Lecture Notes: Action Potential Threshold and Propagation

Key Concepts

• Action Potential Threshold

  • Not all depolarizations produce action potentials.
  • Depolarization must reach a threshold voltage for an axon to fire.
  • Threshold is the voltage where voltage-gated sodium channels open.
  • Typically requires membrane depolarization by 15-20 mV from rest.

• Depolarization Process

  • Increased sodium permeability and influx might precede potassium efflux.
  • Leads to a depolarization wave that activates voltage-gated sodium channels.

All-or-None Phenomenon

• An action potential either happens completely or not at all.
• Requires enough voltage-gated sodium channels to open.
• Propagates quickly down the axon.

Propagation of Action Potentials

• Propagation

  • Spread of the action potential down the axon.
  • Sodium influx causes local currents that open adjacent sodium channels.
  • Positive feedback effect continues down the axon like a domino effect.

• Non-Myelinated vs. Myelinated Axons

  • Non-myelinated axons: successive segments depolarize and repolarize.
  • Myelinated axons: action potential regenerates at specific nodes.
  • Sodium channels near the origin remain inactivated, ensuring one-way propagation.

Refractory Periods

• Refractory Periods
  • Absolute Refractory Period
    • Period during which another action potential cannot be generated.
    • Voltage-gated sodium channels are open but cannot reset immediately.
  • Relative Refractory Period
    • Occurs as voltage-gated sodium channels begin to reset.
    • Hyperpolarization state makes it more difficult to generate action potentials.

Action Potential Frequency and Stimulus

• Stimulus Intensity
  • Action potentials are independent of stimulus intensity.
  • Differentiated by frequency and number of action potentials.
  • Higher frequency indicates a stronger stimulus.

Factors Affecting Velocity of Action Potentials

• Conduction Velocity

  • Dependent on axon diameter and degree of myelination.
  • Larger diameter axons conduct impulses faster.
  • Myelinated axons use saltatory conduction, which is faster than continuous conduction in non-myelinated axons.

• Saltatory Conduction

  • Action potentials jump from node to node (myelin sheath gaps), speeding up transmission.

Types of Nerve Fibers

• Classification by Fiber Type
  • Group A Fibers: Largest diameter, heavily myelinated, fast transmission (e.g., motor fibers to skeletal muscles).
  • Group B Fibers: Intermediate diameter, lightly myelinated, moderate speed.
  • Group C Fibers: Smallest diameter, unmyelinated, slowest transmission.

These notes provide a concise overview of the key elements of action potential threshold and propagation, highlighting important physiological mechanisms and factors influencing nerve impulse conduction.