Notes on Action Potential Propagation

Overview of Action Potential Propagation

• Action potential generation and its propagation along an axon consists of four main phases:

  • Resting Membrane Potential: The stable, negative charge of an axon when not transmitting signals.

  • Depolarization: Occurs when sodium ions rush into the membrane, making the inside more positive.

  • Repolarization: Return to the negative resting potential after a peak in the action potential.

  • Return to Rest: Achieved through the sodium-potassium pump re-establishing the resting state.

Propagation Mechanism

• Propagation of an Action Potential: The mechanism by which an action potential travels along the axon-

  • Initiated at the axon hillock, it progresses toward the axon terminals.

Unmyelinated Axons

• Unmyelinated axons are not insulated with myelin and are often found in gray matter of the brain and spinal cord.

• Characteristics:

  • Short Distance Conduction: Ineffective over long distances due to lack of insulation.

  • Electrical Activation: If the axon hillock is stimulated:

  • It depolarizes, causing sodium influx.

  • This activates the next segment, leading to a domino effect where each segment depolarizes in succession.

  • Once a segment depolarizes, it repolarizes, entering a refractory period where it cannot be activated again immediately.

• Speed Limitation: Because every segment must depolarize, it results in slower transmission (approx. $2 ext{ m/s}$).

Myelinated Axons and Saltatory Conduction

• Myelinated axons, covered with myelin produced by Schwann cells and oligodendrocytes, greatly enhance conduction speed.

• Saltatory Conduction: The action potential appears to jump from node to node along the axon:

  • Nodes of Ranvier: Gaps in myelin where voltage-gated sodium channels are located.

  • Mechanism:

  • When the action potential reaches the first node, it triggers depolarization at this site.

  • This burst of activity travels quickly through the insulated myelin to the next node, where it reactivates the depolarization process.

  • The cycle continues down the axon, with each node re-initiating the action potential.

• Speed Comparison: Saltatory conduction in myelinated axons is significantly faster at about $30 ext{ m/s}$ compared to only $2 ext{ m/s}$ in unmyelinated axons.

Summary of Key Differences

• Speed:

  • Myelinated Axons: Approx. $30 ext{ m/s}$ (efficient for long distances).

  • Unmyelinated Axons: Approx. $2 ext{ m/s}$ (inefficient for long distances).

• Type of Conduction:

  • Myelinated = Saltatory (jumping from node to node).

  • Unmyelinated = Continuous conduction along the membrane of each segment.

• What is an Action Potential?: It's like a small electrical signal that travels along a nerve cell (axon).

• Four Main Parts:

  • Resting Membrane Potential: This is how a nerve stays calm when it's not sending messages. The inside of the nerve is negative (kind of like a battery) when it’s just hanging out.

  • Depolarization: This happens when tiny particles called sodium ions rush into the nerve, making the inside more positive.

  • Repolarization: After being positive, the nerve returns back to being negative again.

  • Return to Rest: A special pump helps bring everything back to normal after the signal goes through.

• How it Travels:

  • The signal starts at the beginning of the nerve and moves all the way to the end.

• Unmyelinated Axons: These are nerve cells that aren’t covered in insulation. They are slower because they need to activate each part one by one.

  • Speed: Moves slowly at about 2 meters per second, like a slow walk.

• Myelinated Axons: These nerve cells are covered in insulation, which makes them super fast.

  • Saltatory Conduction: The signal jumps from one insulated spot (node) to another, like hopping along.

  • Speed: Moves much faster at about 30 meters per second, like a fast run.

• Key Differences:

  • Myelinated nerves are fast and jumpy, while unmyelinated nerves are slow and need to move step by step.