Lecture 7: Myelin

Question 1

Why does white matter in the brain appear white?

Answer 1

because it is composed of myelinated axons which are lipid rich

Question 2

What are the myelinating cells of the peripheral nervous system?

Answer 2

Schwann cells

myelinate in a 1:1 ratio

Question 3

What are the myelinating cells of the central nervous system?

Answer 3

oligodendrocytes

myelinate in a 50:1 ratio

Question 4

What are the two different types of Schwann cell?

Answer 4

myelinating and non-myelinating

Question 5

What is the role of non-myelinating Schwann cells?

Answer 5

bundle together unmyelinated axons

Question 6

Schwann cells myelinate multiple axons within the central nervous system and all nerve fibres are myelinated by Schwann cells.
TRUE/FALSE

Answer 6

false

Question 7

What is the oligodendroglial cell lineage?

Answer 7

oligodendrocyte precursor cells (OPC) -> immature oligodendrocyte -> myelinating oligodendrocyte

Question 8

Is myelin single-layered or multi-layered?

Answer 8

myelin is a multi-layered, compacted membranous sheath

Question 9

How is myelin organised?

Answer 9

into internodes

Question 10

What are the regions of unmyelinated axon called? What are these regions characterised by?

Answer 10

nodes of Ranvier
here there is clustering of voltage gated sodium channels
there are very few voltage gated sodium channels on the myelinated part of the axon

Question 11

What does myelination induce in the axon?

Answer 11

induces architectural rearrangement

Question 12

What are the three distinct molecular domains myelin divides the axon into?

Answer 12

the node of Ranvier, the paranode and the juxtaparanode

Question 13

What are characteristics of the node of Ranvier?

Answer 13

short, unmyelinated segments of axon which contains clusters of voltage-gated sodium channels

Question 14

What is the role of ankyrin proteins in the node of Ranvier?

Answer 14

anchors contents of the node to the underlying actin-spectrin cytoskeleton

Question 15

What are characteristics of the paranodes?

Answer 15

flank the nodes and acts as a junction between noncompacted paranodal myelin loops and the underlying axolemma

Question 16

What is the role of NF155-Caspr-Contactin complexes in the paranodes?

Answer 16

anchor paranodes to the underlying actin-spectrin cytoskeleton

Question 17

What are characteristics of the juxtaparanodes?

Answer 17

between paranodes
contain voltage-gated potassium channels
the periaxonal space

Question 18

What is found in the node of Ranvier, the paranode and the juxtaparanode respectively?

Answer 18

K+ channels, Caspr and βIV spectrin

Question 19

What are different ways of increasing conduction speed?

Answer 19

myelination or increasing axon diameter

Question 20

What is the advantage of having larger axons?

Answer 20

less resistance to current flow

Question 21

What is the advantage of having myelinated axons?

Answer 21

pushes voltage gated sodium channels into clusters which results in the clustering of sodium ions as they are attracted to the exposed negatively charged membrane of the axon
this leads to increased capacitance at the node of Ranvier

Question 22

What does increased capacitance of the nodes of Ranvier lead to?

Answer 22

very rapid activation of multiple voltage gated sodium channels
action potential jumps from node to node which results in very fast conduction

Question 23

What is capacitance at the node of Ranvier and what is capacitance at the internode?

Answer 23

capacitance is higher at the node of Ranvier and lower at the internode

Question 24

How does lower capacitance at the internode affect propagation of the action potential?

Answer 24

lower capacitance at the internode means that a lower change in ion concentration is required to propagate the action potential -> results in greater speed and metabolic efficiency