarizes the membrane, increasing excitability.

    Nerve Excitability

    • Absolute refractory period (ARP): The nerve fiber is completely unresponsive to any stimulus, no matter the strength.
      • Corresponds to the early part of the descending limb of the spike potential (after firing level).
    • Relative refractory period (RRP): The nerve fiber can be stimulated, but only by a stronger-than-threshold stimulus.
      • Corresponds to the late part of the descending limb of the spike potential, until it reaches the firing level.
      • Some sodium channels are still inactivated, and potassium channels are open, causing a greater efflux of positive potassium ions.
    • Supernormal phase of excitability: The nerve fiber is more excitable than normal, responding to weaker stimuli than usual.
      • Corresponds to the negative after-potential (after firing level) and the initial 1/3 of the positive after-potential.
    • Subnormal phase of excitability: the nerve fiber is less excitable than normal, and requires stronger stimuli to be excited.
      • Corresponds to the remaining 2/3rds of the positive after-potential.

    Physical Factors Affecting Excitability

    • Warming: increases excitability by enhancing metabolic reactions needed by the sodium-potassium pump.
    • Cooling: decreases excitability by decreasing metabolic reactions needed by the sodium-potassium pump.
    • Deep pressure, trauma, and injury: Decrease excitability.

    Chemical Factors Affecting Excitability

    • Local anesthetics (cocaine, Novocaine, xylocaine): block sodium channels, reducing sodium permeability.
      • This decreases depolarization and lowers excitability and conductivity.
    • Sodium ions: Increase in extracellular sodium increases depolarization and excitability.
    • Potassium ions: Increase in extracellular potassium decreases depolarization and excitability.
      • This is due to increased potassium efflux.
    • Calcium ions: Calcium binding to sodium channel proteins decreases sodium permeability, lowering excitability and conductivity.
      • High calcium increases excitability.
    • pH changes:
      • Alkalosis: Decreases free calcium levels and increases excitability.
      • Acidosis: Increases free calcium levels, decreasing excitability.
    • Hypoxia, increased carbon dioxide, and ischemia: Decrease excitability in nerve fibers.

    Electrical Factors Affecting Excitability

    • Electrotonic Potentials (Electrotonus): Changes in nerve potential caused by constant sub-threshold galvanic current stimulation.
      • An-electrotonus: Occurs at the anode (positive charge application).
        • Increases the potential difference across the membrane.
        • Results in hyperpolarization and decreased excitability.
      • Cat-electrotonus: Occurs at the cathode (negative charge application).
        • Decreases the potential difference across the membrane.
        • Results in depolarization and increased excitability.

    Factors Affecting Excitability

    • Excitability is the ability of a nerve fiber to respond to a stimulus.
    • Excitability is influenced by different factors like:
      • Refractory Periods
      • Physical factors
      • Chemical Factors
      • Electrical Factors

    Refractory Periods

    • Absolute Refractory Period (ARP): The nerve fiber is completely unresponsive to any stimulus, regardless of its strength.
      • This occurs during the early part of the descending limb of the spike potential (after firing level).
    • Relative Refractory Period (RRP): The excitability of the nerve fiber is partially recovered.
      • Stronger stimuli (more than threshold) are needed to excite the nerve during this period.
      • This occurs during the late part of the descending limb of the spike potential until the firing level.
    • Supernormal Phase of Excitability: The excitability of the nerve fiber is above normal.
      • Weaker stimuli (below minimal) can excite the nerve during this period.
      • This occurs during the negative afterpotential.
    • Subnormal Phase of Excitability: The excitability of the nerve fiber is below normal.
      • Stronger stimuli are needed than usual to excite the nerve.
      • This occurs during the positive afterpotential (initial 1/3 of the ascending limb).
    • Cause of the Relative Refractory Period:
      • Some sodium channels still have not been reversed from their inactivation state.
      • Potassium channels are wide open, leading to a large outflow of potassium ions to the outside of the fiber.

    Physical factors Affecting Excitability

    • Temperature:
      • Warming: Increased excitability due to increased metabolic reactions needed for the sodium-potassium pump.
      • Cooling: Decreased excitability due to decreased metabolic reactions needed for the sodium-potassium pump.
    • Mechanical factors:
      • Deep pressure, trauma, and injury can decrease excitability.

    Chemical factors Affecting Excitability

    • Local Anesthesia (cocaine, Novocain, Xylocaine):
      • They block sodium channels, decreasing sodium permeability.
      • This leads to decreased depolarization and reduced excitability.
    • Sodium Ions:
      • Increased extracellular sodium leads to decreased potassium efflux, decreasing the resting membrane potential (RMP) and increasing excitability.
      • Decreased extracellular sodium leads to increased potassium efflux, increasing the RMP and decreasing excitability.
    • Potassium Ions:
      • Increased extracellular potassium leads to decreased depolarization and decreased excitability.
      • Decreased extracellular potassium leads to increased depolarization and increased excitability.
    • Calcium Ions:
      • Increased calcium binds to sodium channel protein, decreasing sodium permeability and decreasing excitability.
      • Decreased calcium increases sodium permeability and increases excitability.
    • pH Changes:
      • Alkalosis: Decreased free calcium, leading to increased excitability.
      • Acidosis: Increased free calcium, leading to decreased excitability.
    • Hypoxia, Increased CO2, and Ischemia:
      • All decrease the excitability of nerve fibers.

    Electrical factors Affecting Excitability

    • Electrotonic Potentials (Electrotonus): Changes in nerve fibers due to constant galvanic current stimulation of sub-threshold intensity.
      • Anelectrotonus: Application of positive charges on the outer surface of the membrane leads to hyperpolarization and decreased excitability.
      • Catelectrotonus: Application of negative charges on the outer surface of the membrane leads to depolarization and increased excitability.

    Factors Affecting Excitability

    • Absolute Refractory Period (ARP): The nerve fiber is completely resistant to any stimulation, no matter the strength. Excitability is at 0%.
    • Relative Refractory Period (RRP): The nerve fiber's excitability partially recovers. Only stimuli stronger than the threshold will elicit a response. Excitability ranges from 0-100%.
    • Supernormal Phase of Excitability: Excitability is above normal (greater than 100%). Weaker than minimal stimuli can excite the nerve.
    • Subnormal Phase of Excitability: Excitability is below normal (less than 100%). Stronger stimuli are required to excite the nerve.
    • Causes of Relative Refractory Period:
      • Some sodium channels are still inactivated.
      • Potassium channels are widely open, increasing potassium ion flow out of the fiber.

    Physical Factors Affecting Excitability

    • Warming: Increases excitability by promoting metabolic reactions, which are required for the sodium-potassium pump.
    • Cooling: Decreases excitability by reducing metabolic reactions needed for the sodium-potassium pump.
    • Pressure, Trauma, and Injury: Decreases excitability

    Chemical Factors Affecting Excitability

    Local Anesthesia:

    • Mechanism: Blocks sodium channels, reducing sodium permeability.
    • Impact: Decreases depolarization, leading to reduced excitability and conductivity.

    Ions:

    • Sodium (Na+):
      • Increased extracellular sodium increases excitability and conductivity.
      • Decreased extracellular sodium decreases excitability and conductivity.
    • Potassium (K+):
      • Increased extracellular potassium decreases excitability and conductivity.
      • Decreased extracellular potassium increases excitability and conductivity.

    Calcium (Ca+):

    • Decreased Calcium: Increases excitability by reducing sodium channel permeability.
    • Increased Calcium: Decreases excitability by increasing sodium permeability.

    pH Changes:

    • Alkalosis: Decreases free calcium, increasing excitability.
    • Acidosis: Increases free calcium, decreasing excitability.

    Hypoxia, Increased CO2, and Ischemia:

    • Decrease the excitability of nerve fibers.

    Electrical Factors Affecting Excitability

    Electrotonic Potentials (Electrotonus):

    • Changes: Occur in nerves due to sub-threshold stimulation with a constant galvanic current.
    • Types:
      • An-electrotonus: Hyperpolarization at the anode, decreasing excitability.
      • Cat-electrotonus: Depolarization at the cathode, increasing excitability.

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    Description

    Explore the factors affecting the excitability of nerve fibers, including concepts such as absolute and relative refractory periods. Learn how physical factors like temperature influence nerve responses and the role of ion channels in excitability. This quiz will test your understanding of these essential physiological concepts.

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