It is a conundrum. Why should meylin be produced by oligodendrocytes or schwwan calls rather than neurons themselves. It seems that Myelin is indispensible to neuronal function and should be the part of neurons (produced by neurons themselves) rather than an manufacture of other cell moieties?
Any comments out there?
Just to expand on how useful it is to have specialization in cells:
Some neurons that reach from the spinal cord to the feet have axons about a meter long. And those axons have to be maintained: you need pumps to keep the ion concentrations just right, sodium channels to carry the action potential all the way down, neurotransmitters constantly being packaged into vesicles at the very end of the axon, etc. All those proteins, all transcribed from DNA in a nucleus a meter away. All that ATP to power them. All the intracellular transport that gets them where they need to go. It's a very demanding job.
Meanwhile, myelin is made of tons and tons of cell membrane (complete with membrane proteins!) wrapped around the axon. Something on the order of 1 square millimeter of membrane from each myelinating cell. A single cell, making all those membrane lipids and all those membrane proteins, and replacing them when they get old or damaged. And that meter-long axon has something like 10,000 cells each making just one little section of myelin. So if you unwrapped all the myelin from that one neuron and laid it out flat, it would be around 100 square centimeters.
Imagine if the poor neuron had to make enough membrane to cover like half a sheet of paper all by itself! There's no way a single cell could do that, especially on top of all the other things a neuron already has to do. Making myelin would put a huge burden on the neuron.
On the other hand, if 10,000 Schwann cells (or oligodendrocytes in the CNS) make the myelin, each one has a manageable amount of membrane to make. And the neuron doesn't have to do any work--in fact, its work gets easier because the myelin makes transmission more efficient. It can get by with a smaller axon, fewer sodium channels, etc. Plus the myelinating cells secrete growth factors and so on that help the neuron stay healthy.
The goal of myelin is to make it easier for the neurons to do their jobs, and that's only possible if the neurons don't have to make the myelin themselves.
Just to expand on how useful it is to have specialization in cells:
Some neurons that reach from the spinal cord to the feet have axons about a meter long. And those axons have to be maintained: you need pumps to keep the ion concentrations just right, sodium channels to carry the action potential all the way down, neurotransmitters constantly being packaged into vesicles at the very end of the axon, etc. All those proteins, all transcribed from DNA in a nucleus a meter away. All that ATP to power them. All the intracellular transport that gets them where they need to go. It's a very demanding job.
Meanwhile, myelin is made of tons and tons of cell membrane (complete with membrane proteins!) wrapped around the axon. Something on the order of 1 square millimeter of membrane from each myelinating cell. A single cell, making all those membrane lipids and all those membrane proteins, and replacing them when they get old or damaged. And that meter-long axon has something like 10,000 cells each making just one little section of myelin. So if you unwrapped all the myelin from that one neuron and laid it out flat, it would be around 100 square centimeters.
Imagine if the poor neuron had to make enough membrane to cover like half a sheet of paper all by itself! There's no way a single cell could do that, especially on top of all the other things a neuron already has to do. Making myelin would put a huge burden on the neuron.
On the other hand, if 10,000 Schwann cells (or oligodendrocytes in the CNS) make the myelin, each one has a manageable amount of membrane to make. And the neuron doesn't have to do any work--in fact, its work gets easier because the myelin makes transmission more efficient. It can get by with a smaller axon, fewer sodium channels, etc. Plus the myelinating cells secrete growth factors and so on that help the neuron stay healthy.
The goal of myelin is to make it easier for the neurons to do their jobs, and that's only possible if the neurons don't have to make the myelin themselves.
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