Ch 11 Action Potential Propagation and Conduction Velocity

Action Potential Propagation

• After generating an action potential at the axon hillock, it must propagate to the axon terminal where neurotransmitter release occurs.
• Propagation: Similar to the definition of propaganda, it means to spread. The action potential needs to travel the full length of the neuron.
• Upon reaching the terminal, the influx of calcium ions occurs, triggering neurotransmitter release.

Mechanism of Spread

• Depolarized membranes initiate a chain reaction, causing adjacent regions of the axon to depolarize, similar to a wave at a sporting event.
• This is an all-or-none event, ensuring that the action potential reaches the end of the axon without dissipating.

Conduction Velocity

• Refers to how quickly the action potential travels down the axon.
• Influenced by:
• Axon Diameter: Larger diameters increase conduction velocity (larger axons = faster transmission).
• Myelination:
  • Myelinated Fibers: Facilitate saltatory conduction (action potential jumps between nodes), making transmission much faster.
  • Unmyelinated Fibers: Conduct in a continuous wave-like manner, resulting in slower transmission.

Nodes of Ranvier

• Gaps between myelinating Schwann cells or oligodendrocytes.
• Important for saltatory conduction, as ion channels are located here. The action potential jumps from node to node, speeding up the propagation along the axon.

Clinical Application: Multiple Sclerosis

• An autoimmune demyelinating disease that attacks Schwann cells and oligodendrocytes, impairing saltatory conduction.
• This leads to slow transmission of action potentials and can affect motor functions significantly, hindering communication between the brain and the body.