Action Potential Lecture

May 23, 2024

Action Potential Lecture

Introduction

  • Resting potential: How neurons stay inactive (~-65 mV)
  • Action potential: How neurons transmit messages
  • Goal: Understand ion channels, sodium channels, EPSPs and IPSPs

Key Terms and Concepts

Membrane Potentials

  • Depolarization: Membrane potential becomes more positive (reduction in membrane potential)
  • Threshold Potential: Point at which action potential occurs (around -40 to -50 mV)

Ion Channels

  • Voltage-gated sodium channels: Open when membrane potential increases; sodium rushes in, depolarizes membrane
  • Voltage-gated potassium channels: Open in response to depolarization; potassium rushes out, repolarizes/hyperpolarizes membrane

Channel Mechanisms

  • Sodium Channels:
    • M gate: Activation gate, opens fast around -40 mV
    • H gate: Inactivation gate, closes channel slowly like a timer
  • Potassium Channels:
    • N gate: Delayed rectifier, closes channel slowly after it opens

Action Potential Phases

  1. Resting State: ~-65 mV, closed sodium, and potassium channels (except leak channels)
  2. Threshold Reached: Membrane potential rises to -40 mV
  3. Depolarization (Rising Phase): Voltage-gated sodium channels open, sodium rushes in
  4. Overshoot: Membrane potential peaks (~+40 mV), sodium channels begin to close
  5. Repolarization (Falling Phase): Voltage-gated potassium channels open, potassium rushes out
  6. Hyperpolarization (Undershoot): Membrane potential drops below resting potential (~-80 mV)
  7. Return to Rest: Leak potassium channels and sodium-potassium pumps restore resting potential

Refractory Periods

  • Absolute Refractory Period: No new action potential possible (during overshoot and early repolarization)
  • Relative Refractory Period: Requires stronger stimulus for new action potential (during late repolarization and hyperpolarization)

EPSPs and IPSPs

  • Excitatory Postsynaptic Potential (EPSP): Small depolarization, moves membrane potential closer to threshold (often due to sodium influx via ligand-gated channels, e.g., glutamate)
  • Inhibitory Postsynaptic Potential (IPSP): Small hyperpolarization, moves membrane potential away from threshold (often due to chloride influx)

Action Potential Propagation

  • Saltatory Conduction: Rapid jumping of action potentials between nodes of Ranvier, facilitated by myelination (oligodendrocytes in CNS, Schwann cells in PNS)
  • Nodes of Ranvier: Gaps in myelin sheath with high concentrations of voltage-gated sodium channels
  • Myelin Sheath: Insulating layer around axons, increases conduction speed and efficiency

Comparing Nervous Systems

  • Centralized Nervous System (Mammals): Myelinated axons, saltatory conduction, high-speed signal transmission
  • Distributed Nervous System (Invertebrates like squid and jellyfish): Unmyelinated axons, larger axon diameters, slower signal transmission

Toxins Affecting Ion Channels

  • TTX (Pufferfish): Blocks voltage-gated sodium channels, causing paralysis
  • Lidocaine/Novocaine: Blocks pain signals by inhibiting sodium channels
  • Scorpion Toxin: Affects potassium channels and sodium channels, leading to disruption in neuronal function
  • Other Toxins: Betrachotoxin, Veratridine, Saxitoxin, each affecting ion channels in different ways

Application of Knowledge

  • Be able to explain the action potential phases and channel activities
  • Identify conditions for absolute and relative refractory periods
  • Discuss the effects of toxins and local anesthetics on ion channels
  • Understand differences between centralized and distributed nervous systems

Study Tips

  • Review the phases of the action potential and the roles of sodium and potassium channels in each phase
  • Watch the summary video on Canvas for a visual representation of these processes
  • Practice explaining the concepts to ensure a deep understanding