Lecture Notes on Action Potentials

Introduction to Action Potentials

• Resting Potential Recap: Inside of neuron more negative compared to outside.
• Action Potentials: Messages sent by axons.

Graded Potentials

• Definition: Brief, small changes in membrane polarization due to ion concentration changes.
• Types:
  • Hyperpolarization: Inside becomes more negative (e.g., -70mV to -90mV).
  • Depolarization: Inside becomes less negative (e.g., -70mV to -65mV).
• Causes:
  • Chemical messages at synapses.
  • Laboratory: Electrical currents applied to axons.
• Ion Involvement:
  • Potassium exits cell → Hyperpolarization.
  • Sodium enters cell → Depolarization.
• Duration: Short-lived effects.

Action Potentials

• Trigger: Strong enough stimulus causing depolarization beyond the threshold of excitation.
• Threshold: Typically -40 to -50mV for neurons.
• Process:
  • Sodium Channels Open: Sodium enters, causing depolarization.
  • Peak of Excitation: Inside becomes positive (~+30 to +40mV).
  • Repolarization: Potassium exits cell returning membrane polarization to resting state.
  • Hyperpolarization: Due to slow closing of potassium channels.

Refractory Periods

• Absolute Refractory Period: Neuron cannot initiate another action potential.
• Relative Refractory Period: Neuron can initiate action potential if stimulus is strong enough.

All-or-None Law

• Action potentials are uniform in size and speed.
• Action Potential Consistency:
  • At any given moment, either fully initiated or not.

Transmission Speed

• Axon Diameter Influence: Larger diameter → Faster transmission.

Summary

• All-or-None Rule: Action potentials occur with consistent size and speed once threshold is reached.
• Toilet Analogy: Useful for understanding threshold and refractory periods.