Lecture Notes: Graded Potentials and Action Potentials

Introduction

• Graded potentials can lead to action potentials, but not all stimuli or depolarizing events produce action potentials.
• Action potentials are initiated by voltage-gated channels.

Graded Potentials

• Decrease in Strength: They weaken as they move away from the stimulus source.
• Analogy: Similar to firing a gun, where a trigger must be pulled with enough strength (threshold) to fire. Pulling harder doesn't increase the speed.

Threshold

• Definition: The membrane potential at which a voltage-gated channel changes shape.
• Only depolarizing graded potentials move closer to threshold; hyperpolarization is inhibitory.

Neuron Areas of Focus

1. Dendrites: Location of chemically-gated channels responsible for graded potentials.
2. Axon Hillock (Trigger Zone): First encounter of voltage-gated channels, acts as an integrating center.

Types of Graded Potentials

1. Subthreshold Graded Potentials

• Characteristics:
  • Resting membrane potential is at -70 mV.
  • Local membrane potential might reach -40 mV but loses strength.
  • By the trigger zone, potential might be -56 mV (below the threshold of -55 mV).
  • Voltage-gated sodium channels remain closed.

2. Suprathreshold Graded Potentials

• Characteristics:
  • Similar loss of strength occurs.
  • At the trigger zone, potential is above -55 mV.
  • Voltage-gated sodium channels open, converting graded potential to action potential.

Voltage-gated Sodium Channel Conformations

• Three Conformations:
  1. Closed: Activation gate closed, inactivation gate open. No sodium permeability, maintains resting potential, and assesses graded potential strength.
  2. Open: Both gates open, allowing sodium influx and rapid depolarization.
  3. Closed (Inactive): Activation gate open, inactivation gate closed. Occurs at +30 mV, sodium permeability drops.

Voltage-gated Potassium Channels

• Threshold: Opens at +30 mV, closes at -70 mV.
• Function: Allows potassium efflux for repolarization.
• Conformations: Open or closed.

Refractory Period

• Definition: Time for sodium channel to revert from third to first conformation.
• Significance: Allows action potentials to propagate in one direction.

• Further discussion on the refractory period will be covered in a later video.