graded potentials vs. action potentials

Overview of Neural Signaling

  • Focus on ion channels and their role in neural communication, particularly between neurons.

Learning Objectives

  • Describe ion channels and their roles in generating electrical signals (graded potentials and action potentials).

  • Compare and contrast graded potentials and action potentials.

  • Reference pages: Saladin, 8th edition, pages 449-452.

Graded or Local Potentials

  • Definition: Change in membrane potential that is localized to a specific region of the plasma membrane.

    • Varies in Size:

      • Small stimulus = small change in membrane potential.

      • Large stimulus = larger change in membrane potential.

  • Initiation: Triggered by opening gated channels:

    • Voltage-gated channels

    • Ligand-gated channels

    • Mechanically gated channels

  • Graphs Representation:

    • Depolarizing Graded Potential:

      • Membrane potential moves from resting (around -70 mV) to a more positive value (less negative).

      • Example of depolarization caused by a stimulus.

    • Hyperpolarizing Graded Potential:

      • Membrane potential moves further negative from resting potential (more polarized).

      • Example of hyperpolarization caused by specific stimuli.

  • Ion Movements Question:

    • What ions and directions produce depolarizing versus hyperpolarizing potentials?

Action Potentials

  • Definition: A significant change in membrane potential that affects the entire plasma membrane.

  • All or None Principle:

    • Action potentials do not vary in size; either reach the threshold and fire or do not fire at all.

  • Location of Generation:

    • Graded potentials typically occur in dendrites and cell bodies.

    • Action potentials are generated along the axon.

  • Action Potential Phases:

    • Depolarization Phase:

      • Sodium channels open, allowing Na+ ions to enter the cell.

    • Repolarization Phase:

      • Sodium channels close, and potassium channels open, allowing K+ ions to exit.

    • After Hyperpolarization:

      • Membrane potential dips below resting level before returning.

  • Ion Channel Dynamics:

    • Sodium channels are voltage-gated and close during repolarization due to inactivation gates, preventing further Na+ entry until reset.

    • Potassium channels remain open, contributing to hyperpolarization.

Refractory Periods

  • Absolute Refractory Period:

    • A neuron cannot respond to another stimulus at all.

  • Relative Refractory Period:

    • A large enough stimulus can generate another action potential.

  • Importance of Refractoriness:

    • Refers to the directionality and conduction of action potentials in neurons.

Questions for Discussion

  • Effects of blocking sodium channels on cell depolarization.

  • Changes in resting membrane potential based on extracellular potassium concentration.