The myelin is surrounding axons of neurons.
it takes two forms: compact and axons are said myelinated and the second form is surrounding several axons but is "loose" ands the axons are called unmyelinated in that case.
The main difference is the number of axons that are covered.
The myelin is meant to improve insulation, to increase speed of conduction but may decreases the energy cost of the neuron.
This last statement goes against the science we know: Is it possible to improve speed of the action potential without any increase of the energetic expense?
Michel K Herde I agree with the second part of your reply. The number of Schwann cells is higher for "myelinated" axons since "unmyelinated" axons are packed in Remak bundles.
I may disagree with the cost for the neuron itself. The main idea is that the myelinated/fast fibres use less material (ions channels, ATP, ions, etc) than their counterparts slow/unmyelinated.
But have you noticed that the spike duration remains quite long even if the speed increases?
The saltatory conduction AND this duration implies a huge energetic cost that increases with speed IMHO...
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