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. (D)</p> Signup and view all the answers

What defines the magnitude of an action potential?

<p>The difference between maximum and minimum membrane potential. (C)</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. (D)</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. (B)</p> Signup and view all the answers

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

<p>It overshoots the resting level. (C)</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 (A)</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+) (C)</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 (B)</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. (A)</p> Signup and view all the answers

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

<p>93% (C)</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 (D)</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. (D)</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 (C)</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 (D)</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. (D)</p> Signup and view all the answers

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

<p>Absolute refractory period (A)</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. (D)</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 (A)</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. (C)</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 (D)</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. (B)</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. (D)</p> Signup and view all the answers

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

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

How does hypernatremia affect the process of depolarization?

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

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

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

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

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

What happens to nerve excitability during the after hyperpolarization phase?

<p>Nerve excitability decreases below normal. (A)</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. (C)</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. (A)</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. (B)</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. (D)</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. (C)</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. (B)</p> Signup and view all the answers

Which phase of action potential directly follows the latent period?

<p>Depolarization phase (B)</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. (B)</p> Signup and view all the answers

Flashcards

Resting Membrane Potential (RMP)

The difference in electrical potential between the inside and outside of a nerve or muscle cell membrane when it's not actively transmitting signals.

Selective Permeability

A passive force based on the different permeability of the cell membrane to potassium (K+) and sodium (Na+) ions, at rest.

Sodium-Potassium Pump (Na+-K+ Pump)

An active force that pumps three sodium ions (Na+) out of the cell and two potassium ions (K+) into the cell, maintaining the concentration gradient.

Excitability

The ability of excitable tissues, like nerves and muscles, to generate and transmit electrical signals in response to stimuli.

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Action Potential

A rapid, short-lasting change in membrane potential, involving a rapid depolarization followed by repolarization and often a brief hyperpolarization.

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Depolarization

The initial phase of an action potential, where the membrane potential becomes less negative (more positive).

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Repolarization

The return of the membrane potential to its resting negative value after depolarization.

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Hyperpolarization

A brief period after repolarization where the membrane potential becomes even more negative than the resting potential.

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Firing Level

The point at which the membrane potential reaches -55mV, triggering a rapid depolarization phase in an action potential.

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Sodium Influx

Voltage-gated sodium channels open, allowing a massive influx of sodium ions into the cell, causing a steep rise in membrane potential.

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Afterhyperpolarization

A brief period following repolarization where the membrane potential becomes even more negative than the resting potential.

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Depolarization Exceeding 15mV

The change in membrane potential that causes depolarization to exceed 15 mV, reaching the firing level and triggering the opening of voltage-gated sodium channels.

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Positive Feedback Mechanism

The process where the sodium influx increases dramatically during depolarization, causing a rapid reversal of polarity.

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Magnitude of Action Potential

The magnitude of the action potential, which is the difference between the peak of depolarization and the resting membrane potential, typically around 105 mV.

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Passive Cause of Resting Membrane Potential

The membrane's inability to allow negatively charged ions like proteins, organic phosphates, and sulfates to easily cross from inside the cell to the outside.

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Na+ - K+ Pump

An active transport mechanism that pumps sodium ions out of the cell and potassium ions into the cell. It's electrogenic because it transfers more positive charges to the outside, contributing to the resting membrane potential.

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Polarized State

The resting state of the cell membrane, where the inside is negatively charged relative to the outside.

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Hyperpolarized State

A state where the membrane potential becomes more negative than the resting potential.

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Depolarized State

A state where the membrane potential becomes less negative than the resting potential, moving closer to zero.

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Latent Period

The period between the application of a stimulus and the start of depolarization during an action potential.

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Depolarization Phase

The rapid rise in membrane potential during an action potential, caused by the influx of sodium ions.

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Absolute Refractory Period (ARP)

During this phase, a nerve cell cannot generate another action potential, regardless of the strength of the stimulus. This is due to the inactivation of sodium channels, preventing further sodium influx.

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Relative Refractory Period (RRP)

The nerve is partially excitable. A stronger-than-usual stimulus is needed to trigger a new action potential because the membrane potential is still recovering from the previous action potential.

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Supranormal Period

A brief period during which the nerve cell is more excitable than at rest, making it easier to generate an action potential.

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Subnormal Period

A short period when the nerve cell's excitability is lower than usual, making it more difficult to generate an action potential.

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Ionic Basis of Repolarization

The sodium channels are inactivated, preventing further sodium influx; potassium channels open, allowing potassium efflux. This actively restores the negative membrane potential after depolarization.

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Inactive Na+ Channels

Sodium channels are closed and not responsive to a stimulus. They are ready to open again when the membrane potential returns to its resting level.

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Hypernatremia Effect on Action Potential

High sodium levels in the blood facilitate depolarization of the nerve cell membrane.

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Hyponatremia Effect on Action Potential

Low sodium levels in the blood delay depolarization and reduce the amplitude of the action potential.

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Blockage of Voltage-Gated Na+ Channels

Blocking sodium channels prevents the generation of an action potential, as sodium ions are crucial for depolarization.

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Tetrodotoxin (TTX)

A toxin that blocks sodium channels by preventing their activation, thus stopping action potential generation.

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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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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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