Neurophysiology: Action Potential
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Questions and Answers

What occurs at the membrane potential of -55 mv during the action potential?

  • Membrane potential immediately returns to resting level.
  • The membrane potential becomes permanently negative.
  • The rate of Na+ influx decreases.
  • The firing level is reached and Na+ channels start to open. (correct)

    • What is the maximum membrane potential reached during depolarization?

  • 0 mv
  • -70 mv
  • -55 mv
  • +35 mv (correct)

    • Which statement correctly describes the role of Na+ during depolarization?

  • Na+ influx increases due to concentration and electrical gradients. (correct)
  • Na+ influx is balanced by K+ efflux.
  • Na+ is repelled by positive charges inside the cell.
  • Na+ channels close when membrane potential is at +35 mv.

    • What triggers the positive feedback mechanism during depolarization?

    <p>Further depolarization over 15 mv leading to more Na+ channel opening.</p> Signup and view all the answers

    What defines the magnitude of an action potential?

    <p>The difference between maximum and minimum membrane potential.</p> Signup and view all the answers

    What happens after the depolarization phase ends?

    <p>The membrane potential rapidly returns to resting level, potentially overshooting into hyperpolarization.</p> Signup and view all the answers

    What characterizes the depolarization phase of an action potential?

    <p>The rate of depolarization increases markedly after reaching -55 mv.</p> Signup and view all the answers

    During the repolarization phase, what primarily happens to the membrane potential?

    <p>It overshoots the resting level.</p> Signup and view all the answers

    What primarily causes the resting membrane potential in excitable tissues?

    <p>Selective permeability of the cell membrane and ion concentration gradients</p> Signup and view all the answers

    Which ion's permeability is typically greater at rest, leading to a negative resting membrane potential?

    <p>Potassium (K+)</p> Signup and view all the answers

    What is the primary role of the sodium-potassium pump in maintaining resting membrane potential?

    <p>To maintain ionic concentration gradients across the membrane</p> Signup and view all the answers

    Which of the following statements best explains the resting condition of a neuron's membrane?

    <p>The membrane is impermeable to negatively charged proteins.</p> Signup and view all the answers

    What percentage of the resting membrane potential is generated by passive forces?

    <p>93%</p> Signup and view all the answers

    Which of the following best describes the ionic environment of a neuron at resting potential?

    <p>High concentrations of K+ inside and Na+ outside</p> Signup and view all the answers

    What is the impact of an increase in extracellular potassium concentration on the resting membrane potential?

    <p>It makes the resting membrane potential less negative or more positive.</p> Signup and view all the answers

    How is the resting membrane potential directly measured in laboratory settings?

    <p>By placing electrodes inside and outside the membrane</p> Signup and view all the answers

    What is the primary cause of repolarization during an action potential?

    <p>K+ efflux through voltage-gated K+ channels</p> Signup and view all the answers

    Which statement accurately describes the behavior of voltage-gated Na+ channels during repolarization?

    <p>They become inactive due to the closure of the h gate.</p> Signup and view all the answers

    In which period is the nerve completely unable to respond to any stimulus?

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

    What characterizes the opening of n gate in voltage-gated K+ channels?

    <p>It is slower and more prolonged compared to other gates.</p> Signup and view all the answers

    During which period can a new action potential be generated but requires a stronger stimulus?

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

    What is the condition of voltage-gated K+ channels at the end of repolarization?

    <p>Some K+ channels remain open with a slow return to closed state.</p> Signup and view all the answers

    Which period follows the absolute refractory period in the sequential phases of action potential?

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

    What happens to the responsiveness of voltage-gated channels after an action potential?

    <p>They regain responsiveness indicating readiness for a new action potential.</p> Signup and view all the answers

    What characterizes the absolute refractory period (ARP) in nerve excitability?

    <p>Nerve excitability is depressed, preventing any new action potentials.</p> Signup and view all the answers

    During which period is nerve excitability decreased below normal, requiring stronger stimuli for activation?

    <p>Subnormal period</p> Signup and view all the answers

    How does hypernatremia affect the process of depolarization?

    <p>Facilitates the process of depolarization.</p> Signup and view all the answers

    Which substance blocks the activation gate of voltage-gated Na+ channels?

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

    What distinguishes the supranormal period in terms of nerve excitability?

    <p>Nerve excitability is increased, allowing weaker stimuli to trigger excitation.</p> Signup and view all the answers

    What is the effect of hypornatriemia on the action potential?

    <p>Delays depolarization and reduces action potential amplitude.</p> Signup and view all the answers

    In which phase do stronger stimuli than normal become necessary for nerve excitation?

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

    What happens to nerve excitability during the after hyperpolarization phase?

    <p>Nerve excitability decreases below normal.</p> Signup and view all the answers

    What is the primary role of the Na+ - K+ pump in resting membrane potential?

    <p>It is an electrogenic pump transferring more positive charges outside the cell.</p> Signup and view all the answers

    Which of the following correctly describes the depolarized state during an action potential?

    <p>The membrane potential is briefly reversed and becomes positive.</p> Signup and view all the answers

    What occurs during the latent period of an action potential?

    <p>The stimulus travels along the axon to the recording electrode.</p> Signup and view all the answers

    Which characteristic defines hyperpolarization in action potential?

    <p>The membrane potential becomes more negative than the resting state.</p> Signup and view all the answers

    The resting membrane potential is primarily influenced by which of the following factors?

    <p>The inability of negatively charged proteins to exit the cell.</p> Signup and view all the answers

    What is the main event triggered by a threshold stimulus in an excitable cell?

    <p>A transient reversal in membrane polarity.</p> Signup and view all the answers

    Which phase of action potential directly follows the latent period?

    <p>Depolarization phase</p> Signup and view all the answers

    What effect does the Na+ - K+ pump have on the distribution of ions across the membrane?

    <p>It actively transports K+ ions into the cell and Na+ ions out.</p> Signup and view all the answers

    Study Notes

    Course Information

    • Faculty: Medicine
    • Academic Year: 2024-2025
    • Year: 1
    • Semester: 1
    • Module: Human Body Function (HBF) 102

    Action Potential

    • Topic: Action Potential
    • Presented by: Eman Mohamed Ali
    • Department: Physiology

    Objectives

    • Discuss Action Potential and its ionic basis
    • Explain excitability changes during action potential
    • Deduce the effect of plasma ionic concentration changes on action potential

    Introduction

    • Diagram of a neuron including dendrites, nucleus, soma, axon, axon terminal, myelin, and Schwann cell, with node of Ranvier labeled.

    Resting Membrane Potential (RMP)

    • Definition: The difference in electrical potential across the cell membrane of excitable tissues (nerves and muscles) in a resting state.
    • Inside of the membrane is negatively charged relative to the outside.

    How to Measure RMP

    • Diagram showing electrodes placed outside and inside the membrane, with a CRO (Cathode Ray Oscilloscope).

    Causes of Resting Membrane Potential

    • Passive Forces (93%):
      • Selective permeability of the cell membrane.
      • Impermeability of the membrane to negatively charged proteins, organic phosphates, and sulfates inside the cell.
    • Active Force (7%):
      • Na+-K+ Pump: An electrogenic pump that transfers positive charges to the outside, crucial for maintaining RMP (coupling ratio 3/2).

    Passive Causes of the Resting Membrane Potential

    • Permeability of the membrane to potassium (K+) ions is 50-70 times greater than permeability to sodium (Na+) ions at rest.
    • At rest, the K+ ion concentration is higher inside the membrane than outside, while the opposite is true for Na+ ions.
    • The membrane is leaky (through K+ and Na+ leak channels).

    Active Causes of the Resting Membrane Potential

    • Na+-K+ pump is an electrogenic pump with a coupling ratio of 3/2. It transfers more positive charges to the outside, maintaining the resting membrane potential.

    Interactive Question (Page 11)

    • Diagram showing ion movement across a cell membrane; labeling questions 1, 2, and 3.
    • Identifying the name and function of these components.

    Action Potential

    • Definition: A transient reversal in membrane polarity in excitable cells (nerve or muscle) in response to a threshold stimulus

    Phases of Action Potential and its Ionic Basis

    • Latent period
    • Depolarization phase
    • Repolarization phase

    1. Latent Period

    • The time between stimulus application and depolarization.
    • Corresponds to the time stimulus takes to travel along the axon to recording electrode.

    2. Depolarization Phase

    • Slow initial depolarization.
    • Membrane potential reaches -55mV (firing level).
    • Rapid depolarization to +35 mV, with inner surface becoming positive to outer surface.
    • Magnitude of action potential = 105 mv

    Ionic Basis of Depolarization

    • When depolarization exceeds 7mV, voltage-gated Na+ channels open at an increasing rate (m-gate opening).
    • Depolarization exceeds 15mV (membrane potential reaches -55mV) more activation of voltage-gated Na+ channels occurs.
    • Na+ influx increases due to concentration and electrical gradients
    • Positive feedback mechanism causes a dramatic increase in Na+ influx, leading to reversal of polarity.

    3. Repolarization Phase

    • Membrane potential falls rapidly to resting levels
    • Voltage-gated Na channels become inactivated (h-gate closure)
    • K+ efflux occurs due to opening of voltage-gated K+ channels and outward diffusion of positive charges.
    • Opening of n-gates leads to a slower and more prolonged process which can go below resting level

    Ionic Basis of Repolarization

    • Voltage-gated Na+ channels become inactive (closure of the h-gate).
    • K+ efflux due to opening of voltage-gated K+ channels (n-gate opening).
    • The slower opening of the n gate and prolonged K+ efflux completes repolarization.
    • All voltage-gated K+ channels eventually close.
    • Both Na+ and K+ channels regain responsiveness, preparing for the next action potential

    Interactive Question (Page 25)

    • Matching questions to diagrams showing different states of sodium (Na+) channels (inactive, active, and resting voltage-gated).

    Excitability Changes During Action Potential

    • Absolute refractory period (ARP)
    • Relative refractory period (RRP)
    • Supranormal period
    • Subnormal period

    1. Absolute Refractory Period (ARP)

    • Nerve excitability is completely lost
    • Corresponds to the depolarization phase and the early part of repolarization (ascending limb and the upper 1/3 of repolarization).

    2. Relative Refractory Period (RRP)

    • Nerve excitability is partially recovered.
    • A stronger stimulus than normal is required to excite the nerve
    • Corresponds to the remaining portion of repolarization (the descending limb).

    3. Supranormal Period

    • Nerve excitability is elevated above normal.
    • A weaker stimulus than normal can excite the nerve.

    4. Subnormal Period

    • Nerve excitability decreases below normal.
    • A stronger stimulus than normal is needed to excite the nerve.
    • Corresponds to the period after hyperpolarization.

    Effect of Plasma Ionic Concentration Changes on Action Potential

    • Hypernatremia: No effect on RMP, facilitates depolarization.
    • Hyponatremia: No effect on RMP, delays depolarization and reduces action potential amplitude.
    • Hyperkalemia: Decreases RMP, initially increases excitability, but then slows repolarization due to Na+ channel closure.
    • Hypokalemia: Decreases excitability.

    Blockage of Voltage-Gated Na+ Channels

    • Tetrodotoxin (TTX): Blocks Na+ channels by closing m-gates (activation gates).
    • Local anesthetics: Block Na+ channels by inactivating h-gates (inactivation gates).

    Blockage of Voltage-Gated K+ Channels

    • Tetraethylammonium
    • Blocks K+ channels, prolonging repolarization.

    Effect of Calcium Concentration on Action Potential

    • Hypocalcemia: Increases excitability by increasing Na+ influx.
    • Hypercalcemia: Decreases excitability and stabilizes the membrane.

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    Description

    Explore the complexities of action potentials in neuroscience with this quiz. Discover key concepts such as membrane potential, depolarization, repolarization, and the roles of various ions. This quiz will test your understanding of neural excitability and the mechanisms involved in generating action potentials.

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