Lecture 7: Myelin

Question 1

What is the difference between dysmyelination and demyelination?

Answer 1

Dysmyelination is the failure to form normal myelin in the first place. Demyelination is when an axon looses its myelin.

Question 2

What makes up the myelin sheath in the CNS?

Answer 2

Oligodendrocytes.

Question 3

Is myelin known for being protein-rich or lipid-rich?

Answer 3

Lipid-rich.

Question 4

What is it called when the electrical current jumps from node to node in an axon?

Answer 4

Saltatory conduction.

Question 5

In which structure is the white matter located centrally and the grey matter located peripherally? And in which is it the other way around?

Answer 5

In the brain. It is the other way around in the spinal cord.

Question 6

Is the myelin sheath wrapped tightly or loosely around an axon?

Answer 6

Tightly.

Question 7

———— ————
•••••••••••••••••••••••
———— ————
A B C D C B A

Where the dots are the axon and the lines are the myelin sheath, and assuming that the diagram continues laterally, which regions do A, B, C and D represent?

Answer 7

A = internode
B = juxtaparanode 
C = paranode
D = node of ranvier

Question 8

Which protein, whose function can not be compensated for by another protein, is required for compaction of CNS myelin?

Answer 8

Myelin basic protein.

Question 9

Are myelinated internodes longer in the CNS or PNS?

Answer 9

PNS.

Question 10

Unmyelinated axons can conduct signals at up to what speed?

Answer 10

~10m/s

Question 11

Myelinated axons can conduct signals at up to what speed?

Answer 11

~150m/s

Question 12

How does the myelin sheath shape the axon during development?

Answer 12

The myelin sheaths push the Na channels towards the node of Ranvier during formation as they elongate longitudinally.

Question 13

Why is the myelinated section of an axon thicker than at the nodes of ranvier? (No, it’s not including the actual layer of myelin).

Answer 13

Myelin influences phosphorylation of neurofilaments within the axon directly beneath it, causing the neurofilaments to extend arm-like structures, in turn spreading the distance between them and making the overall diameter thicker.

Question 14

The thickness of the myelin sheath correlates with what?

Answer 14

The axonal diameter.

Question 15

What is G-ratio? And what is it in humans?

Answer 15

The ratio between axon diameter and myelin sheath diameter. G-ratio = axon diameter / fibre diameter.
In human it is approximately 0.6

Question 16

Which contains proportionately more lipid than protein, the whole brain or myelin?

Answer 16

Myelin! 70% lipid and 30% protein.

Question 17

What are the four distinct phases of the life of an oligodendrocyte?

Answer 17

1) Birth, migration and proliferation of oligodendrocyte precursor cells (OPCs)
2) Morphological differentiation where the oligodendrocyte establishes an expansive network of processes
3) Axonal contact, leading to ensheathment and generation of compact myelin around target axons
4) Long-term trophic and metabolic support of the encased axon

Question 18

If oligodendrocytes don’t manage to make contact with an axon because there is no space, what happens to it?

Answer 18

It is phagocytosed by microglia. This is common because oligodendrocytes are born in excess, with almost twice the necessary amount being born.

Question 19

Do behavioural experiences influence oligodendrocyte plasticity?

Answer 19

Yes!

Question 20

Do leukodystrophies involve demyelination or dysmyelination?

Answer 20

Dysmyelination. Genetic causation, mainly effecting the CNS white matter.

Question 21

Does multiple sclerosis involve demyelination or dysmyelination?

Answer 21

Demyelination. Involving the CNS.

Question 22

Are Charcot-Marie-tooth and Guillan-Barré syndrome disorders of CNS myelin or PNS myelin?

Answer 22

PNS myelin.

Question 23

Which protein replaced PO in CNS myelin through evolution?

Answer 23

Proteolipid protein (PLP).