Action Potentials on an axon test 3 lecture

Oct 13, 2024

Lecture on Action Potentials in Neurons

Introduction

  • Action potentials are propagated down the axon of a neuron.
  • They start with the summation of graded potentials.
    • Must reach a threshold of -55 mV.
    • Occur at the axon hillock (trigger zone).

Structure of a Neuron

  • Axon: inner yellow portion.
  • Myelin Sheath: surrounds the axon, made of Schwann cells (Peripheral Nervous System).
  • Nodes of Ranvier: gaps without myelin.
  • Axon Terminals: branches at the end of an axon.
  • Synaptic End Bulbs: swellings at the terminal ends.

Voltage Gated Channels

  • Open/close based on changes in membrane potential.
  • Types of Channels:
    • Potassium Voltage Gates: allow potassium (K+) to exit the cell.
    • Sodium Voltage Gates: more complex, require two gates to open for Na+ to enter.
    • Calcium Voltage Gates: located on synaptic end bulbs.

Action Potentials

  • Characteristics:
    • All or nothing phenomenon.
    • Travel the entire length of the axon in one direction: from hillock to synaptic end bulbs.
    • Controlled by positive feedback and voltage gates.
  • Phases:
    • Depolarization: membrane potential becomes more positive.
    • Repolarization: returns to resting potential.
    • Hyperpolarization: membrane potential becomes more negative than resting.

Graded Potentials vs. Action Potentials

  • Graded Potentials (GPs):
    • Decremental (die out).
    • Occur in somas and dendrites.
    • Controlled by ligand-gated channels.
  • Action Potentials (APs):
    • Non-decremental (travel entire length).
    • Occur on axons.
    • Controlled by voltage-gated channels.

Detailed Process of Action Potentials

  • Start at resting potential (-70 mV).
  • Summation of GPs to reach threshold at the hillock (-55 mV).
  • Depolarization Phase:
    • Sodium gates open, Na+ rushes in, potential goes to +30 mV.
  • Repolarization Phase:
    • Sodium gates close, potassium gates open, K+ exits, potential returns to -70 mV.
  • Hyperpolarization Phase:
    • Potassium gates slow to close, potential drops below -70 mV.

Sequential Events in Axon Segments

  • Each axon segment undergoes depolarization, repolarization, and potentially hyperpolarization.
  • Repeats until reaching synaptic end bulbs.

Conclusion

  • Understanding membrane potentials and voltage gates is essential.
  • Recommended resource: WileyPlus interaction on membrane potentials.