Lecture Notes: Action Potentials and Propagation

Overview

• Welcome to Week 3 Lecture
• Importance of attendance in on-campus lectures
    - In-person interactions with instructors and peers
    - Essential for group project formation

Key Concepts from Previous Lectures

• Subject focus: Action Potentials
    - Key definitions and characteristics
    - Membrane potentials
    - Properties of excitable tissues, primarily nerve cells

Action Potentials

• Definition: Brief reversal of membrane potential
    - Transition from negative to positive inside the cell due to ion movement
    - Occurs in response to specific stimuli
    - Key measurements include resting membrane potential at approximately -70 mV
• Waveshape of action potentials represented graphically
    - Y-axis: Membrane potential (mV)
    - X-axis: Time (milliseconds)
    - Action potential occurs over 5 ms

Importance of Threshold Potential

• Threshold potential: Minimum membrane potential required to trigger an action potential (+/- -55 mV)
    - If not reached, graded potentials occur, which do not lead to action potentials (termed failed initiations)

Phases of Action Potential

1. Depolarization Phase
      - Triggered by influx of Na+ ions into neuron
      - Voltage-gated sodium channels open, allowing sodium ions to flow based on concentration gradient (150 mM outside, 15 mM inside)
      - Rapid depolarization leads to peak potentials of +30 to +50 mV

2. Repolarization Phase
      - K+ ions flow out of the neuron via voltage-gated potassium channels
      - This causes the membrane potential to decrease, moving back towards resting potential

3. Hyperpolarization Phase
      - Membrane potential temporarily dips below resting potential due to prolonged opening of K+ channels
      - Gradually returns to resting state (-70 mV)

Propagation of Action Potential

• Mechanism of propagation involves self-sustaining cycles of action potentials along axon
• Each action potential activates adjacent areas of the membrane
    - Unidirectional travel: Follows sequence from axon hillock to terminal
    - Refractory periods prevent backward propagation

Refractory Periods Explained

1. Absolute Refractory Period
      - No action potential can be generated regardless of stimulus strength
      - All sodium channels remain inactive during this phase

2. Relative Refractory Period
      - A stronger than usual stimulus can initiate another action potential
      - Sodium channels return to closed state and are capable of opening, but the membrane is hyperpolarized below resting potential

Visual Representations

• Importance of diagrams in understanding action potentials and required modifications in permeability (i.e., sodium and potassium)
• Different y-axes to represent distinct aspects of action potential and permeability
    - Red for action potentials
    - Green and blue for sodium and potassium permeability respectively

Analogy and Contextual Understanding

• Analogy: Mexican Wave
    - Represents action potentials propagating along neurons
    - Similar to how waves ripple through a crowd
• Emphasis on understanding action potentials in terms of complete cycle events (not isolated)

Hypothetical Scenarios for Experimental Understanding

1. Stimulus applied in middle of axon will propagate action potential in both directions
2. Simultaneous stimuli at both ends will produce action potentials that meet in the middle but will not propagate past each other due to refractory periods

True/False Reflections

• Action potentials are all-or-nothing events: True
• Action potentials produced only when the change in membrane potential reaches threshold: True
• Action potentials can vary in size: False (they are consistent)
• Action potentials decrease in size away from point of initiation: False (they maintain size)

Conclusion

• Importance of active engagement and asking questions during study
• Encouragement for further discussion in subsequent lectures and group studies