Lecture Notes: Absolute and Relative Refractory Periods

Introduction

• Lecturer: Leslie Samuel
• Series: Interactive Biology TV
• Episode: 12
• Topic: Absolute and Relative Refractory Periods

Voltage-Gated Sodium Channels

• Voltage-gated sodium channels have three states:
  • Closed
  • Open
  • Inactive
• Structure:
  • Activation gate
  • Inactivation gate
• Functioning:
  • Stimulus causes the activation gate to open
  • After 0.5 to 1 millisecond, the inactivation gate automatically closes

Action Potential

• Triggered when membrane potential goes above the threshold
• Voltage-gated sodium channels open, allowing sodium ions (Na+) to rush in
• Membrane potential becomes more positive

Absolute Refractory Period

• Definition: Period during which no new action potential can be initiated, regardless of the stimulus
• Occurs because voltage-gated sodium channels are either open or inactive
• Key Points:
  • The absolute refractory period (ARP) prevents another action potential while one is already underway
  • No further stimulation can reopen the already open or inactive sodium channels

Relative Refractory Period

• Definition: Period following the absolute refractory period during which a new action potential can be initiated, but requires a stronger stimulus
• Process:
  • During repolarization, potassium ions (K+) rush out, causing the membrane potential to decrease
  • Voltage-gated sodium channels begin to reset to a closed state
  • Initially, only a few sodium channels are reset, necessitating a stronger stimulus for another action potential
  • As more channels reset, the required stimulus strength decreases
• Ends when all sodium channels are reset to the closed state

Summary

• Absolute Refractory Period: Sodium channels are open or inactive; no new action potential possible

• Relative Refractory Period: Sodium channels start resetting; new action potential possible with stronger stimulus

• Closing Remarks: The distinction between the absolute and relative refractory periods is crucial for understanding neuronal signaling dynamics.

End of Lecture