Propagation of the Action Potential Along the Neuron
The development of an action potential in one part of the neuron in turn creates action potentials in adjacent regions of the neurolemma going down the axon toward the terminals. The impulse spreads over the neurolemma like a wave (as mentioned before). One could picture this as a line of dominoes standing on end. If one domino is pushed the entire line of dominoes will fall, one after the next, going in one direction.
Video 1. View the YouTube video "Domino Analogy in a Neuron" (opens in new window)
The velocity of conduction down the neuron depends on the neuron's diameter and whether or not the neuron is myelinated (has a myelin sheath). Large diameter fibers conduct impulses faster than small diameter fibers. Think of a hallway with lots of students getting out of class at the same time. Which hallway width (or axon diameter) would allow faster movement of the crowd? The larger hallway does, as the larger axon width (diameter) allows for more movement of ions.
Cells that are myelinated exhibit very fast conduction due to a process called saltatory conduction. Saltatory conduction describes how the impulse 'jumps' from node of Ranvier to node of Ranvier. Saltatory means dancing or jumping, so I think of salsa dancing when I see this word and it makes me remember what this term refers to on the axon. The Schwann cells that wrap around the axons contain a substance called myelin. The myelin acts as an electrical insulator and therefore, the action potential must 'jump' these myelin sheaths to get from node to node.
Animation 1. View the YouTube animation "Saltatory Conduction" (opens in new window)
This is a much faster means of conduction and is also less energy expensive for the cell, because only the sodium channels that are at the small nodes have to be opened. Just as it is much faster to cross a room by taking giant steps instead of baby steps, the saltatory signal is faster and uses less energy. A signal that would have to open all of the sodium channels all the way down the axon (continuous conduction)would take a lot longer to get to the end. If one were to take baby steps to get across the room, instead of giant steps, it would take a lot longer to get there.
Video 2. View the YouTube video "Conduction Analogy" (opens in new window)
The conduction speed also depends on temperature - warmer body temperature allows for faster movement of action potentials versus a body that is in hypothermia.
Video 3. View the View the YouTube video "Characteristics of Life - Responsiveness" (opens in new window)
