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As far as I am aware, most of the central nervous system consists of myelinated axons and most of the peripheral consists of unmyelinated.

What is the reason for this?

Would it not be more efficient to have them all myelinated?

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In this answer I'm building off of @Fizz's pointer to the nice paper by Laurence Knowles, "The Evolution of Myelin: Theories and Application to Human Disease" [1].

First -- I just want to point out that your question is adaptationist, which means that it assumes that myelin is on some axons and not on others for a reason. Whereas maybe evolution just hasn't gotten around to it yet, even though myelinating all axons would actually be beneficial.

That said, here are two of the main downsides of myelination:

  1. It might cost energy. This one is a bit controversial, but at least one study has found [2] that while myelin decreases the energetic cost of firing an individual action potential, the whole apparatus involved in myelin (e.g., in the CNS, the oligodendrocyte, its processes, and the sheath itself) actually costs more energy than it would cost to just open all of those ion channels without saltatory conduction.

  2. Myelin and its associated oligodendrocytes/Schwann cells take up space. This is likely to be especially important in areas of the nervous system where there are a lot of neurons packed into a small area.

So from the adaptationist perspective, the question is whether the benefits of myelinating an axon, such as increased conduction velocity, increased conduction fidelity, axon protection, and supplying the axon with nutrients, are worth the downsides of taking up space and possibly increasing net energy use.

  1. http://www.ashdin.com/journals/JEM/235996
  2. http://www.jneurosci.org/content/32/1/356
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  • $\begingroup$ Not to mention, when neurons are tightly packed, there is little need for increasing the transmission speed. It's not like an alpha motor neuron where you need to send a signal a long distance. $\endgroup$ – forest Mar 22 '18 at 23:15
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Would it not be more efficient to have them all myelinated?

If the transmission of weak signals would be highly efficient, this could be painful - and distracting (e.g.: also see https://en.wikipedia.org/wiki/Allodynia). However, it is nice to be able to sense weak signals that persist ("itching").

Schmelz et al. 1997, J. Neurosc. show that inefficient unmetylated neurons transmit the signal that we perceive as "itching", if the weak signal persists.

Note that - as indicated in above publication - neurons that sense an immediate environmental threat to which we should respond immediately (e.g.: if an object that we grab is extremely hot or extremely cold) are more efficient.Their activity will generate a strong sensation of immediate "pain" - and consequently call us to take an immediate behavioral response and to avoid this threat.

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Myelinated neurons are neurons whose axons are surrounded by myelin; the myelin has has an insulating effect and allows the axons to conduct neural impulses faster - but at some metabolic cost, so neurons are not myelinated unless there is a significant advantage to they're being able to conduct faster.

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