Axon diameter and myelination
So far we have looked at action potentials as if they were occurring at only one particular location of the neuron's membrane, in the axon hillock. In reality action potentials start at this location, but then get propagated down the axon to produce neurotransmitter release at the synaptic terminals.
Several factors contribute to the speed of this propagation. Two of the main factors are axon diameter and myelination.
Let's start with axon diameter: action potentials propagate much faster in thicker axons than in thinner ones. As an analogy, think about water going through a pipe. If the pipe is very narrow, then almost all of the water is in contact with the pipe wall, encountering a lot of resistance to its flow. If the pipe is thicker, though, proportionally less water is creating resistance with the pipe walls and therefore the flow is faster.
Myelination divides the axon into insulated segments, making the channels on those portions of the membrane unable to open. When an action potential travels down a myelinated axon, it skips between the gaps left by myelin, called Nodes of Ranvier. By jumping in between these gaps, conduction is a lot faster.
Effect of axon diameter and myelination on conduction velocity
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Fill out this table with the values from the simulation. Hovering over a point in the graph will give you the x and y values.
|Axon diameter (µm)||Conduction velocity (m/sec)|
- Export the spreadsheet with the data, and use it to find the line of best fit for the non-myelinated and myelinated data (note that the non-myelinated data is not really linear, but for the purpose of this comparison the linear approximation is good enough).
- Suggest why, for very small axon diameters, there is little benefit of myelination for conduction velocity.